Abst-1

Identification OF THE Limestone Reservoirs in Mumbai High (North) Oil field AS Naturally Fractured ONES and analysis of field performance UPto 2000-01

Kottilil Narayanan

Formerly Chairman, DGH Council 

The field went on production in 1976 and a peripheral water flood was initiated in 1984.  From 1989 a staggered line drive flood was initiated within the oil pool.  Production performance of the L3 reservoir of Mumbai High North oilfield had indicated that extensive fractures of reservoir scale together with equally extensive unsealed faults with minor throws had negatively affected the effectiveness of the water flood programme. 

The author, when examining the 3D data of the field in ONGC’s Institute of Reservoir Studies in the year 2000, identified the first direct evidences of such faults and fractures on horizon and time slices of the data. 

The Mumbai structural uplift is characterized by a major gravity low.  This is not surprising as the structural high lies on the western side of the aborted early Tertiary rift to the east – oceanic crust is present only to the east of the Mumbai structural high - within the Cambay rift itself.  Several wells on the Mumbai High have met, possibly Archaean, crystalline basement without crossing Deccan basalt flows.  The COB to west is located perhaps only at the Kori/Laxmi High.  The eastern margin fault limiting the oil pool is, on seismic evidence of flower structures, reversal of tilt along the long axis of the structural high, etc., to be a right lateral transcurrent fault. 

The reservoir scale fractures and the several unsealed faults with minor vertical throws affecting the Miocene carbonates of the field are both considered to be consequent on the major transcurrent movement along the western margin fault of the Cambay rift.  Similar fractures and faults are also expected to be present within the crystalline basement and these could account for the presence of oil accumulations in the basement in Mumbai High North.

The oil in the fractures and unsealed faults provided the bulk of the early production of oil from L3 reservoir of Mumbai High North field.  The networks of faults and fractures were also the first to be swept by injected water.  The production performance, particularly the production rates and producing gas-oil-ratios of wells, indicates that the bulk of the production from the matrix porosity was by solution gas drive until around 1990 or so even though a peripheral water flood was started in 1984 and a pattern flood in 1989.  Co-current and counter current imbibitions started playing a part only later.   The latter process became more important than the former as soon as the entire fracture and fault network volume was flooded out.  It is worth pointing out that more than 16 years back AB Das Gupta had indicated that if remedial action is not taken up soon, imbibition could become the only process for recovery from the L3 reservoir of the field.

The porosity due to natural fractures and faults has been estimated to be a minimum of 3% of total porosity.  The operator’s estimate of IOIP for L3 of BHN was entirely for matrix porosity.  As of the end of fiscal 2000-01recovery of 110.492 million standard cubic metres stock tank oil has been produced and this is now considered to be only 15.9 % of IOIP in total pore volume.  In terms of recovery from the matrix pore volume, the recovery of around 77 million scm stock tank oil becomes only 11.7% of the oil in the matrix pore volume.  The actual volume of the fractures and unsealed faults could be much larger than assumed in this paper (instead of 3% it could be, say 5 to 7 % of total porosity) and then, to that extent, recovery to 2000-01 from matrix IOIP will be lesser.

Recent work has shown that if co-current imbibition is the main process, recovery from naturally fractured carbonates can be higher than recovery from Berea type sandstones.  Now that all the vertical fractures and unsealed faults are filled with injected water one of the main processes working in L3 of Mumbai High North is counter-current imbibition and, therefore, ultimate recovery from the matrix pore volume is expected to be very low.  The current rate of annual production is less than 0.7% of IOIP and economic recovery could at best be in the 20-22% range of matrix IOIP (before the offshore structures rust away and become unsafe for operations).  Noting that the operator‘s current exploitation strategy does not cater for the presence of conductive fracture/fault networks, all hopes of recovering more than 20 to 22% of oil in the matrix pore volume are totally unrealistic. 

For improved recovery the water flood should have been carefully calibrated to the geometry of the fracture network.  This can perhaps still be done if the flood geometry could be modified after a careful mapping of the fracture/fault network and co-current imbibition made to become the dominant process consequent on suppression of counter-current imbibition.

No investment decision should be taken on the basis of numerical simulation because results of such work would be erroneous on account of inadequacy of detailed descriptions of unsealed faults with minor throws, reservoir scale fractures and unsealed mega-joints in the reservoir model. The main recommendation is to immediately suspend water injection, recycle all gas to the gas caps, and carry out well planned pilot projects. 

Recent experimental work has shown that injection of miscible fluids (gas) has the advantage of the miscible gas avoiding the fractures and entering the matrix pore spaces.  As current industry thinking is that even the most recently developed dual permeability algorithms do not work correctly in simulating naturally fractured carbonate reservoirs it is essential to stop the ongoing water flood and start on pilot field projects for testing whether miscible injection would work in L3 reservoir of the Mumbai field.

Abst-2

 A Review of the Petroleum Systems of the northern offshore Cauvery Basin

Neil Oates

Hardy Oil, London

This paper describes the main features of the petroleum system of the Cauvery Basin, offshore the east coast of India. Particular emphasis is placed on how the acquisition of new 3D seismic in 2005 has lead to an improved understanding of potential hydrocarbon trapping mechanisms and the probable areal extent of reservoirs.

Hardy Oil & Gas has been active in the exploration of the Cauvery Basin offshore the east coast of India since 1996 when it acquired the interests of Vaalco in licences CY-OS/90/1 and CY-OS/2.  The CY-OS/90/1 licence contains the PY3 oilfield, discovered in 1988, for which Hardy is the operator responsible for bringing the field onstream in late 1996. Subsequently redevelopment took place in 2003 which involved the successful implementation of water injection to provide pressure support, the first such scheme offshore India. The PY3 field reservoir is the upper Cretaceous age Nannilam sandstone and the oil is relatively light at 49°API. The only other significant hydrocarbon discovery in the offshore area is the PY1 gasfield operated by Hindustan Oil Exploration Co. (‘HOEC’). This accumulation occurs within fractured and weathered granitic basement and is expected to be developed very shortly. Significantly sub-commercial volumes of oil have been discovered in the Eocene overlying this field.

The CY-OS/2 licence consisted initially of all the exploration acreage surrounding the PY3 and PY1 accumulations. However over time after a number of mandatory relinquishments the currently retained portion consists of 2 separate areas in the north and south respectively (total 859km2). These areas were those deemed by Hardy from their earlier studies to possess the greatest prospectivity. Currently the CY-OS/2 partnership consists of Hardy (operator, 75%) and GAIL (25%).

Approximately 12,OOOkm of 2D data was available at the time Hardy took over the licence and of this 1381km has been reprocessed. In addition 170km2 of 3D data was available over the PY3 field area. 17 wells had been drilled in the CY -OS/2 licence all of which were plugged and abandoned although several had encountered oil and/or gas shows. 10 wells had been drilled in the PY3 area of which 5 discovered oil.  In 1998 170km2 of 3D seismic was acquired over the PY1 field area some of which extended into the CY-OS/2 exploration block.

During its period of operatorship Hardy has drilled 2 exploration wells (with a third imminent at the time of writing) and acquired 653km2 of 3D seismic in the CY-OS/2 licence in September to October 2005.  The data was processed as a prestack time migrated volume with velocity analysis performed at 500m x 500m intervals. During the processing much emphasis was placed on surface related multiple elimination

Abst-3

Problems in the Thrust-belt Imaging – A case study from Upper  Assam

K.L.Mandal, D.S.Manral and B.J.Reddy

Geophysics Department, Oil India Limited, Assam

In any thrust-belt, seismic data quality is generally poor due to complex geological settings associated with such areas. The imaging becomes more complicated due to scattering of seismic energy because of complex geological settings. This is further aggrevated; as such areas are logistically difficult and often covered by boulder beds. This makes the seismic acquisition more difficult and seismic data very noisy in thrust belt areas. In the paper we present the imaging problems of seismic data along the ‘NagaThrust’ in Upper Assam Basin. In this area, so far, the success in exploration is limited primarily due to inaccurate and poor imaging of the thrust. Across this thrust there are lateral velocity variations in shallow as well as in deeper formations. At places rapid changes in elevations also been observed in this area. All these in combination lead to poor and inaccurate images. In order to obtain better image of this thrust a different approach is taken at each step in processing. In our study, the meaningful velocity analysis/modeling and better statics solution applied along with the pre-stack depth migration has helped immensely to improve the subsurface image close to real geological setup.

Abst-4

Seismic expression of the Canyon Fill Facies and its Geological Significance- A Case Study from Ariyalur-Pondicherry Subbasin, Cauvery Basin, India

S.K.Roy Moulik, G.K.Prasad

KDMIPE, ONGC,Dehradun

Two-way time contour mapping at the top of Cretaceous-Tertiary boundary (K/T boundary) and the isochronopach of Sequence K3C reveal a canyon network at the K/T surface of Aryalur-Pondicherry Sub-basin, Cauvery basin, located at the southeast coast of India. At the end of Cretaceous period there was tectonic reorganization resulting uplift causing marine regression throughout the basin. This caused incision of exposed shelf and the formation of submarine canyon at the shelf edge. Subsequently canyon was filled during Late Paleocene times. Five different types of canyon fill facies have been identified based on the seismic expressions of the canyon fill. Integrating the available well data with seismic, lithological interpretation of different types of canyon fill facies have been made and presented diagrammatically in this paper. Somewhere it is sand rich, at some places shale rich and sometimes consists of alternate sand and shale facies. The porosities and permeabilities of the sands are quite good as evidenced by the testing data. A conceptualized depositional model of these sands has been postulated. This model shows that sands were deposited in submarine canyon set up under the influence of gravity driven mass transport processes like slumps/slides and debris flow.

Abst-5

Using curvature attributes in 3D seismic data interpretation

Satinder Chopra* and Kurt Marfurt**

*Arcis Corporation, Calgary

 **University of Houston, Houston

Identification of subtle faults and predicting fractures associated with folds and fractures is one of the major goals of careful seismic interpretation.  With the common use of 3D surface seismic data since the early 1990s, dip magnitude and dip azimuth have been used for enhancing faults that are difficult to see.  Coherence attribute, measuring lateral changes in seismic waveforms and amplitude, also proved to be a useful for achieving this goal.  Recently, horizon-based curvature attributes have proved useful in delineating faults and predicting fracture orientation and distribution (Roberts (2001), Hakami et al., (2004)). There are different curvature measures that can be used, and each has its own characteristic property.  More recently, volume curvature computation has been introduced (Al-Dossary and Marfurt (2006)), which has several significant advantages over horizon-based curvature, not least of which is circumventing the need to pick horizons through which no continuous surface exists.  In this presentation, we discuss the usefulness of curvature attribute applications to 3D surface seismic data.

Abst- 6

Genetic depositional modeling through seismic sedimentology- A case study from Mahanadi Offshore Basin, East Coast of India

R.N.Dwivedy, Janardhana Vema, R.K.Phukan

ONGC, Dehradun

An attempt was made to understand the genetic depositional processes in Mahanadi Offshore Basin of East coast of India through seismic sedimentology. A regional sequence stratigraphic framework was reconstructed by integrating 2D seismic and a few well data. Seismic attribute studies were made in two 3D volumes. Paleo geographical set up was deciphered within Forced regression unit and Low stand system tract within Neogene sections. Relative amplitude standout and its spatial distribution was inferred as Basin floor Fan (BFF) within Late Miocene to Early Pliocene section. The erosional remnants and minor channels also could be delineated within the Forced Regression packages. The paper outlines a novel approach of application of seismic sedimentology in the exploratory area, where a limited well control is available.

Abst-7

AVO ATTRIBUTES FOR EVALUATION OF GAS-HYDRATES IN THE MAKRAN ACCRETIONARY PRISM

Kalachand Sain and Maheswar Ojha

National Geophysical Research Institute, Hyderabad

The crossplot of AVO intercept (A) and gradient (B) (A-B attribute) can be used to detect free-gas irrespective of hydrates saturations across a bottom simulating reflector (BSR). However, estimation of hydrates and free-gas needs knowledge of their distributions within the sediments. Theoretical computations of A-B attribute for various gas-hydrates modes applied to a P-wave seismic data set in the Makran accretionary prism reveals 10-15% hydrates saturation underlain by 2.5-4.5% uniformly distributed free-gas.

Abst-8

DIRECT DETECTION OF HYDROCARBON USING MARINE CONTROLLED SOURCE ELECTROMAGNETIC SOUNDING

Samir Kumar Dhar,  Jayanta Basu

ONGC,  Ankleshwar

Marine Controlled Source Electromagnetic Sounding (CSEM) is a geophysical exploration method which can identify buried resistive layers underlying deepwater conductive sediments. It is known from conventional petrophysics that hydrocarbon saturated sediments is having higher resistivity value than water saturated sedimentary rock. Therefore, accurate identification of resistive sedimentary layer at the subsurface may detect directly a hydrocarbon pool. The present work attacks the problem of identifying the resistive layer by interpreting the CSEM response. Selection of suitable field specific data acquisition parameter is another point of concern in this work, to achieve noise free interpretable quality CSEM data.

In this work, an attempt has been made to determine the field specific data acquisition parameter for a deep water exploration block situated at eastern coast of India. A set of synthetic model response curves are generated by simulating different possible values of acquisition parameters to select their applicable range of values for data acquisition. A 2D petrophysical forward model is constructed by considering the available geological information of the exploration block. CSEM response is then simulated over this field specific model with realistic data set. The forward modeled response is clearly showing the presence of resistive layer buried in the conductive host rock. The response anomaly is further enhanced with increase in the resistivity contrast between the layers. The validity of the modeled response is compared with an actual CSEM response curve acquired from a field where the input geological information is kept same as that of the field.

This inversion modeling technique can identify a resistive layer at the subsurface, its resistivity value and its lateral extent from CSEM response. It is also possible to predict the possible depth of occurrence of the resistive layer by overlaying marine seismic section with CSEM response. By knowing the exact cause of increase in resistivity within the sediments, one can conclude about the nature of fluid present within an entrapment. Thus, marine CSEM survey in combination with marine seismic may help to locate the hydrocarbon bearing trap before any exploratory well drilling in a block. Moreover deep water exploratory well costs are enormous and associated risks are also very high. Proper implementation of this emerging exploration technology is having a huge significance in deepwater petroleum economics. The success of this technology lies in good quality data acquisition and its proper geological and petrophysical analysis which is the core idea behind this work.

Abst-9

RE-LOOK ON THE PROSPECTING  OF UTTARANCHAL FOOTHILLS IN OIL’S ACREAGES IN GANGA BASIN

Akshaya Kumar, Bedanta Pd. Sharma

Oil India limited,Duliajan(Assam)

The hydrocarbon exploration in Himalayan foothills, in particular and Ganga basin, in general has so far proved futile through oil/gas shows have been reported from many parts of the basin. Limited exploratory drilling, so far, has not encountered the much expected Eocene source rock i.e. Subathu Formation, which are reportedly exposed in several thrust sheets in the area. Oil India Limited (OIL), for the first time conducted seismic surveys (dynamite) in geologically complex and extremely rugged terrain of the Uttaranchal foothills. Processing and interpretation of the date, particularly application of advanced techniques elucidated the hydrocarbon prospectivity of the study area from the exploration point of view. Identification of Triangular zones (triangle zones having thrust-stack prospects with apparent closure as well as new plays, etc.) which appear analogues to many other oil/gas bearing triangular zones around the world, have renewed interest in the area. Triangular zones in the study area are made-up of stacked south-travelling thrust sheets and their north-vergent back thrust, build on a detachment at top of Vindhyan where the target is Dharamsala (reservoir) and Subathu (source and reservoir).

The area merits a sustained exploration programme. The present paper highlights focus on the exploration in triangular zones for deep as well as shallow  likely hydrocarbon prospects, which is expected to result in the discovery of hydrocarbon in Uttaranchal foothills. These concepts are expected to open up a large new vista of exploration plays in this hitherto unrewarding basin.

Abst-10

CGG EYE D

Patrice Canal

CGG

This year marks 75 years of  CGG research playing a leading role in extending the frontiers of what geophysical technology can do. The inter-related family of CGG’s advanced seismic products and methods is known as Eye-D. The  paper outlines the current state of progress of our heavy  investment in Eye-D technology for data  acquisition and processing, which is providing ever clearer imaging to guide exploration drilling and reservoir management.

Applications of Eye-D technology shown include  full wavefield data acquisition on land and offshore, using new methods such as HPVA Vibroseis recording. Improved imaging in mountainous zones at reduced cost is illustrated, as is a  new use of seismic to make gasfield development less hazardous.

Improved offshore acquisition using new-generation solid streamers is described, as are Eye-D methods for seabed acquisition and processing using buried cable and autonomous node systems.  Examples of advanced Eye-D processing for pre-planning, denoising,  velocity estimation and multiple attenuation are presented.

The contribution now being made by time lapse or 4D seismic is demonstrated in terms of  the information it brings and its proven benefit over cost to oilfield operators.

Abst-11

Net Sand Estimation in Deep Water Turbidite Sands, Bonga Fields, Offshore Nigeria

 Raghu Chunduru and Peter Nordstrom,

Shell International Exploration & Production, Houston, USA

Lateral prediction of net pay sands and the uncertainty of these predictions play a  fundamental role in the life cycle of hydrocarbon production from exploration through field development. To this end, quantitative seismic inversion and attribute based methods are playing a key role in the lateral prediction work process flow.  In this presentation, we present some of the challenges, assumptions, strengths and weaknesses associated with the methodologies and their impact on Static and Dynamic modeling and commercial drivers for the deep water turbidite Fields, offshore deepwater Nigeria.

