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 withchallenges of low (< 1 l/s) groundwater yields and failed boreholes, suspected to be due to poor knowledge of thesubsurface sequence and the structural setting. This has necessitated a detailed assessment of the groundwaterpotential of the basin using an integrated geophysical technique. Ground magnetic profiling at 10 m spacing; longspread 2D Dipole-Dipole resistivity imaging with dipole length of 20 m; and forty-eight large spread SchlumbergerVertical Electrical Soundings (VESs) were carried out along six traverses ranging in lengths from 1.12-4.51 km thatcut across prominent remote sensing/aeromagnetic derived lineaments. The magnetic anomalous zones were usedto constrain the locations of the Dipole-Dipole profiles while the 2D resistivity images influenced the locations ofthe VES stations. The magnetic intensities varied between -761 and +636 nT. Thin and thick dyke magneticanomalies 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 withwidth extent of between 40 and 250 m and depth extents >100 m. Four geoelectric layers comprising thetopsoil/laterite, weathered layer, partly weathered/fractured basement and the fresh basement were delineated. Thepartly weathered/fractured aquifer has resistivity and thickness ranges of between 25 and 1472 ohm-m and 4.5 and117 m respectively. The weathered layer and partly weathered/fractured layer are the major aquifer units with thelatter having a percentage of occurrence of 56.25% as against 43.75% for the former, indicating prospect for highergroundwater potential due to enhanced transmissivity and porosity of the partly weathered/fractured basementcolumns. This was corroborated by an increased groundwater yield of 1.4 l/s from a 60 m deep borehole drilledwithin one of the resistivity anomalous zones which penetrated the weathered layer aquifer and the upper segmentof the partly weathered/fractured basement. It is concluded that the basin has prospect for high groundwaterpotential, in areas where the basement is weathered and fractured to deep depth, and the fractures areinterconnected over a large area, to influence transmission and storativity of the groundwater. Recommendeddrilling depths may exceed 100 m compared with previous reliance on relatively shallow boreholes.