Mapping of aquifer units in a complex geologic terrain using natural electric field and electrical resistivity techniques

Abstract

Aquifer mapping in a typical complex geologic terrain like Kobape, Southwestern Nigeria is essential to meet the needs of freshwater for domestic and industrial purposes. Natural Electric Field (NEF) measurements over eight traverses and Vertical Electrical Sounding (VES) at ten points were carried out for aquifer mapping in the area. PQWT-TC150 model of a typical water detector, which works by integrating the principle of nuclear magnetic resonance, magnetotelluric and induced polarization methods is used for the NEF measurement. It images the subsurface to a depth of 150 m while the VES was carried out to show the geoelectric layers. The results of NEF revealed the presence of confined aquifer units while the VES showed the subsurface structure to be of 3 to 4 layers viz: topsoil, sandstone, fractured layer, and fresh basement. The corresponding resistivity ranges of the geoelectric layers are 341–3596 Ωm, 1378–4333 Ωm, 635–1000 Ωm, and 3721–56382 Ωm while their thicknesses ranged from 0.5–1.6 m, 0.8–25.4 m, 2.2–19.8 m, and undeterminable fresh basement. The overall aquifer mapping in the complex geologic terrain that straddles the boundary of crystalline basement rocks and a sedimentary basin using integrated technique of NEF and VES revealed various confined aquifers. The NEF significantly showed station twenty on traverse three has a good aquifer within a fractured zone at a depth of 15 m and the same region is verified by the result obtained at VES 3, which has a fractured layer of 19.8 m thickness.