Geophysical Investigation of Basement Rock Structures around a proposed Dam Site,

Abstract

A combination of carefully selected geophysical surveys comprising very low frequency electromagnetic (VLF-EM) and electrical resistivity (VES) methods was employed to study the basement rock structures in a proposed dam site within University of Ilorin campus. Data acquired in the survey were processed to inverted subsurface geological model, 2D conductivity structures and geo-electric cross-sections for the area. The results were interpreted to delineate the subsurface rock and structure, the number of geo-electric layers, and to select a suitable area for the construction of dam extension. The four geo-electric layers delineated correspond to lateritic top-soil layer, weathered rock layer, fractured rock layer, and the fresh basement rock. The VLF-EM and resistivity cross-section revealed a series of competent and weak zones. The weak zones are characterized by weathered/fractured rocks, and they correspond to areas of low resistivity anomalies in VES survey and high conductivity anomalies in VLF-EM survey. Competent rock zone corresponds to area of high electrical resistivity anomalies and low conductivities in VLF-EM tomogram. Depth to fresh basement rock in the weak zone ranges from 7.8 – 14.8 m. The competent rock zones correspond to high resistivity anomalies in electrical resistivity cross-section and low conductivity anomalies in VLF-EM tomograms. Depth to fresh basement rock in the competent zone ranges from 3.3 – 6.8 m. the competent zone recommended for the construction of the dam extension.