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

K. Choudhury ¹, D. K. Saha ¹, D. C. Ghosh ¹

Affiliations
1 Central Geophysics Division, Geological Survey of India, Calcutta - 700 016, IN

Abstract

Mixing of subsurface fresh and brackish/saline water has given rise to pollution in the aquifer and consequent environmental problems in the urban and semi-urban areas of the Howrah District of West Bengal. Also, overdrawal of groundwater has adversely affected the water table resulting in increase in salinity and possible threat to the civil structures. In order to suggest some remedial measures to arrest further deterioration of the environmental problems due to excessive and random pumping of groundwater, vertical electrical soundings (VES) using Schlumberger configuration have been carried out at 70 locations. Geoelectrical sections, isoresistivity maps at different depth levels as well as total transverse resistance (TTR) map of the area have been prepared. Analysis of these interpreted sections and maps have been able to (1) ascertain the resistivity and geohydrological characteristics of the subsurface layers, (2) map the aquifers and saline/brackish groundwater zones and (3) suggest measures for safeguards against groundwater pollution. Further, prospective groundwater zones and an old channel of Bhagirathi river are identified within [he highly conducting brackish deltaic area. The litho-sections brought out by the interpretation of survey data uould be useful to the city planners for land use, foundation studies and related utilities.

Keywords

Groundwater, Vertical Electrical Sounding, Geoelectrical Section, Gangetic Delta, West Bengal.

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P. R. Pant ¹, B. N. P. Agarwal ², K. Choudhury ³, S. P. Dasgupta ³

Affiliations
1 Geological Survey of India, 12-13-369, 'Sai Krupa', Tarnaka, Secundrabad-500 017, IN
2 Department of Applied Geophysics, Indian School of Mines, Dhanbad-826004, IN
3 Central Geophysics Division, Geological Survey of India, 27, J.L. Nehru Road, Calcutta-700016, IN

Abstract

Qualitative interpretation of the regional Bouguer anomaly map of the Deccan Trap region of peninsular India, based on the anomaly size, its gradient and minor contribution from the shallow crust assessed by the results of other geophysical methods, suggests that the major part of the these anomalies have deep seated origin with 'lows' corresponding to the zones of subsidence and 'highs' to the uplifts involving the crust. Deep faults along the west coast and segmented faults demarcating the Narmada rift valley and Narmada-Son Lineament (NSL) have been delineated. Spectral analysis, matched filter technique for deconvolving gravity effect of sources at a particular depth and a single interface gravimetric inversion scheme have been utilised to delineate density discontinuities and their 3D configurations. The deconvolved gravity effect due to the deepest density interface at a mean depth of 117 km seems to reflect lithospheric variations and is used to distinguish the stable crust from the metastable one. The Moho relief map, apart from structural features, exhibits a good agreement with the results by Deep Seismic Sounding (DSS) in Koyna area and across NSL.

The block-wise analyses reveal higher average depths to Moho and Conrad, which are typical of stable shield crust, in the southern part whereas the northern blocks associated with rift tectonics exhibit considerably thin upper crust (5.8 to 12.6 km) with another crustal interface at a depth in the range of 16.3 to 26.8 km. The latter depth forms the base of the middle crustal layer i.e., top of the lower crustal layer comprising lower mantle derivatives. These inferences are based on the correlation between Bouguer anomaly and upper crustal thickness and layered velocity structure brought out by DSS data across the NSL. Filtered Bouguer anomaly maps have further facilitated the interpretation and better demarcation of structural features inferred from qualitative interpretation.

Keywords

Geophysics, Bouguer Anomaly, Deep Crustal Structure, Gravity Inversion, Deccan Trap.

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D. K. Sawa ¹, K. Choudhury ¹, P. K. Chakraborty ¹

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

Abstract

A number of saline/brackish water zones are present in the subsurface around the Kolkata metropolis. Mixing of fresh and brackish groundwater has created environmental problems in certain areas. Vertical Electrical Soundings (VES) employing Schlumberger configuration have been deployed in the eastern and southern Kolkata metropolis for delineating the subsurface saline water zones. Interpretation of VES data has brought out disposition of saline/brackish and fresh water zones at different depth level which would be useful in the proper drawal of groundwater with minimum risk of saline contamination. Resistivity surveys have also delineated clay formations which act as barriers for saline water percolation or transmission. Aquifer zone at some depths south of Bhangar canal is vulnerable for saline water contaminations large part of this area is occupied by brackish/saline water in the subsurface. It has been found that the area north of the canal does not contain any appreciable saline water in the subsurface and may be considered environment friendly it is further inferred that the saline water was trapped inland in the Holocene times during marine transgressions causing salinity of groundwater at several places.

Keywords

Saline Water Zones, Vertical Electrical Sounding, Kolkata Megacity, West Bengal.

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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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