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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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H. N. Siddiquie ¹, S. M. Karisiddaiah ¹, V. Subrahmanyam ¹

Affiliations
1 National lnstitute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

The total magnetic intensity map of the area shows that the inner shelf to a depth of 50m is marked by a zone of magnetic closures. The occurrence of offshore ortho-amphibolites and the qualitative interpretation of the magnetic map indicate the extension of onshore rocks (or the amphibolites and associated rocks of Precambrian (?) and intrusions of Deccan Trap age) under a thin cover of sediments to the offshore to a depth of about 50 m.

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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

Affiliations
1 Geological Survey of India, AMSE Wing, Bangalore - 560 078, IN

Abstract

No Abstract.

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A. S. Subrahmanyam ¹, G. P. S. Murty ¹, K. V. L. N. S. Sarma ¹, K. Mohana Rao ¹, N. P. C. Reddy ¹, M. M. Malleswara Rao ¹, V. Subrahmanyam ¹, P. Suneetha Rani ¹, A. Anuradha ¹, K. S. R. Murthy ¹

Affiliations
1 National Institute of Oceanography, 176, Lawson's Bay Colony, Visakhapatnam- 530 017, IN

Abstract

Earth tremors of low to moderate intensity (3.0 to 5.5) recurring over last century in and around coastal and shelf regions of Visakhapatnam has prompted us to acquirc and study the geophysical data between coastal regions of Kakinada and Kalingapatnam in western part of Central Bay of Bengal (17°N - 18°30'N) for understanding the recent seismicity of the region. Preliminary analysis of magnetic and shallow seismic data reveals the presence of some prominent land-Ocean tectonic lineaments, reactivation of which is considered to be responsible for tremors in the region. Total field magnetic anomaly map of the study area shows two major anomaly lineations such as: (a) N-S trending medium amplitude (˜250 nT) lineation off Visakhapatnam which is considered to be offshore fragment of converging point of two onshore lineaments. Also the lineament is associated with- five earth tremors of magnitude 3.7 to 4.3 at 40 m water depth. Impact of tremors is also observed in the overlying sedimentary strata in the form of vertical faults. (b) A NE-SW high amplitude anomaly zone (˜820 nT) off Kalingapatnam trending NE-SW, which is in close proximity with the epicentral location of earthtremors of 5.5 magnitude. This lineament can be spatially correlated to onshore NW-SE trending structural lineament indicating the probable reactivation of offshore segment resulting in the earth tremors Off Vizianagaram, reactivation of preexisting structural lineaments associated with the NW-SE trending Kandivalasa river fault may be the source of tremors at 20 m water depth. Magnetic basement contour pattern changes from NE-SW to NW-SE off Visakhapatnam. It may be inferred that the basement is bounded by two fault controlled lineaments passing through epicenters of earthtremors off Visakhapatnam and Vizianagaram. Basement structure shows down faulted basement (2 to 3 km) underlying the epicentral location along the lineaments trending NW-SE in the innershelf of visakhapatnam. Shallow seismic data also indicate steepiy dipping sub bottom fault in the study area of Visakhapatnam. South of Visakhapatnam, shallow seismic records indicate vertical faults and shearing reflectors in the lower sedimentary layers. From the deformed lower sedimentary layers. It may be considered that the region might have undergone seismic activity in the recent past, but presently there are no reported tremors in this region. Even undeforrned top sedimentary layers suggest lack of recent tectonic activity between Visakhapatnam and Kakinada in coastal and shelf regions. It may be summarized from the preliminary analysis of the geophysical data that the earth tremors observed north of Visakhapatnam are caused mainly due to reactivation of tectonic lineaments whereas south of Visakhapatnam it is relatively free from any seismicity. Based on the felt reports, tremors are attributed to shallow focus origin. Detailed investigations from multidisciplinary approach like geological, geomorphological, geophysical and remote sensing observations coupled with seismic information in the study area will throw more light on the micro seismic activity and related recent tectonics of this coastal region of east coast of India.

Keywords

Tectonic Lineaments, Magnetic anomalies, Microseismic activity, East coast, Andhra Pradesh.

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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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V. Subrahmanyam ¹

Affiliations
1 National Institute of Oceanography, Dona Paula, Goa 403004, IN

Abstract

The isomagnetic anomaly map (1: 3,00,000) reflects conspicuous E-W, NW-SE, WNW-ESE, NE-SW and N-S trends which are similar to that of the onshore Dharwar trend. The map also shows short wavelength, high amplitude anomalies off Coondapoor, broad wavelength anomalies off Mangalore and relatively smooth zone further south to Kasaragod. A prominent WNW-ESE structural element (fault) south off Coondapoor is observed in the three eastern profiles (A-A', B-B' and C-C') which was not reflected in the western profiles (D-D', E-E' and F-F') and this can be extrapolated towards the adjacent coast to join an onshore structural element trending in the same direction. This structural element perhaps represents the offshore extension of the fault at Udipi. The broad wavelength anomalies off Mangalore and Malpe may be generated from sources composed at a depth of around 4 km. The observed magnetic smooth zone south off Mangalore is interpreted in terms of insignificant magnetization contrast in the crystalline basement.

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