Author(s): M U Igboekwe, H E Ohaegbuchu
This paper is aimed at using up-hole method of seismic refraction survey to investigate the thickness of the weathering layer as well as the velocity of seismic waves through the layer. The first events to arrive at the geophones on a refraction profile had travelled by refraction along the top of the high speed zone just below the weathering layer. Using the time- offset curves of the first arrivals for shots from different charge depths; the thickness of the weathering layer was calculated by conventional seismic refraction methods. Such a survey was carried out using a TD500 top drive drilling machine mounted on a 6 × 6 m truck. The depth of the weathering layer as well as the velocities of seismic waves in the weathering layer and the refractor, were calculated from critically refracted arrivals using flat layer models. Using the time-intercept method for interpretation, the thickness of the low velocity weathering layer in the area varied from 15 to 20 m with a regional average of 18.15 m. The p-wave velocity in the uppermost layer varied from 1385 to 800 m/s with an average value of 1092 m/s. This is a direct experimental proof of low velocity zone confirming the weathered nature of the area.