Shallow Seismic Refraction Survey for Foundation Characterization at a Proposed Infrastructure Site in Udi, Enugu State

Udi Local Government Area of Enugu State is an active zone for infrastructure development, yet geotechnical characterization using geophysical methods remains underutilized in the region. This study employs shallow seismic refraction to characterize foundation conditions at a proposed infrastructure site in Udi by determining compressional wave velocity distribution, estimating weathered layer thickness, and classifying subsurface competence. Six seismic refraction profiles were acquired using a 12-channel seismograph with geophones spaced 5 m apart, and shot records were generated using a 5 kg sledgehammer source. Time-distance graphs were constructed and interpreted using the plus-minus method and reciprocal time technique. Compressional wave velocities in the first layer range from 280 to 620 m/s, consistent with loose lateritic soils and weathered material. Velocities in the second layer range from 1,200 to 2,400 m/s, suggesting partially saturated or compacted clay-rich sediments. A competent refractor with velocities exceeding 3,600 m/s is encountered at depths between 8 m and 22 m across the site. Zones of anomalously low velocity gradients are interpreted as potentially weak foundation units that may require reinforcement. Bearing capacity estimates derived from velocity-density relationships suggest moderate to good foundation suitability across much of the site, with localized zones of concern. These results provide site-specific engineering data to support structural design decisions. The study underscores the utility of seismic refraction as a cost-effective complement to conventional borehole investigations for infrastructure planning in Enugu State.

Keywords: seismic refraction, foundation characterization, compressional wave velocity, weathered layer, Udi