Journal of Geological Society of India (Online archive from Vol 1 to Vol 78)

Mita Rajaram ¹, S. P. Anand ¹, T. S. Balakrishna ²

Affiliations
1 Indian Institute of Geomagnetism, New Panvel, Navi Mumbai - 410 218, IN
2 0il and Natural Gas Commission, Priyadarshini, Sion, Mumbai - 400 022, IN

Abstract

Advancement in instrumentation technology, accurate aircraft positioning using GPS coupled with sophisticated computers and data processing and imaging techniques have improved the resolution of airborne magnetic surveys manifold which can now provide Super-High resolution aeromagnetic maps. This has catapulted into several countries repeating their magnetic surveys with the improvement in technology especially in regions of viable resources Unfortunately, India still does not have a complete aeromagnetic survey of the whole country even at the reconnaissance level. In this paper, we have put together all a variable ground and marine magnetic data collected by different agencies, to complement the available aeromagnetic data over India and its contiguous regions and produced a composite magnetic anomaly map at an altitude of 5000 feet. It has been our endeavour to show the importance of a magnetic a normally map in bringing out the structural details related to tectonic activity by looking below the sediments, Deccan traps, etc From the composite magnetic anomaly map it appears that there is a region that has been subjected to high tectonic activity, running across the Centre of the Indian subcontinent from the western offshore to the Brahmaputra valley in the east, dividing the country into a northern and southern block. It is hoped that the National Agencies will take up the aeromagnetic coverage of India, on a priority basis and survey the entire region where aeromagnetic coverage has not been undertaken, including the Deccan trap covered regions A magnetic anomaly map produced by uniformly distributed data collected on a single platform will certainly provide more reliable information.

Keywords

Crustal magnetic anomalies, Aeromagnetic survey, Tectonic activity, Long wavelength, Magnetic anomaly map of India.

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S. P. Anand ¹, Vinit C. Erram ¹, Rajaram Mita ¹

Affiliations
1 Indian Institute of Geomagnetism, Colaba, Mumbai - 400 005, IN

Abstract

Ground magnetic surveys were conducted over the Mahanadi basin covering an area of 21,000 km². Both total field and vertical field data were collected at 5 km interval, mainly to study the deeper regional features. Total field anomaly map prepared after correcting for the IGRF and external field variations, shows a combination of NE-SW, E-W, and NW-SE trends. To identify magnetic sources related to different depths, various transformations have been applied to the total field anomaly map. The second vertical derivative and downward continued map showed that the NE-SW and E-W trends are related to ridges and depressions of the coastal basin, implying that these are shallow features. The deeper features evidenced from upward continuation showed NW-SE trends. Thus, the Mahanadi delta is composed of two structural units associated with different tectonic events. These NW-SE trends may possibly be associated with the extension of Mahanadi graben underneath the coastal basin. The study reveals that the shallow NE-SW to E-W trends are superposed on the deeper NW-SE trends formed prior to the break-up of Gondwanaland, and the former trends resulted due to the dismemberment of Gondwanaland. A comparison with DSS studies conducted over Mahanadi delta and Lambert graben of East Antarctica showed that the'deeper features can be correlated with an intracrustal layer related to a velocity of 6.4 km/s-6.6 km/s at an average depth of 6.5 km, seen in both the areas. Analysis of aeromagnetic data over Lambert graben of East Antarctica will help reconfirm our observation.

Keywords

Magnetic Anomalies, Crustal Structure, Mahanadi Basin, Eastern Ghats, Lambert Graben, Gondwanaland.

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