Assessment of subsoil characteristics by electrical resistivity method for engineering, groundwater and environmental impact at a proposed estate in a basement complex terrain—a case study from Southwestern Nigeria

Abstract

We present an integrated site investigation of a proposed estate with a view to generating baseline data required for safe design and construction of structures and groundwater schemes, installation and protection of utilities, and post-construction environmental issues. An electrical resistivity survey involving 1D and 2D imaging techniques was involved. The 1D resistivity imaging involved the Schlumberger Vertical Electrical Sounding (VES) with half current electrode spacing of 1–125 m, whose data were interpreted for layer resistivities and thicknesses using segment-by-segment curve matching and 1D forward modeling with W-Geosoft/WinSev 5.1 code. The 2D dipole-dipole resistivity imaging utilized 20 m dipole length and an expansion factor (n) ranging from 1 to 5. The 2D data were inverted to 2D resistivity images with DIPRO software. The 1D and 2D interpretation models delineated five subsurface layers comprising topsoil, fill, laterite, weathered basement, and fresh basement. The weathered basement is the main aquifer with thicknesses ranging from 11.9 to 64.4 m with prospect for groundwater development. The essentially lateritic subsoils in the upper 10 m with resistivity values generally > 450 Ωm are adjudged moderately to highly competent. The low-porosity/permeability lateritic layer overlying the weathered layer aquifer constitutes a sealant against infiltrating pollutant with the groundwater significantly protected. Except within the stream channel, the soils/subsoils are practically non-corrosive.