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

M. V. Ramana ¹, V. Subrahmanyam ¹, L. V. Subba Raju ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403 004, IN

Abstract

Total magnetic field intensity measurements along two parallel NW-SE tracks from the Direction Bank to the Kori Bank show correlatable magnetic anomalies. High frequency positive anomalies observed northwest of Kathiawar coast terminating at about 21°N are interpreted due to the magnetic basement i.e, basalts at a depth of about 2 km. A low amplitude negative anomaly observed around 21°N (between 69°E and 70°E) may be due to the westward extension of the Narmada-Son lineament. Absence of significant anomalies south of these high frequency anomalies may be due to thin Traps or greater thickness of sediments. High amplitude and broad wavelength negative anomaly observed off Bombay (l9°N, 71.5°E) is interpreted as due to contact or faulting of Precambrian formations and Deccan Traps.

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K. V. L. N. S. Sarma ¹, M. V. Ramana ¹, G. P. S. Murty ¹, V. Subrahmanyam ¹, K. S. Krishna ¹, A. K. Chaubey ¹, M. M. Malleswara Rao ², S. L. Narayana ²

Affiliations
1 National Institute of Oceanography, Donapaula, Goa - 403004, IN
2 National Institute of Oceanography, Regional Centre, Visakhapatnam-530023, IN

Abstract

Analysis of the total intensity magnetic data over the western Andaman shelf has been carried out for determination of depth to magnetic basement using the Analytical signal and Werner Deconvolution methods. The basement depths are refined with optimisation procedure of iteration modelling. The depths Derived from these methods match well with the acoustic basement mapped by the seismic reflection survey across the Andaman shelf. The interpretation by these methods demonstrates the rapid utility in virgin areas forderiving the first order basement configurationquickly. The gradual slope of the basement from west to east on the western Andaman shelf may be attributed to the ongoing process of the subduction of the older crust in the Sunda Trough.

Keywords

Marine Geophysics, Andaman Sea.

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M. V. Ramana ¹, V. Subrahmanyam ¹, K. V. L. N. S. Sarma ¹, B. T. V. Seshavataram ²

Affiliations
1 National Institute of Oceanography, Donapaula, Goa-403 004, IN
2 Oil and Natural Gas Commission, Sire Mansion, Mount Road, Madras-600 006, IN

Abstract

Close grid masine magnetic surveys in the vicinity of a drill well site PH 9-1 in Palk Ray revealed that the area is characterized by smooth magnetic field except for a local anomaly caused by a lost wellhead. The smooth magnetic field is attributed to the deep burial of Precambrian granitic basement devoid of any charnockite intrusions. The seismic reflection records of the study area show >3200 m thick sediments over the basement.

Keywords

Marine Geology, Geophysics, Palk Bay, Cauvery Basin.

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V. Subrahmanyam ¹, M. V. Ramana ¹, L. V. Subba Raju ¹

Affiliations
1 National Institute of Oceanography, Dona Pau]a, Goa 403 004, IN

Abstract

Geophysical studies of the southwestern continental margin revealed significant surface and subsurface topographic highs (ridges) trending NNW-SSE to NW-SE beyond the shelf break. Residual magnetic anomaly map depicts prominent NNW-SSE, NW-SE and E-W trends. The magnetic anomaly map also reveals three different provinces, A, B and C, based on magnetic signatures, confined to inner shelf, outer shelf and lower shelf. A prominent E-W trending structural Jineament with a NW-SE trending offset is observed cutting across the inner shelf and the lower shelf. Model studies of the geopotential data over the topographic rise indicate about 18 km thick mafic basement of continental origin at a shallow depth (∼800 m). The gravity model studies indicate a thick consolidated sediment layer deposited over the horst and grabenlike features within the granitic layer. The occurrence of ridges parallel to the shelf edge shows the similarity of the western Indian continental margin with the other typical and extensively studied passive margins.

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