Application of shallow seismic refraction and 2D electrical resistivity imaging to site investigations

Abstract

A Geophysical study involving 2D electrical resistivity imaging and shallow seismic refraction was conducted to provide support data for engineering site characterization. The case was a potential building site coterminous to the Business School in the precinct of the Kwame Nkrumah University of Science and Technology, (KNUST) Kumasi. The data of interest included depths to bedrock, the thickness and the degree of compaction of the weathered overburden and the variability in the top soil. Collocated resistivity and p-wave velocity data were acquired along fourteen traverses each of length 240 m oriented in the north to south direction. The inter-profile separation was 10 m. CVES resistivity data were collected using the multi-electrode ABEM Terrameter SAS 4000 Lund imaging system in the Wenner configuration with a basic array length of 12 m (minimum electrode spacing of 4 m) and was rolled-along to cover the total profile length. Along the same traverses, the ABEM Terraloc Mk 6, a 24 channel recording system, with a 9 kg energy source and 10 Hz electromagnetic geophones placed 5 m apart were used to collect the p-wave seismic refraction data. The apparent resistivity data were inverted with the RES2DINV software. The soil column (the very near surface) varied in thickness up to 5 m and consists of mostly sandy-clay/lateritic soil with resistivity between 200 Ω.m and 600 Ω.m. Within this layer, there are high resistivity enclaves which are speculated to be loose sand or poorly compacted materials. This resistivity layer is almost coincident with the first of the three acoustic layers identified by the seismic refraction. This zone has average p-wave velocity of 760 m/s. The seismic refraction again delineated a weathered zone which is thicker at the northern section of the site but tapers as it approaches the southern part, extending from 5 to 25 m and 10 to 15 m respectively at the northern and southern parts. The average p-wave velocity is 1300 m/s. The bedrock was delineated depths > 24 m at the north but at shallower depth of about 12 m at the south. The bedrock has p-wave velocity within the range of 3759 m/s to 5321 m/s, with a mean seismic velocity of 4300 m/s. On the resistivity sections, the bedrock was not evident except at the southern part close to the stream.