GEOPHYSICAL INVESTIGATION OF BEDROCK RELIEF FOR DAM ADEQUACY OF THE OSARA DAM USING ELECTRICAL RESISTIVITY METHODS

Mepaiyeda, S.^{1, 2}, Jethro, M.A¹ and Olonilebi, P .O.³

¹Department of Minerals and Petroleum Resources Engineering, School of Engineering, Kogi State Polytechnic, P.M.B 1101, Lokoja, Kogi State, Nigeria

 ²Department of Geology, Faculty of Science and Agriculture, University of Fort Hare, Private Bag X1314, Alice, South Africa

³Department of Building Technology, School of Environmental Technology, Kogi State  Polytechnic, P.M.B 1101, Lokoja, Kogi State, Nigeria

Corresponding author e-mail and phone; geospiritonline@yahoo.com, (+234) 8036490137

ABSTRACT: Geophysical investigation to assess the integrity and extent of damage was carried out at an existing dam site in North central Nigeria using the electrical resistivity method to determine the nature of the subsurface along the dam axis and identify general geologic conditions related to the selection of the dam site. Vertical electrical sounding (VES) measurements were taken along 8 stations at 50m interval across the dam axis, with maximum current electrode separation, (AB/2) of 40m, measured parallel to the geologic strike at each point. Results obtained from the qualitative and quantitative interpretation of the apparent resistivity values showed a 3-layered geo-electric structure namely the topsoil with high resistivity (> 300 ohm-m), weathered basement and the bedrock with low resistivity (< 100 ohm-m). There is a general decrease of resistivity with depth across the 8 stations. The geo-electric section in an E-W direction across the dam axis showed a zone of extensive weathering and erosion characterized by low resistivity values (17ohm-m to 42 ohm-m), at depths of 15-20m between VES 4 and VES 5 across the dam axis. It is recommended that to prevent seepages and differential settlement along the dam axis and nearby structures, filling of the dam beds with competent earth materials to block possible seepage pathways for water should be done in order to prolong the life span of the dam usage amongst other remediation measures.

KEYWORDS; Dam, apparent resistivity, bedrock, geo-electric section, seepages