The OML 118 block is located 120 km offshore Nigeria and lies in water depths ranging from 900 to 1500 m. The area is characterized by N-S trending shale ridges and iapers separated by shale withdrawal synclines. It is operated by Shell Nigeria Exploration and Production Company Limited (SNEPCO), under a PSC arrangement, on behalf of the license holder, the Nigerian National Petroleum Exploration (NNPC), in partnership with Exxon, Agip and Total. Bonga turbidite reservoirs were deposited in a range of deepwater depositional environments.  These include: incised channel fill complexes, channel-lobe, and amalgamated channel or channel/levee systems. To date, several wells have been drilled encountering primarily oil in reservoir levels of Miocene age.   Attribute based methods and Multi-stack simultaneous inversion results are used in detailed reservoir characterizations and are the basis for subsequent dynamic flow simulations of               Bonga.

A blind test was performed by estimating net sand prior to the drilling of one of the Bonga appraisal wells. The predicted net sand thickness from the multi-stack inversion results gave a good match to the net sand thickness encountered at the well, clearly demonstrating the reliability of multi-stack inversion in lateral prediction. Currently, most of the static and dynamic models that are built use inversion results for the majority of the reservoir sands in greater Bonga area. The proposed inversion scheme and methodologies are also being implemented at various projects within Shell Technology E & P.

Abst-12

Fluid Contacts as exploration and development tool : a case Study

N.K Khatri, R.T Arasu , Syam Mohan V, Harilal, Birbal Singh  and V. Rangachari

GEOPIC, ONGC, Dehradun

The fluid contacts are flat in depth domain. If lateral velocity variations at contact level are not there, they have to be flat in time domain as well. Reservoirs having fluid contacts become easy targets for exploration and development irrespective of type of entrapment i.e. structural or stratigraphic or both. Reflection events generated from the fluid contacts are unconformable with the ones from the beds along dip direction. Contacts may be close to conformable on seismic section when seen in strike direction of the beds. As the seismic section is picked up in directions other than the strike of the beds, the dip of the beds becomes quite evident and also the flat spot unconformable with them. This understanding gives the very basis for identifying them on seismic data. Fluid contacts show the single value of contour on structure map of the surface indicating the presence of one  type of fluid above the contact and other type  below it.

The dipping sand and shale sequence with upward seal provided by unconformity has given multilayered reservoirs in this study area. One such interpreted flat spot was drilled as an exploratory target and found to be gas reservoir and flat spot as gas water contact. Extent and vertical limit of the hydrocarbon pool could also be mapped from the fluid contacts to help development strategy. A few more flat spots have been established as fluid contacts. Encouraged by the results more flat spots are being mapped as an exploratory target.

Abst-1 3

Hydrocarbon potential of  ultra Deep water  areas  of  Mahanadi Basin

V.K.Rao and M.K.Bhargava

Reliance Natural Resources Ltd, Delhi

The hydrocarbon potential of ultradeep water areas of Mahanadi Basin in the east coast of India   falling in the proximal part of Bengal Fan   has been  evaluated from the available data. Extensive geophysical investigations   carried out earlier have indicated  presence of thick pile of  more than 4 kms of Bengal fan sediments underlain by  about 12 kms thick  continental rise sediments.

Whereas the fan system   post dates  collision of India  and Asia  and uplift of Himalayan  orogeny ,the continental rise sequences represent the synrift and post rift  episodes of  separation of Antarctica and Australia  from the Indian mass during Cretaceous times. The basinal  area  is predicted to be underlain by Lower Cretaceous Oceanic crust.

The scarce seismic data   available   in this part of the Basin has been studied . Gravitational collapse structures due to   sediment loading   at the edge of continental slope,particularly in the proximal part of  Bengal fan  could be a major structural style. Some of the possible play types as deduced from   regional  seismic sections  indicate presence  of channel levee complex, overbank deposits and basin floor/slope fans. The recently acquired seismic data by GX Technologies of USA   have indicated possible DHI in some areas.

The envisaged petroleum systems are discussed and   the hydrocarbon potential is estimated to the tune of  20.5   TCF of gas  based on analogous settings for this frontier exploration province.

Abst-14

Low-Frequency Dielectric Dispersion and Microwave Dielectric Properties of Dry and Water Saturated Limestones of Jodhpur Region

V P Singh, R J Sengwa* and Ashok Soni

Well Logging Services, Oil and Natural Gas Corporation Limited, Rajahmundry

*Microwave Research Lab., Deptt. of   Physics, J. N.  Vyas University, Jodhpur

The complex dielectric constant of 17 different grades of dry and water-saturated limestone samples from the Jodhpur region were investigated in the frequency range of 100 Hz to 100 kHz and also at 10.1 GHz, at room temperature. The Cole-Cole dielectric dispersion model is used to compute the values of low-frequency-limit dielectric constant, high-frequency-limit dielectric constant, dielectric strength, and principal value of the relaxation time for each dry limestone sample. The chemical composition, sample bulk density, and porosity of each sample studied were determined to ascertain which of these factors govern their dielectric parameters. Analysis of the low-frequency dielectric constant confirms that the sample chemical composition and bulk density are the major factors governing the dielectric constant values, but for water-saturated samples, the frequency-dependent dielectric constant values are also sensitive to porosity and grain size. The AC conductivity of all the dry samples increases with the increase in frequency from 100 Hz to 100 kHz, but water-saturated samples have comparatively high conductivity and some samples have values that are independent of the frequency.

The microwave dielectric constant of dry samples depends mainly on the amount of the constituents of the oxide, in their chemical composition, and on sample bulk density. The contribution in the measured dielectric constant values of each oxide present in the sample is identified by using their density-reduced values of dielectric constant. The porosity-dependent mixing equations for solid and fluid components of the system were also found suitable for the determination of the dielectric constant of water-saturated limestone samples at microwave frequency.

Abst-15

Reservoir Monitoring Through Wireline Logs

 V. Gopala Rao, S.L.N. Rao, U.C. Das, T.K.Dutta and R.N.Chakraborty

HLS Asia Ltd., New Delhi

Oil fields of upper Assam have been monitored using Reservoir Monitoring Tool (RMT) of Halliburton. It is based on Carbon / Oxygen ratio principle measured through inelastic mode of interaction between high-energy neutron and nucleus. A supplementary support to this measurement is also provided by the gamma ray spectrum recorded from the capture mode of nuclear reaction. Water saturation values estimated through open hole logs and C/O method (recorded after a time lapse of nearly four years) have been compared which indicate a pretty good match at the top part of oil zone, while in the bottom part, oil seems to have been replaced by water and OWC moved up by 15m. The original OWC was at 2782m and the present OWC is at 2767m. The perforation interval 2763-2767m is now very close to the water zone, which is the reason for higher water cut. Ultrasonic cement evaluation using Halliburton’s Circumferential Acoustic Scanning Tool (CAST) also indicated no channeling behind casing.

Abst-16

Sea Bed Logging - A tool for risk mitigation in Hydrocarbon Exploration

R.K.Khanna and P.S.Rao

ONGC, Mumbai

The resistivity of the hydrocarbon saturated formation is significantly higher than the surrounding non-hydrocarbon bearing rocks. This property of hydrocarbon saturated rocks is made use in identifying the subsurface resisivity anomalies using the Sea Bed Logging technology as a tool and accordingly the survey is designed. Large current of the order of thousand amperes generated onboard a vessel is discharged through the electrodes kept apart by few hundred meters(source). The refracted electrical energy from the subsurface layers and the induced magnetic energy in the earth’s magnetic field is measured by the receivers spaced at regular intervals along a profile while the source is moving from one end to the other end of the profile. The variations in the refracted electrical and magnetic energy measured and compared with that of a reference receiver kept closer to the survey area. The recorded data is plotted in amplitude Vs offset and phase Vs offset. Anomalies observed in the plots are carefully analysed and integrated with available seismic data to reduce risk, there by improve the success ratio of the drilled locations.

Abst-17

NEW TECHNIQUE FOR THE DETERMINATION OF API GRAVITY OF OIL FROM LOGS

....................................................................................................................................................................................................................................................................... P. Ramalingam

IRS, ONGC, Ahmedabad

Normally three types of porosity logs are recorded in the open hole. They are used to determine the porosity individually and also porosity and lithology from their cross plots. Apart from this, three types of ratios, namely M, N, and P can be computed from these logs. The M and N ratios are used in M-N cross-plot to determine the lithology of the formation. The present study deals with a new cross plot using the Density log vs P and Density log vs N. It was observed that clustering of points occurred on these cross plots. It was also observed that there exist a correlation between these clusters and API gravity of oil produced. The study was made using the data of the wells belonging to Cambay and Assam & Arakan basins of India. The results show that this method can be used to estimate API gravity of oil directly from the log before testing a formation.

Abst-18

Diagnosing Poor Cement Bonding using combination of Ultrasonic Logs AND Sonic Logs for successful Cement Squeeze Operations in Oil Wells – A case study of  Upper Assam

A. Narayan and  P. Borah

Oil India Limited, Assam

This paper provides a case-study of the evaluation techniques used to assess the cement sheath by combining the sonic logs and ultra-sonic image logs which aided in successful cement squeeze operations in production wells of Hapjan, Dikom & Nagajan Oilfields of Oil India Limited in Upper Assam. Sonic data acquired using CBL provides only the average cement sheath behind the casing. Ultrasonic image tools include a rotating transducer which enables it to provide a 360o image of the cement bond around the casing. It incorporates a real time statistical variance plot which provides a clearer picture of the cement sheath. Although Ultrasonic image tools have many applications, this paper focuses on its use in evaluating cement sheath only. Ultrasonic measurements evaluate the cement sheath by measuring the acoustic impedance (in MRayls) between the casing & the cement whereas sonic tool evaluates the cement by measuring only the attenuated amplitude of the sonic signal in terms of compressive strength and bond index. The significant contrast of acoustic impedance between liquid phase & that of the cement behind casing measured from ultrasonic tool gives the quality of cement sheath. The Acoustic impedance of gas can range from 0-0.8 MRayls, gas cut liquid can range from 0.38-1.15 MRayls, water and mud can range from 1.15-2.3 MRayls, liquid-to-solid transition can range from 2.3-2.7 MRayls, light weight cement can range from 2.7-3.85 MRayls, most of the cement can range from 3.85-5.0 MRayls and hard solid materials can have 5.0 MRayls and above. Each zone of the cement fill-up between the casing & the reservoir rock around the casing circumference has been identified by the ultrasonic acoustic images which contains a combination of different images of cement, water & formation fluids in different colours. As a result, the acoustic impedance image displayed by ultrasonic logs gives the random pattern of cement sheath, liquid & gas which can easily detect channels & fractures. In this case study, combination of interpreted sonic & Ultrasonic logs provided clear indication of channels/ poor cement fill-up in production wells of Hapjan, Dikom & Nagajan oilfields of Oil India Limited. Based on these evidences, isolation repair operations were carried out by cement squeezing at selective zones through open perforations. Post squeeze logs were then recorded to evaluate the success of the cement squeeze jobs. Subsequent testing of these wells allowed quick pay off as production rates improved.

Abst-19

RESERVOIR SCREENING USING IATROSCAN TLC-FID AND GAS CHROMATOGRAPHY AND IDENTIFICATION OF OIL, OIL-WATER CONTACTS, TAR MATS AND RESIDUAL OIL SATURATION IN THE ASSAM PETROLEUM ACCUMULATION

G.C. Datta, P. Sivan, and R.R. Singh

KDMIPE, ONGC, Dehradun

Formation evaluation experts usually have little difficulty in interpreting wireline logs to access the type of reservoir fluids in sand and shale sequences. This assessment is usually accomplished by log data which detects low hydrogen and low electron densities typical of gas zones and repeat formation tester (RFT), which uses both the pressure gradient and sample acquisition technique to evaluate reservoir fluids. Some times log data exhibits a false indication of oil and gas shows due to poor hole conditions and effect of chemicals used in the drilling fluids, specially glycol, diesel, enhanced mineral oils (EMO) etc. To avoid these problems an improved technique has been developed for the identification of pay zones on the basis of combined thin layer chromatography - flame ionisation detection (TLC-FID) of the Iatroscan and gas chromatography (GC) finger prints of residual hydrocarbons from side wall core (SWC). This technique is more accurate and cost effective input for reservoir fluid characterisation.

The important parameters need to identify the nature and composition of the reservoir fluid include the bulk composition of the sidewall core extract ( content of saturate, aromatic and nitrogen, sulphur and oxygen compounds ), amount of extractable organic matter (EOM) in mg/g of rock, GC finger print and the ratio of pristane (Pr) to nC17 . These parameters integrated with geological and geophysical (wire line logs) evidence, yield more accurate and reliable formation evaluation criteria.

Finger prints of oil-saturated sands are typical of topped (C15+) whole oils, whereas finger prints of hydrocarbons extracted from the gas-dominated sands show an abbreviated hydrocarbon distribution more typical of gas condensates. Finger prints of extracts from water-productive intervals show a further abbreviated signature, suggesting that high molecular weight hydrocarbons did not migrate in to these intervals.

The GC and GCMS analysis of SWC extracts from Assam basin have been carried out for the identification of pale-oil zones. Oil finger printing of SWC extracts in the wells KR-A and LW-B of Tura Formation of Paleocene age clearly indicates accumulation of hydrocarbons in the zones. The finger prints of hydrocarbons extracted from the gas dominant sands show the distribution more typical of gas condensates. In the well KR-A tar mat and oil water contact is also identified.

Abst-20

Stable carbon isotope geochemistry of natural gases from Upper Assam Basin, India

A.K. Mittal, H.C. Pande, A. Raina, and              R.R. Singh

KDMIPE,ONGC, Dehradun

Upper Assam Basin is the shelf part of Assam-Arakan Basin, which is a known hydrocarbon producing province in northeastern India. The sedimentary cover in the basin consists dominantly of Tertiary sequences (about 7000 m) resting unconformably over the granitic Precambrian basement. The present work is an attempt to genetically characterize the natural gases from some fields of Upper Assam Basin on the basis of their molecular and stable carbon isotopic compositions.

The study reveals that gases are of thermogenic origin and generated from a source within oil-window. Gases are generally enriched in C2+ hydrocarbons, suggesting association with liquid hydrocarbons.

Gases from Kopili (Eocene), Barail (Oligocene), and Tipam (Miocene) reservoirs from Lakwa, Geleki, Demulgaon, Changmaigaon, and Banmali fields are genetically related and have been generated from a source of low to moderate maturity (methane d13C, ˜ -45.0 to  -48.0‰). 

Mixing of bacterial gas is prevalent in Rudrasagar, Safrai, Charali and Amguri fields (methane d13C in Rudrasagar ranging from -59.4 to -52.4 ‰). Lighter ethane isotope compositions in many gases from Rudrasagar field point towards contribution from local immature/early mature sources in this area.

Basement, Basal Sandstone (E. Eocene to Paleocene), Kopili (Eocene) reservoirs of Borholla-Changpang fields and Sylhet (Eocene) reservoir from Khoraghat field in South Assam Shelf, on the other hand exhibit much heavier methane isotopic compositions (d13C methane ˜ -36.0 to -38.0 ‰) suggesting a more mature source different from the gases of Lakwa, Geleki and Rudrasagar fields of Upper Assam Valley.

The Bokabil (Miocene) gases in Khoraghat and Nambar area in South Assam Shelf, however, are similar to Kopili, Barail and Tipam reservoir gases from Upper Assam Valley fields and suggest generation from a similar source at lower maturity.

The study thus reveals occurrence of two groups of gases, which in turn points to the existence of mainly two petroleum systems in the basin. This observation is in conformity with the findings of the oil study in the area. However, scattering in isotopic data of gases indicates some maturity and organic facies variation of the sources and that the local sources (immature to early mature) have also contributed towards hydrocarbon (especially gas) generation in the basin. 

The study also indicates that bacterial degradation is prevalent in shallower pays in Tipam sandstone reservoirs of Lakwa (TS-1 and TS-2) and Geleki (TS-2 to TS-5) fields.

     Further, similarity in the isotopic composition of Sylhet and Basement gases from southwest of the basin (Khoraghat and Borholla-Changpang fields) to northeast (Tengakhat field) suggests that more such hydrocarbon pools may be available between these two geographical extremes within Sylhet and deeper reservoirs. Therefore, traps within Sylhet and deeper formations may be the future targets of exploratory drilling for reserves accretion in the basin.

Abst-21

STABLE CARBON ISOTOPE GEOCHEMISTRY: A DIAGNOSTIC TOOL FOR UNDERSTANDING THE ORIGIN OF UNONVENTIONAL GAS DEPOSITS

A. Raina. A, H.C.Pande, P. Sharma,A.K. Mittal

KDMIPE, ONGC, Dehradun

Isotopic ratio mass spectrometry has long been recognized as the diagnostic tool to analyze petroleum fractions and products. Owing to its versatile capabilities of producing ions by the phenomenon of Electron Impact(EI) , Field Ionization and many more, the techniques finds its application in the analysis of almost every complicated samples related to source of hydrocarbons.Furthermore,due to the tremendous technological growth in the separation of components of hydrocarbons  along with latest available computer based workstations with on line and real time data generation, the stable carbon isotopic analysis of any species of hydrocarbon has become possible. Continuous flow GC-IRMS and EA-IRMS, an ultimate state of art technology, is being currently used at our work center for understanding the genesis of new frontiers of hydrocarbon sources: CBM gases and Gas Hydrates.

In present time, the environmental requirements of energy sources have advanced to the forefront along with their economic demands. Because of this fact, the role of natural gas along with unconventional resources of natural gas has increased sharply in recent years. Some of the prospective areas of these unconventional resources are Gas hydrates and CBM gases. The objective of this study is to unravel the molecular composition and history of the coal bed gases and gas hydrates found in Indian sedimentary basins.

