Comparison of Compressional-wave Velocity-depth Profiles from Surface and Downhole Detectors in the Near Surface in the Southeast Niger Delta, Nigeria

Seismic refraction survey was conducted in the south-east Niger Delta at 27 stations using surface-detectors, and estimated near-surface compressional-wave velocities. At each station, a borehole was drilled with casing down to 60-m depth, and determined downhole compressional-wave velocities. Compressional-wave velocities estimated by surface-detectors were then compared with those estimated by downhole survey. The acquisition system includes 0.2kg dynamite, 12-channel-geophone and 11-channel-hydrophone receiver cables for surface-detectors and downhole surveys respectively. The soil layers in the Niger Delta has 2 distinct layers specified as follows, namely, top weathered layer of loose sand sediments with compressional wave velocity ranging from 213 to 763 ms-1 with an average of 503ms-1. Weathered-layer thickness ranges from 2.7 to 16.50m, with an average of 8.1m. Consolidated-layer compressional wave velocity ranges from 1580 to 1906ms-1, with an average of 737ms-1. A vertical velocity gradient of 30.5s-1 was calculated for the area. There exists a 7-18% difference between the velocities determined by the surface-wave and downhole methods. The surface-wave method gives spatially-averaged velocities along the line of traverse coincident with the geophone spread. In the case of downhole seismic method, velocities are specifically applicable only to the borehole location. The implication is that downhole measurements can be adversely affected by the local borehole factors and conditions resulting from casings and excessive collapse around the flushed-zone of the borehole. The spatially-averaged velocities and depths estimated by surface-detector method are more reliable for the site-specific characterization for reflection seismology, geotechnical engineering and groundwater.