Geophysical Investigation of an Earth Dam: A Case Study of Agba Dam, Ilorin Southwestern Nigeria

Authors: Ijaleye OT, Olayinka AI, Adamu LM, Ugbena KG

DOI Info: N/A

ABSTRACT

Geophysical investigation of Agba dam, Ilorin, Southwestern Nigeria was undertaken to assess the vulnerability of the dam to seepage. The field survey involved very low frequency electromagnetic (VLF-EM) and vertical electrical sounding (VES) techniques with Schlumberger electrode configuration. VLF-EM survey at 10 m and 20 m spacing were conducted along the axis and flanks. Forty VES were carried out on four traverses established in NW–SE direction along the dam axis and NE–SW directions along the right and left flanks. Results of VLF-EM showed conductive zones indicating possible fractured basement. VES data revealed three to four geo-electric layers; topsoil, having resistivity range of 149–1282 ?m with a mean of 448±315 ?m, thickness of 0.4–4.2 m with a mean of 2.0±0.8 m, lateritic hardpan with resistivity of 924–1614 ?m and a mean of 1235±269 ?m, layer thickness of 1.6–3.2 m with a mean of 2.3±0.5 m, weathered and fractured basement with a resistivity of 12–115 ?m with a mean of 51±29 ?m, thickness of 2.5–21.2 m with a mean of 9.4±4.9 m. Fresh basement had a resistivity of 111–1831 ?m with a mean of 643±380 ?m. Possible fractured basement occurred in areas of resistivity <400 ?m. Areas of resistivity >400 ?m indicated fresh basement rock. The study revealed that the basement rock expected to serve as a sealing barrier for water in the reservoir shows some degree of fracturing which if not properly checked and manage, could lead to the collapse of the dam in the future.

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