Evaluation of hydraulic conductivity of subsoil using electrical resistivity and ground penetrating radar data: example from Southwestern Nigeria

Abstract

Groundwater seepage has being linked to the cause of building failures in Erinle area, Ede metropolis, SW Nigeria. A comparative study of the hydraulic conductivity (K) of first two subsoil or subsurface layers underlying the area of investigation was carried out using data acquired from ground penetrating radar (GPR) and electrical resistivity (ER) surveys. Ground penetrating radar (GPR) and electrical survey (ER) were performed along eight traverses over the shallow subsurface. From these a total of eight radar sections and geoelectric sections were developed along an electrode separation of 80 m. The results from the radar section and geoelectric section show that the area under investigated is made up of three stratigraphic layers (laterite topsoil, weathered layer, bedrock). The radar sections show that the second subsurface layer (weathered layer) or subsoil is pervious as compared to the others. The weathered layer shows relatively smooth reflections suggesting high groundwater flow which accounts probably for the groundwater seepage observed. Hydraulic plots and Porosity confirms the pervious nature of the weathered layer as oppose to the other two layers as expressed by high hydraulic conductivity (K) values of 1.0 × 10−5–7.4 × 10−6 m/s and high porosity values of 2.16–5.06 which makes it liable to groundwater seepages. Both the ER and GPR surveys confirmed that the second subsurface layer (weathered layer) is more prone to groundwater seepage especially in traverses (1, 2, 3 and 5) where the relative thickness of the first layer (lateritic topsoil) is less than 5 m and in traverses (4, 6, 7 and 8) having a resistive layer (basement). This study shows the importance of geophysical investigation in detect of groundwater seepage and post-foundation investigations.