JOURNAL OF GEOPHYSICS

AEG aims at publication of the results of original research in the field of Geophysics. Scientific contributions in the shape of original research articles, short communications, case histories involving developments in methodology, techniques, instrumentation, interpretation etc., and discussions on the papers are published by the

Association in its Journal and seminar proceedings.

 AEG  started publication of the Journal in 1980. The Journal of Geophysics is a Quarterly Journal and is currently in its XXVII volume  The Journal attracts scientific papers from a large number of earth science institutions  in India and abroad.

CONTENTS OF JULY, 2006, Vol.XXVII   No.3

Tectonic history of major geological structures of peninsular India and development of petroliferous basins and eruption of Deccan and associated volcanics

K.S. Misra

Submarine canyon and deep-water depositional setting in offshore Bengal basin

Ravi Bastia, S.K.Chakraborty, Jaydip Guha, B.Mishra, P.Gupta

Interpreting self-potential anomalies caused by  two-dimensional inclined sheet like-structures using maximization of a likelihood function

M.Tlas and J.Asfahani

An interpretation of gravity anomalies over the Gongola arm, upper Benue trough, Nigeria.

E.M.Shemang and W.R.Jacoby

Exploration of Potash deposits by magnetic and deep electrical resisitivity methods in Nagaur basin, Thar desert, India

Babu Lal

Geophysical investigation for detection of cavities at Dowlaiswaram arm of Godavari anicut, Andhra Pradesh

R.S.Ramteke, C.Subba Rao, N.Ghosh

Tectonic history of major geological structures of peninsular India and development of petroliferous basins and eruption of Deccan and associated volcanics

K.S. Misra

 Abstract

 The present study highlights the salient features and tectonic history of major geological structures which transact the Peninsular India. These structures include grabens and rifts as well as lineament and tectonic zones.  They seem to have played very significant role in development of sedimentary basins which host coal bearing Gondwanas and marine petroliferous Mesozoic and Tertiary rocks. Evidences are provided to suggest that these structures mainly experienced continued extensional tectonics since Proterozoic times.  This extensional tectonics resulted in development of elongated basins along them. Other characteristic features invariably associated with these structures are high relief and heat flow, high density of lineaments with associated neotectonic and seismic activity as well as gravity and magnetic anomalies.

            Deposition of very thick sedimentary sequences culminated  in volcanism  both during upper Proterozoic and upper Mesozoic times.   Volcanism appears to be due to decompressional melting related to basinal faults progressively reaching to the critical depths.  Melting of lithosphere generated felsic lavas in the initial phase, followed subsequently by typical tholeiitic flows. In areas where these structures are found to be intersecting each other, much thicker deposition of sedimentary rocks and more  pronounced  volcanism  have  taken  place due to the compounding effects of extensional tectonics.  Further, high heat flow, high density of lineaments and enhanced neotectonism and seismicity are also noticed in the vicinity of intersectional areas.  The present author believes that these compounding effects are very significant not only in development of depocentres but also in supply of organic material from continental areas to these deltaic regions.  Apart from this, the upwelling heat along associated downward progressing extensional faults both before and after the volcanism was largely responsible for providing the kitchen for distilation of oil, both during the Mesozoic and Tertiary times.  Furthermore, the suitability of trap rocks as cap rock in formation of Mesozoic oil traps is also suggested.

Submarine canyon and deep-water depositional setting in offshore Bengal basin

Ravi Bastia, S.K.Chakraborty, Jaydip Guha, B.Mishra, P.Gupta

Abstract

              Bengal Basin is located along the north east coast of India having an offshore continuation covering approximately an area of 40,000 km2 in the Indian waters within the Bay of Bengal. Thick Neogene sedimentary sequence in the study area located in the southern part of offshore Bengal Basin was fed by the Ganges, Brahmaputra, Mahanadi, Subarnarekha, Baitarani and Brahmani rivers. The sediments in the present day shelf area exhibit a complex depositional history with unique juxtaposition of shallow water deltaic sediments and deep water sediments in the form of incised canyons filled up with deep water channel-levee complex deposits.

               A detailed study was carried out on the morphology and architecture of the deep water depositional elements in this area utilizing around 4000 lkm 2D seismic, 1786 km2 of 3D seismic and data from 12 drilled wells. Study of special seismic attributes such as spectrally decomposed amplitude and ‘sweetness’, etc corroborated by wire-line log, mud log and bio-stratigraphic data from the drilled wells lead to the understanding of a depositional model comprising of the following elements and environments with characteristic morphology and architecture.

              Shelfal Canyon cut and fill sequences with geometries like - simple “V” shaped cuts with fills in the form of low to moderate sinuous channels or complex canyon cut and fills showing multiple canyon cuts caused by mass-wasting with intervening fills in the form of deep water to sub-aerial channels and deltaic progradational deposits.

