High resolution aquifer characterization using crosshole GPR full-waveform tomography

Limited knowledge about the spatial distribution of aquifer properties typically constrains ourability to predict subsurface flow and transport. Here we investigate the value of using high resolution full-waveform inversion of cross-borehole ground penetrating radar (GPR) data for aquifer characterization. Bystitching together GPR tomograms from multiple adjacent crosshole planes, we are able to image, with adecimeter scale resolution, the dielectric permittivity and electrical conductivity of an alluvial aquifer alongcross sections of 50 m length and 10 m depth. A logistic regression model is employed to predict the spatialdistribution of lithological facies on the basis of the GPR results. Vertical profiles of porosity and hydraulicconductivity from direct-push, flowmeter and grain size data suggest that the GPR predicted facies classifi-cation is meaningful with regard to porosity and hydraulic conductivity, even though the distributions ofindividual facies show some overlap and the absolute hydraulic conductivities from the different methods(direct-push, flowmeter, grain size) differ up to approximately one order of magnitude. Comparison of theGPR predicted facies architecture with tracer test data suggests that the plume splitting observed in a tracerexperiment was caused by a hydraulically low-conductive sand layer with a thickness of only a few deci-meters. Because this sand layer is identified by GPR full-waveform inversion but not by conventional GPRray-based inversion we conclude that the improvement in spatial resolution due to full-waveform inversion is crucial to detect small-scale aquifer structures that are highly relevant for solute transport