Geological-Geophysical Investigations for Hydrological Studies in a Basement Complex Terrain, Southwestern Nigeria

Geological field mapping and vertical electrical soundings (VES) were conducted in Igbo-Ora, southwestern Nigeria in order to unravel the subsurface structures, as part of the preliminary investigations for groundwater resources assessment, development and management in a crystalline basement terrain, southwestern Nigeria. The geological survey was carried out to produce a local geological map with spatial distributions of different basement rocks and their structural trends. Metamorphic and igneous rocks make up 90 and 10%, respectively, of the rocks in the study area. They include the banded gneiss, biotite granite gneiss, quartzite/quartz-schist and granitic intrusions of varying grain sizes. Twenty-five VES surveys were conducted within the biotite granite gneiss terrain of the area, using Schlumberger array, providing layering and geoelectrical parameters. Three geoelectric layers delineated from the VES 1D inversion models are clayey sand/sandy clay top soil (overburden), partly weathered or fractured basement and fresh basement. The corresponding inverse model resistivity values ranges are: 209.7–2298.0, 45.1–346.2  and 1013.7–33,124.0 Ωm with bottom depths ranges of 0.9–2.9 and 4.6–42.0 m, respectively. The topmost clayey sand/sandy clay layer will serve as the protective layer, while the saturated portion of the partly weathered or fractured basement, at depth, favors groundwater exploration and development in the study area.