FIELD GPR IMAGING OF FLOW AND TRANSPORT IN A DISCRETE FRACTURE: CAPABILITIES AND LIMITATIONS

Fractured rocks exhibit heterogeneous hydraulic properties hindering remediation of contaminated sites. Geophysical imaging can be used to complement hydraulic testing by investigating rock volumes and fluid properties away from boreholes. We present results of controlled field ground penetrating radar (GPR) imaging experiments designed to test the capabilities and limitations of the method for characterizing flow and transport in a discrete fracture. Although conventional GPR surveying has been used in the field to image fractures and monitor saline tracers, we find that subsurface imaging can benefit significantly from: a) Consideration of GPR signal polarization to account for the directionality of the electromagnetic wavefield relative to fracture channel geometry; b) Acquisition of cross-polarized signal components in order to capture the full scattered EM wavefield; c) Examination of independent polarization components to help quantify subsurface heterogeneity; and d) Examination of signal phase to delineate electrically conductive tracers or contaminants in fractures. We present three-dimensional, multi-polarization, time-lapse GPR imaging of dipole flow and natural gradient saline tracer tests through a discrete sub-horizontal fracture 7.6 m below surface with an estimated hydraulic aperture of 0.5 mm. We find good agreement of GPR imaging fracture connectivity (channeling) and transport with observations from independent hydraulic, tracer and heat transfer tests conducted at the same site. We conclude that GPR imaging has the capability to image accurately heterogeneous flow and transport in fractures, but multi-polarization acquisition is necessary in order to relate accurately fracture and fluid properties to GPR observations. Currently, commercially available GPR instrumentation can be adapted for multi-polarization surface surveying but cannot be deployed in boreholes.