Geoelectric investigations in Bakreswar geothermal area, West Bengal, India

Abstract

A cluster of hot springs in Bakreswar geothermal area, which belongs to the Chotanagpur Gneissic Complex in the eastern part of Peninsular India, is characterised by varying temperature and similar chemical composition.

Vertical electrical sounding (VES) investigations in and around Bakreswar reveal the presence of two to four prominent lithologic layers under prevailing hydrodynamic conditions. The intermediate weathered zone and the fractured rocks constitute a single aquifer system of varied hydraulic conductivity under water table condition. Lithology and groundwater conditions, as inferred from the VES, as well as hydrological studies, are in agreement with the nearby bore hole lithologs. Water table contours accompanied by VES findings of the region indicate that the occurrence and movement of groundwater take place mostly within the weathered and fractured rocks under unconfined condition. 1D interpretation of VES results reveals few promising groundwater potential zones in the eastern part of the region. Wenner resistivity profiling, coupled with VES and geological studies, indicates the presence of a nearly N–S striking buried fault providing passage for hot water to emerge in the form of springs.

Introduction

Bakreswar geothermal area (23°52′48″N:87°22′40″E) is one of the few groups of geothermal areas in the Chotanagpur Granite Gneiss Plateau of the eastern part of the Indian Peninsular Shield (Fig. 1). The geothermal areas in the terrain are characterised by surfacial manifestation of a cluster of springs with varied temperatures (35–88°C) and similar chemical compositions (Deb and Mukherjee, 1969). The springs mostly issue out of fractures in a reactivated composite mass comprising predominantly granitic rocks (Precambrian) with an E–W belt of sparsely occurring sedimentary outliers of Gondwana formation (Lower Permian to Middle Jurassic).

Several workers have investigated Bakreswar group of hot springs from time to time, dealing with their geological mode of occurrence and chemical composition Ghosh, 1948, Mukherjee, 1967, geochemistry (Chowdhury et al., 1964), genesis Chaterji and Guha, 1968, Deb and Mukherjee, 1969, natural gases with special reference to helium emanation Chatterjee, 1972, Ghose and Chatterjee, 1980, Ghose et al., 1989, Ghose et al., 1994, isotopic composition Ghose and Chatterjee, 1978, Majumdar et al., 1998 and geothermal energy potential Mukherjee, 1982, Majumdar et al., 1992, Mukhopadhyay, 1996.

Most of the previous works are restricted to surfacial studies and little is known about the subsurface geology related to the hot spring activity. On the basis of geophysical magnetic and electrical studies done by the Atomic Minerals Division (Calcutta) in the Bakreswar geothermal area, Nagar et al. (1996) have suggested the presence of a N–S trending fault extending for about 2 km in this area. Details of their investigations are not, however, published.

The weathered and fractured rocks present a good resistivity contrast with compact basement crystallines, and the structures, such as joint, fault, dyke, etc., in a geological terrain impose marked anomalies on the resistivity profiles. Hence, electrical resistivity investigations involving Schlumberger sounding and Wenner profiling are carried out in and around the Bakreswar geothermal area in conjunction with geological studies to (1) identify various lithologic layers for examining the nature of aquifer systems, (2) probe into the hydrology of the springs, and (3) detect the possible major fault zone or buried fractures leading to the hot water emergence in the form of springs.