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 delin- eated 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 and1013.7–33,124.0Xmwithbottomdepths 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 ground- water exploration and development in the study area