Abstract:
Hydrogeophysical study of parts of charnockite terrain of Akure, Southwestern Nigeria was carried
out using the electrical resistivity method. The study involved vertical electrical sounding
technique using the Schlumberger array, and static water level measurement in 102 hand-dug
wells. The area is underlain by the Precambrian Crystalline Basement Complex rocks with
lithology consisting largely of charnockite with some granite and migmatite gneiss bodies. A total
of one hundred and fifty two (152) vertical electrical soundings were carried out with seven (7) of
them parametric. The geoelectric parameters obtained were iterated using interactive computer
software. The interpreted results enabled the determination of overburden thickness, longitudinal
unit conductance and coefficient of anisotropy while the static water measurement was 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 values ranges between 748 Ω-m
and ∞ while the vadose zone thickness values range between 0.5 and 11.3 m. The groundwater
potential of the area was classified based on the overburden thickness and resistivity values. Areas
with resistivity values between 100 to 800 ohm-m, and overburden thickness greater than 45 m,
between 15 and 45 m, and less than 15 m were classified as high, medium and low groundwater
potential zone respectively. A modified geologic classification of the study area using coefficient
of anisotropy (λ) was attempted. Areas with low anisotropy value less than 1.2 were classified as
charnockite while areas with higher anisotropy value greater than 1.2 were classified as granite
and migmatite gneiss complex. A correlation of the field geological map with the coefficient of
anisotropy map presents a good correlation especially within areas underlain with charnockite,
while low level of correlation observed around the northeastern flank is possibly due to inferred
geological contact determination during geological fieldwork.