Barakar Formation, which is the chief contributor of the quality coal seams in most of the coal fields confined to the Permian Gondwana basins, have an estimated coal reserves of about 200 billion metric tones with respect to CBM exploration. The desorbed gas samples from  some of the potential areas like Jharia,Bokaro,North Karanpura of West Bengal have been analyzed for their chemical composition and stable carbon isotopic composition to probe into the coal bed methane potential of this area. Study reveals that the gases desorbed from the cores of coal seams have appreciable amount of methane, more than 80% in most of the cases. The gases have been genetically characterized by stable carbon isotopic values of methane that vary from -71.8 to -22.7 per mil. The isotopic values are indicative of varying proportion of biogenic as well as thermogenic gases generated during coalification process.

The geochemical analysis of core samples derived from the KG offshore basin has resulted in identifying the presence of gas hydrates. The desorbed gas samples from the cores have been analyzed for their chemical composition to probe into methane potential of the gas hydrates. Study reveals that gases have appreciable amount of methane, more than 95% with little of ethane, propane and CO2 in the area where gas hydrates are localized. The stable isotopic studies of adsorbed gases will be carried on continuous flow isotopic ratio mass spectrometer (CF-IRMS), recently installed at KDMIPE, Dehradun.

The study will contribute to the understanding of the formation and origin of these gases in general and to some extent provide information to estimate the future gas production characteristics in these frontier fields.

Abst-22

Seismic reflection amplitudes constraining the Lakshmi Gas Field reservoir model, Gulf of Khambat, India

Alan Atkinson, Shiju Joseph, David L Sturrock, Paul Fejer, John Holroyd 

Cairn Energy (India) PTY Ltd., Gurgaon

The Lakshmi Gas Field is located in the Gulf of Khambat off the West coast of India. It was discovered in 2002 and has been on production since 2003. The field is situated in the Cambay Basin, a failed Palaeocene rift filled with shallow marine clastics.  The trapping mechanism at the Lakshmi Field is four-way structural closure which was formed in the late Miocene to Pliocene by structural inversion during convergence of the India and Eurasia tectonic plates. The main reservoirs are the sandstones in Miocene Babaguru (around 800m TVDSS) and Tarkeshwar Formations (around 1000m TVDSS).

Babaguru reservoir sands are interpreted to be bar sands deposited in an open marine shore face setting with good reservoir characteristics and continuity, an environment broadly similar to the present day Gulf of Khambat. They are seismically mappable and exhibit bright amplitudes with polarity reversals characteristic of gas bearing interval. Tarkeshwar reservoir sands are believed to be discrete tidal channels deposited in a delta plain environment. From structural considerations, Lakshmi field which is comprised of two culminations LA and LB with a minor low separating them are part of a much larger four-way closure. Geological interpretations aided by seismic strongly suggests laterally extensive reservoirs. Production performance of certain sands confirmed the barrier between LA and LB which necessitated reviewing the interpretations to assess the gas volumes for production planning. However, there was no clear seismic evidence of a sealing fault separating LA and LB. The seismic data were reprocessed to aid re-mapping for the second phase of development well drilling, with the objective of better understanding the gas distribution and production history.

After reprocessing, four high quality angle stacks were created. These datasets indicated clearly the distribution of gas on the mid and far angle stacks, confirming the modelled AVO response. Additionally, the near angle stack revealed more limited distribution of the sands than had previously been thought. Mapping of one reservoir in particular, within the Babaguru formation (which was interpreted as a single parasequence), suggests an environment of deposition consistent with estuarine tidally modified bar sands, similar to present day analogue from Gulf of Khambat. In addition, the high quality of seismic image and agreement with modelled response allowed the depth conversion of this low relief field to be driven by seismic amplitudes: the maximum limit of amplitudes, where governed by structure, could be tied to the known gas water contact. These observations resulted in the mapped field OGIP conforming to the production history and pressure depletion predictions, thereby saving unnecessary development wells. In addition, a potential exploration well was shelved after it was seen to be targeting uneconomic gas reserves. Development drilling was confined to reservoir sand targets which were defined on the basis of seismic amplitudes. This second phase of development drilling has now been completed, all wells achieving their objectives.

Abst-23

GAS GEOCHEMISTRY AS TOOL FOR RISK REDUCTION IN HYDROCARBON EXPLORATION

H.C.Pande, Ashok Raina, Pankaj Sharma and A.K.Mittal

KDMIPE, ONGC, Dehradun

Geochemical characterization of gas seeps and natural gas produced from the subsurface reveals the origin of the gas. Mainly two distinct processes generate gaseous hydrocarbons in a reservoir, bacterial activities during diagenesis and thermal cracking of petroleum precursors, which produce biogenic and thermogenic gases respectively. Whereas bacterial gas is mainly enriched in methane, thermogenic gases contain an abundance of higher hydrocarbons in varying proportions. Chemical composition and stable isotopic characteristics of also gives an insight into the association of the gas with a liquid hydrocarbon pool and its maturity.

Natural gases of biogenic as well as thermogenic origin and mixed nature are encountered all over the world and in Indian basins in varying proportions. Study of a gas seep studied in the past from a tube well in Rahatgarh in MP, India, revealed its biogenic origin and a gas collected from Bengal basin well was characterized as thermal high maturity thermal gas based on their chemical composition and stable carbon isotopic signatures.

Whereas bacterial gases reported from seeps and tube wells may be of commercial importance to a small as well as large extent, associated wet gases produced from the subsurface may lead to a commercial discovery when integrated with other geological and geophysical data.

Abst-24

Petroleum geochemistry of Bombay Offshore Basin

S. Pahari, Meenu Varshney, Harvir Singh, IVSV Prasad and R.R.Singh

KDMIPE, ONGC, Dehradun

Source potential of Tertiary sedimentary deposits of Bombay offshore Basin have been evaluated to identify the potential source rocks, assess their maturity, timings of hydrocarbon generation and   hydrocarbon charge potential.

The classic sediments in the lower Eocene - Paleocene section are the principal source rocks across Bombay offshore basin. In addition, marginally mature potential source rocks within Oligocene in Tapti Daman area and within Neogene in DCS and deeper towards basin exist. High source potential index (SPI), characterize the lower Eocene to Paleocene section at Surat-Mahim­Vijaydurg depressions. Low source potential index in the lower Eocene to Paleocene section has taken place at Saurashtra low, Shelf Margin area and Murud and Rajapur lows. But, in Lower Miocene - Middle Eocene section, source potential is more at depressions of Saurashtra low and Shelf Margin areas to Surat-Mahim-Vijaydurg depressions.

The maturity modeling indicates that At Mahim graben, the top and base of middle Eocene has attained maturity threshold (0.5%VRo) at recent past and at 11 mybp and the base of Paleocene has attained maturity equivalent to 0.75 % VRo at 4-5 mybp. At Vijaydurg graben, the base of mid Eocene has attained maturity threshold (0.5%VRo) at 11 mybp and the base of Paleocene has attained maturity equivalent to 0.75 % VRo at 4-5 mybp. At South Bombay Low, the base of Paleocene, mid Eocene and Oligocene attained 0.75% VRO at 10 mybp, 4mybp and recent past respectively. At Murud depression, the base of Oligocene and base of lower Miocene attained 0.75 % VRo at 7 mybp and recent past respectively. Top of lower Miocene has not yet attained 0.75 % VRo but attained 0.5% VRo at 1-2 mybp. The base of mid Eocene (Bassein) and base of Paleocene (Devgarh-Panna) has attained 0.75% VRO at 8 and 14 mybp respectively; at present they are in gas window.

Lower Eocene-Paleocene source rocks in the Surat-Mahim-Vijaydurg depression and south Bombay low has sourced most Bombay offshore oils. Additional contribution of the Oligocene source rocks towards some Tapti-Daman and Bombay High oils and lower Miocene sources that exist in Saurashtra and shelf margin areas towards the DCS oils is possible. In shelf margin depression, Miocene source rocks may also charge reservoirs. The modeling reveals that whereas at Mahim and other eastern depressions the lower Eocene-Paleocene source rocks are appropriately mature, at Saurashtra low, Murud and other western depressions the lower Miocene source rocks have attained adequate maturity.

The significance and usefulness of this work is to understand and improve the prediction of the occurrence of oil and gas, to provide exploration leads through geochemical characterisation and thermal maturity modeling for future exploration and to understand the possible reasons for exploration failure in Saurashtra low and Shelf margin area. It also helps in reducing the risk when exploring for petroleum.

Abst-25

Microbial Prospecting of Hydrocarbons in Jamnagar sub basin, Saurashtra, Gujarat

M.A.Rasheed, D.J.Patil, and A.M. Dayal

National Geophysical Research Institute,  Hyderabad

Microbial Prospecting of hydrocarbons is a sure exploration method based on the detection of anomalous population of hydrocarbon oxidizing bacteria in the surface soils, which indicate the presence of subsurface oil and gas accumulations. The use of specialized microbial techniques to detect the presence of various groups of methane, ethane, propane and butane oxidizing bacteria can reliably differentiate between prospective and non-prospective areas. In the present study Propane-oxidizing bacteria have been considered as indicator microbes. Microbial Prospecting of hydrocarbons method was applied in Jamnagar sub-basin, Saurashtra, Gujarat. A total of 150 soil samples were collected on a grid of 1 x 1 km. The propane oxidizing bacterial count in the soil samples of the studied area was found to vary from zero to 6.86x 105 cfu/gm of soil sample. The overall bacterial count was found to be significant. Two microbial blooms were identified of high propane oxidizing bacterial concentration and mapped in the area.

Abst-26

SEISMIC VELOCITIES AND ESTIMATION OF GAS-HYDRATES ACROSS A BSR FROM AVA MODELING IN THE WESTERN CONTINENTAL MARGIN OF INDIA

Maheshwar Ojha and Kalachand Sain

National Geophysical Research Institute, Hyderabad

The bottom simulating reflector (BSR) commonly used marker for gas-hydrates investigation, has been identified by seismic experiment in the western continental margin of India (WCMI). Seismic velocities hold a key to understand the origin of BSR and for quantitative assessment of gas-hydrates. Here we show a new approach of A V A modeling to estimate seismic velocities across various reflectors including the BSR in the WCMI. The result reveals the P-wave velocity of 2.25  km/s and S-wave velocity of  0.985 km/s above the BSR. This corresponds to a Poisson ratio of 0.382 and hydrate saturation of  -30%.    The comparison of estimated P-wave velocity (1.77 km/s) above the hydrated sediment to that (1.78 km/s) below the BSR implies that the origin of BSR is mainly due to gas-hydrates.

Abst-27

Elastic properties of hydrate bearing sediments using effective medium theory

Ranjana Ghosh, Maheshwar Ojha and               Kalachand Sain

National Geophysical Research Institute, Hyderabad

Accurate and detailed seismic velocity structure of gas-hydrates bearing sediments may be determined by careful analysis of controlled source seismic data. Most attempts to predict hydrates concentrations from estimated seismic velocities have been based on semi­-empirical formulae and/or simple effective medium theory, which can not take into account the effects of sediments micro-structures and anisotropy. We intend to propose a method that can take into the effects of these. Since shales make up 75% of majority of sedimentary basins, an effective medium theory is used to relate the seismic properties of a clay-rich hydrates bearing sediment to its porosity, mineralogy, micro structural features and hydrates saturation. The method considers two possible end members for the distribution ;of gas­ hydrates in the pore spaces: (i) hydrates are unconnected and located in the pore voids without appreciable grain contact - non-contact model and (ii), hydrates are connected forming cements around the grains - contact model. The scheme is transversely isotropic to allow anisotropy due to alignment of clay platelets, and is based on a combination of self consistent approximation (SCA) and differential effective medium theory (DEM).

Abst-28

Geochemical Investigations of the Oils and the Source Rocks of Upper Assam, India: Implications for Oil-Oil and Oil-Source Correlation

B. G. Goswami, A. K. Bhatnagar, and R. R. Singh

KDM IPE, ONGC,Dehradun

Geochemical composition of the Barail, Kopili, Tipam and Sylhet oils and the extracts of potential source rock samples from the different fields of the Upper Assam area have been examined in the present study in order to carry out oil-oil and oil-source correlation.

Stable carbon isotopic compositions and biomarker data indicate that oils in the Tipam (Early to Middle Miocene), Barail (Late Eocene to Early Oligocene) and Kopili (Middle to Late Eocene), reservoirs are genetically correlated across the Lakwa, Geleki, Rudrasagar Safrai and Charali fields. However, oils from the Sylhet Formation (Middle to Late Eocene) of Panidihing area and the Basal Sandstone (Eocene) oil of the Lakwa field are distinctly different .and do,: not correlate to other oils in the study area. Barail coal-shale and Kopili shale sequences are identified as potential source rocks. Both strata contain organic matter predominantly derived from higher plants like flowering and resinous plants and similar organofacies exist in both Barail coal-shale and Kopili shale sequences.

An oil-source correlation study shows that both Barail and Kopili rock extracts are genetically correlatable with the Barail, Kopili and Tipam oils. No rock extract from the ~tudy area is correlatable with the Sylhet oil, which is genetically correlatable with the Eocene 0)1 of the area lying outside the existing boundary of the study area in northeast/east direction~ Therefore areas, further northeast/east from the present location in Panidihing appears to be more promising targets for oil exploration in Sylhet Formation.

The study suggests the presence of four source rock-reservoir pairs in the Upper Assam: the Kopili-Kopili, Barail/Kopili-Barail, Barail/Kopili- Tipam and Sylhet- Tura/Sylhet source ­reservoir pairs. It was observed that both Kopili and Barail shales in a few wells in the Sonari area have entered the oil window but are relatively less mature than the oils. It is expected that the actual hydrocarbon kitchens containing more mature source sediments are likely to be present in the adjoining Naga-Schuppen belt. Therefore, areas in and around the Naga-Schuppen belt, wherever better reservoir facies and favourable entrapment conditions are expected, will be preferred targets for further exploration.

Abst-29

Uranium Exploration worldwide: History, target styles, target areas and current activity

Stewar Taylor

President, Mega Uranium Ltd., Australia 

These are exciting times for the uranium exploration industry worldwide. Due to the ever-increasing global demand for energy, rising oil prices, concerns over security of oil supply and the need to reduce greenhouse gas emissions, there has been a renaissance of the nuclear energy industry. After around 25 years of depressed uranium prices caused mainly by the abundance of secondary supplies, the uranium spot price per pound U3O8 has risen without interruption from $US10.90 in June 2003 to $US56 (23.10.06), and further significant increases are forecast. This remarkable price increase is due to rising demand, declining inventories, the probability that Russian HEU secondary supplies will be curtailed in 2014, mine production problems in 2003 which indicated that supply is fragile, and the paucity of deposits ready to bring into production in the short term due to low investment in the industry over the past 25 years.

The uranium price rise has sparked a major resurgence of uranium exploration worldwide. A mere three years ago few companies were conducting uranium exploration – now over 400 companies hold uranium exploration projects. In Australia, the number of ASX-listed companies involved in uranium exploration has risen from 5 in 2003 to 124 at present. Currently companies are conducting uranium exploration on 40 countries worldwide, but the activity is focused on the major uranium producing countries and those containing the bulk of the world’s known uranium resources  – Canada, Australia, Namibia and the USA. Other popular targets for uranium exploration companies include Mongolia, Argentina, Peru, Zambia, Tanzania, South Africa and Niger. The principal uranium target styles sought are unconformity-related, sediment-hosted and iron oxide Cu-Au-U.

This is the third phase of uranium exploration activity. In the first phase from 1947 until 1962, exploration was conducted mainly by prospectors and discoveries were mainly of small outcropping deposits. The second phase, from 1970 until 1985, was characterized by the participation of many multinational major/junior companies with large budgets, a much more widespread exploration effort worldwide, the progressive use of more sophisticated techniques, equipment and concepts leading to the detection of uranium mineralisation at ever greater depths, and the discovery of the western world’s major known uranium resources, including Ranger (1970), Jabiluka (1971), Yeelirrie (1972), Olympic Dam (1975) in Australia, and Rabbit Lake (1968) and McLean Lake (1980) in Canada.

      Key features of the current exploration phase, which commenced in 2005, are the sudden emergence of numerous junior companies, an emphasis on reexamining uranium deposits and anomalies delineated in the previous exploration phase, a greater detection capability for blind deposits through more advanced instrumentation and concepts, and a shortage of geologists with expertise in uranium exploration.           

Abst-30

On the effective use of integrated study of aero-space data for targeting heavy mineral placers - A case study from Krishna - Godavari coast, Andhra Pradesh, India

K. Jagannadha Rao, R. Muralidharan, P.K.Srivastava, T.Desapati, A.K.Chaturvedi, and Anjan Chaki

Atomic Minerals Directorate, Begumpet, Hyderabad

Integrated study of Airborne Gamma Ray Spectrometric (AGRS) and Remote Sensing datasets was carried out over an area of 25,000 sq. km along the alluvial plains and coast of Andhra Pradesh with an objective to identify potential zones of heavy mineral concentration. Anomalous thorium zones above a threshold of 30 ppm were demarcated from the AGRS data considering the invariable association of monazite with other heavy minerals. Marine and fluvio-deltaic landforms were mapped from satellite image. Integration of the high thorium zones with favourable marine landforms resulted in identifying 36 potential sand bodies with extensions of 1-25 km in length and 1-8 km in width. 

The delineated sand bodies include the known Kakinada heavy mineral deposit.  Ground confirmation and auger hole sampling upto 1.5m depth from twenty-seven potential sand bodies revealed presence of 2 - 62% Total Heavy Minerals (THM).  Besides the known high heavy mineral concentrations along beaches, four new inland placer- rich bodies associated with palaeo-strand lines and rear dunes were also located with THM content ranging from 14-62% at Gudapalli, Nakshatranagar, Chollangipeta, and Pattigondi. The heavy minerals identified mainly are ilmenite followed by pyriboles, sillimanite, garnet, zircon, rutile and monazite.  The technique establishes the validity and effective integrated use of AGRS and Remote Sensing to locate favourable heavy mineral placer concentrations.

