DC resistivity survey for the assessment of groundwater potential in Oba Akoko, Southwestern, Nigeria
A DC resistivity survey of Oba Akoko in southwestern Nigeria was carried out over the three major rock units of the Migmatites; granite, granite-gneiss and grey-gneiss underlying the study area. The objectives of the study included the delineation of the subsurface geological/geoelectrical sequences and aquifer unit(s) and types with a view to assessing prospective site for groundwater potential rating of the area. Fifty nine Schlumberger vertical electrical sounding data were acquired. The inter-electrode spacing (AB/2) varied from 1 to 100 m with maximum spread length of 200 m. The ground resistance measurements were made with ABEM SAS 1000 resistivity meter. Seven distinctive sounding curve types were obtained from the data inversion/interpretation techniques. These included H, A, HA, AA, HK, QH and AK curve types. Maximum of four geological/geoelectric layers were delineated; the top-soil, clay/clayey sand/sandy clay weathered basement, partly weathered/fractured basement and fresh basement with resistivity values that ranged from 20 - 360 Ωm, 25 - 850 Ωm, 476 - 979 Ωm and infinity ohm-m respectively, while the thickness values ranged from 0.4 - 3.8 m, 1 - 17 m, 4 - 17 m respectively, and depth to rock head of 24.9 m. Isopach maps of the weathered layer and overburden showed spatial distribution of the thickness with a relatively thick weathered/fractured basement not less than 10 m as basement depressions or pockets of closures. The thickness pattern showed uneven basement bedrock interface. It may, therefore, be concluded that groundwater potential of the study area is generally low arising from shallow depth to basement or thin regolith and few fracture density in places in the area. The localized fractured bedrocks and weathered basement columns/basement depressions arising from the deep weathering of the parent rocks may have constituted the main sustainable aquifer/groundwater storage units for consumption via borehole groundwater abstraction in the area.
Keywords: Geology, Mapping, Geoelectrical, Resistance, Delineating