Seismic Refraction Survey for Engineering Site Characterisation in Enugu State

This study employs seismic refraction surveying to characterise subsurface engineering properties at a proposed construction site in Enugu State, South East Nigeria. Foundation design for multi-storey buildings in the Enugu Cretaceous sedimentary terrain requires reliable knowledge of depth to competent bedrock, overburden compressibility, and layer velocity contrasts that conventional borehole investigations may incompletely capture at economical cost. Seismic refraction offers complementary subsurface information through measurement of compressional wave velocities in near-surface geologic layers. This study deploys a 24-channel seismograph with 4.5-hertz geophones at 3-metre spacings along four profiles at a proposed institutional development site in Enugu North local government area. Forward shots and geophone hammer hits generate the seismic signals. Travel time data are analysed using the time-distance intercept method and reciprocal method. Layer velocities and depths are computed and correlated with available borehole data from an adjacent construction site. Findings reveal a three-layer velocity model with a weathered overburden velocity of 380 to 520 metres per second, a partially weathered intermediate layer at 1,200 to 1,800 metres per second, and a fresh shale-sandstone bedrock refractor at 2,800 to 3,400 metres per second. Bedrock depth varies from 4.2 to 11.8 metres across the site, indicating significant lateral variation requiring differential foundation design. Soft zones identified in the northwest corner of the site require soil improvement treatment. The study recommends seismic refraction as a cost-effective engineering site investigation complement to standard geotechnical borings.

Keywords: seismic refraction, engineering site characterisation, bedrock depth, Enugu State, P-wave velocity.