Abst-31

ON THE EFFICACY OF ELECTROMAGNETIC (Turam) TECHNIQUE FOR URANIUM EXPLORATION OVER ROHIL-KHATUNDRA TRACT OF SIKAR DISTRICT, RAJASTHAN

R.Srinivas*, V.Ramesh Babu*, M.N.Chary* and Rajiv Vimal**

*Atomic Minerals Directorate for Exploration and Research, Department of Atomic Energy, Hyderabad, **Jaipur

Detailed electromagnetic (Turam) survey was conducted over Rohil-Khatundra tract of Sikar district, Rajasthan where fracture controlled and albitite-hosted uranium mineralisation in association with sulphides occurs in the rocks of Delhi Super Group. The conductors identified from this survey persist over a strike length of 1 km as discrete parallel bands, which are intercepted in many boreholes and found to be associated closely with significant uranium mineralization. Study of EM response at two frequencies of 105and 315 Hz revealed persistency of these conductors at depth. Also, Induced Polarization (I.P)/ Resistivity survey, in the village portion of Rohil where EM survey could not be taken up due to cultural noise, is utilized effectively. The above results are discussed in detail in this paper.

 Abst-32

DELINEATION OF URANIUM POTENTIAL ZONES IN GWALIOR BASIN, INDIA USING MULTI- SENSOR DATA SETS

B.V.S.N.Raju, Abhinav Kumar, Anupam Goswami*, A.K.Chaturvedi and C. Shanti Kumar

Atomic Minerals Directorate, Begumpet, Hyderabad

*R.K. Puram, New Delhi

Gwalior basin forms an E-W trending elongated basin covered by Gangetic Alluvium in the north and east, by Kaimur Group sediments in the west and underlain by Bundelkhand granite in the south. IRS LISS3 and PAN Satellite images were used for the interpretation of geomorphology, geology and structure of the Basin. Airborne radiometric and magnetic survey data was processed, images generated and interpreted. The output data has been organized in to vector, raster and tabular data. The maps showing geology, structure and uranium occurrences were digitised as vector data and georeferenced. The data related to map features were tabulated and made interactive by using unique ID for each feature. Digital enhanced satellite, Airborne gamma ray spectrometric (AGRS) and magnetic images were incorporated in the form of raster data.

The world’s known high grade uranium deposits are spatially  associated with Proterozoic rocks and faults. Atomic Minerals Directorate for Exploration and Research (AMD) has intensified uranium exploration activities in Proterozoic basins of India. The Proterozoic Gwalior basin was evaluated for uranium potential by the integration of available data sets using Arc GIS. Thematic Maps on detailed scale (1:25000) were generated by integration of the geology, structure, ground uranium occurrences and AGRS uranium anomaly zones. Based on the thematic maps and uranium exploration knowledge Uranium potential zones were delineated.

Abst-33

TARGET DELINEATION FOR URANIUM EXPLORATION IN THE SOUTHERN MARGIN OF BHIMA BASIN –                                A GEOPHYSICAL CASE STUDY

J.K.Dash, S.Sethuram, V.Ramesh Babu and M.N.Chary

Atomic Minerals Directorate, Begumpet, Hyderabad

Proterozoic basins constitute one of the most important repositories of uranium province across the world. Bhima basin in India is one among those situated in the southern fringes of Deccan synclise, which has geophysically received less attention in

the early years in comparison to other Puranas such as Cuddapah, Vindhyan, and Kaladgi etc. possibly due to the absence of any economic mineral deposits. Recent studies on airborne radiometric data followed by surface radiometric checking reported a wide radioactive zone near Ukinal (Lat.16045’45”N, Long.76039’59”E), Gulbarga district, Karnataka which extends intermittently all along the major EW trending Gogi-Kurlagere fault over a stretch of 15 kms proximal to the southern margin of Bhima basin. In course of detailed ground checking all along this faulted margin, occurrence of significant uranium mineralisation hosted by both fractured granites and brecciated limestone at Gogi (Lat.16045’N, Long.76045’E) evoked considerable interest among the geoscientists due to its richer grade and shallow depth environment. Targeting this mineralized host in a complex structural set-up under soil cover turned out to be a challenging task and prime objective of the geophysical investigation at Gogi.

Electromagnetic (TURAM), Induced Polarisation (IP) and resistivity methods were chosen for exploration in view of the close association of disseminated sulphides and carbonaceous matter with the mineralized rock. EM response from the measurements of Field Strength Ratio (FSR) and Phase Difference (PD) suggests multiple conductor

environments in the area, the stronger one (in terms of higher PD and FSR) being due to shale and the moderate due to sulphides in the host rock. Time domain IP survey in conjunction with resistivity facilitated in defining the nature of conductors.

A combined analysis of EM and IP responses enabled distinction of the conductors based on their nature. Of these, the sulphide rich zones, delineated both within the basin and basement are considered favourable and promising targets for uranium mineralisation. Nevertheless, the shale boundary delineated by EM method supplemented the planning of exploration programmes since the uranium mineralisation occurs close to it. Based on these results, subsequent drilling intercepted significant uranium mineralisation in the boreholes.

Abst-34

Airborne magnetic data – A source of subsurface geological information for Uranium Exploration: A few case studies over some proterozoic sedimentary basins of India

M. Bandyopadhyay

Atomic Minerals Directorate,Begumpet, Hyderabad

Airborne survey was first done in India by AMD in the year 1956 for   exploration of uranium. At the initial stage it was aimed to locate near surface Uranium anomalies.  Acquisition of magnetic data along with radiometric data started from the year 1972. So far, AMD has acquired valuable aeromagnetic data amounting to nearly 5.2 lakh sq km area, spread over important Uranium Provinces of India. Quality of the aeromagnetic data has improved through the ages with the application of new technologies. The most recently collected aeromagnetic data collected by heliborne survey over parts of Vindhyan basin accurately sampled subtle magnetic anomalies at the basement.

Aeromagnetic survey is an effective tool for studying basement features of Middle Proterozoic sedimentary basins. Nonmagnetic (low susceptibility) sediments overlying the basements of distinctly higher susceptibility makes this possible. Study of Aeromagnetic maps over Bhima, parts of Chhattisgarh, Bijawar, Gwalior and Vindhyan basins has deciphered many important geological features viz. basement morphology, movements of basement blocks along faults and continuation of fracture/fault associated with higher value of uranium below the sediment.  AMD will continue to cover more and more areas by airborne survey in future days to meet the exploration challenges imposed on it with better quality airborne multi parameter geophysical data.

Abst-35

The VTEM airborne electromagnetic system-benchmarking continuous improvement via repeat surveys over time

Ken Witherly, Richard Irvine

Condor Consulting, Inc.

The VTEM helicopter time domain electromagnetic prospecting system was introduced into commercial service in late 2002. From that time, the manufacturer has continued to improve the system in terms of geophysical performance and operational reliability. The main technical changes have been in transmitted power and the noise levels in the overall system and in the receiver coil assembly. As the changes and improvements have been introduced in the last four years, the VTEM system has re­flown a number of sites, thus allowing field benchmarking of system changes to be performed. The outcomes of four repeat surveys are examined and discussed in terms of evolving geophysical performance.

While it is in the nature of all equipment manufacturers to make claims that their latest product or model is ‘new and improved’, these claims are often quite difficult to establish without some form of impartial and geophysically sound benchmarking. In a somewhat unique situation, the VTEM system has flown four sites in the past four years thus allowing for comparison of system performance with time.

The VTEM system uses the concentric loop geometry; a main transmitter loop (currently 26 m in diameter but can be varied) with a 1.1 m diameter receiver coil situated in the center. The basic system design.

Abst.-36

Crustal architecture of Gujarat and adjacent areas using aeromagnetic data

B.K.Sahu

GSI Training Institute. Bandlaguda, Hyderabad

About 65% of the total area of Gujarat is covered by aeromagnetic surveys of difierent resolution carried out between early seventies and late nineties. The aeromagnetic data of Gujarat and adjacent areas has been compiled, processed and analyzed to understand the crustal architecture of the region. The datasets were upward/downward continued to a common platform of 1000 m, corrected for IGRF for the corresponding epochs, merged together and gridded with 500mx500m cell size. The total intensity anomaly, vertical derivative and analytic signal maps were used to demarcate different tectonic elements of the region. Quantitative estimation of the depth of burial of the magnetic sources (basement rocks) was carried out through Euler deconvolution. A basement configuration map was prepared from the Euler depth solutions. The magnetic data, especially the alignment of Euler depth solutions, has helped in defining the eastern and western boundaries of the Cambay Basin precisely. A number of deep-seated linear structures including the northwest-southeast trending coastal fault in Saurashtra have been interpreted. A concealed east-west dyke swarm in central Saurashtra was clearly brought out. Clustering of depth solutions (5km and above) and corresponding high analytic signals at several locations throughout Saurashtra indicates existence of isolated circular bodies interpreted as buried basic/ultrabasic plugs. The ambiguity pertaining to the boundary between the basement gneisses and the southern Aravalli supracrustals is resolved. Though, interpretation of aeromagnetic anomalies in a regional scale has helped in understanding the crustal complexity of the area, non-availability of data over a major part of Kachh is a hindrance in interpretation of deep seated geological structures that are responsible for high seismicity in the region.

Abst-37

Airborne Magnetic Method in Regional Mapping and Exploration of Natural Resources: An Overview

S.P.Anand* and Mita Rajaram** 

*McPhar, New Delhi

** Indian Institute of  Geomagnetism, Navi Mumbai

In the present article we give examples/case histories citing the utility of magnetic method for the regional mapping, identification of concealed minerals deposits and for the exploration of ground water. Regional aeromagnetic surveys conducted over a part of the Dharwar craton and Southern Granulite Terrain is discussed. High resolution airborne magnetic surveys conducted over Singhbhum Uranium Province, India for the identification of concealed uranium deposits and Pamiaba Basin, Northeast Brazil, for ground water exploration is briefly outlined.

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terraTEM TM

THE modern TEM SYSTEM

Roger Henderson

Alphageo Instruments, Sydney

terraTEM is very advanced transient electromagnetic survey system using the latest developments in electronics and computing. It was designed by professional geophysicists with many years of experience using TEM instruments and optimising their suitability for field use. Its great range of options makes it a versatile system suitable for a wide range of applications from near-surface to deep exploration. These applications include environmental, geotechnical, archaeological, groundwater, mineral, geothermal and petroleum surveys. The very fast sampling provides for high resolution and the large storage capacity permits a large number of soundings to be acquired in a short time. The one small case incorporating the receiver and transmitter and the battery pack that can both be carried by one person are all that are needed to conduct field surveys.

Abst-39

ELECTRICAL STRUCTURE OF THE UPPER CRUST OF CENTRAL INDIAN SUTURE ZONE, NARMADA - SON  LINEAMENT, DECCAN TRAPS, SOUTHERN GRANULITE REGION, AND EASTERN DHARWAR CRATON

T .Harinarayana

National Geophysical Research Institute, Hyderabad

Cratons and mobile belts are the major tectonic features in Indian peninsular continental crust. To study the crustal structure, major initiatives are taken by DST under deep continental study (DCS) programme. As a part of such a programme NOR!, besides other studies, has taken up wide band magnetotellurics, to map the electrical structure of the upper crust on a regional scale. Long geotraverses have been covered in southern granulite terrain (SGT), Deccan trap (DT) region, central Indian shear (CIS) zone, Narmada-son lineament (NSL) zone and eastern Dharwar craton (EDC). It is observed that upper crustal character are different in these regions. Continent-continent collision tectonics is evidenced in SGT, distinct resistivity changes is observed across CIS in upper crust, correlation of surface faults with conductive features are seen in NSL region and the crustal signature in EDC. Additionally, these studies have brought out a relation between upper crustal rocks and the development of seismicity in the region. In the present paper, the upper crustal signatures of the above five different regions are compared and the importance on the study of electrical resistivity of crustal rocks is discussed to understand the seismotectonics of the region.

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FLUID SATURATION EFFECTS ON ELECTRICAL ANISOTROPY

I. Aigbedion

Geophysics/Physics Department, Ambrose Alli University, Ekpoma,Nigeria

  This paper describes a simple model for calculating the coefficient of anisotropy as a function of hydrocarbon saturation for a laminated formation composed of alternating layers dominated by macroporosity and microporosity. The model is not limited to laminated sand-shale formations but applies generally to formations with vertical variability in capillarity. The degree of anisotropy depends on difference in resisitivity between the individual thin beds or laminations, which depends in part on their fluid saturation.

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Phase tensor and phase sensitive strike analysis of MT data from Deccan trap area – an evaluation

B. P. K.Patro*, S. V. S. Sarma** and H. Brasse***

* National Geophysical Research Institute, Uppal Road, Hyderabad  ** Association of Exploration Geophysicists, Tarnaka, Hyderabad  ***Freie Universitat Berlin, Germany

Determination of directionality and dimensionality is an essential prerequisite for subsequent modeling. Several powerful tools are available for analyzing MT impedance tensor. There exists many types of distortions of telluric fields caused due to near-surface conductivity inhomogeneities and they may occur in different environments of regional conductivity distribution (Bahr, 1991). Indeed no simple and unique mathematical expression exists that can be applied in all cases to separate local and regional contributions to observed impedance tensor. Bahr (1991) proposed seven classes of telluric distortions for the evaluation of measured impedance tensors that facilitate an understanding of the dimensionality of the medium at different depth levels. The concept of Phase tensor that has been introduced recently (Caldwell et al., 2004) does not require any assumptions about the dimensionality of the underlying conductivity distribution. The phase tensor is unaffected by the galvanic distortion. In the present study we have computed the phase tensor properties (a, b and strike) and also Bahr’s classification parameters (k, S, m, h and strike) of the MT data obtained from the Deccan trap covered area from western part of the Peninsular India.  We made an attempt to evaluate the results of application of these two approaches to the real data. Both these approaches yielded similar results while the phase tensor approach seems to have given a stable strike direction. The general strike direction of N45oW for the data on Sangole - Partur and near NS direction for the Koyna profiles are consistent with those obtained earlier (Sarma et al., 2004; Patro et al., 2005) for the entire data sets using other approaches like Groom and Bailey (1989), Smith (1997), McNeice and Jones (2001) and as with the regional geology.

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NATURE OF BOUGUER GRAVITY ANOMALY AND GONDWANA BASINS OF RANGPUR PLATFORM, NORTHWEST BANGLADESH

Idris Miah

* Superintending Geophysicist, Geological Survey of Bangladesh

Gravity survey played an inevitable game in discovering Gondwana basins including coal in Rangpur platform, northwest Bangladesh. The relation between Bouguer gravity anomaly map and Gondwana basins of the area is very much distinct. Deciphers of this relation and establishment of the findings will be very much interesting for discovery of geological history as well as economic minerals of the area.

 Regional Bouguer anomaly map of Rangpur platform bears an especial nature for Gondwana basins. It delineated a number of low gravity closures, which are interpreted as basin like structures within the basement complex. Among these basins some contain Gondwana sediments and a few of them are coal bearing. Rest of the basins is reserved for the preservation of Tertiary sediments only. Natures of Bouguer anomaly map of the Gondwana basins of Rangpur platform are different from that of the other basins of the area. Most probably previous tectonic activity or activities are responsible for this specific signature of the Bouguer anomaly map as well as the formation of Gondwana basins and also the deposition or preservation of Gondwana sediments. Orientation of the gravity low closures over the basins of Rangpur platform is the indicator of Gondwana basins. NS or NNW-SSE oriented low gravity closures having close space contour in the east, point out the presence of basin like structures of that direction. These structures are responsible for the preservation of coal bearing Gondwana sediments. Modeling of low gravity closures having close spacing contour indicated the presence of N-S running steep faults. The shapes of low gravity closures and close contour line in the east of the closures suggested that the subsurface structures might be single-wall grabens or half grabens having major faults of similar orientation in the east.

Drillings at Barapukuria, Badarganj, Burirdoba, Dighipara and Khalaspir proved above findings. All these Gondwana basins are NNW-SSE or NS oriented and their major faults of same orientation running through eastern banks.  So NNW-SSE or NS oriented low gravity closures having close spacing contour lines in the east of Bouguer anomaly map of Rangpur platform suggested the presence of Gondwana half-grabens of that direction, whose major faults running in the east of same orientation prospective for Gondwana coal.

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A Bouguer anomaly computation algorithm with variable

density option

Rambhatla G. Sastry

Department of Earth Sciences, I.I.T, Roorkee

The Bouguer anomaly computation with the inclusion of variable densities honoring surface geology is a long-felt need. This aspect assumes fundamental importance for gravity data acquired over high relief. Here an algorithm for Bouguer correction with variable densities scheme is proposed.

The processing of gravity profile along Mahe-Sumdo- Tso Morari of Ladakh Himalaya is attempted with this algorithm. A maximum difference of about 50 - 70 mGal in the final Bouguer anomaly is observed between data processed through normal procedure with uniform Bouguer density (=2.67gm1cc) and by proposed one with variable density. This underlines the importance of the proposed scheme.

