Resolving scales of aquifer heterogeneity using ground penetrating radar and borehole geophysical logging

Abstract

Accurate and reliable characterization of aquifer heterogeneity remains one of the foremost problems in hydrogeology. In this study, ground penetrating radar (GPR) and borehole geophysical logging are used to investigate scales of heterogeneity present locally (<500 m laterally) within an outwash deposit comprised of inter-bedded and cross-bedded sands and gravels of glaciofluvial origin. At a small scale (<15 m laterally), gamma log data in adjacent boreholes show evidence of fining upward sequences, occasional coarsening upward sequences, and abrupt changes in grain sizes, which appear to be laterally continuous at scales of 10 m. At the site scale (<500 m laterally), GPR profiles show a strong reflection interpreted as the water table. Reflectors in the unsaturated zone are more clearly defined than those beneath the water table due to signal attenuation within the saturated sediments. Undulating to discontinuous reflectors at scales of 10–15 m are interpreted to result from interbedded and cross-bedded sands and gravels. A few laterally continuous horizontal to sub-horizontal reflectors, which extend at least up to 360 m, are interpreted as unconformities, based on evidence of gravel bars, truncation of underlying units, as well as scour and fill features in a nearby gravel pit exposure. Overall, the integration of these two geophysical methods provided evidence of unit correlation at the two scales of investigation.