Use of electromagnetic profiling and resistivity sounding for groundwater exploration in the crystalline basement area of Igbeti, Southwestern Nigeria

A combination of electromagnetic (EM) profiling and vertical electrical sounding (VES) has been employed to investigate the prospect for groundwater development in the basement complex area of Igbeti and its environs, Southwestern Nigeria. The study area is northwest of Ilorin and north of Ogbomoso. The bedrock geology comprises a suite of metasediments, gneisses and intrusive granites. The field survey was conducted at nine locations. In each case, an EM profile 150 m in length was measured with the Geonics EM-34-3 equipment; the coil spacing was 20 m and data collected for both the horizontal and vertical dipole modes. A Schlumberger VES with a maximum half-current electrode spacing (AB/2) of 133 m, was subsequently conducted at the location of the anomaly identified on the EM profile. The results indicate that there are three main targets for groundwater in this area. These include the weathered zone, the fractured zone and the vertical dykes. In four of the VES locations the resistivity of the prebasement layer is less than 50 ohm-m and this has been interpreted as a clayey lithology. On the other hand, the resistivity of this layer is higher than 85 ohm-m in four other VES locations, indicative of a sandier lithology. Moreover, the VES indicated a fairly shallow overburden thickness at less than 20 m. The fractured gneiss sequence could be inferred from the VES data as relatively low model resistivity (less than 1500 ohm-m) for the geoelectric basement. The vertical dykes were inferred from the large separation between the horizontal and vertical dipoles on the EM conductivity profiles, coupled with negative readings on the vertical dipole curves. The boreholes drilled in the area, based on results of the geophysical surveys, penetrated a sequence of topsoil/laterite, moderately weathered granite, fractured gneisses and fresh bedrock. The approach described in this paper is shown to be rapid and cost-effective in terms of location and assessment of groundwater resources in crystalline rock terrains. KEY WORDS: Electromagnetic profiling; resistivity sounding; groundwater exploration; Igbeti area; southwestern Nigeria. Global Journal of Geological Sciences Vol.2(2) 2004: 243-25