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Gravity and magnetic surveys in the northern part of the                   Cauvery basin

D.Bhaskara Rao, K.V.V.Satyanaryana and A.Praveen Kumar

Andhra University, Visakhapatnam

The gravity and magnetic data along three profiles across the northern part of Cauvery basin have been collected and the data is interpreted for basement depths.  The first profile runs from Cuddalore to Villupuram, and the second starts from Port Novo and ends at Neyveli and the third runs from Tarangambadi to Vridhachalam. Gravity profiles are interpreted using variable density contrast with depth. The density contrast function with depth has been constructed using well log density data obtained from ONGC Chennai. From the Bouguer anomaly, the residual anomaly is constructed by graphical method with well data as control. The residual anomaly profiles are interpreted using prismatic as well as polygonal models. The maximum depths to the basement are obtained as 3.0 km, 3.8 km and 4.6 km for the first, second and third profiles respectively. Magnetic anomaly profiles are also interpreted using the dyke model. The total field magnetic anomaly profiles, in general, consist of number of peaks and the individual peaks are interpreted as intrusive bodies into the sedimentary basin using dyke models.

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Gravity studies over the volcanic region of Saurashtra Peninsula and Central India: some inferences about regional tectonics

Bijendra Singh* , M.R.K.P. Rao*, A.P. Singh*,  D.V. Chandrasekhar*, M.B.S.V. Rao*, V. Vijaya Kumar, D.Ch. V. Raju, V.M. Tiwari*, K. Arora, S.K. Prajapati** and T. Chandrasekhar*

*National Geophysical Research Institute, Uppal Road, Hyderabad **Indian Institute of Geomagnetism, Mumbai

Gravity group of National Geophysical Research Institute (NGRI) has carried out extensive and detailed gravity measurements over the Saurashtra Peninsula and Narmada-Tapti region of Deccan volcanic province as a part of integrated geophysical surveys for the delineation of subtrappen Mesozoic sediments. Bouguer anomaly map of this region depicts number of short wavelength anomalies of varying amplitudes superposed on large wavelength regional anomalies due to deep seated sources. The short wavelength having large amplitude circular gravity highs observed over the region are caused due to igneous intrusion of Deccan origin where as small amplitude gravity lows reflect subtrappean sediments. On the other hand, large wavelength relative gravity lows observed over the topographic rise in central Saurashtra, Over Ajanta hills towards the south of Tapti and over the Vindhyan mountains north of Narmada River bears an inverse correlation with the topography indicating mass deficiency beneath the excess topography load due to isostatic compensation. However, the most conspicuous feature of the map is the E-W trending relative gravity high aligned over the Satpura mountain belt. This in itself is an anomalous signature and suggests under compensation or presence of subsurface load beneath the mountains.

To understand the regional tectonics, joint analysis of gravity and topographic data has been carried out. The fact that near zero free-air anomalies in regions of elevated topography points towards state of complete isostatic compensation provides an unbiased estimate of isostatic regional field from the corresponding Bouguer anomalies. Using this concept, regional isostatic anomaly map and regional compensated topographic map are prepared. Superposition of these maps indicates negative correlation. From the comparisons of residual (uncompensated) topographic map (obtained by subtracting the regional elevation from the observed elevation) with that of regional and observed topographic maps, it is observed that the entire topography of central Saurashtra, the Ajanta Hills and the Vindhyan Plateau are compensated where as the Satpura mountains are largely uncompensated. Presence of an E-W trending uncompensated linear mountain belt in the southern part of Saurashtra Peninsula suggest continuation of Satpura trends towards west over the southern part of Saurashtra Peninsula. Thus, the analysis of gravity and topographic data has brought out two very distinct state of isostatic compensation prevailing in this region. The topographic mass on either side of Narmada-Tapti region are fully compensated and may indicate pre-Trappean topography. On the other hand, uncompensated topography in line with Satpura trend might have originated after the Deccan volcanism and appears to contribute significantly to the seismicity of the this region.

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THE FALLACY OF BOUGUER ANOMALY IN GEOPHYSICAL EXPLORATION AND THE NEW CONCEPT OF THE THEORY OF GRAVITY ANOMALY

M.Kesavamani, C.Ramachandran,

M.V.R.Krishna rao,R.M.C.Prasad and P.K.Rammohan

Geological Survey of India, Bandlaguda, Hyderabad

The Bouguer anomaly, in vogue, is really an enigma because it is believed to indicate both the gravity field/mass and the density variations. The theory of gravity states that gravity field is proportional to the mass distribution irrespective of the density of the sources. However, the vertical gradient of gravity field indicates the negative density contrast or the density of the causative source. Hence, if the Bouguer anomaly brings out the mass distribution, it cannot indicate the density variations. On the other hand, if it indicates density variations, it cannot reveal the mass distribution and the measured field should be proportional to vertical gradient of gravity. The primary object is to relate the difference of, obtained observed gravity (gobs) on the surface of the earth and the theoretical value of the force of gravity on the spheroid (gTh), on to the geoid equi-potential surface for a comparison.

In contrast to the conventional gravity anomaly with conceptual corrections, we define the normal gravity anomaly as vertical gravity anomaly (VG) which is the vertical difference [VG= (gobs –gTh.)], between the observed gravity on the surface of the earth and the theoretical gravity on the   geoid. This is proportional to vertical gradient of gravity and indicates the apparent density variations. This is similar to the calculation of Vertical gradient of gravity in free air correction.

The Bouguer anomaly defined by the equation (BA)= [g obs + (Free-air Correction (FC) - Bouguer correction (BC) + Terrain correction (TC) –g Th] can be written for simple Bouguer correction as [BA= VG + 0.3086* h - 0.04190* h*s ] or [BA/h=VG/h+0.3086-0.04190*s].  This is the actual obtained anomaly after corrections at station level. This is the difference between the observed gravity and the theoretical gravity with variable h. We show that the Free- air anomaly (FA) and Vertical gravity anomaly (VG) are the extremes of Bouguer anomalies (BA) for the assumed densities of Bouguer slab. When density of the Bouguer slab (s) = 0; the BA = FA= [VG+ 0.3086*h] or [BA/h= FA/h= VG/h+0.3086]. Also, when s= 7365kg/m3, the BA= [VG,] or [BA/h = VG/h]. These anomalies indicate different signatures for the same causative source at station levels. However, When h is constant i.e., on a horizontal surface, the VG, FA and BA show similar anomalies as that observed in plain areas. These anomalies are proportional to non-normalized vertical gradient of gravity with change in background levels separated by the correction factor. So, the gravity anomalies indicate only the density variations and not the gravity field.

Integrating VG with h gives the actual gravity field, called here, as Simple gravity anomaly (SG) at station level which indicates the mass variations. In this case where the VG is not normalized, it is numerically equal to negative of VG. Hence, Simple gravity [SG= - VG].  As the measured anomaly is the vertical differences, the station level VG and SG anomalies can easily be transferred on to the geoid surface without any corrections. By logic, the anomalies obtained on uneven surface should be a mirror reflection of the anomalies when transferred on to the even surface. That is, the observed data on the uneven ground surface equals to negative of the measured field on the even surface. The VG and SG at geoid level are respectively equal to negative of VG and SG on the ground surface. Therefore, station level VG (SLVG) is equal to geoid level SG (GLSG) which is the gravity field on the geoid. Similarly, station level SG (SLSG) is equal to geoid level VG (GLVG) which indicates the density variations on the geoid. Also, for a comparison on a horizontal datum, the free air correction factor (FC) may be used to project the station anomalies on a common datum in free air. If FC is used as a correction factor for the increase in height and added to the station anomalies, all the anomalies show similar signatures as that obtained in VG. However, if FC is used for upward continuation and the correction is subtracted, because of the decrease of natural vertical gradient with height, all the anomalies show similar signatures as obtained in SG, revealing the mass distribution.

The Bouguer anomalies indicate the negative density contrast areas as “highs” and positive contrast areas as “lows”. Therefore, the Bouguer anomaly with conceptual corrections, referred to as the gravity field, is a fallacy in geophysical exploration because it is proportional to Vertical gradient of gravity. So, the difference in the gravitational forces at two levels is proportional to vertical gradient of gravity and hence decreases with height. Consequently, the gravity field/ mass increases with height. Our differing fundamental perception of the concept of theory of gravity anomaly delineates the density and mass variations at both the station and geoid levels, and minimizes the ambiguity in geophysical exploration.

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GEOPHYSICAL SURVEYS OVER BUNDELKHAND GRANITES OF JHANSI-LALITPUR DISTRICTS, UTTAR PRADESH FOR LOCALISING THE TARGET AREA FOR MINERAL POTENTIAL

Amar Singh, Narendra Singh, H.P Mishra,                     S.P Chaube, M.K.Rai, S  K. Singh, R S Acharya,              R P  Rai

Geophysics Division, Geological Survey Of India, Lucknow

The paper presents the results of regional gravity-magnetic surveys in Bundelkhand Granitoid Complex falling in peripheral region of Aravali Craton in Jhansi - Lalitpur districts, Uttar Pradesh. The results of gravity-magnetic survey along six traverses at 200 m station interval in Bansi area, Lalitpur district UP have also been discussed. Numerous gravity high closures supplemented by magnetic high as well as low closures have been delineated. The most striking feature in Bansi area is cross ultramafic dykes in the northern vicinity. These areas have assumed significance in the light of Jog area close to the gravity and magnetic closures. Though the large tract of Aravali craton passes through the alluvial covered areas yet its proximity to Panna-Majhgaon diamondiferous, mineralized belt of Rajasthan and the mineralized area of Sonbhadra district covering southeast portion of Uttar Pradesh, appear target areas for mineral/diamondiferous potential of the craton.

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MINERAL POTENTIAL OF THE DHANJORI METAVOLCANICS EAST SINGHBHUM, JHARKHAND

L.K. Das, K.K.Dasgupta and M.K. De

 Geological Survey of  India, Kolkata

Deep Electrical Profiling (DEP) has proved to be very useful in identifying buried conductive anomalies in Dhanjori basin. 2-D modeling of such anomalies indicated a near vertical conductive body (80 ohm m) within the metabasics of Dhanjori basin. A deep stratigraphic cum structural borehole has been drilled vertically upto a depth of 768 m over this anomaly zone which proved flat lying metavolconic assemblages grading from acidic to ultra basic in composition with interlayering of tuff and sediments. At least four different flows have been identified following the classical top and bottom criteria followed in stratigraphy.

The bore hole lithology is in itself a revealation as it is different from the expected lithostratigraphy of Dhanjori basin. The metabasics are highly sheared and striated and the propensity of shearing increases with depth.

Deep Electrical Sounding (DES) has been successful in estimating the thickness of the lithopackage which is within 1 km.

SP, resistance, natural gamma ray logs run up to 750 m depth indicate 37 distinct conductive zones, varying from a fraction of a meter to several meters. Some of these  are sulphides with pyrite, chalcopyrite, covellite and arsenopyrite assemblages. Density and chargeability values measured in the laboratory on selected core samples corroborate the above findings.

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ELECTRICAL AND PEXMEL RESPONSES OVER A SULPHIDE BODY IN SHYAMI-KI-DHANI AREA, SIKAR DISTRICT, RAJASTHAN

Dinesh Gupta*B.Banerjee**B.B.Bhattacharya***

*GSITI, Zawar Centre, 15-16 Jhalana Doongri, Jaipur

**Formerly with Geophysics Division, GSI Kolkata.

 *** S.N.Bose National Centre for Basic Sciences

Massive sulphide mineralisation with total sulphide content (TSC) about 25-30% in visual estimate was reported from the debri of a dug well dump near village Shyami-ki-Dhani in Neem-ka-Thana tehsil of Sikar district  of Rajasthan. Integrated Geological, Geophysical & Geochemical surveys were carried out in the area. In addition, to identify the type and abundance of metallic mineral present in the study area, PEXMEL (Partial Extraction of Metallic Ions by Electrolysis) survey was also carried out over the geophysical & geochemical anomaly zone. The results of the synergistic approach are presented in this paper.

The study area falls in the Survey of India toposheet No. 45M/14. The formations consists predominantly of low grade metamorphosed calcareous, arenaceous and argillaceous sediments belonging to Ajabgarh Group of Delhi Supergroup. Major lithounits exposed in the study are carbonaceous phyllite, marble intercalated with calc schists and gneisses.

 Surface geophysical surveys employing SP, IP, resistivity, and magnetic methods were carried out in the area. Significant anomalies have been recorded in this area.

Results of PEXMEL survey over test traverse in this area are indicating 1025 NCC units for Zn over a background of 100 NCC units; and 150 NCC units of Pb over a background of 30 NCC units.

A rock sample analyzed from well dump gives the following values: 1.3% Zn and 0.3% Cu.

Surface Geophysical surveys employing electrical methods along with PEXMEL method has been successful in identifying sulphide body’s extension and type of metallic sulphide.

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STRUCTURES AND TECTONICS OF KOPILI VALLEY AREA AS SEEN IN THE LIGHT OF GRAVITY - MAGNETIC DATA

A. Hakim, B. Pathak and S. M. Syiem

Geological Survey of India, Shillong

Gravity - Magnetic survey in and around Nagaon district, Assam have revealed a significant gravity anomaly of the order of about -8 mgals. A basement depression towards northern part of the area around Juria - Rupahi and Gerua Ati could be traced. The NW – SE trending linear gravity gradients and vertical as well as total field magnetic anomalies from Raha to Puranihatkhola is observed in close proximity to ‘Kopili’ lineaments (faults?). These Geophysical signatures may have some bearing with the basement tectonics in the valley. Similarly, the NE – SW trending steep gravity gradients and related magnetic features passing through Misa – Samaguri – Nagaon and Baropujia/Raha in west and beyond is also found  proximity to ‘Kalang fault’. The gravity ‘high’ with complex circular features found in Bouguer anomaly map around Kampur  and towards Borapani Reserve Forest could be the disturbances in the subsurface resulting from neotectonic activities in the area

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AN INTEGRATED APPROACH TO DELINEATE PULLAMPET SUB-BASIN BOUNDARY IN THE SOUTHERN NALLAMALAI FOLD BELT ANDHRAPRADESH, INDIA

I.V.Reddy,M.S.Reddy and G.Satyanarayana

Geological survey of India,Hyderabad

The Nallamalai Fold Belt (NFB), fonns a part of the middle to late Proterozoic intracratonic Cuddapah Basin consists of rocks belonging to Nallam~hi Group of Cuddapah supergroup. The Nallamalai Group comprises of a sequence of quartzite (BairenkondaINagari Fonnations) overlain by argillaceous sediments with bands of dolomite and quartzite(Cumbum/Pullampet Formations).The Pullampet sub-basin boundary was delineated on the basis of new findings or observations made recently in Mandavi river and Cheyyair river valley sections. While in the northwestern part, the Pullampet sub-basin is in contact with the Papaghni sub-basin, towards central and eastern part, it is in contact with the Cumbum sub-basin. In the western and the southern parts, it directly rest unconfonnably over the Peninsular Gneissic Complex. This geological interpretation is further supported by geophysical and geochemical data.

Based on total intensity aero magnetic and airborne electro-magnetic data around east-west trending Cheyyeru river and north-south trending Patagunjan river respectively, it is possible to distinguish the rocks occurring on either side of the rivers are different and represented as Cumbum and Pullampet Fonnations. Major oxide and trace element data of the rocks in these two fonnations are different and supports the above geological interpretation.

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EVIDENCES OF A POST CRETACEOUS TRANSPRESSIONAL REGIME INTERPRETED FROM THE MORPHOTECTONIC FEATURES IN THE CONTINENTAL SHELF BETWEEN THE RUSHIKULYA AND KUSHABHADRA RIVERS, ORISSA COAST, INDIA

B. K. Nandi, R. Singh, S. R. Samadder, G. K. Dutta, B. P. Pal and L. K. Das

Geological Survey of India, Marine Wing, Kolkata

Shallow seismic reflection data along several coast perpendicular traverses off Chilka Lake in Orissa, have brought out the dispositions of the seabed and a few sub-seabed reflectors along with an important morphological upwarp extending over 115 km with a relief of 3-8m associated structural breaks. These breaks are mostly aligned in NE-SW direction.

 Free-air (FA) gravity anomaly of the offshore and onshore region between the Mahanadi and Godavari basins clearly brings out an N-E trending gravity high or crustal thinning along the Eastern Continental Margin of India (ECMI). This FA anomaly and the geometry of the shore line is in sharp contrast to the gravity anomaly as well as the shore geometry down south, where the ECMI suddenly turns N-S along the 80° E meridian. Superimposed on the gravity high referred above, a profound gravity low has been identified in the shelf area off Chilka Lake. Fortuitously, this gravity low zone is found to be an approximately 5.4 km wide canyon/channel cut, in the shelf area, ahead of the shelf-break, with a vertical relief of 400 m from the seafloor. The actual relief must be much larger, a part of which is presently filled up by sediments. Obviously presence of such NE-SW trending morphotectonic unit in this part of the shelf area is a very significant features in the shelf morphology which is absent down south.

Presence of an upwarp in the sea floor of the dimension of 115kmX8m associated with a reverse fault on the sea ward side with identical down throw of 8m, conclusively proves a compressional tectonic regime in the post Cretaceous period following the emplacement of the 85° E ridge. Formation of the SW-NE trending canyon in the sea floor is an evidence of strike slip movement in the same direction. This is further corroborated from the focal plane solutions of earthquakes north and south of the area. Thus the morphotectonic units mapped in this zone in the continental shelf off Orissa coast is an ideal example of a transpressional regime following the emplacement of the 85° E ridge.

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A RE – EXAMINATION OF THE CRUSTAL STRUCTURE ACROSS NARMADA – SON – LINEAMENT (NSL), MADHYA PRADESH, CENTRAL INDIA USING MAGNETOTELLURIC AND GRAVITY MODELING

D.C. Naskar, L.K. Das, R.K. Majumdar, K.K. Roy and Kalpan Choudhury

Geological Survey of India, Kolkata

The subsurface geology and tectonics of parts of Central India bounded by longitudes 74° E to 81° E and latitudes 20° N to 24° N is largely unknown . The area falls in and around the Narmada–Son Lineament (NSL) zone and is characterised by several major tectonic/geological provinces, e.g. the Archaean complex, the lower Proterozoic Bijawar/Mahakoshal group, the mid/upper Proterozoic Vindhyan sediments, the Permo–Carboniferous Gondwanas, the Cretaceous Bagh, Lametas, Deccan traps and the Recent alluvium.

