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.

本文报告了一个案例研究，其中结合使用地震折射和电阻率调查来调查西非尼日利亚阿萨大坝失水过多的原因。大坝失水过多是过去 7 年伊洛林人民供水短缺的原因，尤其是在旱季。该研究旨在利用两种地球物理方法调查大坝失水原因并定位大坝失水点。对大坝基础周围采集的数据进行处理，以对大坝的地下及其周围环境进行建模和成像。大坝地下的电阻率模型显示，从近地表到约 30 m 的深度，水平距离为 80-220 m，呈导电带形式的高渗透水饱和部分。在与大坝垂直的两条剖面线上存在类似的结构，证实了导电区并暗示了导电结构的 3 维性。根据沿大坝轴线获取的地震折射数据计算出的速度模型显示，在大坝中段，距离近地表 80-200 m 至约 20 m 之间存在低速牛轭结构。大坝地下低速结构与高导结构的存在和对应关系，表明存在水饱和的高渗透岩石，被解释为大坝基础过度渗水的点。该研究建议应使用钻孔岩心和钻屑更详细地研究渗流区域的内部几何形状，