Using two- and three-dimensional georadar methods to characterize glaciofluvial architecture

Abstract The threat of pollution in the shallow subsurface has led to an increasing need to understand how complex heterogeneities in gravel aquifers influence groundwater transport. To characterize these heterogeneities, we have conducted extensive two-dimensional (2-D) and three-dimensional (3-D) ground-penetrating radar (GPR or georadar) surveys in two quarries within the Rhine valley of northeastern Switzerland. The quarries comprised gravel and sand deposited in a proglacial braided river system that followed the maximum Wurm-stage glaciation. After the surveys, the terrace walls beneath the two study sites were photographed as they were being excavated. By combining information extracted from the 2- and 3-D georadar images with the outcrop photographs, it was possible to correlate georadar facies with the various glaciofluvial architectural elements. The dominant elements were scour pools formed at or near the confluences of two stream channels and horizontally bedded and massive gravel sheets deposited during moderate to high water flow conditions and exposed during low flow conditions. Architectural elements were generally elongated parallel to the mean flow directions of the ancient river system. Variations in the strike of their long axes reflected lateral and vertical changes in the local flow direction. Time slices showed structural trends not evident on 2-D georadar images and photographs. Data interpretation was quicker, more complete and less ambiguous when georadar facies analyses were based on a combination of georadar vertical profiles and time slices than when based on georadar vertical profiles alone.