Electrical Resistivity Sounding for Subsurface Delineation and Evaluation of Groundwater Potential of Araromi Akungba-Akoko Ondo State Southwestern Nigeria

Abstract

Electrical resistivity sounding of Araromi area of Akungba Akoko was conducted with a view to delineating the subsurface units and evaluating the groundwater potential of the area. Forty (40) Schlumberger vertical electrical resistivity soundings were acquired with ABEM SAS 1000 Resistivity Meter. The electrode spacing (AB/2) was varied from 1 – 65 m with maximum spread length of 130 m. The interpretation of the data was quantitative, and involved partial curve matching and computer iteration technique using WinResist software.

Three to four distinct subsurface geologic layers were identified from the geoelectric layers, aided by borehole lithological logs. These included a lateritic clay/sandy clay/clayey sand/sandy topsoil, clayey weathered layer, partially weathered/fractured basement and the fresh basement with resistivity and thickness values of 3-174 ohm-m and 0.7-1.5 m; 196-472 ohm-m and 3.9-20.1 m; 533-742 ohm-m and 1.8-4.2 m; and infinity ohm-m respectively.. The depth to bedrock varied from 0 to 20.1 m. The bedrock relief was generally uneven, with bedrock elevations that varied between 312 and 346 m above sea level. The bedrock relief map delineated two major parallel basement depressions striking approximately NW-SE as the major groundwater collecting centers. The isopach map of the overburden showed generally thin overburden (< 15 m). The weathered/fractured layer constituted the dominant aquifer units with tendency for low groundwater potential rating arising from the clayey and thin nature of the overburden. Future groundwater resource development in the study area is considered feasible in few places characterized by relatively thick and sandy column of the overburden units.

Keywords: Schlumberger, Sounding, Electrode, Resistivity, Thickness, Groundwater, Rating