2-D Seismic Refraction Tomography Investigation of a Sewage Treatment Site

Abstract

Geophysical investigation involving seismic refraction tomography (SRT) was conducted at a sewage treatment site located at Ahmadu Bello University Zaria with the objectives of determining the overburden thickness and velocity of the subsurface layers in the area, and delineating any geological structures, such as fracture zone, that may pose a threat to the safe running of the Sewage lagoon system in the area.. Five seismic profiles, with three running N-S and two running E-W, were acquired over the earthen-dikes separating the lagoons with the aid of a 24-channel digital Seismograph and a 5 kg sledge hammer striking a rubber plate. Tomography models were generated for each of the profiles with the aid of ray tracing through a wave-front inversion method that is based on finite difference approximation of the eikonal equation. Geological artifacts, as a result of noise observed in the tomography models of some of the profiles, were removed by filtrations that include manual editing of model’s physical parameters and iterative model updating through simultaneous iterative reconstruction techniques (SIRT). The result of the investigation shows that the area consists of three subsurface layers; an undifferentiated overburden layer with a P-wave velocity range of 891 m/s to 1421 m/s, a partially weathered zone with P-wave velocity range of 3010 m/s to 5129 m/s and underlying fresh granitic basement with P-wave velocity range of 5704 m/s to 7762 m/s, while the overburden thickness in the area varies from an average of about 18 m in the northern part to about 55 m in the southern part. The investigation also revealed a fracture zone within the underlying granitic basement, which occurs at about 60 m below the earthen-dike located at the centre of the study area.

Keywords: 2-D Seismic, Refraction, Tomography, Sewage Lagoon