Hydrogeophysical characterization of transport processes in fractured rock by combining push-pull and single-hole ground penetrating radar experiments
Résumé
The in situ characterization of transport processes in fractured media is particularly challenging due
to the considerable spatial uncertainty on tracer pathways and dominant controlling processes, such
as dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. We
attempted to reduce this uncertainty by coupling push-pull tracer experiments with single-hole
ground penetrating radar (GPR) time-lapse imaging. The experiments involved different injection
fractures, chaser volumes and resting times, and were performed at the fractured rock research site
of Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions we used both fixed
and moving antenna setups in a borehole that was isolated with a flexible liner. During the fixedantenna
experiment, time-varying GPR reflections allowed us to track the spatial and temporal
dynamics of the tracer during the push-pull experiment. During the moving antenna experiments,
we clearly imaged the dominant fractures in which tracer transport took place, fractures in which
the tracer was trapped for longer time periods and the spatial extent of the tracer distribution (up to
8 meters) at different times. This demonstrated the existence of strongly channelized flow in the
first few meters and radial flow at greater distances. By varying the resting time of a given
experiment, we identified regions affected by density-driven and ambient flow. These experiments
open up new perspectives for coupled hydrogeophysical inversion aimed at understanding transport
phenomena in fractured rock formations.
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