Evaluation of Layer Velocity Disparity and Lithological Changes within the Subsurface using High Resolution Downhole Seismic Method for Site Characterization

The aim of this research work is to evaluate the velocities distribution and lithological changes within the subsurface, necessary for site characterization at a construction site located in the oil rich Niger Delta of Southern Nigeria using downhole seismic method. The downhole seismic survey was carried out with an offset distance of 3 m, at an interval of 1 m to a depth of 30 m. The seismic velocities which ranges from 457 m/s to 1972 m/s for the p wave velocity, and 291 m/s to 1066 m/s for s wave velocity connotes that there are two layers of unconsolidated and consolidated sand  in the region under investigation. The range of Poisson’s ratio 0.16 to 0.23 determined for the unconsolidated sand layer and 0.24 to 0.29 for the consolidated sand layer, gave a clear confirmation that the region under investigation is made up of two lithological unit of sand, from the surface down to a depth of 14 m, and from 15 m to 30 m. The determined shear, bulk and Young’s modulus were able to delineate the unconsolidated and consolidated sand layers by registering low values down to a depth of 14 m. It is imperative to conclude that the area under investigation is characterized with two lithological units, the saturated and unsaturated sand layer of different elastic parameters. It was also recommended that proper engineering remediation is required to improve the strength of material down to a depth of 14 m, before erecting any structure.