Geophysical and Borehole Studies for Optimum Drilling Depth and Groundwater Yields in a Typical Basement Terrain, South-western Nigeria

This study embraced the integration of 20 vertical electrical sounding (VES) and 15 borehole log datasets to determine optimum drilling depth and yields of boreholes in part of Akungba - Akoko, southwestern Nigeria. Resistivity soundings interpretation was aided by borehole litho-geologic information. Results revealed line-to-line correlation of three-five distinct subsurface geological/geoelectrical profiles to include; the topsoil, sandy/clayey weathered/partially weathered basement, fractured basement/ fresh basement. The thickness and resistivity of the weathered basement column ranged between 1.9 - 15.8 m and 26 - 579 ohm-m respectively. Regoliths, with a mean thickness of 9.97 m (approximately 10 m), constituted thin aquitards developed on the basement. The combination of regoliths and the near–surfaced interbedding multiple thin layers of the fractured basement up to 30 m depths gave lower yields of less than 1.0 l/s to shallow boreholes. These drilled depths were those constrained at the instance of the poor index of lithologies exhibited by the sounding results, thus rendering the thin fractured beds undifferentiated from the thick resistive basement. Depths for high groundwater yields of 1.15 – 2.56 l/s were recorded for deeper borehole depths of 46 - 70 m, with a mean of 55.3 m. It can therefore be concluded that optimum groundwater yields not less than 2.0 l/s were recorded for boreholes that took full advantage of the weathered lithology and structures at deeper depths of 60 - 65 m. The study further asserts that beyond the mean depth of 55.3 m, the deposits become more compacted, low fracture density with groundwater yields wholly retarded, thereby leaving the optimum drilling depth to 70 m, as the target of most groundwater prospects for optimum yields of deeper boreholes in the area