Applications of Vertical Electrical Sounding (VES) and Two-Dimensional (2-D) Imaging Methods for Groundwater Exploration and Basement Configuration at GOFAMINT Church Camp, Ibadan, Southwestern Nigeria

Abstract

Groundwater exploration has been a critical problem in the basement complex
terrain underlain by hard rocks which have low porosity and permeability. This low
yielding terrain has created problem of water scarcity within the region. The study
area falls within this region and has been affected by this problem. The occurrence
of groundwater in this terrain is localized and concentrated within weathered zone
and network of fractures. A total of thirty Vertical Soundings (VES) were taken with
some of the VES point on the 2-D profiles mainly to probe further the conductive
anomalies. Apparent resistivity values were plotted against AB/2 on bilogarithmic
paper and was later curve matched to obtain the layer parameters. This serves as
an input to the computer iterative algorithm to create the model of the subsurface.
A total of two 2-D imaging profile were taken to visualize the subsurface in two
dimensions. An inter-electrode spacing of 10 meters was employed with six levels.
From the study, the results of the VES reveals that some of the fractures might be
dry with exception of few which are promising hydrogeologically. Also, the VES
results revealed the following geoelectric layers ranging from top soil, clay, lateritic
pan, weathered zone, fractured basement/fresh basement. The top soil has
thickness ranging from 0.2 to 3.0 meters with mean of 1.9 meters and resistivity
ranging from 63 to 836Ωm with mean of 301Ωm. The lateritic clay has thickness
ranging from 3.1 to 5.7 meters with average of 4.4 meters and resistivity between
60 to 351Ωm with mean of 154Ωm. The hard lateritic pan has thickness of 0.6 to
3.9 meters with mean of 2.2 and resistivity of 1369 to 2568Ωm with mean of
1936Ωm. The weathered layer has thickness ranging from 1.1 to 36.4 meters with
mean of 9.0 meters and resistivity ranging from 11 to 380Ωm with mean of 87Ωm.
The fresh rock (rock boulder) has thickness of 4.4 meters and resistivity of 2094Ωm.
Fractured / fresh basement has an infinite thickness and resistivity ranging 377 to
5564Ωm with mean of 1921Ωm. Depths to basement top were determined at each
VES points and were used to model the basement topography. The 2-D
pseudosection of profile 01 present nearly horizontal layers indicating uniformity
in resistivity at each layer while the profile 02 shows few structural heterogeneities
mainly at western and central part of the site with characteristic dome.
The low resistivity value, thick overburden and fractured bedrock constitute the
aquifer unit. Also, series of basement depression could also serve as water
collection center and hence prominent area of groundwater abstraction. There is
also no evidence of mineralization from the result obtained.