Integrated geophysical investigation of subsurface structures and groundwater potential in Ipogun Drainage Basin, southwestern Nigeria

Abstract

Public water supply within the Ipogun Drainage Basin is grossly inadequate and in most towns nonexistent.
Attempts at mitigating the inadequacy through drilling of shallow/moderately deep boreholes have met with
challenges of low (< 1 l/s) groundwater yields and failed boreholes, suspected to be due to poor knowledge of the
subsurface sequence and the structural setting. This has necessitated a detailed assessment of the groundwater
potential of the basin using an integrated geophysical technique. Ground magnetic profiling at 10 m spacing; long
spread 2D Dipole-Dipole resistivity imaging with dipole length of 20 m; and forty-eight large spread Schlumberger
Vertical Electrical Soundings (VESs) were carried out along six traverses ranging in lengths from 1.12-4.51 km that
cut across prominent remote sensing/aeromagnetic derived lineaments. The magnetic anomalous zones were used
to constrain the locations of the Dipole-Dipole profiles while the 2D resistivity images influenced the locations of
the VES stations. The magnetic intensities varied between -761 and +636 nT. Thin and thick dyke magnetic
anomalies were observed. The characteristics of the magnetic modelled fault/fractured zones include thin-tomoderate overburden thicknesses (< 20 m), width extent of between 20 and 255 m and vertical/near-vertical dip.
The 2D resistivity model-delineated-fault/fractured zones correlated well with the magnetic anomalous zones with
width extent of between 40 and 250 m and depth extents >100 m. Four geoelectric layers comprising the
topsoil/laterite, weathered layer, partly weathered/fractured basement and the fresh basement were delineated. The
partly weathered/fractured aquifer has resistivity and thickness ranges of between 25 and 1472 ohm-m and 4.5 and
117 m respectively. The weathered layer and partly weathered/fractured layer are the major aquifer units with the
latter having a percentage of occurrence of 56.25% as against 43.75% for the former, indicating prospect for higher
groundwater potential due to enhanced transmissivity and porosity of the partly weathered/fractured basement
columns. This was corroborated by an increased groundwater yield of 1.4 l/s from a 60 m deep borehole drilled
within one of the resistivity anomalous zones which penetrated the weathered layer aquifer and the upper segment
of the partly weathered/fractured basement. It is concluded that the basin has prospect for high groundwater
potential, in areas where the basement is weathered and fractured to deep depth, and the fractures are
interconnected over a large area, to influence transmission and storativity of the groundwater. Recommended
drilling depths may exceed 100 m compared with previous reliance on relatively shallow boreholes.