Geotechnical investigation of near surface lithological unit using downhole seismic method

Abstract

Downhole seismic survey stands as one of the best method for determining the velocity distribution within different lithological unit, and to infer the different lithology within these layers. The aim of this research work is to make use of downhole seismic method for site characterization, determination of velocity distribution and establishment of the lithology within the various layers. Four boreholes were investigated for the purpose of this research work. The methodology employed involves putting a hydrophone inside the borehole, with the energy source on the surface at an offset distance of 3 m from the borehole. A stack of 5 shots were use to generate seismic signals that were detected by a hydrophone and recorded with a seismograph. The recording was carried out at interval of 3 m within the borehole. The result of the survey revealed that the range of p waves velocity within the survey area is from 995 m/s to 2695 m/s, and the s wave velocity ranges from 582 m/s to 1489 m/s. It was identify base on their velocity that the various lithologies within the boreholes are clays, wet sand and saturated shale. The poisons ratio which ranges from 0.24 at the top layer to 0.28 at the bottom, gave an indication of relative thick overburden made up sandyclay, underlain by consolidated layer of sand. The values of poisons ratio which increased with depth ruled out the possibility of velocity anisotropy within the various lithological unit.