ABSTRACT
In hard rock groundwater exploration, the aim of subsurface characterization is to delineate the weathered zone and its saturated thickness, saprolite, basic intrusives, structures and saturated fractured zones occurring up to ≈200m depth. Also, it is to decipher fracture orientation and connectivity and the water content, flow and quality. It is likely that information on these aspects will be obtained through geophysics which basically provides subsurface physical property distribution or variation. The delineation of the weathered zone, mostly within 30m depth, generally does not pose any problem. The conventional DC resistivity sounding or imaging is adequate for the purpose. The problem starts with the investigation of deeper horizons to which one must look for a better yield and potable water; the delineation of underlying productive saprolite whose thickness and character vary and that of deeper fractured zones which are generally thin (a few centimetres to a few metres). As far as resistivity of the saprolite is concerned, under normal situation with fresh water saturation, it is mostly associated with intermediate values compared to the weathered zone and underlying compact formations and represents a layer with transitional increase in resistivity with depth. Detection and characterization of this layer by the resistivity method alone is quite difficult if it is ‘thin’ (thickness compared to depth of occurrence), because of transitional resistivity, non-uniqueness (equivalence) and poor resolution leading to a range of equivalent models. Similarly, it is difficult to quantify the parameters of thin conductive localized fractured zones in highly resistive compact bedrock due to suppression and decreased resolution even with reduced scale of measurement. Like DC resistivity, the commonly used EMmethods also suffer from similar limitations and are generally used in combination. Integrating the DC and EM results with the inferences drawn from remote sensing, geological and borehole information, hydrogeochemical data, other geophysical methods and techniques and combining the interpretational procedures sequentially or through joint inversion can improve resolution and minimize (not eliminate) non-uniqueness or uncertainty in target characterization and depth estimations. The integration of methods or a multi-method approach has almost become a necessity to solve critical hydrogeological complexities holistically (Chandra et al., 2002a).
