Geophysical Characterization of the Basement Rocks and Groundwater Potential Zones Using Electrical Resistivity Sounding Technique

Abstract

This study characterizes the basement rocks and delineates groundwater potential zones in a basement complex terrain of Adeyemi College of Education, Ondo, Southwestern Nigeria due to its erratic and complex nature of groundwater development. Vertical Electrical Sounding (VES) technique adopting Schlumberger configuration was used to acquire the apparent resistivity data using a resistivity meter. Ten VES stations were established with 100 m spread for half-current electrode spacing. The data were subjected to partial curve matching and computer based iterative interpretations. The generated sounding curves revealed heterogeneous nature of the subsurface geological sequence; topsoil (132-467 Ωm), sandy clay (101-370 Ωm), lateritic clay (347-1133 Ωm), weathered layer (128-750 Ωm), fractured basement (554-5045 Ωm) and fresh basement (14244-24744 Ωm). The results delineated weathered and fractured basements as the aquifer units for the study area. The composite thicknesses of the aquifers were used to characterize the subsurface into high and low groundwater potential zones. VESs 2, 6, 7, 8, 9 and 10 revealed adequate overburden thicknesses (26.5-58.7 m) for groundwater storage, and they were recommended for groundwater exploitation. VESs 3 and 5 revealed low thicknesses (7.1-19.3 m) of the aquifer units, and were regarded as low groundwater potential zones. VESs 1 and 4 revealed fresh basement with thin overburden thicknesses (13.3-10.3 m) and no evidence of fractured and groundwater bearing zones. This study generally revealed high groundwater prospects, and efforts to sink boreholes had been abortive in the past due to lack of adequate knowledge as regards the heterogeneous nature of the subsurface.

Keywords: Basement complex, Heterogeneous nature, Vertical Electrical Sounding, Apparent resistivity, Overburden thickness, Groundwater potential