Investigating aquifer structure in a low-latitude crystalline basement complex of southwestern Nigeria using radial vertical electrical sounding

Abstract

The ability of radial vertical electrical sounding (RVES) to detect anisotropy caused geologically by fracturing, jointing, layering, and rock fabrics was employed to investigate the structure of hard rock aquifers in Oyun-Asa basin. This was facilitated by the need to hydrogeologically characterize the aquifers in an attempt to meet increased demands on the potable water supply due to population explosion, increasing industrialization, and agricultural growth in the area. This was accomplished by using the anisotropy models obtained from the variation of computed coefficient of anisotropy and its orientation with depth to characterize hydrogeological properties of the weathering profile developed over different lithology in the area. The geometric mean resistivity values were also computed and calibrated to assess the degree of weathering. The coefficient of anisotropy values obtained from best-fit ellipses of the polar apparent resistivity plots of RVES data varied between 1.01 and 3.00 along different directions. The lower and upper range values, 1.01–1.20 and 1.21–3.00, were attributed to heterogeneous and homogeneous anisotropy, respectively. The heterogeneous anisotropy revealed pre-existing heterogeneities within the lithology at different depths and could have enhanced the weathering process locally. The homogeneous anisotropy indicated the fissured layers comprise three fissure sets: horizontal or stratified fissured layer (SFL), sub-vertical, and vertical. Six types of homogeneous anisotropy models whose coefficient of anisotropy (homogeneous) values and orientations vary with fracture intensity and depth were also identified. The geometric mean resistivity ranged between 82 and 2337 Ωm indicating varied degrees of weathering and its dependency of different lithologies in the study area. Aquifers in Oyun-Asa basin are localized, depending on extent of weathering and fissuring of the underlying rocks. The presence of SFL known for its contemporaneity with weathering process indicates the hard rocks in Oyun-Asa are in different weathering phases.