Detection and characterization of near surface structures using seismic refraction around Shika, Nigeria.

Abstract

A geophysical investigation was carried out at Shika, using seismic refraction method. The aim was to delineate, map and characterize the different strata of the subsurface within the flanks of a dam. In order to achieve this aim, varying geometric spreads were used with geophone spacings of 5m, 4m, 3m and 2m successively for different traverse on the same profile. The Generalized Reciprocal Method (GRM) results gave average velocities of 1356 ms^-1, 2543 ms^-1 and 4065 ms^-1 for the overburden, weathered basement and fresh Basement, respectively. In some profiles, the model suggests a sedimentary thickness of not less than 20m 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 the velocities of hidden and undetected layers within the overburden and weathered Basement respectively. These overburden deposits consisting of consolidated and unconsolidated conglomerates were identified from the velocity values and correlated with the available borehole log data collected from within the area. In addition, a picture of the subsurface layering was successfully mapped from the refraction results despite the geologically complex environment.

Keywords: Offset distance, Refractor, Forward and Reverse rays, Velocity analysis function, Generalized time depth, Depth sections