keywords: Very low frequency-electromagnetic, electrical resistivity, pumping test, geoelectric properties
An assessment of three failed boreholes that were earlier drilled at Ibusogboro Area of Ibadan was carried out using integrated Very Low Frequency-Electromagnetic (VLF-EM) and Electrical Resistivity (ER) methods. The aim was to investigate the causes of the borehole failure and to identify suitable site for groundwater development based on geoelectrical properties of the subsurface layers. Sixteen (16) VLF-EM profiles were used to delineate linear geologic structures within the study location. Seven (7) Vertical Electrical Soundings (VES) stations were occupied at the identified lineament zones. Four geoelectric units were delineated. These include the topsoil, weathered/partially weathered layer, fractured basement and the fresh bedrock with resistivity values which ranged between 91 and 698 Ωm, 20 and 213 Ωm, 55 and 225 Ωm, and 149 and 1399 Ωm, respectively. The result of VLF-EM revealed that the failed boreholes were located on unfissured fresh basement rock. The VES carried out at the VLF-EM delineated lineament zones revealed fractured basement at four locations. Test boreholes (BH) 4 and 5 were drilled at VES 2 and 6 to penetrate the fractured basement and yields of 2300 and 4400l litres/h were acquired through pumping test, respectively.
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