Geophysical Evaluation of Subsurface Geology for Foundation Purposes at Wukari, Northeastern Nigeria, Using Electrical Resistivity Method

Geophysical investigation using the electrical resistivity method was conducted at a proposed engineering site in Wukari, northeastern Nigeria, to evaluate the subsurface geology for foundation purposes. The main objective of the investigation was to delineate the subsurface stratigraphy and the geological structures beneath the study area that may influence the safe development of the proposed site. Vertical electrical sounding (VES) surveying technique, which utilizes the Schlumberger electrode array, was employed to acquire 1D resistivity data at 40 stations in the area. The data were acquired at regular station intervals of 100 m along four profiles, including three East-West parallel profiles and a perpendicular North-South profile of approximately 1 km each in length. The survey revealed the study area to be underlain by three stratigraphic units, comprising (1) clayey-sand to lateritic topsoil with an average resistivity of 1750 Ωm and thickness of about 12 m, (2) a middle layer, predominantly clay/shale, with an average resistivity of 28 Ωm and thickness that increases from approximately 20 m in the south to about 100 m towards the north, and (3) a moderately to highly indurated sandstone bedrock that dips at approximately 20 degrees to the north and characterized by high resistivity values with an average of 3414 Ωm. The survey also revealed the overburden thickness to be increasing from about 35 m in the south of the study area to about 100 m in the north, while near-surface structures such as fold, fault and fractures that can influence the safe development of the proposed site were also detected beneath the study area.