Delineating groundwater potential zones (GWPZ) in part of basement complex of Southwestern Nigeria using integrated geophysical methods

Abstract

Groundwater occurrence in basement areas is very erratic, and mapping out suitable zones for groundwater exploitation in such terrain largely depends on the choice of the geophysical technique adopted. Groundwater potential zones (GWPZ) within the Jericho area of Ibadan, Southwestern Nigeria, were investigated using an integrated geophysical (electromagnetic profiling and electrical resistivity) approach. Vertical dipole (VD) and horizontal dipole (HD) surveys for the electromagnetic profiling (EMP) were executed along eleven (11) E-W profile lines 250–400 m long. The electrical resistivity methods included horizontal electrical resistivity profiling (HRP) using a current electrode (AB) spacing of 45 m at a 15 m measurement distance and 2D-multichannel resistivity tomography (MRT) with a minimum 5–7 m dipole length depending on the extent of each traverse line. Sixty-two (62) vertical electrical sounding (VES) were carried out at selected ground points with maximum AB/2 = 100 m. The EMP results reveal conductivity values for VD ranging between − 214 and 336 mS/m (average = 21 mS/m) and HD between − 24 and 56 mS/m (average = 14 mS/m). Furthermore, apparent resistivity values for HRP range between 575 and 14 Ωm (average = 134 Ωm), reflecting a moderately conductive subsurface lithology. The 2D MRT section revealed three to five subsurface layers, with aquifer thickness of 1–30 m. VES results reveal H, AK, HK, HA, QH, and HKH curve types revealing multiple layering systems (3–5 geoelectric subsurface layers) with varying resistivities and thicknesses (topsoil = 14–599 Ωm, 1–2 m; lateritic clay = 13–72 Ωm, 0.44–19 m; quartzitic/pegmatite unit = 288–1070 Ωm, 1–37 m; weathered saprolite = 10–2950 Ωm, 4–18 m; and fractured/fresh basement rock = 472–16,908 Ωm, 1–43 m). About 66.2% of investigated VES points represent potential drill points with apparently thick weathered overburden units and fractured underlying bedrocks. Conclusively, the eastern, western, and northeastern parts of the study area are classified as good GWPZs. This study has revealed the effectiveness of the combination of geophysical techniques for groundwater investigation in difficult geological terrains.