Groundwater Prospecting and Exploration in a Low Potential Hard Rock Aquifer: Case Study from Ogbomoso North, South-western Nigeria

Very Low Frequency Electromagnetic (VLF-EM) and Electrical Resistivity methods has been employed to unravel the localized nature of groundwater occurrence in Ogbomoso North, southwestern Nigeria. This was aimed at addressing the growing demand for groundwater use in the area underlain by migmatite gneiss with minor intrusions of pegmatite and quartz veins. Nine profiles comprising five VLF-EM and four electrical resistivity profiles were conducted parallel to the two major azimuths in the area.  The anomalous zones identified on the profiles were further investigated by twelve Vertical Electrical Soundings (VES). Lithologic logs from three boreholes were also used to ground truth the geophysical findings. Quantitative interpretation of VES data showed that the Geoelectrical Succession comprise three to five layer earth models. The aquifer units are localized comprising weathered layer and fractured basement. The lowest yield of 280.6LPH was obtained from well bored on VES point having three-layer earth models made up of resistive-conductive-resistive geoelectric succession. Well bored on VES points having more than three-layer earth models and comprising alternating band of resistive-conductive geoelectric succession have more yield ranging between 520.8LPH and 590.4LPH.  Furthermore, there was strong correlation between geoelectric succession and lithologic sections obtained from the drill cuttings of the bored wells. Keywords: Groundwater Exploration, Geophysical Methods, Lithology Logs, Fractured Basement, Crystalline Rocks