GPR Characterization of Discrete Fractures and Monitoring of Channeled Flow: Elucidating the Forward Model

Abstract

Ground penetrating radar (GPR) is an effective geophysical method for imaging fractures. Of interest to hydrogeologic studies of fractured aquifers is quantification of fracture aperture distribution and flow channeling. We present work relating GPR signal response to fracture aperture variability, water salinity changes and flow channeling. We show that characteristic and quantifiable reflected radar signal amplitude and phase responses relate to fracture aperture and fluid salinity. Radar signal amplitude increases as fracture aperture increases and as fluid electrical conductivity increases. Radar reflection phase is relatively insensitive to aperture change (at frequencies lower than 200 MHz) but highly responsive to fracture water electrical conductivity changes (up to 1 S/m). Contrary to conventional thin‐layer theory expectation, lower frequency radar signals exhibit greater sensitivity to changes in fluid electrical conductivity than higher frequency signals. Three‐dimensional multi‐polarization reflection imaging shows that aperture variability and flow channeling introduce significant polarization effects to the radar wavefields that need to be accounted for in order to quantitatively relate GPR reflection response to fracture aperture and water salinity. Increasing fluid salinity along flow channels results in increasing polarization effects on the recorded signals offering an additional GPR attribute to varying fluid salinity. Improved understanding of the forward problem of the response of GPR signals to fracture properties is advancing our ability to relate geophysical observations to fractured rock hydrologic properties. Saline tracer tests monitored by GPR revealed 1 to 1.5 m wide flow channels trending across the survey area. The spatial scale of these channels corresponds roughly to hydrodynamic dispersivity measured from interwell saline tracer breakthrough. GPR “amplitude breakthrough” provided estimates of mass transport along the fracture that are in good agreement to estimates derived from chemical monitoring mass breakthrough in boreholes. This work shows that GPR imaging offers the capability to define the geometry of flow channeling and reduce the uncertainty of transport predictive modeling in bedrock groundwater systems.