Investigating the cause of excessive seepage in a dam foundation using seismic and electrical surveys—a case study of Asa Dam, West Africa

A case study is reported in this paper, where a combination of seismic refraction and electrical resistivity surveys is used to investigate the cause of excessive water loss in Asa Dam, Nigeria, West Africa. Excessive water loss in the dam has been the reason for the shortage in water supply being experienced by the people of Ilorin, especially in the dry season, in the last 7 years. The study aims to use two geophysical methods to investigate the cause and locate the point of water loss in the dam. Data acquired around the dam foundation were processed to model and image the dam’s subsurface and its surrounding. Electrical resistivity models of the dam’s subsurface revealed a high-permeable water-saturated section in the form of a conductive zone at a horizontal distance of 80–220 m, from near-surface to a depth of about 30 m. The presence of a similar structure on two profile lines perpendicular to the dam confirmed the conductive zone and suggested 3-dimensionality of the conductive structure. Velocity model computed from seismic refraction data acquired along the dam axis revealed the presence of a low-velocity oxbow structure at the mid-section of the dam, between distance 80–200 m from near surface to about 20 m. The presence and correspondence of a low-velocity structure with a high-conductivity structure in the dam’s subsurface, an indication of water-saturated high-permeable rock, was interpreted as the point of excessive water seepage in the dam foundation. The study recommends that the internal geometry of seepage area should be study in greater details using borehole cores and cuttings, in order to design an appropriate anti-seepage rehabilitation intervention.