FRACTURE IMAGING AND MONITORING OF CHANNELED FLOW IN THE FIELD USING GROUND PENETRATING RADAR

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. We present an overview of ground penetrating radar (GPR) imaging for characterization of fractures and monitoring fluid flow. 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.

Improved understanding 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. 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. Time-lapse GPR revealed field scale imaging of flow channeling variability resulting from varying orientation hydraulic gradients. 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.