Geophysical surveys comprising gravity and magneto telluric (MT) sounding have been carried out along two profiles in the area. Rapid Relaxation Inversion technique (Smith and Booker, 1991) generated by the inversion of (ñTE, öTE), (ñTM, öTM) and (ñTE+TM, öTE+TM) has been used for 2D modeling of the MT data while qualitative interpretation of the MT data has been carried out using the TE and TM mode apparent resistivity and phase pseudo–sections (Swift, 1967). Qualitative and quantitative interpretation have been carried out for the above data sets as well as for the Bouguer gravity map of the area which includes 2D gravity modeling of two profiles using  the GEOSOFT (Oasis Montaj), Version 6.0, north – west geophysical associates of Corvallis, Oregon.

The principal objectives of the research investigation are the delineation of both the shallow and deep geological structures in the crust and the upper mantle, the dispositions of the different rock units in the area specially at depth and to demarcate the zones of rifting of the continent in the area through mapping of the highly conducting rocks in the crust and the upper most mantle. Geophysical investigations have indicated several interesting results.

The rifting of the continental crust is evident in MT results from the large extent of the conductive zone encompassing the lower crust and the upper most mantle. Lithosphere–asthenosphere boundary at 110–120 km depth has been deciphered from the MT survey.  The Moho depth north of A is comparatively less what is interpreted over the concealed Vindhyan basin below the trap which is around 37 km . It is rightly so as has already been discussed by Kaila et al. (1985) through their DSS work. Again there is a substantial Moho upwarp around the Satpura horst structure. The presence of the Narmada rift has thus also been established from the iinterpretation of the gravity data due to shallow crustal thickness . From TE and TM mode vertical cross section it is evident that the lower crust north of Chhegaon Makhan and the upper mantle is distinctly made up of relatively conductive rocks, in both the TE and TM modes. This is in sharp contrast to the crustal signature over the Satpura horst, where it is more resistive, up to a depth of almost 20 km. These may represent dehydrated exhumed lower crustal rocks in the Satpura mobile belt. A Moho upwarp has been interpreted from the gravity model around Jabalpur  which created the Proterozoic rift on which the Mahakoshals and the Vindhyans were deposited. Further, geophysical surveys have identified the various subsurface formations and their thickness in different areas.

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Geophysical Investigation for New Coal Prospect in Ulia - Sankargarh Area, Surguja District, Chattisgarh

D.C. Naskar, A.K. Lahiri, C.S. Venkiteswaran,  L.K. Das  and P.K. Chakraborty

 Geological Survey of India, Kolkata

Geophysical surveys employing gravity, magnetic (VF) and resistivity sounding have been conducted in Ulia-Patna-Jagima area bounded by Lat: 23°20' to 23°25' N, and Long: 83°30' to 83°40' E over a virgin Gondwana prospect. The known coal fields in the vicinity of this area are Bishrampur and Tatapani-Ramkhola. The investigation was taken up to delineate the location and the thickness of the coal bearing Gondwana formation in the area.

 The interpreted vertical resistivity depth section are shown for easy comprehension.  The coal bearing Gondwana formations are inferred, to occur at sounding locations 2, 3, 4, 5 and 7 respectively. The Gondwana formation is characterized by resistivity ranging from 50-400 Ohm-m. The thickness varies from 282-442 m. In almost all the soundings with large electrode separations the resistivity sounding curves begin to rise asymptotically at an angle of 45° suggesting highly resistive substratum indicative of basement below the Gondwana. Three gravity-magnetic (GM) traverses perpendicular to the strike of the exposed coal outcrops have been observed in the area. A Bouguer anomaly low of -10 mgal is observed over the Gondwana lithic fill. In order to understand the variations of Bougue~ anomaly in term of basement topography, 2D gravity modeling has been done over Ulia­Patna-Jagima profile using GMSYS program module of GEOSOFT software package.

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Role of Multi component in fracture mapping

Ashish Kumar Singh. Pawan Kumar Singh.               Amit Singh

Banaras Hindu University, Varanasi

Where reservoir rock is very impermeable (limestones. cherts, dolomite, etc.), ftactures may provide all or most of its porosity and effective permeability .Studies of producing oil wells in ftactured limestones have determined a single fracture, under favorable reservoir conditions, can provide sufficient permeability to yield good amount of oil per day. Consequently, mapping locations of high intensity ftactures and determining their orientation/lateral extent could be of great value in reservoir development, especially for locating horizontal well sites. Multi component surveys (nine components) have proven effective in delineating ftacture strike, density, and lateral extent. P-waves are difficult to use for ftacture prediction as these are more likely to be affected by changes outside the reservoir than inside. Vertically traveling S­waves have a horizontal particle motion. If the motion is parallel to the cracks, it will encounter a stiff material; whereas if it is perpendicular, it will encounter a flexible material. This change in rigidity of the rock causes a change in the velocity. However, high acquisition and processing costs, limited availability of S-wave sources, and volume/quality of S-waves rendered such surveys unpopular. P-S converted waves were deemed a cost-effective alternative and were investigated in the 1980s. The reasons were simple. P-S converted waves are (1) generated with a compressional source, (2) less labor intensive,(3) expected to contain the same information as S-waves, and (4) more practical in 3-D surveys (reduced data volume and cost). In practice, however, dealing with P-S waves is more cumbersome than with non converted waves. Through careful survey design/acquisition/processing, anisotropy parameters can be successfully estimated from P-S converted waves.

Abst-56

IDENTIFICATION OF TRAP THICKNESS SANDWICHED BETWEEN RAJAHMUNDRY SAND STONES AND TIRUPATI SANDSTONES BY GEOELECTRICAL METHODS

V. Vijay Kumar

A.P. State Groundwater Board, Hyderabad

 Geophysical methods have been used to identify various subsurface features in the field more so the aquifer boundaries in ground water studies.The geographical area of east Godavari dist can be broadly divided into three stratigraphic units viz  Rajahmundry sandstones belonging to upper tertiary group,  deccan traps  and Tirupati sandstones belonging to lower  tertiary group. The ground water potentials in the Tirupati sandstones aquifers are higher when campared to Rajahmundry sandstones as observed from the bore/tube wells drilled in the disrtict. The traps occur in-between these two formations. The depth of occurrence and the thickness of the trap zone vary from place to place in the district. Hence the identification of this trap zone is very important and a challenging task since this helps not only in drilling but also in deciding the aquifer to be tapped for ground water exploitation. Vertical Electrical Sounding (VES) surveys have been conducted to study the subsurface features. The success of this method and the data from litho logs of drilled wells together with electrical log data have been analyzed to understand the behavior of the trap formations in the district. Few resistivity imaging sections and also depth sections were prepared for comparative study.

Abst-57

SITE AMPLIFICATION STUDY IN THE GARHWAL HIMALAYAN REGION BY USING CODA WAVES

 Babita Sharma*, S.S.Teotia**Dinesh Kumar**

*ISR, Gandhinagar ,**Kurukshetra University, Kurukshetra

Vertical components of twenty four earthquakes came in the Garhwal region in 1989, are taken to study site amplification using coda waves by using reference site method in the frequency range of 3 Hz to 24 Hz. For this purpose four sites(DHR, POR, SUR, CHK) are chosen in the area. Taking POR as reference site, relative site amplification is estimated at other three sites. Site amplification factor in the frequency range of 3 to 24 Hz at DHR, CHK and SUR vary from 1.11 to 2.25, 1.48 to 3.72 and 1.37 to 1.68 respectively. By this analysis it is observed that the site amplification is more at the site having softer rock geology. At DHR and CHK, where sandstone is present, the variation in site amplification is more. On the other hand at SUR, the presence of Quartzite implies relatively less site amplification variation.

Abst-58

Tectonic structure of the Guhagar-Chiplun region based on ground magnetic data

Vinit C. Erram and Gautam Gupta

Indian Institute of Geomagnetism,Navi Mumbai

      Morphometric data of the Konkan coast region of Maharashtra reveals the presence of a number of NNW-SSE trending fractures. One of the prominent fractures, as clearly depicted from satellite imagery is seen to be developed from the Vashishti in the North to the Shastri river in the South. In order to delineate the fractures and fault zones and to ascertain their depth, ground magnetic studies have been carried out in the Guhagar and Chiplun region. The ground magnetic data revealed the presence of some fractures parallel to the Arabian coast; prominent amongst them are at Shringar Tali and Rampur, having depths of 2.5 and 2.8 km respectively. It has been inferred that these fractures penetrate through the basaltic and the Dharwar rocks and form a part of the fracture system-paralleling fault.

Abst-59

Geospot vis-à-vis Geoelectricity

T.K.Das and A.Chaudhuri

Centre for Space Physics,Kolkata

Self  potential (SP) electric anomalies are explained on the basis of geospot model. The Piezoelectric effect occurs in the crustal rocks in presence of the geospot. SP anomalies are caused due to this Peizoelectric effect.

 Self  potential (SP) electric anomalies as observed by the researchers from different countries like Russia, Greece, China, Japan and the United States are found to last upto several days before an earthquake. Their correlation with earthquake occurrence has been emphasized as  possible means for predicting earthquakes.  Several models come up parallely to explain these observations, including streaming potential effect, Peizoelectricity and resistivity variation. In this paper we have tried to explain SP electric anomalies in relation to a geospot which is a strong magnetic field zone formed at the core-mantle boundary of earth.

Abst-60

STRUCTURAL INFERENCES FROM TOTAL MAGNETIC INTENSITIES IN THE GADAG REGION: A CASE STUDY

Ramadass G* and Veeraiah B**

* Department of Geophysics, Osmania University, Hyderabad

** National Geophysical Research Institute, Hyderabad

2-D total field intensity magnetic observations were made over an area of approximately 2750 Sq Km with maximum N-S and E-W extents of 55 and 50 Km respectively in the geologically and tectonically interesting Gadag region with a view to obtain a clearer perception of the structural configuration of the region. From qualitative analysis of the total magnetic intensity data, several tectonic features – deep seated faults and magnetic linears in the region were identified. Quantitative evaluation consisted of modeling. Eleven profiles were digitized from the low pass filtered output of the magnetic data and modeled for the regional magnetic interface.  The computed depths to this interface along each profile were digitized and contoured to obtain its subsurface topography.  This magnetic interface was found to vary between depths of less than 1 km to 8 km. Further, from results of forward modeling of the anomalous body within the schistose zone in the region, it was found that the banded iron formation (BIF) occurred at depths ranging from 300 to 1300 meters.

Abst-61

Density Determinations Using Nettleton Methods—A classic Case Study  in the foot-hills of the Himalaya, Himachal Pradesh

D .Saha, S.M. Chatterjee,  D.Sar

Oil and Natural Gas Corporation Limited

Approximation and selection of correct average density of the underlying rocks  in the Himalayan foothills  as acquisition parameter for Precision Gravity Survey and for regional interpretation of the sediments is very important.  The present study was carried out using raw field gravity data of  one of the investigation of Himachal Pradesh to evaluate critical density of the rocks within local range of topography between 200-1600mtrs above Mean  sea level .  The density of 2.6gm./c.c. has been found to be most promising average surface density of the rocks using different density curve of rocks ranging from 2.0 to 3.0 gm./c.c and critically examining their behavior with their respective free-air and elevation values.

Abst-62

Applying Geophysics to Environmental and Engineering Problems: A Texas Sampler

Jeffrey G. Paine

Bureau of Economic Geology, The University of Texas at Austin

Themes common to successful applications of geophysics to near-surface problems include understanding the environmental or engineering issue, enumerating physical properties that might serve as relevant proxies, designing and conducting a field experiment to measure the properties (taking into account such things as target size, depth, and physical property contrast), and interpreting the results in the dual context of the measurements themselves and what they tell us in our specific application. Following this basic approach, we have applied several geophysical methods (particularly EM and seismic) to various near-surface problems in Texas over the last several years. In our most recent study, we used helicopter-borne EM instruments to acquire “streambed induction logs” along a major river that identified salinized stream segments and helped quantify natural and man-induced contributions to stream salinity. Other recent examples address global issues such as salinization, groundwater exploration, wetlands mapping, transportation infrastructure, and hydrogeologic site characterization.

Abst-63

APPLICATION OF GEOELECTRICAL METHODS IN HYDROPOWER

PROJECTS OF NEPAL

Surendra Raj Pant

Tribhuvan University,Kathmandu, Nepal

Until mid-90s conventional seismic refraction method was the only subsurface geophysical investigation in hydropower projects of Nepal. Usually proposed study areas lie in the mountain and are inaccessible for vehicular transportation. Very high heterogeneity of surface material creates poor energy coupling for both source and receiver. Because of the high-energy content and easy transportability, explosives were the preferred source for the generation of seismic waves.Due to the rebel activities in the recent years, however, the government has tightened regulations  on transporting and using explosives. This situation has forced the hydropower projects to search for alternative geophysical methods in place of seismic refraction. Electrical resistivity tomography (ERT) was selected as a possible replacement method. In the initial stage of the application of ERT, various electrode arrays and data acquisition approaches were evaluated for data quality and subsurface resolution. Experience in different projects reveals that the pole-dipole electrode arrangement with data levels at different electrode spacing is suitable for better resolution of the subsurface material. Other geoelectrical methods that used to study the subsurface are the self-potential (SP) and mise-á-masse methods. SP-method was used to localize the leakage and outflow area in the existing dam and mise-ámasse method was used to assess the groundwater flow velocity in the proposed dam area. The results of the investigation indicates that these methods can furnish most of the subsurface information needed for a hydropower project

Abst-64

An Improved Methodology For Identification of Ground Water Potential Zones in a Typical Khondalitic Terrain

B. Venkateswara Rao

JNTU, Kukkatpally, Hyderabad

Integrated studies for evaluation of ground water potential in a typical khondalitic (garneti ferrous sillimanite gneiss) terrain is accomplished by proposing an improved methodology involving numerical weights and ratings assigned to various geophysical and geomorphic parameters leading to a computation of ground water potential Index (GWPI) of a given site.  It is found that the GWPI of any site should be 35 and above in order to have 75% success rate of wells in a given khondalitic terrain with an yield norm of 8000 litres per hour (LPH) per a well.  This methodology is an improved one in the sense that the weights are tested for their validity unlike in the earlier methods proposed for similar purposes.  The studies indicate that the ground water potential zones are located on gently sloping uplands covered by either shallow buried pediplains or washplains situated between the lineaments or streams. In addition, the potential areas should have basement depths either between 20-30 m or 40-45 m with an aquifer resistivity range 25-65 ohm m. Aquifers in the low lying areas near streams became kaolinised and acting as barriers to accumulate ground water in the flat-upland areas.

Abst-65

Comparison of resistivity imager and self potential applications in groundwater investigation in hard crystalline terrain of                   Sri Lanka

S.U.P. Jinadasa

, National Aquatic Resources Research and Development Agency (NARA), Crow Island, Colombo,  Sri Lanka

A geological and geophysical study was carried out to investigate groundwater accumulations in the hard rock terrain in Monaragala, Sri Lanka. It is important to identify the subsurface structures in the area for studying groundwater bearing formations. In the past, traditional methods were widely used for identification of regional geological structures. Information from geological methods alone does not provide reliable clues to groundwater accumulations. These methods appear to be suitable for detection of subsurface structures only. Therefore, it is important to carry out different geophysical survey methods to confirm these results.

The named Timbiriya area at Monaragala district Sri Lanka was selected to conduct this research. Geological map and aerial photo interpretation show the subsurface structures in area. The well-defined lineaments run along the Mandappan Oya and its tributaries in Timbiriya area could be clearly identified. Small-scale linear structures could be identified in Malgastalawa and Nakkala areas.

During this study resistivity imager system and self-potential methods were carried out parallel to each other to get better comparison of both methods. Site selection was done with the assistance of magnetic survey, which was conducted in the previous year in Monaragala area. These locations were identified as geologically weak zones which favourable for groundwater accumulation by magnetic and geological methods.

Possible subsurface structures could be identified by resistivity method as well as the self­potential methods. But magnetic data interpretations do not always tally with the resistivity and self-potential interpretations. Comparison of resistivity and Self potential is always given better result than resistivity imager survey alone.

Abst-66

Time-Domain Electromagnetic Soundings to Characterize Water Quality Within a Freshwater/Saline-Water Transition Zone, Estancia Valley, New Mexico-A Reconnaissance Study

Sachin D. Shah, Wade H.Kress, Lewis A.Land

USGS, Texas, USA

During July 2005, The U.S. Geological Survey, in cooperation with the New Mexico Bureau of Geology and Mineral Resources, conducted a reconnaissance study in an area of about 525 square kilometers in the Estancia Valley in central New Mexico USA to characterize water quality using time domain electromagnetic (TDEM) surface geophysical soundings. Seven TDEM soundings were collected along a south-south-easterly profile about 19.3-kilometer long in which apparent electrical resistivity was measured to depths of about 80 meters. The study area is composed of deformed Paleozoic and Mesozoic rocks overlain by Cenozoic (Quaternary) sediment. Paleozoic and Mesozoic-age strata generally dip eastward and Quaternary sediment overlies progressively younger strata from west to east. Subsurface geologic units contain water of varying quality, from fresh to slightly saline. Faults and fractures characterize the subsurface. A Geonics Protem 47 system using a 60-meter by 60-meter square loop transmitter antenna was used to collect the TDEM soundings. For each sounding, voltage data were averaged and evaluated statistically using a computer program called NTEMA VG v. 3.04. The principle finding of this study is the potential relation between faulting and the occurrence of freshwater and slightly saline water, based on the configuration of resistivity along the TDEM profile.

