Generalized reciprocal method applied in processing seismic refraction data from the basement terrain of Zaria, Northwest Nigeria

Abstract

A geophysical investigation was carried out at Shika, near Zaria, using seismic refraction method; with the aim of analyzing the data obtained using the generalized reciprocal method (GRM). The technique is for delineating undulating refractors at any depth from in-line seismic refraction data consisting of forward and reverse travel-times. An in-line geometric spread with optimum geophone distance of 25m was used. The result obtained showed that the optimum XY spacing, the upward and downward traveling segments of the rays to each geophone emerged from near the same point on the refractor. This method proved to be particularly well-suited for situations involving large lateral velocity variations and highly irregular refractor surfaces. By using computer processing techniques and advanced interpretational features of GRM, it has been possible to derive a picture of the subsurface layering from the refraction results even in geologically complex environments. However, average velocities of 1356 m/s, 2309 m/s to 2543 m/s and 4065m/s were obtained for the Overburden, weathered Basement and Basement respectively. The borehole log data correlated perfectly with the depth sections as far as the
thickness and lithology of the strata from the surface to the basement, is concerned. Subsurface features such as loose ground and low velocity zones were identified and mapped correctly. Finally, it was evident from the processed data that this technique is best suitable for investigating and mapping these geologic features which are of great significance in engineering and geotechnical studies.