Fracture fluid flow properties investigation using GPR and hydraulic testing methods

Characterization of the fluid flow properties of fractures is of particular interest because fractures rapidly transmit fluids, such as groundwater, contaminants and hydrocarbons. We investigate the use of ground-penetrating radar (GPR) and hydraulic testing to characterize groundwater flow variability along two prominent horizontal fracture planes in a carbonate aquifer. Three-dimensional radar reflection surveying provided highresolution imaging of the two horizontal fractures located below the water table. Radar reflector amplitude variation of up to one order of magnitude is observed along the fracture surfaces. Ana1,ytical computations of reflection coefficients and radar finite difference modeling of saturated-fracture centimeter-scale aperture variation are in agreement with the observed radar data amplitude variation. Thus, radar surveying identified centimeterscale mechanical fracture aperture variation and mapped flow conduits and flow barriers in three dimensions. Hydraulic slug testing, conducted previously at the test site, provided hydraulic conductivity measurements for each of the two fractures at seven well locations within the area covered by the 'GPR data. The slug tests suggested submillimeter to millimeter scale hydraulic fracture apertures at the seven well locations. The apparent discrepancy of one order of magnitude of fracture aperture determinations (cm vs. mm scale) arises from the different properties measured by the two methods. GPR signal amplitude responds to the mechanical fracture aperture, i.e. the actual size and shape of the conduit. A slug test yields a one-dimensional point measurement of actual fluid-flow and assigns an average property value for the surrounding area. This study suggests that integration of GPR and hydraulic testing methods offers the potential for improved determination of the fluid-flow properties of fractured, anisotropic geologic formations.