LITHOLOGIC CONTROL FOR GPR PROFILES IN A BEACH-RIDGE DEPOSITIONAL ENVIRONMENT

Ground penetrating radar (GPR) profiling was used to delineate the subsurface architecture and radar facies of a prograding beach-ridge system on the flank of the Fraser River delta, southwestern British Columbia, Canada. To correlate the radar reflection patterns, facies boundaries and radar discontinuities with lithological evidence, vibracoring in fine sediments, and flight augering in gravels was carried out. Vibracoring (7 cm core) in the beach ridges yielded continuous cores but depths of penetration were limited by the occurrence of gravel and typically halted at 2 m). Flight augers (15 cm diameter), on the other hand, were drilled through the total thickness of the beach-ridge system into the underlying tidal-flat sediments. The drawback of flight auger samples for detailed sedimentological analysis is that lithologic detail can be partially or completely destroyed by the rotation of the flights during drilling. To complement the drilling data and provide additional parameters to correlate radar signatures with lithology, cone penetration test (CPT) logs were obtained. These logs provide continuous records of sediment resistance to penetration by a steel rod (tip resistance in MN/m2 ) and variations in pore water pressure (m) within the beach sediments so penetrated. These logs, widely used in engineering, can be used to more finely interpret the correlative links between lithology and GPR images of the beach ridges.