An integrated geophysical approach for post foundation studies at Edunabon town, southwestern Nigeria

An integration of geophysical methods including electrical resistivity and seismic refraction was used to investigate the possible causes of the cracks observed on some buildings at Edunabon, southwestern Nigeria. A total of three 2D electrical resistivity imaging (ERI) traverses and eight vertical electrical sounding (VES) were acquired using dipole–dipole and Schlumberger electrode configurations, respectively. The 2D ERI and VES data were processed and inverted to obtain the subsurface models. Seismic refraction data were also acquired along eight traverses using a 24-channel ABEM seismograph with geophone spread of 2 m, and integrated with electrical resistivity data, to adequately delineate the geomaterials responsible for the localized structural defects in the area. The results of electrical resistivity method revealed four geoelectric layers including topsoil, clayey material, weathered basement and fractured/fresh basement. However, the geosection models of seismic refraction revealed two geophysical layers (overburden and basement bedrock) with characteristics that corroborate the electrical resistivity results. The overburden has an average velocity and thickness of 405 m/s and 7.1 m, respectively, which is diagnostic of weak/unconsolidated materials, presumably clayey material. The integrated results revealed that the observed cracks on the buildings may be due to the foundation soil that is composed of incompetent materials (clayey materials) which may compress under differential effective stress. This study shows that the integration of geophysical methods can identify the causes of the differential subsidence of engineering structures.