Abstract

Geophysical investigation involving the electrical resistivity method has been undertaken along three proposed dam axes trending NNW-SSE (335° WCB) across ENE to WSW flowing Owuruwuru River at Ikere Ekiti Southwestern Nigeria. The study is aimed at evaluating the feasibility of the area for establishing a small earth dam and reservoir. The electrical resistivity method involving the Wenner and Schlumberger arrays for horizontal profiling and Vertical Electrical Sounding (VES) techniques was adopted. The study area is underlain by the Precambrian Basement Complex rocks of Southwestern Nigeria. Three Wenner profilings were undertaken along each axis with electrode separations of 10, 20 and 30 m respectively. Forty-four VES locations were occupied within the study area. The resistivity profiles showed that the northern flank of the stream (right abutment) is characterized by relatively lower resistivity values (77 to 327 Ω-m) with respect to those obtained at the central and southern (left abutment) flanks (83 to 1077 Ω-m). The resistivity profiles thus present a general morphology of the concealed basement in form of low relief on the northern flank with respect to high relief on the southern flank. Geoelectric sections generated from VES results showed that the dam site is underlain by clayey sand topsoil, sandy clay weathered basement, partially weathered/fractured basement and the presumably fresh bedrock. Materials of the overburden (topsoil + weathered basement) are generally thicker (3.4 to 19.3 m) on the northern flank of the stream channel and thinner (0.9 to 10.3 m) on its southern flank. The fractured bedrock delineated on the southern flank is confined and poses minor threat to the proposed dam. However the thicker overburden beneath the northern flank and thin overburden on the southern flank present incompatible structure that may initiate uneven settlement of the dam embankment thus making the site unsuitable for earth dam construction.

Key words: Dam axes, horizontal profiling, vertical electrical sounding, overburden, fractured bedrock.