Subsurface geological, hydrogeophysical and engineering characterisation of Etioro-Akoko, southwestern Nigeria, using electrical resistivity tomography

ABSTRACT Noninvasive geoelectrical subsurface characterisation provides screening of the earth medium to resolve complexity in subsurface geology caused by inhomogeneity of the overburden strata and bedrock architectures. The subsurface geological, hydrogeophysical and engineering conditions of Etioro-Akoko, southwestern Nigeria, were investigated using electrical resistivity tomography (ERT). This was aimed at unravelling the hydrogeodynamic and litho-structural complexity that are responsible for low groundwater yield in wells/boreholes and foundation failures. Field 2D resistivity data sets of the study area were inverted and used to produce geotomographic models for detailed insights into the complex subsurface geological setting. Results of the 2D resistivity inverted models showed three to four distinct layers; the topsoil, weathered layer, partially weathered/fractured bedrock and fresh bedrock. Bedrock structures occasioned by fracturing and deep weathering of the bedrock were delineated with resistivities and thicknesses ranging from 40 to 950 Ωm and 10–25 m, respectively. The fracture systems; F1, F2, F3, F4 and F5 in NW-SE, NNW-SSE, NE-SW and ENE-WSW orientations act as the major groundwater collecting centres in the area. The orientations and geometries of these geologic features are the manifestations of structural deformation of the underlying geology. Fourteen hand-dug well and four borehole points were proposed based on the ERT results. Conversely, the localised bedrock structures and oscillating bedrock topography were suggestive of potential threats to the foundations of engineering structures in the studied area. Reinforcement of concrete foundations at certain sites where ERT suggested that the underlying strata were not capable of bearing loads was recommended as well. This study has offered a detailed understanding of the subsurface geological disposition for sustainable groundwater development and siting of durable civil engineering structures in the studied area and other areas with typical complex geological settings.