Integrated resistivity surveys for delineation of fractures for ground water exploration in hard rock areas

Acute shortage of ground water in hard rock areas is well known. Ground water occurs in limited areal extent in secondary porosity generally developed due to weathering, fracturing, jointing, faulting etc. within the hard rock formations. These structural changes (fractures etc.) are sparsely distributed in the hard rock areas. Gradient profiling followed by geoelectrical sounding has been successfully utilized to delineate fractures saturated with water in hard rock areas of Gurudev Nagar, Mirzapur District, India. To begin with, the Gradient profiling (GP) survey was conducted along nine profiles with fixed current electrodes separation of 600 m and different potential electrodes separations for all the points lying between central region of the profile (about 33% of the total spread) at a station interval of 5 m. Eventually a comparative study of Wenner profiling and Gradient profiling along a specific profile line exhibits superiority of later over former. Based on the analyses of the response of GP survey, eleven prominent ‘low’ resistive points were selected. Geoelectrical sounding (GS) survey was then carried out at these locations. The interpreted sounding results show presence of fractured to highly-fractured sandstone. A test bore hole drilled at one of the locations yields a continuous discharge of ground water over 20,000 l/h. Present study clearly demonstrates that the GP survey is a powerful initial technique to identify the presence of fractures in hard rock areas and subsequent application of geoelectrical sounding at selected locations is very helpful for quantitative investigations.