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

D. K. Saha ¹, D. C. Naskar ¹, P. M. Bhattacharya ¹, J. R. Kayal ¹

Affiliations
1 Geological Survey of India, Central Geophysics Division, 27, J.L. Nehru Road, Kolkata - 700 016, IN

Abstract

A geophysical field investigation, deploying Magnetotelluric (MT) and Deep Electrical Resistivity Sounding (DES), and analysis of the seismological data were carried out in the Shillong Plateau and Assam Valley area for deciphering the south dipping hidden Oldham fault at the plateau and valley boundary. Bilham and England (2001) reported that the great 1897 Shillong earthquake was caused by the pop-up tectonics of the Shillong Plateau between the two bounding faults, the south dipping hidden Oldham fault to the north and the well known north dipping Dauki fault to the south at the Plateau and Bengal basin boundary. The MT survey was carried out using SAMTEC-2 MT instrument, and the DES was conducted by unilateral equatorial arrays with 5-10 km separation between the transmitting and receiving dipoles. A total of 14 DES and 21 MT soundings were observed in the area. The DES delineated different subsurface layers down to a depth of about 1.5 km. It is observed that the depth of the high resistivity basement (granite gneiss/Archaea.n gneiss) increases towards north of Brahmaputra river; the depth varies from 150111 to 650 rn. To the south of Brahmputra, on the other hand, the depth of the basement is shalIower, it varies from 56m to 340 m. The MT soundings were conducted along three parallel north-south traverses in the area : Traverse I along Dauki-Kaurbaha, from Bangladesh border to Bhutan border, Traverse I1 dong Chherapunjee-Chhagaon and the Traverse I11 along Dainadubi-Dalgama, across the boundary of the ShilIong Plateau and Assam Valley, The traverses I1 and 111 were too shon due to local problems. Interpretation of the 21 MT soundings reveals a conductive horizon at a deeper lcvel (7-8 km) only at the Kulsi (Traverse 11) and at the,Dalgama (Traverse 111) stations, to the north of the proposed Oldham fault. These limited MT observations, however, do not resolve the south dipping hidden Oldham fault.

The seismological data recorded by the permanent network in the Shillong Plateau and Assarn Valley area during the last decade (1990-1997) are analysed. A intense seismic activity is observed in the Plateau region. A north-south depth section of the earthquakes across the Plate'au and the valley, suggests that the most intense seismic activity beneath the Plateau is bounded by two major tectonic features, the north dipping Dapsi thrust, western extension of the Dauki fault, and the south dipping Oldham fault. A detailed gravity survey and a deeper MT survey along longer traverses across the Oldham fault may shed more light on this hidden structure.

Keywords

Magnetotelluric, Deep Resistivity, Oldham Fault, Shillong Plateau, Assam Valley.

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V. Srinivasan ¹, D. K. Saha ², D. C. Naskar ², P. M. Bhattacharya ², J. R. Kayal ²

Affiliations
1 Geological Survey of India, Op TNPK, Chennai, IN
2 Geological Survey of India, Kolkata, IN

Abstract

No Abstract.

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L. K. Das ¹, B. Das ², S. N. Chowdhury ³, D. C. Naskar ³, G. Karunakar ³, S. K. Dey ²

Affiliations
1 GSI, Marine Wing, DK 6, Salt Lake, Kolkata - 91, IN
2 GSI, seminary Hills, Nagpur, IN
3 GSI, CGD, 27 JLN Road, Kolkata - 16, IN

Abstract

Gravity-Magnetic mapping and resistivity sounding have been conducted in a grid pattern of 50 m × 100 m and 200 m × 200 m respectively over one square km area between Tokapal and Duganpal villages on Jagdalpur-Gidam road about 18-20 km from Jagdaipur township. The objective has been to map the eastern and the northern extension of an already known kimberlite sill in the area which is exposed in a pit and intersected in some drill holes at shallow depth to the west of the area surveyed.

The Bouguer anomaly (BA) map reveals a near circular peak marginal high of + 0.5 mGal around 0/N100 station over the background value of -59 mGaI in the area surrounded by an envelope of +0.3 mGal, almost in a N-S disposition between the traverses W350 and E100 stretching from the northern end down to S400 grid. Another such N-S feature, somewhat subdued (+0.3m Gal), is recorded in the BA map, in the northeastern part of the area bounded by traverses E200 and E400 and the base line. It is interesting to note that this sill like body is exposed around station E200/S300 where no BA high is detected. However, in the magnetic (VF) map, this sill body is picked up as a dipolar anomaly. Joining the axes of the dipolar lows and highs, the magnetic kimberlite body is yet again resolved into two near circular bodies as recorded in the gravity map. A gap in the continuity of the kimberlite sill around E200 is also indicated.

