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
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
SELF-POTENTIAL ANOMALIES BY NONLINEAR INVERSION
Shalivahan, Bimalendu B. Bhattacharya and Mrinal K. Sen
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.
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.
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.
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.
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.
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.