Abst-67

An investigation of groundwater condition by Geoelectric and Geochemical methods: A case study in northern part of Sagar Island, South 24 Parganas, West Bengal, India

R.K.Majumdar, D.Das

Jadavpur University, Kolkata

Integrated geoelectric and geochemical investigation are carried out in the northern part of Sagar Island region to assess the prevailing groundwater condition and chemical quality of groundwater. Geologically, the area is constituted of alluvial and marine sediments of quaternary age, which is underlain by the vast thickness of tertiary sediments. Vertical electrical soundings (VES) in the area of investigation show mostly five layers consisting of topsoil, saline water, brackish water, clay layer and fresh water bearing zones. The YES findings show potential freshwater bearing zone of appreciable thickness at depths from 175 m to 220 m under confined condition. The results of YES studies significantly correspond with the borehole data and a litho-resistivity relationship is established for this area. Chemically the fresh groundwater is Na-HC03 type with TDS ranging from 465 mgll to 560 mg/l. Chemically the water is safe for drinking and domestic purposes with low to medium SAR values ranging from 4.00 to 6.74. The seawater contamination (SWC) values for these water samples are significantly low.

Abst-68

GEOPHYSICAL STUDIES TO ASSESS THE COMPETENCE OF BEDROCK ALONG CHIKAR DAM ALIGNMENT, PAR-TAPI-NARMADA LINK PROJECT, DANG DISTRICT, GUJARAT

H.P. Mishra, Babu Lal, R.L.Regar, Amar Singh,, M.K. Rai, P Sharma, N. Singh, Sanjai K.Singh

 Geological Survey of India, Sector E, Aliganj, Lucknow

The paper presents the results of Geophysical surveys conducted along the Chikar dam alignment in district Dang, Gujarat. The Par-Tapi-Narmada Link Project of National Water Development Agency (NWDA) is first Water Resources Management Scheme involving inter basin transfer of Par, Nar, Tan, Ambika, Khapri and Purna rivers through Tapi (Ukai reservoir) and Narmada (Sardar Sarovar reservoir) to Kutch region in the state of Gujarat. Under this programme, geophysical surveys employing seismic, electrical resistivity and magnetic techniques were conducted for the delineation of bedrock configuration and the geological structural features present in the vicinity of the dam alignment. The Chikar dam site is located across river Ambika near village Chikar that is situated about 12 km from Wagai township on Wagai-Saputara road. Geophysical surveys have successfully delineated the bedrock topography along the dam and energy dissipation axes. Two prominent linear features are deciphered across the alignment at the right bank of the river running almost parallel to the river Ambika. The bedrock charecterised by P wave velocity4800-5000 m/s and electrical resistivity 60-1200 Ohm-m appears to be competent. Subsequent drilling carried out in the riverbed have corroborated the geophysical results.

Abst- 69

GEOPHYSICAL INVESTIGATION IN COMBATING ENVIRONMENTAL PROBLEM: RIVER BANK EROSION IN MURSHIDABAD AND MALDA DISTRICTS, WEST BENGAL

B. Sarkar , K. N. Prasad and L. K. Das

Geological Survey of India, Kolkata

Electrical resistivity survey using sounding technique has been extremely helpful in determining the subsurface distribution of flood deposits and younger alluviums upto a depth of 60-80m in Maya-Fazilpur sector of Murshidabad district as well as in Goloktola-Gopalpur-Balutola areas of Maida district, West Bengal. The disposition of clay/silt/sand and coarse sand beds were interpreted from the results of the sounding curves at surface, 3m and 10m depths. A positive correlation of advancing erosional front of the rivers has been established from the study with the disposition of sand bars at 3m and 10m subsurface depth. Some remedial measures are also proposed

Abst-70

ANALYSIS OF GEOELECTRICAL AND HYDROLOGICAL PARAMETERS FOR DECLINATION OF POTENTIAL AQUIFER ZONES IN THE PARTS OF GUWAHATI URBAN AREA, ASSAM

   B.Pathak and A.Hakim

Geological Survey of India, Shillong

A relationship between geo-electrical and hydrogeological parameters has been used to analyse the characteristics of aquifer zone and their location in the urban areas of Guwahati. Logarithmic values of total transverse resistance evaluated from vertical electrical resistivity soundings plotted against the logarithmic value of transmissivity of the aquifer, gave a linear relationship between them. Based on the total transverse resistance, and hydraulic transmissivity, a map with potential aquifer zones has been prepared, which exhibits four prominent zones with low to very high ground water potential. The poor aquifer zone is characterised by total transverse resistance < 4000 ohm sq.m where hydraulic conductivity is found to be very low k = 1 m/day to 9 m/day. This zone is comprised primarily of clay with fine sand. The intermediate aquifer zone is characterised by total transverse resistance ranges from 4000 to 8000 ohm sq.m and hydraulic conductivity k = 9.08 m/day to 22.5 m/day. On the other hand, good or high to very high aquifer zone k 2.22 m/day to 116 m/day and k = 116 m/day to 161 m/day is delineated to the west of Dipar beel and the area around Azara,Mirza, Palasbari and Bhattapara village.

Abst-71

 A new scenario of crustal thermal structure of central India

S.N. Rai and S. Thiagarajan

National Geophysical Research Institute, Hyderabad

The Narmada-Son-Lineament (NSL) and its surrounding regions in central India is characterized with the presence of numerous hot springs, feeder dykes for the Deccan Traps and seismicity all along its length.  Physical properties such as elasticity, density, thermal/electrical conductivity of subsurface rocks are affected by variation in the subsurface temperature which in turn affects most of the geological processes such as lithospheric deformation, earthquake genesis, crustal magnetization, geothermal processes etc.  Therefore, knowledge of subsurface temperature field is essential to understand such processes of a region.  The present work deals with the 2-D thermal modeling to delineate the crustal thermal structure in the region lying between 740 30’ to 800 30’ E and 200 30’ to 240 30’ N trending in the SW-NE with NSL more or less in the centre.  Only conductive mode of heat transport which is dominant in the crust is considered.  The 2-D modeling approach needs 2-D crustal structure upto the Moho and P-wave velocity distribution for construction of the model and for estimation of the heat production in the middle and lower crust, respectively by using an empirical relationship between heat production (A) and P-wave velocity (Vp). 2-D crustal structure and P-wave velocity distribution in the central India given by DSS studies along the four DSS profiles running across the Narmada-Son Lineament are used in the present study.  The heat production in the upper layer is estimated from the concentration of radioactive elements (U, Th, K) of surface rocks and by using exponential model. The computed results are constraint by surface heat flow values, Brittle-Ductile(B-D) transition zone in the lower crust and Curie depth of the region.

Numerical results reveal that the conductive surface heat flow value in the region under investigation varies between 46 to 49 mW/m2.  Out of which 19-23 mW/m2 is the contribution from the mantle heat flow and remaining from within the crust.  The Curie depth is found to vary between 40 to 46 km and is in close agreement with the Curie depth 40±4 km estimated from MAGSAT data.  The depth of 4500C isotherm is the depth of lower crustal Brittle-Ductile transition representing the depth of seismicity.  In the present study the depth of B-D transition zone is found to vary between ~33-37 km which is in close agreement with focal depths of two deep focused Satpura (1938) and Jabalpur (1997) earthquakes.  The Moho temperature varies between 500 to 5800C.  It indicates that the central Indian region is characterized by low mantle heat flow which in turn makes the lower crust brittle and amenable to the occurrence of deep focused earthquakes. Based on the present and some other studies an inference can made that a major part of the peninsular India comprising of the Bastar craton, Wardha Pranhita Godavari graben and the central part of NSL region under investigation forms a single heat flow province in respect of heat flow from the upper mantle which is around 23 mW/m2.  Variation in the surface heat flow value in these areas is mainly caused by variation in the radioactive heat production in the upper crust.

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Tsunami of  26th  December, 2004: Impact in Southern India and Explanations

V. Pratap and S.C Sonare

Oil & Natural Gas Corporation, Mehsana

A great megathrust earthquake of magnitude M9.3 struck the Sumatra trench more than 2000kms to the south east of Chennai in the Indian Ocean triggering a devastating tsunami wrecking havoc of hitherto unknown proportions in Southern India and Sri Lanka. The major brunt of the devastation was borne besides the Sri-Lankan coast by Tamil Nadu and South Coastal Kerala. The havoc rendered specifically to sea board off Chennai, Nagapattinam, Colachel and Karunagapally in the west coast appears surprising at the outset. A geological appraisal, however, throws insight on those factors responsible for the devastation due to tsunami.

The eastern sea board off Chennai and Nagapattinam being located closer to the source area had attendant severe devastations while areas farther away near Colachel and Karunagapally had damages of lesser extent. Prominent radials emanating from Cambay node, one passing through Cuddapah Basin to Chennai further extending into Bay of Bengal and another to Gulf of Mannar through Dharwar Craton influenced the migration of the tsunami wave onto the east coast and the Achancoil Shear zone entry into the Arabian Sea must have influenced the navigation onto the west coast. The ocean current pattern, ocean floor topography, and the earth’s rotation played a dominant role in deflecting the tsunami waves onto the west coast. The continuing active  NNE drift of the Indian plate following its collision with the Eurasian plate along with progressive increase southward in relative velocity of drift in Peninsular India resultant of welding of the northernmost subcontinent also added to the specific landfall points bearing the damaging power of tsunami waves. Sri Lanka shielded the south-eastern part of Tamil Nadu from receiving direct tsunami waves. The devastations have occurred in areas with comparatively lower electrical surface conductance. The combined effects of these significant geological factors led to steering the powerful tsunami generated waves onto these preferred seaboard points. 

Abst-73

Determination of seismic source model and site effects of recent strong motion Himalayan earthquakes

A. Joshi and K. Mohan

Kurukshetra University, Mehsana

          The Garhwal Himalaya has witnessed two major earthquakes, namely the Uttarkashi (1991) and the Chamoli (1999) earthquakes in the recent past, which has been recorded in the strong motion array.  The source spectra of these two recent Himalayan earthquakes has been determined and compared with theoretical source spectra. The comparison of theoretical source models suggests the suitability of the Atkinson model to represent these Himalayan earthquakes. Using the parameters of best-fitted theoretical spectra corner frequencies for the Uttarkashi and the Chamoli earthquake are estimated as .085 and .17 Hz, respectively. These corner frequencies corresponds to the stress drop of 26 bars and 60 bars for the Uttarkashi and the Chamoli earthquake, respectively which matches with the observational value of stress drop in the Himalayas.  Using three component data of these two earthquakes site amplification has been computed at nine recording sites.  It is seen that the site response is capable of explaining the characteristics of recorded strong ground motion during these earthquakes.

Abst-74

Correlation of geoelectrical and dynamic cone penetration test results for geotechnical investigation

Kumari Sudha, M. Israil, S. Mittal, J. Rai

Indian Institute of Technology Roorkee

          Electrical Resistivity Tomography (ERT) and Dynamic Cone Penetration Test (DCPT) have been recorded at two sites in Uttar Pradesh, India, for geotechnical investigation. The ERT were recorded using 72 electrodes deployed at electrode interval of 5m with total profile length of 355m each. RES2DINV inversion software was used to generate the electrical image of subsurface from recorded data. Total twenty eight DCPT tests were conducted around ERT profiles to investigate the detail soil mechanical properties. The soil mechanical resistances obtained by DCPT have been correlated with the electrical resistivity at two selected locations. It has been found that electrical resistance is linearly related with the soil strength. The investigation suggests the use of high resolution electrical imaging technique in geotechnical investigations. The application of ERT for geotechnical investigations is economic and faster in comparison to the conventional method used in civil engineering.

Abst-75

Preliminary Results from GPS station at Kolkata

Mallika Mullick, Dhruba Mukhopadhaya ,            Bikash C.Poddar

Center for Study of Man and Environment(CSME), Kolkata

  Crustal deformation study using GPS geodesy is being carried out by different research groups in the shield region and in the northwestern and northeastern parts of the Himalayan belt. In the less explored  region of North Bengal foothills and the Gangetic delta, we have set up GPS stations in the foothills and sub-Himalayan ranges of the Darjeeling-Sikkim Himalayas and in Kolkata in the Lower Gangetic plains. The stations in the foothills were occupied in the campaign mode and in each station the data were collected continuously over a period of 4-5 days. In the station at Kolkata (named CSME, 220 26' 57.21” N : 88025’30.39” E) the data were collected continuously in two stretches extending over 45 days and 75 days respectively. The Sikkim earthquake on February 14, 2006 took place during this second phase of data collection. A preliminary analysis of the data from station CSME is presented in this report.

           Data have been collected using Zephyr Geodetic antenna and Trimble 5700 receiver set. The GPS data has been processed using GAMIT/GLOBK suite of softwares (Version 10.2) which are a comprehensive suite of programs developed jointly by MIT, Scripps Institute of Oceanography and Harvard University . A successful GAMIT run with the GPS data from station CSME  along with the data from IGS stations IISC, KIT3 KUNM, LHAS and PIMO has yielded the satellite orbits, atmospheric zenith delays, earth orientation parameters  and the three dimensional relative positions of the ground stations with loose constraints .  The  ‘quasi observations ‘ obtained as GAMIT output has been passed on to GLOBK  which is a Kalman filter that combines these solutions . Time series of daily solutions of site position of CSME (Fig 1) and other IGS stations  have been obtained and also the time series for baseline lengths between pairs of stations. The scatter of the time series has been minimised to attain greater accuracy by using suitable sites with good a priori coordinates for stabilization while running GLOBK. The time series of the baseline distance vectors  CSME-LHAS, CSME-IISC, CSME-KUNM and CSME-KIT3 have been plotted. The tectonic implications are discussed.

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A PRELIMINARY SEISMIC CHARACTERISATION RELEVANT TO RISK AND DISASTER MANAGEMENT OF ANJAR TOWN, KACHCHH DISTRICT IN GUJARAT, INDIA

Babu Lal, L.N.Singhand S.L.Singh

Geological Survey of India, Jaipur

     Disaster of earthquake depends upon the man-made buildings/structures having foundation on seismically weak geological formation and structure which collapse causes killing of life of living there under. Anjar town in Kachchh district of Gujarat in India located in the conjunction area of E-W trending Katrol Hill Fault (KHF) and N-S lineament was worst damage in the Bhuj quake of 26 January 2001. Seismic characterization of the Anjar township area obtained from seismic refraction surveys demarcates the safer and risk locality important role in development plan concern with earthquake disaster management

 Analysis of seismic data have revealed that subsurface layer below soil cover characterized with P-wave velocity in the range of 1500-1850m/s corresponds to compact soil/weathered rock and forms a sustainable foundation quality formation for civil developments/housings construction. The seismic characterization map has indicated that most parts of the surveyed area are represented by higher order seismic velocity ranging 1650-1850m/s where civil development projects appear to be safer. The higher risk for civil projects can be expected in three localities represented by lower seismic velocity below 1650m/s. In consequent, worst damage in parts of the Anjar town falling over one of such seismic risk locality was observed in this earthquake. An adequate engineering consideration for foundation building in such risky locality is imperative in disaster management. Thickness of this foundation formation is observed about 10m in the western part. In this area a layer having velocity range 2500-4800m/s that corresponds to sandstone underlies this layer. The importance would be given to this third layer formation for foundation in this risk locality.

Abst 77

STRAIN FORMATION PROCESSES IN HYPOCENTRAL ZONE OF LATUR EARTHQUAKE

V.K. Rao

National Geophysical Research Institute, Hyderabad

Multidisciplinary approaches have become essential to understand the complexities involved in strain accumulation in intraplate region of stable shields. The geophysical signatures observed in and around Latur earthquake region in Maharashtra viz., presence of a low velocity zone, high conductivity, intersecting gradient of Bouguer anomaly (localized nature), low heat of flow and thin crust can best be explained in terms of stress accumulation in the hypocentral region of 30th September, 1993 Latur earthquake in the backdrop of strike-slip fault system.

A geochemical model has been proposed here, which involves the serpentinization process, based on the integrated geophysical data in conjunction which the regional and local geology of Latur region. An attempt has also been made to explain the latur earthquake in terms of asperity.

Abst-78

Repeat microgravity survey in Sikkim–Darjeeling Himalaya for  crustal deformation study

A.K.Lahiri,  P.K.Chakraborty and N.P.Singh

Geological Survey of India, Kolkata

Sikkim – Darjeeling Himalaya is seismically active and has experienced a number of earthquakes. Such phenomenon has been observed  in an active geodynamic regime as a results of which profound microgravity changes normally occur in the area. Such changes are attributable either to the change in elevation and or resultant mass transfer at crustal depth or both. In this paper, we report such  gravity changes through a series of repeat microgravity measurements between May, 2001 and December, 2005 across a known thrust zone, the Main Boundary Thrust (MBT), in the Sikkim-Darjeeling Himalaya. Microgravity data were collected along three selected traverses covering a network of 40 gravity stations at an average sampling interval of 8 months. A CG-3M microgravimeter with 1 microGal resolution was used for the periodic gravity measurement.

          Anomalous change (more than ±40 microGal) in repeat gravity values are observed in three zones. These are 1) in the northern end of the Siliguri – Gangtok (N-S) traverse between Ranipul and Gangtok with a maximum temporal variation in gravity value of -174 microGal. 2) From Peshok to Takdah along the SW – NE traverse with a maximum change of -290 microGal, and the third lies over Jorethang along the NW – SE traverse with maximum anomaly of -44 microGal.