            The correct understanding of the complex clastic reservoir architecture in the above depositional setup leads to successful exploration of hydrocarbon entrapped in these reservoirs

An interpretation of gravity anomalies over the Gongola arm, upper Benue trough, Nigeria

E.M.Shemang and W.R.Jacoby

Abstract

A regional gravity survey of the Gongola Arm of the Benue trough was carried out with the objective  of determining structures of both local and regional interest. The results of the gravity interpretation showed that the area of study is characterised by negative Bouguer anomalies that trend in the NE-SW direction and range in values from -75 to -15 mGal with an average of -42 mGal. A first order polynomial was used to approximate the regional anomalies in the area. The residual gravity anomaly map shows anomalies that range in values from -32 to +20 mGal. 2-D modelling of the residual gravity anomalies suggests that the area has a horst and graben structure, with the grabens being as large as 60 km in width and filled with sediments that are up to 4.5 km thick. Results also suggest the existence of high-density rocks of basic composition at depth. The general indication of the structure of the Gongola arm of the Benue trough is that it is a half graben (width ca 60 km) bounded by normal faults.

Interpreting self-potential anomalies caused by  two-dimensional inclined sheet like-structures using maximization of a likelihood function

M.Tlas and J.Asfahani

Abstract

An interpretatation method based on the constrained, nonlinear mathematical optimization concept has been implemented in order to interpret self-potential (SP) anomaly  due to a two-dimensional inclined sheet like-structure. The geophysical problem related to such a structure is mathematically formulated as a constrained, nonlinear optimization problem. This problem is thereafter transformed into an unconstrained, nonlinear optimization one by using an exponential penalty function. This unconstrained, nonlinear optimization problem can then be solved by Hooke and Jeeves direct search algorithm in order to estimate the geophysical parameters of the sheet-like structure, i.e., depth of the centre of the sheet  z , inclination angle q, half length of the sheet  a and electric dipole moment k . The  proposed new interpretation  method has been first tested on a theoretical example with different random noise. A very close agreement has been obtained between theoretical and computed parameters. The validity of this method is also tested on a practical field example, where the observed field data was previously analyzed by different geophysical interpretative methods. The agreement between the results obtained by the proposed technique and those obtained by other geophysical methods is good. A statistical analysis has also been carried out to demonstrate the accuracy and the efficacy of the proposed interpretation method.

Exploration of Potash deposits by magnetic and deep electrical resisitivity methods in Nagaur basin, Thar desert, India

Babu Lal

Abstract

The regional magnetic (vertical field) and Deep Resistivity Sounding (DRS) surveys conducted in parts of Bikaner district, Rajasthan have brought out structural configuration of the basin and disposition of the Jodhpur sandstone group critical for the exploration of potash-rich deposits.

              Spectral analysis carried out for 80Km long magnetic profile has yielded three interfaces.  The paramagnetic Jodhpur sandstone group overlain by diamagnetic halite is the shallowest magnetic interface while other two deeper interfaces are for granitic and basaltic horizon.  The resistive bottom layer obtained in DRS also invariably corresponds to the Jodhpur sandstone group, which forms the floor of evaporite and provides important information for planning exploratory drilling.

            Magnetic survey has revealed the presence of an N-S trending ‘high’ located at northeast of Bikaner, suggestive of a ridge type structure, which acts a barrier resulting in the development of Bikaner sub-basin having potash deposit.  Further magnetic survey has brought out E-W trending successive step fault with northerly downthrown side.  This indicates the deepening of basin towards north.  Significantly, Jodhpur formation  group has been identified at depth by DRS with indication of halite dominated evaporite in Kalu-Hansera area lying northern parts of the survey area.  Geophysical results supported by drilling indicated potash deposits in the deeper part of the basin.

Geophysical investigation for detection of cavities at Dowlaiswaram arm of Godavari anicut, Andhra Pradesh

R.S.Ramteke, C.Subba Rao, N.Ghosh

Abstract

              Geophysical investigations comprising potential gradient profiling, Schlumberger profiling and seismic refraction survey were carried out at Dowlaiswaram arm of Godavari anicut, Andhra Pradesh to detect cavities, if any, under the masonry structure of the anicut.  Two continuous gradient profiles and seven Schlumberger profiles were taken on the downstream side of the anicut in Cistern 1 and 2.  Six seismic refraction profiles were also taken in Cistern 2, where high resistance zone was delineated by electrical survey.

              The result of potential gradient profile indicated three weak zones/cavities in Cistern 1 and two in  Cistern 2.  The existence  of these weak zones at the same place was also confirmed by Schlumberger profiling method.

         The result of seismic refraction survey did not show any weak zone / cavities which may be due to a low velocity zone lying under high velocity layer.