Seismic refraction tomography imaging of high-permeability zones beneath an earthen dam, in Zaria area, Nigeria

Within the subsurface, a few highly-permeable zones may dominate the overall flow regime, strongly influencing the seepage of water through the flanks of an earthen dam. In this paper, an attempt is made to provide a frame work for the characterization and mapping of subsurface channels for water seepage in the vicinity of dams. To obtain detailed information on these subsurface channels, Seismic refraction data were acquired along the two flanks of the dam, each with five profiles, some of which run parallel and perpendicular to the axis of the dam. The acquired data were processed with tomography inversion methods. From the result, more than five different layers were identified within the area. In some profiles, the models suggest thick deposits of sediments overlying the basement. The sporadic velocity gradient within the shallow zones is attributed to the discontinuous and heterogeneous nature of the overburden layers. Consequently, there are high velocity regimes whose values are comparable with those of the unweathered basement. These high regimes may be due to the presence of highly dried and consolidated laterite in the overburden. However, the identified subsurface channels were consistent with low velocity zones indicating high permeable channels in form of loose ground within the overburden. The borehole log data correlated perfectly with the tomography sections as far as the thickness and lithology of the strata from the surface to the basement, is concerned. Hence these potential channels for seepage within the shallow subsurface of the dam, were mapped and identified as loose ground. Therefore, the method proved excellent for delineating these seepage channels within the subsurface of an earthen dam.