f GPR CHARACTERIZATION OF DISCRETE FRACTURES AND MONITORING OF CHANNELED FLOW: ELUCIDATING THE FORWARD MODEL

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. Threedimensional 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.