Geophysics in Groundwater Exploration and Aquifer Mapping in India

Abstract

Integrated approach of hydrogeology and geophysics in groundwater exploration, aquifer mapping, assessing groundwater salinity, designing water wells, estimating hydrogeological parameters and monitoring hydrogeological processes is the domain of Groundwater Geophysics. In India, it is in practice for more than eight decades, initiated by Mysore Geological Surveys in 1930s, followed by GSI in 1945, then CSIR-NGRI, CGWB, State GWDs, Universities, IITs, NGOs and private agencies. Recently for aquifer mapping CGWB got heliborne geophysical surveys conducted by CSIR-NGRI in 6 pilot areas of varied hydrogeology, as also surface geophysical surveys in 15 States by WAPCOS. Application of geophysical methods for aquifer mapping does not vary much from that for groundwater exploration, except for the economic selection of appropriate method(s) for the hydrogeological terrain vis-a-vis systematic coverage of a much larger area at an appropriate scale of measurement to produce a realistic and precise aquifer grid map. Multiple-technique geophysical applications have been made in diverse hydrogeological terrains of hard rocks, cavernous limestones, coasts, alluvial plains, sedimentary basins, deserts and hills. The objective has been to delineate the weathered and deep fracture zones in granite gneisses, deep aquifers in fractured basaltic flows, saturated cavities in limestone, freshwater aquifers in mainland coastal tract and islands, deep fresh water aquifers in sedimentary basins, inland groundwater salinity, deep buried coast parallel freshwater channels, palaeo-channels of rivers and lost rivers in desert, flood plain aquifers, groundwater pollution and identification of impermeable clay barriers in arsenic contaminated areas, alternative aquifers in water stressed urban areas and assessment of subsurface conditions causing land subsidence in overexploited areas. Also, efficient applications have been made in drought, earthquake, coastal cyclone and tsunami affected areas. The geophysical methods have also been used in timelapsed mode for monitoring the quality of groundwater which may form a major input for remediation. The interpretation of subsurface geophysical data is contextual, therefore a variety of issues of resource conservation, protection and augmentation have also been addressed. The geophysical methods can be used on surface, in boreholes and also airborne. The most commonly employed geophysical method is electrical resistivity. It is followed by application of electromagnetic, magnetic and seismic methods and in places gravity method. The other electrical methods used are Induced Polarization (IP), Self Potential (SP) and Mise-a-la-Masse (MM). Since, methods and techniques at places may not independently help decipher the subsurface conditions adequately because of inherent limitations, they are integrated. The modern techniques like electrical resistivity imaging (ERT) which automatically combines resistivity sounding and profiling, ground penetrating radar (GPR) and emerging nuclear magnetic resonance (NMR) technique are in practice but being expensive, used only sporadically. The helicopter borne electromagnetic and magnetic surveys are conducted for high density data generation in less time. The inferences drawn from geophysical surveys may not always match with the borehole results and therefore for hydrogeological transformation, which is crucial in aquifer mapping, the geophysical parameters are standardized through borehole information and forward modeling. Also, the terrain related vital hydrogeological attributes form an input, because interplay of attributes controls the geophysical response. It has to be clearly conceptualized that hydrogeophysics has to start from hydrogeology and end in hydrogeology (Fig. 1).