Assessment of anomalous seepage conditions in the Opa dam embankment, Ile-Ife, Southwestern Nigeria

An integrated geophysical survey, involving the vertical electrical sounding (VES), dipole-dipole electrical horizontal profiling, spontaneous potential (SP) profiling and magnetic profiling, was carried out along the embankment of the Opa dam located within the campus of the Obafemi Awolowo University, Ile-Ife, Nigeria. The aim of the investigation was to map possible anomalous seepage zones along the embankment of the dam. The geoelectric section revealed three distinct layers, namely the clayey sand/lateritic (cap rock) topsoil, the clay/sandy clay weathered layer (core) and the basement (bedrock). The dipole-dipole delineated anomalously low resistivity zones suspected to be seepage zones within stations 8 and 15 (40-75 m), 19-25 (95-135 m), 26-32 (130-160 m) and 38-42 (190-210 m) from the northwestern end of the embankment. SP total field and gradient profiles identified two major interfaces within the anomalously low resistivity zones at around stations 14 (70 m) and 20 (100 m). These are the major seepage zones. Verdant vegetation on the downstream side of the embankment and location of water ponds at the toe of the embankment around the identified seepage zones are field confirmation of the anomalous seepage. KEY WORDS: Anomalous seepage; Assessment; Opa Dam Embankment. Global Journal of Geological Sciences Vol.2(2) 2004: 191-19