GEOELECTRIC SOUNDING FOR GROUNDWATER EXPLORATION IN THE CRYSTALLINE BASEMENT TERRAIN AROUND ONIPE AND ADJOINING AREAS, SOUTHWESTERN NIGERIA

Onipe and adjoining areas are underlain by PreCambrian basement rocks and the dominant rock types are banded gneiss and quartzite. Sixteen (16) geoelectric soundings (VES) using the Schlumberger array were conducted across the area. Field data acquired were interpreted using partial curve matching approach and iterative resistivity sounding interpretation software, IPI2Win Version 3.0.1. The results show three to four geoelectric layers, namely: the topsoil (84 to 1165 ohm-m), weathered basement rock (22 to 436 ohm-m), fractured basement rock (156 to 2973 ohm-m) and fresh basement rock (3419 to 13695 ohm-m). The weathered and / fractured basement rocks constitute the aquifers or aquiferous zones in the area. Two dominant geoelectric sounding (VES) curve types identified are HA and H. Different hydro-resistivity maps produced from the geoelectric sounding (VES) data interpretation are total transverse resistance map, 3-D surface total transverse resistance map, total longitudinal conductance map, 3-D surface total longitudinal conductance map, basement relief map and 1-Grid Vector basement relief map. The reflection coefficient and resistivity contrast values computed from geoelectric sounding interpretation results as well as the various hydro-resistivity maps depict that the best points for siting wells or boreholes in the area are VES stations 3, 4, 7, 8, 9, 10, 13 and 14.