Field GPR Monitoring of Flow Channeling in Fractured Rock

Fractures control the flow of fluids in rocks with important implications for groundwater resources, contaminant transport, geothermal resources, sequestration of carbon dioxide, and the development of unconventional hydrocarbon resources. However, fractured rocks exhibit heterogeneous hydraulic properties that are difficult to characterize using conventional hydraulic testing methods. Flow channeling caused by fracture aperture variability is known to result in preferential pathways of rapid contaminant transport in aquifers and poor sweep efficiency in geothermal reservoirs. Time-lapse ground-penetrating radar (GPR) experiments were conducted at the Altona Flat Rock fractured sandstone field site using surface reflection to monitor saline tracer flow through a water-saturated subhorizontal bedrock fracture at a depth of 7.6 m below surface. Three-dimensional (3-D) GPR grids were acquired, each covering approximately a 100 m2 area at a 0.25 m x 0.5 m trace spacing. Radar data were acquired at 50 MHz and 100 MHz frequencies using broadside and cross-polarized dipole antenna pairs oriented parallel and orthogonal to the survey grid lines. Dipole flow hydraulic tests established by re-circulation of saline traced formation water between injection and pumping boreholes were used to set-up controlled flow of variable salinity tracers and variable direction hydraulic gradients. Natural gradient saline tracer tests were also monitored using GPR. Comparison of GPR reflection amplitudes between background clean water (9.3 mS/m) and traced water of 200 mS/m, 400 mS/m and 700 mS/m fluid electrical conductivity under East-West oriented dipole flow showed overall reflection strength increase with increasing fluid electrical conductivity. Amplitude differencing between each of the saline tracer tests and background reveals 1 m to 1.5 m wide flow channels trending across the survey area and the flow dipole field. North-South oriented dipole tests result in channeled flow patterns distinctly different than the East-West dipoles and natural gradient saline tracer tests. Time-lapse GPR is revealing, for the first time, field scale imaging of flow channeling variability resulting from varying orientation hydraulic gradients.