Application of Varying Geometric Spreads in Seismic Refraction Studies to Characterize the Overburden Strata on the Flanks of Zaria Dam, Northwestern Nigeria

A geophysical investigation was carried out on the flanks of Zaria Dam using Seismic Refraction Tomography. The aim was to analyze the data provided to characterize the different strata of the subsurface within the overburden. To this end, varying geometric spreads were used with optimum geophone spacing of 5 m reduced to 4, 3 and 2 m successively for different traverses on the same profile line. Using the Tomographic inversion, it was possible to derive the picture of the subsurface layering mapped from the Tomograms despite the geologically complex environment. The results gave velocities of 1356, 2543 and 3743 m/s for the overburden, weathered basement and fresh basement, respectively. However, the depth of probe differed as the geophone spacing varied. In some profiles, the model suggested a sedimentary thickness of not less than 20 m overlying the basement. The results showed that as the geometric spread is reduced, the shallow structures within the overburden strata become enhanced. Hence, making it possible to recover hidden and undetected layers with such ranges of velocities as 778–998 m/s and 2197–2439 m/s in the overburden and weathered basement, respectively.