          Seasonal change in ground water level in this hilly terrain may induce limited effect on the gravity variation as the surface run off is maximum due to high slope. Direct comparison of gravity variation with elevation change is not possible as the measurement for precise elevation change was not made.  The anomalous temporal change in gravity may thus  be attributed jointly to geological process involving redistribution of subsurface mass along the planes of detachment as also  the change in topography. It is estimated that the change in density contrast by .005 gm/cc for a vertical cylinder with density of  2.67 gm/cc and radius of 1km with depth extension of 10 km , representing the intrusive body at the contact, could produce -170 microgal anomaly which is comparable to the observed gravity change over the station at Gangtok.

A 2–D subsurface geologic model has been assumed for the N-S Bouguer gravity profile indicating a variable basement depth from 3.5 km to 12 km. To the north of MBT the Daling formation comprising slates, phyllite, dolomites, quartzite and the Darjeeling gneiss form the supracrustal rocks above the basement. To its south Younger sedimentary rocks like the Siwaliks and Gondwanas are seen. The Bouguer anomaly shows a gradual fall from south to north with a few plateau like signatures in between. These small gravity highs featuring like a plateau are interpreted as intrusives.

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 Application of Q-Marine Technology in ONGC

D. Rai, R. K Khanna

Oil & Natural Gas Corporation, Mumbai

ONGC has deployed Q-Marine Vessels in west and east coast of India during 2005-06 field-season for high resolution 3D data acquisition. The technology was focussed primarily on minimising the noise content of recorded seismic data and improving survey repeatability. A popular thrust in the seismic method has been the pursuit of resolution, which requires high frequencies. In towed streamer surveys, a key phenomenon influencing the recordable frequency range is ghosting. The amplitude spectrum of the ghost filter possesses notches at zero Hz and at regular intervals along the frequency axis depending on the depths of the source and streamers. The first non-zero notch is typically considered to be the upper limit of useful bandwidth. Forcing that first non-zero notch to occur as high on the frequency axis as possible is often a key objective in survey design. If the sea state is such that swell noise is present, the severity of that recorded noise worsens with shallower depths of streamer tow. Single-sensor sampling has enabled the swell noise problem to be addressed more effectively. With single-sensor technology, the trace interval is sufficiently dense to allow adaptive velocity filters to be effective in data processing.

The implication is that single-sensor technology can tolerate the recording of more swell noise, thereby enabling the streamers to be towed at a shallower depth, resulting in better resolution. This high-resolution implementation of technology has been the key strategy for these surveys.

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RESISTIVITY VARIOMETER FOR SEISMOLOGICAL STUDIES

K.R. Ramanuja Chary*and N. Ramanujam, K. Nathakiri Murugan and A. Anthony Ravindran

* Integrated Geo Instruments & Services (P) Ltd., Hyderabad

Post Graduate Department of Geology and Research Center, Chidambaram College, Tuticorin

The hypo central regions of earthquakes undergo precursory changes in structure and some physical properties like earth’s magnetic field, geoelectric current, resistivity of subsurface formations, groundwater levels, hydro chemical characteristics etc. Hence continuous monitoring of such parameters will enhance the knowledge of the on-going processes in the earthquake zones and may possibly provide and supplement some important information for the earthquake prediction. An attempt has been made to design, fabricate and establish a semi-permanent Resistivity Variometer station for continuous recoding of subsurface resistivity variations at Erattupetta (Lat 9o47’29.8” and Long 76o47’20.5”) near Idukki Hydro Electric Project, where frequent tremors occur. The preliminary study of temporal variations of subsurface electrical resistivity indicated anomalies a few hours before the occurrence of a tremor.

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STRATIGRAPHY,  PETROLOGY, GEOCHEMISTRY AND DEPOSITION OF THE VOLCANICLASTIC SEDIMENTS OF THE UPPER CREACEOUS BIBAI FORMATION, WESTERN SULAIMAN THRUST-FOLD BELT, PAKISTAN

Abdul Tawab khan

University of Balochistan, Pakistan

The volcaniclastic rocks of the Upper Cretaceous Bibai Formation are exposed through out the Ziarat district within the western part of the Sulaiman Thrust-Fold Belt east of the Quetta Syntaxes (Fig. 1 & 2). The rare in-situ volcanic rocks of the Bibai Formation are composed of basic volcanic rocks  and volcaniclastic sediments (Table. 1), deposited by various processes of sediment gravity flows on the western margin of the Indian Plate and indicate deposition by turbidity currents in over bank (-levee) complex between channels. The mudstone, possessing occasional thin sandstone and siltstone beds in the lower part and profusion of shallow marine fauna in the upper part, indicate deposition in lower fan /basin plane conditions and also an overall swallowing-up trend of the succession. Limestone interbedded with volcaniclastic facies in lower part of the formation.  The stratigraphic succession of the area (Table 1) includes the Wulgai Formation, Shirinab Formation, parh group, Bibai formation, Fort Munro formation, Mughal Kot Formation, Pab sandstone, Dungan Formation, Ghazij Formation, Kirther Formation and the Siwalik group.  Detailed stratigraphy, petrography and geochemical analyses of clasts of the volcanic rocks and sandstone were carried out to determine the origin and deposition of volcaniclastic sediments of the Bibai Formation. XRF data of both major and trace elements were plotted in various discrimination diagrams of the volcanic and associated intrusive rocks. It suggests that the volcano clastic sediments of the Bibai Formation were derived from a volcanic terrain composed of alkali basalts originated by hotspot volcanism.

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Inversion of VSP travel time data

S. Benjamin

Oil & Natural Gas Corporation, Mehsana

Velocity measurements and modeling play a crucial role in the analysis and interpretation of seismic data for exploration and exploitation of hydrocarbons. Vertical seismic profiling (VSP) provides the best in situ method of measurement of seismic velocities.

The direct arrivals in a VSP record gives the travel times from the top of the well bore to the receivers placed inside the well. From these travel times the interval velocities can be computed. However, a native computation of interval velocities may not give the actual sub surface interval velocities due to noise in the data and errors in travel time picking.

In order to compute the interval velocities consistent with errors in travel time data a damped least-squares inversion is resorted to. In this paper the LSQR algorithm is used for inversion.

The interval velocities computed can be used for better velocity modeling required in seismic processing and interpretation as well as in special studies such as pore pressure prediction  

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Attribute extraction from VSP data – A case study from Upper Assam

Aranya Bhaduri and Rahul Dasgupta

 Geophysics Department, Oil India Limited, Duliajan, Assam

VSP data is a link between the surface seismic and well data. Different attributes from VSP can provide an insight to various important properties of formations. In this study, detailed work was done to extract velocity and attenuation from VSP data of Upper Assam. The extraction of velocity is a comparatively simple exercise. On the other hand, estimation of attenuation is relatively complicated issue. In this case, log spectral ratio method was used for estimation of attenuation. Straight line fit in the log spectral ratio vs. angular frequency plots were taken for the computation of quality factor (Q). The VSP data used is a zero offset VSP. Thus, the first arrivals from the surface to the sensor gave a direct measure of the average velocities to that depth.  The variations of amplitude spectrum with depth were also studied. Formation top depths of Girujan, Tipam (Oligocene-Miocene), Barail (Oligocene), Kopili (Eocene-Oligocene), Prang, Nampuk, Lakadong-Theria (Eocene) determined from electrical logs and core cuttings (while drilling) for the same well were taken for correlation with the attributes (attenuation, average Q, interval Q, average velocity and interval velocity). Average and interval Q and average and interval velocities showed prominent trends. 

The study of the two attributes (velocity and attenuation) suggests that in upper Assam basin, the two attributes can be correlated which in turn can reflect significant formation characteristics. The detailed results will be presented in the paper.

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Systems for geophysical data quality evaluation in the field and exploration

  Y.N.T. Seshagiri Rao

National Geophysical Research Institute, Hyderabad

A field geophysicist can use a system for quality evaluation of data so that he can intensify his survey activities in that particular profile.    There are two modules integrated in this project. The first module is Embedded Linux System aiming at normalizing and formatting the geomagnetic data as a convenient input to Stuttgart Neural Network Simulator (SNNS). The second module is back propagation algorithm with momentum in SNNS package used for the automation of pattern classification of geomagnetic quiet data or stormy data. The neural network is trained using 114 sets of   hourly values of geomagnetic data, which have been plotted and the curves obtained. The smooth or quite variations and disturbed variations are identified.    A few sets of magnetic data are used to test the neural net, which yield satisfactory results. The various techniques used in this application are mapping, training, simulation, adjustment of weights and testing. This system is conveniently adaptable to other geophysical data to format and classify the patterns with back propagation algorithm, is used in the present study. The aim of the technique is to train the neural network such that its response to a given set of the inputs is as close as possible to a desired output. The weights used on the connections between different layers have significance and the NN learns by charging weights used to compute an activation function.

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Modeling and classification of litho-facies from well log data: A neural network approach

R.  K. Tiwari  and Saumen Maiti

 National Geophysical Research Institute, Hyderabad

Classifying litholog boundaries from borehole data is complex and non-linear problem. This is due to the fact that several factors, such as pore fluid, effective pressure, fluid saturation, pore shape etc affect well log signals and thereby limit the applicability of linear mathematical techniques. For instance, multivariate statistical methods such as principle component and cluster analyses and discriminant function analysis have invariably been applied for the study of borehole data. These techniques are, however, semi-automated and require a large amount of data, which are costly and not easily accessible every time. Further the existing methods are also very tedious and time-consuming, particularly when dealing with noisy and complex borehole data. It is, therefore, imperative to search for an appropriate nonlinear technique, which could evade these difficulties. The modern data processing and modeling approaches based on the artificial neural network (ANN) techniques are inherently nonlinear and completely data-driven requiring no initial model and hence provide a useful alternative perspective for dealing with such a complex and non-linear geophysical problems.

      Here, a novel approach based on the concept of super self adapting back propagation (SSABP) neural network has been developed for classifying lithofacies boundaries from the well log data. The SSABP learning paradigm has also been applied to constrain the lithofacies boundaries by parameterzing three sets of well log data i.e., density, neutron porosity and gamma ray obtained from the German Continental Deep Drilling Project (KTB). A multi-layer perceptron (MLP) neural networks model was generated in a supervised feed-forward mode for training the published core sample data. To demonstrate the effectiveness of our methodology, we compare our findings with existing results and test the robustness of the algorithm in the presence of colored noise in the data Our analyses demonstrate that the SSABP based approach renders a robust means for the classification of complex litho facies successions from the KTB borehole log data and thus may provide useful guidelines for understanding the crustal inhomogeneity and structural discontinuity in many other critical tectonic regions.

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Structure and Tectonics of Cuddapah basin – Insights from geophysical images

P.Nyna Renu* and Y. Sreedhar Murthy**

*Association of Exploration Geophysicists, Hyderabad

** Department of Geophysics, Osmania University, Hyderabad

The Proterozoic Cuddapah basin has intrigued geoscientist from times immemorial about its origin, structure, tectonics and mineral wealth. The image technology developed by the authors have enabled all the geoscientists to have insight into the geophysical maps which hither to were the monopoly of the specialist. The images of the gravity maps of India brought to light a number of features which are so far not known to the geoscientists or are at variance with the known geology and tectonics of the country. The north-south line from Karakoram to Laccadive Chagos ridge passing through Rajasthan, Gujarat and Mumbai; the Chennai-Mumbai linear and a number of other features seen on the gravity images need further explanation.

The well known crescent shaped Cuddapah basin appears as a rectangular block in the gravity images and occupies an area much larger than the known proterozoic basin extent. The aeromagnetic image of the basin clearly brings out the extension of the magnetic linears under the sedimentary cover. The mineralization with in the Cuddapah basin occurring mostly along the sedimentary and igneous contacts appears to correlate with specific linears. The Cuddapah basin is also seen as the mosaic of a number of sub-basins.  

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Geology and Structure in and around Chattisgarh basin - Some constraints from geophysical imaging

S.V.S. Sarma*, P. Shanthi*  and                                     Y. Sreedhar Murthy**

* Association of Exploration Geophysicists, Hyderabad

Department of Geophysics, Osmania University, Hyderabad

Chattisgarh basin, an intracratonic Proterozoic sedimentary basin of the Indian peninsula, occupies an important place in the Bastar-Bhandara craton from the viewpoint of understanding the geology, structure and tectonics of the region. An attempt is made to relate the available geophysical responses in the region to known geological and structural features. Gravity modeling studies suggest the extension of the Sonakhan greenstone belt further south into the eastern half of the Chattisgarh basin. This belt apparently acts as a divide, with the major Chattisgarh basin located on its west and the other smaller basins Ajumhar, Bastar, Sabari and Ampani lying on its eastern side forming a linear belt. In the light of the results of available Geophysical studies and those obtained from the present gravity modeling together with the known surface geology, a lithological pattern depicting the surface geological and structural features is deduced for the region in and around the Chattisgarh basin. This is characterised by near NNE-SSW trending, parallel linear zones of: Granite-Greenstone - sediment-Greenstone belts, extending over distances of a few hundred kilometers.  The eastern boundary of the Chattisgarh basin, characterized by well defined Gravity lows as reflected in the shaded relief maps of Bouger gravity as well as Zero Free Air based gravity is inferred to represent a mega fracture zone, closely related to the tectonics of the adjacent Eastern Ghat mobile belt. It is also inferred that the crustal column in the region close to the chattisgarh basin, on its western side, characterized by very high resistivies amounting to several thousands of ohm-m as revealed from magneto telluric studies, represents a dry Granite-Gneissic basement.

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A RIGOUROUS GEOSTATISTICAL ANALYSES OF THE SELF POTENTIAL SIGNALS TO DELINEATE THE CONTAMINANT PLUMES

Tanvi Arora*, Shakeel Ahmed* and André Revil**

* IFCGR, NGRI, Hyderabad

** CNRS-CEREGE, Dept. Of Hydrogeophysics and Porous Media, Aix-en-Provence, France

 Accurate mapping of the redox potential and the electrical conductivity of groundwater is important in delineating the shape of a contaminant plume. Organic matterrich contaminant plumes, e.g., associated with leakages from municipal landfills, are the source of electrical potential variations at the Earth’s surface termed self-potential anomalies.

The plume can be regarded as a natural geobattery, in which the source current results from degradation reactions of the organic matter by the growth of micro-organisms. The selfpotential depends on groundwater flow (electrokinetic contribution) and redox conditions (electroredox). The electrical source associated with this electroredox process adds to the

contributions associated with  (1) the electrokinetic conversion of ground water flow and (2) membrane or diffusion potentials associated with concentration gradients of the ionic species present in the pore water. Once removed the electrokinetic contribution, a correlation between in situ redox potentials measured in piezometers and the electrical potential measured at the ground surface using non-polarisable electrodes exists. A geobattery model can explain how organic matter-rich contaminant plumes behave as geobatteries and are the source of a natural electrical field. This field can be recorded at the ground surface and used to map the redox potential of the leachate plume using the self-potential method as an efficient non-intrusive technique. Using the variation of the piezometric head in the aquifer, the electrokinetic contribution is removed from the SP signal. A good correlation is obtained between the residual SP data and the redox potential values. This relationship is used to draw a redox potential map over the overall contaminated site.

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USE OF HYDROGEOMORPHOLOGICAL AND RESISTIVITY TECHNIQUES FOR GROUNDWATER RECHARGE STUDIES in HATFAR NALA BASIN SOLAPUR DIST. MAHARASHTRA

S.M.Sable,R.S. Valsankgkar, A.B.Narayanpethkar

Sholapur University, Sholapur

Hattur Nala basin located on Survey of India toposheet number 470/14 on the scale of 1:50,000. The basin lies between latitudes 17°32’N and 17°40’N and longitudes 750Sl’E and 75°56' E, covering approximately 80 sq.kms area. Geologically the basin belongs to Ambenali formation of Wai subgroup of Deccan Basalts. The drainage is dendritic and the drainage analysis show favorable conditions for groundwater recharge at selected locations. The basin shows multi aquifer system. Aquifers formed by basal clinker formation show relatively low permeability and those represented by soft zeolitic basalts show relatively higher permeability. Permeability is enhanced along the lineaments. Transmissivity, Storativity and Specific capacity values obtained from pump tests reflect the same results.

Electrical resistivity sounding data reveled two, three, four and five layer earth sections and the resistivities correlated well with that of surface and subsurface geology.Apparent resistivity contour maps for different electrode separations represented variation in the distribution of apparent resitivities in the basin and the low and high anomalous regions became evident. Using all the above-analyzed data, sites for surface and subsurface artificial recharge/rainwater harvesting structures have been suggested in the basin. This may provide as a guideline for similar site selection studies elsewhere.

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Geology and Genesis of Fullerene Bearing Shungitic Rocks, Mangampeta Area, Cuddapah Basin, Andhra Pradesh

K.S.Misra

Geological Survey of India, Nagpur

The uniqueness of the volcano-sedimentary sequence in the vicinity of Mangampeta, within the Cuddapah basin is discussed.  The sequence of Paleoproterozoic is known for hosting the world’s largest bedded barite deposit.  The present study has broughtout that the interlayered carbonaceous slates are fullerene bearing shungitic rocks.  Both C60 and C70 forms of fullerene are identified in these rocks by laser desorption/ionization spectrometry.  The organic carbon content in these rocks ranges from 5% to 13%.  This reporting of fullerene bearing shungitic rocks is first of its kind from any geological sequence in India.

The entire volcano-sedimentary sequence is bedded and layering is prominently seen from microscopic to very regional scale.  The significant units include dolomites, black carbonaceous slates now identified as shungitic rocks, quartzites, varigated slates and vitric crystal tuff.  The crystal tuff unit has high silica 81% to 87%, fine grained aphanitic texture with flow structure.  Varying amounts of organic carbonaceous material is also present in all the units of this sequence.  Development of deep sedimentary basin due to rifting of thick lithospheric slab, exhalative and fumarolic activity along deep fractures having connection with mantle sources are enumerated along with the economic potential of this volcano-sedimentary sequence.