Hydrogeophysical study of parts of charnockite terrain of Akure Southwestern Nigeria

Abstract

Hydrogeophysical study of parts of Charnockite terrain of Akure, southwestern Nigeria was carried out using the electrical resistivity method. The study involved one hundred and fifty two (152) Schlumberger geoelectric soundings (with seven (7) of them parametric)) alongside static water level measurements in 102 hand-dug wells. The Precambrian Crystalline Basement Complex rocks underlying the area consist largely of Charnockite with some granite and migmatite-gneiss bodies. The interpreted results enabled the determination of overburden thickness, longitudinal unit conductance (S) and coefficient of anisotropy (λ). The static water level measurements were used to determine the vadose zone thickness and groundwater flow direction. The overburden thickness in the study area ranges between 2.0 and 66.9 m; bedrock resistivity range between 748 and ∞ Ω-m while the vadose zone thickness ranges between 0.5 and 11.3 m. The groundwater potential of the area was classified based on the overburden thickness (h) and resistivity values (ρ). Thus, areas characterized by 100 ≤ ρ ≤ 800 Ω-m with h ≥ 45 m are classified as high, while areas presenting 15 ≤ h ≤ 45 m are classified as medium and areas of h ≤ 15 m are classified as low. A modified geologic classification of the study area using λ values was undertaken with λ ≤ 1.2 areas classified as Charnockite while areas with λ > 1.2 classified as granite and migmatite-gneiss complex. A correlation of the field geological map with the λ map presents a good correlation especially within areas underlain by Charnockite.

Keywords: Charnockite, resistivity, groundwater, overburden thickness, longitudinal unit conductance and coefficient of anisotropy