Contents of
October, 1998 - Vol. XIX No.4
CONTOURS TO IMAGES - PART II:
AEROMAGNETIC IMAGE OF WESTERN PART OF CUDDAPAH BASIN AND ADJOINING
CRYSTALLINES- A CASE STUDY
V. Babu Rao, Y. Sreedhar Murthy and K. Govindarajan
ENGINEERING GEOPHYSICAL
SURVEYS: AN INTEGRATED APPROACH FOR SITE AND ROUTE SELECTION OF OFFSHORE OIL
TERMINAL IN THE GULF OF KUTCH, A CASE STUDY
Ranjit Rath, C.C. Babu and V.V. Reddy
SPECTRAL ANALYSIS OF GRAVITY
ANOMALIES OF THE NORTH-WESTERN PART OF BANGLADESH
Md. Babul Islam, Shaikh Enayetullah, Md. Syed Mostafizur Rahman and Md.
Rezwanul Islam
IDENTIFICATION OF ZONES OF
CORROSIVE GROUNDWATER USING RESISTIVITY METHOD
K.K. Sharma and S. Jayashree
INTERPRETATION OF
SELF-POTENTIAL ANOMALIES BY NONLINEAR INVERSION
Shalivahan, Bimalendu B. Bhattacharya and Mrinal K. Sen
SUBDUCTION IN THE INDO-BURMA
REGION: RELATIONSHIP BETWEEN THE WBZ ZONE AND THE P,T & B AXES
S.P. Satyabala
CONTOURS TO IMAGES - PART II:
AEROMAGNETIC IMAGE OF WESTERN PART OF CUDDAPAH BASIN AND ADJOINING
CRYSTALLINES- A CASE STUDY
V. Babu Rao*, Y. Sreedhar Murthy** and K. Govindarajan***
*National Geophysical Research Institute,
Hyderabad - 500 007
**Dept. of Geophysics, Osmania University, Hyderabad - 500 007
***Association of Exploration Geophysicists, Hyderabad - 500 007
Abstract
Display of Geophysical data in general,
and aeromagnetic data in particular, in image form enhances the visual
perception by several orders of magnitude in comparison to the conventional
contour maps. The recent advances made in the fields of digital data and image
processing and display technologies have succeeded to bring out the full
potential of the aeromagnetic surveys as a mapping tool and as a pre-eminent
component of all the major exploration programs. Availability of geophysical
data set in an equi-spaced rectangular grid form is an essential pre-requisite
to utilize the advantages of the state-of-the-art technologies. Digitization of
the available Indian aeromagnetic data set is thus a priority towards this
goal.
Using the Auto Trace and Digitising
Methodology evolved by us the aeromagnetic data set over the Proterozoic
Cuddapah basin and the adjoining crystallines is taken up as an illustrative
case study and the magnetic images in grey-scale, in colour and as shaded
relief are presented and their advantages over the conventional contour maps
are outlined.
It is pointed out that this is first of
the NGRI aeromagnetic data set to be brought out in a user-friendly display
mode. The magnetic data is corrected for the International Geomagnetic Reference
Field (IGRF) in contrast to that of the aeromagnetic map published earlier. The
efficacy of the Shaded relief map in mineral exploration with particular
reference to Diamond exploration is illustrated.
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ENGINEERING
GEOPHYSICAL SURVEYS: AN INTEGRATED APPROACH FOR SITE AND ROUTE SELECTION OF
OFFSHORE OIL TERMINAL IN THE GULF OF KUTCH, A CASE STUDY.
Ranjit Rath, C.C. Babu and V.V. Reddy
Engineers India Limited, Emgineers India Bhavan, 1, Bhikaiji Cama Place, New Delhi - 110066
Abstract
Offshore oil terminals are built in
strategic locations to facilitate the import of crude oil, which forms a major
share of the current demand in the present Indian Petroleum industry. Because
of its low installation and maintenance cost, high and easy cargo handling
capacity and minimum gestation period in comparison to the other inshore import
facilities, such as Port & harbor jetties, it does possess an edge over the
others. An offshore terminal could be a Single Point Mooring (SPM) or a Multi
Point Mooring(MPM) system depending on the required cargo expected to be
handled and the site conditions.
The present paper discusses various
facets and the associated pros and cons prior to the selection of a suitable
site for a proposed offshore oil terminal of Single Point Mooring type (SPM) in
the Gulf of Kutch and finalisation of the associated submarine pipeline route.
The design criteria and site selection of the SPM, prior to fabrication and
installation, were finalised by undertaking detailed engineering geophysical
surveys and oceanographic investigations in and around a proposed area. This
has helped in understanding the prevailing geological condition i.e. both
seabed and sub-seabed geology, oceanographic and environmental parameters.
