GEOPHYSICAL INVESTIGATION OF PROPOSED DAM SITE ALOMG RIVER ADUNIN, OGBOMOSO, SOUTHWESTERN NIGERIA.

ABSTRACT Dams are vital structures that store water for consumption, irrigation, electricity production and industrial purposes.The design and construction is very important to prevent the collapse of the structure .Thirty (30) Vertical Electrical Sounding points were investigated. Very Low Frequency profiling at 5m inter station along five profiles were carried out. The interpretation of geophysical data were achieved using the following software; Res 2D, Win Resist, Surfer 11, Arc GIS 3.3 and Excel. Six resistivity sounding curve types were obtained from the study area and these are the H, HA , KH, HKH, and KHA curves. The geologic sequence beneath the study area is composed of top soil, weathered basement, fractured basement, and fresh basement. The topsoil is composed of clayey-sandy and sandy-lateritic hard crust with resistivity values ranging from 339-1971 ohm-m and thickness of 0.7-1.3 m. The weathered basement resistivity is within the range of 98-3026 ohm-m and thickness value of 0.3-7.0m.The fractured basement layer resistivity values vary from 163–4049 ohm-m while its thickness ranges from 2.7 to 17m. Fresh basement has resistivity value range of 228 to14793 ohm-m. The bedrock is highly fractured beneath the river axis and extended toward the left abutment while fresh basement is closer to the surface at the right abutment.The fractures beneath the proposed dam axis may pose serious threat to the dam in term of future seepage. The dam should be shifted toward the right abutment by 20m and excavation should be made up to depth of about 5m, the materials so removed could be used for the construction of the embankment and the filter materials