Geo-electrical exploration for groundwater in a hard rock region of Hyderabad, India
Geo-electrical methods are employed very commonly in geohydrological investigations, as they are more economical and effective than other geophysical techniques. The direct current resistivity (DCR) method is an effective tool in groundwater exploration, geothermal studies, civil engineering applications, and in monitoring water pollution and contamination. To achieve these objectives, conventional Schlumberger and Wenner soundings and electrical resistivity tomography (ERT) are currently used worldwide. Like other geophysical methods, interpretation of DCR data provides a non-unique solution of a real geological model. In recent years, two types of technique have been developed to interpret DCR sounding data. The first is the ‘direct method’ that requires no information on the number of layers, and the apparent resistivities are assumed to be true resistivities (Koefoed 1979; Zhody 1989). In the second method, known as the ‘indirect method’, an initial-guess model is required for initialization of the inversion of observed data (Jupp & Vozoff 1975). Non-uniqueness of the interpretation makes it difficult to select the best model that is closest to the real geological model. Results of interpretation of DCR sounding data become more ambiguous with increasing depth. It is very difficult to interpret layer resistivity and thickness with sufficient accuracy, and both synthetic and field examples show that the resolution of thin layers (having thicknesses less than one-tenth of their depth) is particularly difficult (Singh 2003). The properties of a thin conducting layer can be determined in terms of longitudinal conductance, and those of a resistive layer by transverse resistance (Yungul 1996). In the present study, layer models have been obtained using a stable iterative algorithm proposed by Jupp and Vozoff (1975). The effectiveness of the resistivity method in the determination of aquifer parameters was demonstrated by Rijo et al. (1977). Several factors that create problems in the detection of an aquifer/conducting layer, like the presence of a conducting surface layer, effects of anisotropy, and the screening effect of overlying layers, have been discussed in detail by Singh (1998a). It is a well-established fact that ambiguity in interpretation of resistivity data increases very rapidly with increase in depth (Singh 1998b). Singh (2003) suggested a new approach to the detection of hidden aquifers in hard-rock regions based on resistivity data transforms. The resistivity method is applied to determine the groundwater potential of an area and to study the relationship wells. 3D maps of depth and thickness of the aquifer in Osmania University Campus (OUC) were prepared in order to study the variation in the level of the water table. The role of the thickness of the aquifer in water resources management, and the natural/ artificial recharge of the aquifer, has been studied in detail. The main objective of the present study was to delineate the subsurface distribution of groundwater in the OUC. In addition to this, the relationships between the surface/subsurface layer parameters and the yield of existing boreholes and the recharge of the water table were examined. This study has also proved very useful in identifying new sites that are suitable for groundwater exploitation. Location and geology of the area The study area is located between latitudes 17º 24' 30” and 17º 25' 30" north, and longitudes 78º 27' and 78º 29' east. It is an area of 3.1x 2.9 km2 covering the entire OUC. It is a typical hard-rock region where water is found in small pockets within fractures. The whole area is covered by granitic soil, and granitic rocks are also exposed at several locations. Known as the Hyderabad granites, the rocks exposed in this area of slightly elevated topography are of Archæan age.