Characterization of Anomalous Transport in Fractured Rock through the Imaging of Push-Pull Experiments Using Single-Hole Ground Penetrating Radar Reflection Data
Résumé
Interpretation of push-pull data is limited by the absence of a clearly defined length scale, information on subsurface heterogeneity, and the assumption that ambient flow is negligible. We address these issues through a series of push-pull experiments with saline and deionized tracers at a granitic experimental research site in Ploemeur, France. The experiments involve different injection fractures, chaser volumes and resting times. Monitoring is performed in a neighboring borehole (~6m away) by single-hole ground penetrating radar (GPR), in both moving and fixed antenna settings. The GPR borehole is isolated from the tracer using a flexible-liner. We clearly image the dominant fractures in which tracer transport takes place and the spatial extent of the tracer plume at different times, which varies from a few meters to more than 10 meters away from the injection point. The differences between time-lapse images are subtle and consistent and in good agreement with the concentration data. We identify fractures in which the tracer is trapped for longer time periods and postulate that these fractures are partly responsible for the observed tailing behavior. By increasing the chasing volume, we see the tracer moving further away from the injection point and occupying different fractures, but our ability to image the tracer retrieval is decreased. By varying the resting time of a given experiment we identify regions of density-driven flow, heterogeneous advection, and ambient flow. Deionized water minimizes density effects, but the response is more quickly attenuated compared with saline tracers. The fixed antenna acquisitions provide data of unprecedented quality that display temporal dynamics in agreement with the concentration data down to very low concentrations. This data set offers very exciting perspectives for coupled hydrogeophysical inversion aimed at understanding anomalous transport in fractured rock formations.