Based on the data collected during the survey, the SPM location was finalised
at a water depth of 33.7m below Chart Datum, keeping in view the size and
draught requirement of the cargo to be handled. Further, a suitable submarine
pipeline route, from the SPM to the Land Fall Point(LFP) that would further
connect to the Crude Oil Tank farm on the shore, was also selected out of two
proposed alternatives. As these factors are site specific, no general guideline
is applicable and thus require an intrinsic approach, meticulous observation
and interpretation of the collected data by specialists for economical and
successful design, installation and operation of the system.
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SPECTRAL ANALYSIS OF GRAVITY ANOMALIES
OF THE NORTH-WESTERN PART OF BANGLADESH
Md. Babul Islam, Shaikh Enayetullah, Md. Syed Mostafizur Rahman and Md. Rezwanul Islam
Dept. of Applied Physics and Electronics, Rajshahi University, Rajshahi - 6205, Bangladesh
Abstract
The gravity anomalies of the
north-western part of Bangladesh have been analyzed for geophysical
interpretations. Various useful existing spectrum-based data processing and
interpretational techniques have been used. It is found from the spectral
studies that most of the signal powers are associated with the deep-seated
structures. The presence of two different ensembles of causative sources
(shallow and deep) has been detected and their average depths have been
estimated. The estimated depth of the deep-seated sources with an analytical
interpretational approach (Smith rule) is found to be in good agreement with
the average depth of the ensemble of the deep-seated sources with the spectral
interpretational technique. The estimated depths of the deep and shallow
sources are also compared with the available borehole, geological and
geophysical interpretations. The deep-seated sources are the intrabasement
intrusives producing the major regional gravity anomalies and the shallow
sources are the Pre-Cambrian basement rocks in the study area.
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IDENTIFICATION OF
ZONES OF CORROSIVE GROUNDWATER USING RESISTIVITY METHOD
K.K. Sharma and S. Jayashree
Department of Applied Geology, University of
Madras Guindy Campus, Madras - 600 025, India
Abstract
Increased demand for water have
stimulated development of underground water resources. The suitability of groundwater
for domestic, industrial and irrigational purposes is determined by its
quality. When groundwater is distributed through either iron pipes or concrete
pipes, the characteristics like corrosion and encrustation are to be assessed
in detail in order to protect the supply system.
In this paper, the importance of
resistivity methods in demarcating the zones of corrosive groundwater is
outlined. As a case study, zones of corrosive groundwater in the Vaigai basin
have been demarcated using resistivity method. This has been correlated with
the available geochemical data and it has been found that resistivity studies
can help in not only identifying the corrosive zones; but also in identifying
the formation that contributes to this corrosion and hence help in limiting the
depth of well drilling.
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INTERPRETATION OF
SELF-POTENTIAL ANOMALIES BY NONLINEAR INVERSION
Shalivahan*, Bimalendu B. Bhattacharya and Mrinal K. Sen
* Dept. of Applied Geophysics, Indian School
of Mines, Dhanbad - 826 004, India
** Institute for Geophysics, The University of Texas at Austin, 8701 N Expwy,
Austin TX 78759-8397 USA
Abstract
The method of nonlinear inversion by
Very Fast Simulated Annealing (VFSA) – a variant of Simulated Annealing (SA),
has been used to interpret self potential data, both synthetic and observed.
The model parameters are generated from a Cauchy-like distribution which
enables a fast cooling schedule. The VFSA requires a temperature for each model
parameter and the temperature to be used in the acceptance criterion may be
different from the model parameter temperature. The algorithm for VFSA has been
developed for simple targets like sphere, cylinder and sheet. The algorithm has
been first tested on theoretical models. Subsequently field data have been
considered for which the interpreted results by other methods are available for
comparison. The agreement between the results obtained by VFSA and other
methods is good.
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SUBDUCTION IN THE INDO-BURMA REGION:
RELATIONSHIP BETWEEN THE WBZ ZONE AND THE P,T & B AXES
S.P. Satyabala
National Geophysical Research Institute, Hyderabad
Abstract
The Indo-Burma region (longitude
92ºE-98ºE & latitude 20ºN-28ºN) is a subduction zone where the Indian plate
underthrusts the South-Eastern Asian plate. Harvard Centroid Moment Tensor
solutions of 65 earthquakes, which occurred in this region during 1977-1996,
are analyzed to examine the relationship of the T/B/P axes of the Moment Tensor
with the geometry of the surface of the Wadati-Benioff Zone (WBZ). The
earthquakes in this region have focal mechanisms of mixed type with a large
number of them having a predominant strike-slip or thrust components. This
study shows that the WBZ, defined by the eastward dipping inclined zone of
earthquakes up to 200 km depth under the Indo-Burman ranges, has a predominant
downdip tensional stress regime, which is typical of intermediate depth
seismicity of active subduction zones.