3D Euler's deconvolution solutions arrived from both the gravity and the magnetic (VF) data assuming a sill type body have been extremely useful in demarcating the geometry of the causative body at three different depth levels.The resistivity in a grid pattern shows the evidence of buried kimberlite in the area characterized by a marginal resistivity high (30-130 0hm.m). A 3D subsurface map of the kimberlite sill using ROCK WORKS has been prepared from the resistivity data.

Several 2-D G-sMe ctions have also been presented constraining these from the results of resistivity survey. Such sections will immensely help in targetting future drill holes in the area. The 3D Euler's deconvolution solutions and the 3-D configuration of the Kimberlite body have lent the third dimension in prospecting the covered kimberlites through geophysical mapping.

Keywords

Kimberlites, 3D Configuration, Gravity-Magnetic Mapping, Resistivity, Indravati Basin, Bastar District, Chhattisgarh.

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D. C. Naskar ¹, L. K. Das ², A. K. Lahiri ¹

Affiliations
1 Central Geophysics Division, Geological Survey of India, 27 J.L. Nehru Road, Kolkata-700016, IN
2 Central Geophysics Division, Geological Survey of India, 27 J.L. Nehru Road, Kolkata -700016, IN

Abstract

Gravity, magnetic survey and deep resistivity soundings along three profiles in Ulia-Hargawan area, Surguja district, Chhatisgarh provide the basement topography beneath the outcropping coal bearing Gondwana rocks. Deep resistivity soundings confirm that the overlying Gondwana rocks represents the Barakars with resistivity range of 50 Ohm-m to 400 Ohm-m, and thickness varies from 1 m to 442 m. The Archean basement is characterized by still higher order of resistivity in the area. The magnetic vertical field anomaly brought out some lower order magnetic response over the Gondwana rocks which is represented by the Barakars, because the overlying sediments are non magnetic. The amplitude of magnetic anomaly over the Gondwana lying between Ulia-Jagima is 200 nT to 230 nT in the area. One high amplitude magnetic anomaly of 500 nT has been observed just to the immediate west of Jagima which may be due to an intrusive body. Higher amplitude of magnetic anomaly is observed over the basement. Fluctuations in magnetic responses are observed at a few locations which may be due to the presence of basement faults. These are also corroborated with resistivity results.

The gravity profile and the resultant 2-D gravity modeling along Ulia-Patna-Nawapara brought out one basin a! structure which is corroborated by magnetic and resistivity results. The extension of this basin has also been inferred, whose probable direction is south of Ulia to north of Jagima. The thickness of Gondwana is interpreted to be 1 m to 450 m which is more or less corroborated with resistivity results. Two sharp contacts have been interpreted, one to the immediate west of Ulia village and the other to the immediate east of Jagima village respectively which are the boundaries between the Permo-carboniferous Gondwana and the Archaean metamorphic basement. High gravity values are observed over the high density metamorphic rocks.

Keywords

Gondwana, 2D model, Chhatisgarh.

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D. C. Naskar ¹, K. Choudhury ¹, K. K. Roy ², O. P. Singh ¹, R. K. Majumdar ³

Affiliations
1 Central Geophysics Division, Geological Survey of India, Kolkata - 700 016, IN
2 Department of Geology and Geophysics, IIT, Kharagpur - 721 302, IN
3 Department of Geological Sciences, Jadavpur University, Kolkata - 700 032, IN

Abstract

Geoelectrical survey using direct current Schlumberger and equatorial dipole-dipole resistivity sounding techniques and collinear dipole-dipole traversing have been carried out along Borgaon-Sanwer profile in Madhya Pradesh. The area is covered by Deccan basalts overlying the Gondwana, Vindhyan and Bijawar sediments and the basement of Bundelkhand granite. The traverse, having a length of about 160 km, cuts across the Narmada river near Omkareswar. 2D model of 1D interpretation of Schlumberger and equatorial dipole sounding data reveals that the thickness of Deccan Trap varies from a few tens of metres to more than one km. Bundelkhand granite marks the high resistivity basement. Formations of intermediate to low resistivity are the Gondwana, Vindhyan, Bijawar and Quaternary sediments. A fault at Chhaigaon-Makhan has been delineated from the plot of resistivity pseudo-section, which is also corroborated by the findings of spectral analysis of magnetic data. 2D modelling of magnetic data has also confirmed the presence of the fault. The interpretation has also brought out the variable thickness of Deccan Trap between Borgaon and Deshgaon of 200 m to 900 m.

Interpretation of the sounding curves in general reveals high resistivity Bijawar rocks occurring at shallow depth of less than one km in the area south of the Narmada river, whereas to the north of the river the Bijawar occurs at a greater depth. It may, therefore, be inferred that the course of the Narmada river is coincident with a postulated fault plane. The general findings of resistivity survey are mostly compatible with the interpretation of the gravity survey conducted earlier in the area. Besides these, some additional geological structures are also delineated.

Keywords

Resistivity Sounding, Deccan Trap, 2D Model, Spectral Analysis, Bijawar, Central India.

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