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

V. P. Singh ¹, O. P. Singh ¹, C. L. Singh ¹

Affiliations
1 Department of Geophysics, Banaras Hindu University, Varanasi - 221005, IN

Abstract

Detailed ground magnetic survey over a part of the Vindhyan basin around Damoh and Jabalpur (40 km either sides of Hirapur-Mandala DSS profile) has been carried out. The magnetic field anomaly map is characterised by dense contours in trap covered areas near Jabalpur where as the Vindhyans show sparsely distributed contours with several lows. Spectrum analysis of total ground magnetic field anomalies reveals the average thickness of Vindhyans to be 3.9 km and depth of Conrod to be 15km. The average thickness of Vindhyans is in conformity with the Deep Seismic Sounding results of the region. The thickness of Vindhyan sediments along four profiles ranges from 1.5km to 5km. The maximum thickness of Vindhyan sediments at Jabera has been derived to be 4.8km which is close to the results of Oil and Natural Gas Corporation Ltd., Dehradun.

Keywords

Geophysics, Vindhyan Basin, Magnetic Anomaly, Jabalpur, Madhya Pradesh.

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V. P. Singh ¹, O. P. Singh ¹, C. L. Singh ¹

Affiliations
1 Department of Geophysics, Banaras Hindu University, Varanasi - 221 005, IN

Abstract

Detailed ground magnetic surveys over parts of Archaeans. Satpuras and Chhattisgarh basins belonging to different ages around Mandala-Raipur has been carried out. Detailed investigations provide depth of magnetic sources, basement structural trend. nature of intrusive bodies and thickness of sediments in the basins. The basement structural trend along the five profiles in Archaeans (AA'and BB'), Satpuras (CC') and Chhattisgarh (DD' and EE') have been derived which ranges from 1.5 to 3.5 km. The Archaean and Chattisgarh basins show two dyke like features at depth of 0.28 and 1.26 km. It also reflects upwarping of the basement along the profiles.

Keywords

Geophysics, Intracratonic Basins, Archaeans, Satpuras, Chhattisgarh, Magnetic Anomaly.

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O. P. Singh ¹, Pankaj ¹, M. Bhattacharya ¹

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

Abstract

A study of travel times for the P_g, P^* and P_n waves is conducted selecting three traverses over the Shillong Plateau, Mikir Hills and Naga Thrust zone respectively in the Northeast India region. Seismic velocities and thickness of the upper crust, lower crust and depth to the Moho are estimated. These traverses are selected through the location of the maximum number of seismic stations and the epicentres of the earthquakes in the study area. About 136 earthquakes, with magnitude range 2.0-5.0 and depth range 5-30 km, recorded during the period 1986-1999 are used. The P_g, P^* and P_n velocities (km/sec) are estimated to be 6.04-6.21,6.51-6.62 and 8.47-8.65 respectively in the region. The corresponding thickness (km) of the upper crust, lower crust and depth of the Moho are 16-19, 27-30 and 40-50, respectively. The presence of high P_n wave velocity (8.47-8.65 km/sec) in the region is possibly representing a colder and denser lithosphere.

Keywords

Seismic Velocities, Cristal Structure, Naga Thrust, Mikir Hills, Shillong Plateau, Northeastern India.

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J. R. Kayal ¹, Sagina Ram ², O. P. Singh ¹, P. K. Chakraborty ¹, G. Karunakar ³

Affiliations
1 Gelogical Survey of India, Central Geophysics Division, 27, J.N. Road, Kolkata - 70001 6, IN
2 Geological Survey of India, Northern Region, Lucknow - 226 024, IN
3 Geological Survey of India, Eastern Region, Salt Lake, Kolkata - 700 094, IN

Abstract

The aftershock sequence of the Chamoli earthquake (m_b 6.3) of March 28, 1999 provides an opportunity to study the aftershock characteristics and the seisrnotectonic structure in the Garhwal Himalaya. Detailed analysis of the spatial variation, the frequency-magnitude distribution (b-value), the activity decay-rate (p-value) and fault-plane solutions of the aftershocks throw new light on the aftershock characteristics and seismotectonics of the region. The observations suggest that the aftershock generating process and the b- and p- values changed with time, space and magnitude-ranges. The main shock occurred on the Basement Thrust by a thrust-fault mechanism. The aftershocks, on the other hand, are triggered by the seismogenic faults to the south of the Main Central Thrust (MCT) by thrust as well as by strike-slip faulting, and all the events occurred above the plane of detachment.

Keywords

Main shock, Aftershocks, Seismicity, Fault-Plane Solution, Seismotectonics, Chamoli, Garhwal Himalaya.

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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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