Groundwater Investigation Using Combined Geophysical Methods

A geophysical investigation involving the seismic refraction and the vertical electrical sounding (VES) electrical resistivity methods was carried out around Ajebandele quarters, Ile-Ife, Osun State, Southwest Nigeria. The study was carried out with a view to determine the subsurface layer parameters (velocities, resistivities and thicknesses) and use same to categorise the ground-water potential of the area.

Five seismic stations were shot and six vertical electrical sounding locations, along each traverse for the refraction, a total of five (5) traverses, with traverses one and two trending north-south (N-S) and traverses three, four and five trending east-west (E-W). Geophone separation of 2m was used and an offset distance of 4m along each traverse. A 12 channel BISON seismograph was used in recording the values of the arrival times at the geophones positions. The vertical electrical sounding stations were located based on the results of the refraction survey. The VES technique was employed for the resistivity method. A total of six VES stations were occupied with electrode separation (AB/2) varying from 1m to 65m. An ABEM terrameter SAS 300C was used in the survey. The results of interpretation of the seismic refraction time-distance graph and VES curve were used to construct geo-sections showing the subsurface layers, their corresponding velocities and thicknesses.

The interpretation of results from seismic refraction showed three layer formations, the top layer with thickness ranging from 1.3m to 3.6m and velocities ranging from 352m/s to 827m/s. The velocity of the second layer ranges from 678m/s to 2105m/s with a thickness of 6.1 to 9.4m and velocity of the third layer ranging from 2222m/s to 3229m/s. A discontinuity representing a fracture is observed on traverse four (4). Maximum of five (5) layers were delineated with H, HK, AA and HAA curve types and showed the bedrock in the area occurs quite close to the surface. The topsoil had resistivities ranging from 105Ωm- 412Ωm to a depth of 0.5m-1.4m. The weathered layer had resistivities ranging from 102 Ωm- 224 Ωm to a depth of 3.4m- 6.2m and the fresh bedrock had resistivities ranging from 1161Ωm- ∞. In VES four (4), a fractured zone is observed at a depth of about 15m. Although the number of layers delineated differ, both methods indicate viable aquifer at a fractured zone along traverse four.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California