Application of Ground Penetrating Radar and Electrical Resistivity Techniques for Subsurface Stratigraphic Mapping in Southwestern Nigeria

The frequent building collapses in Nigeria have been attributed to lack of pre-construction investigations that will assist engineers obtain in-situ geotechnical information and the structural subsurface settings are often ignored; and when done, it is haphazardly. To address this issue and demonstrate the importance of such survey, a combined Ground Penetrating Radar (GPR) and Vertical Electrical Sounding (VES) data was acquired in a part of Southwestern Nigeria. A 200 MHz antenna was used for the data acquisition along four traverses. The data were subjected to standard GPR processing techniques, and attributes analyses such as instantaneous-frequency, amplitude and phase. Also, for comparative and engineering characterization purposes, longitudinal conductance and coefficient of anisotropy were computed from the VES results and used for determining the competency of the bedrock. From the GPR results, it was observed that the subsurface mapped is characterized by an erosional truncated, low angle, southerly dipping, tangential reflections. Further, stratified rocks dipping at an angle of 32º occurs between 1.0 to 4.5 m depth in all the GPR sections; these strata were truncated by topsoil at shallow depths. Also, some of the sections depict ancient channel structures that have a dimension of 70 m by 40m. The resistivity data suggest that the study area is characterized by four distinct geoelectric sequences. These sequences are comprised of topsoil, which is composed of clayey-sand to lateritic clay whose thickness ranges between 0.25 m – 8.12 m; weathered bedrock with a thickness of 3.84mto 12.61m; stratified bedrock with a thickness ranging between 0.33m – 7.51m and the fresh bedrock. These results reveal that the study area is characterized by a complex subsurface geology. The area has low to moderate longitudinal conductance and anisotropy coefficient values which suggest that incompetent to semi-competent bedrock exists beneath the subsurface. A good correlation between the GPR and resistivity derived thicknesses was established.