GPR Phase Response to Fracture Saline Tracers

Flow in fractured rock is highly heterogeneous and difficult to predict. Groundwater, geothermal and hydrocarbon resource development requires knowledge of fracture fluid flow properties. Geophysical methods such as seismic, electrical and electromagnetic, have been used to image fracture systems. Ground penetrating radar (GPR) has been shown to image fractures and monitor saline tracers in groundwater aquifers. Previous studies have considered primarily the amplitude response of GPR signals. In this work we examine the phase response of GPR in a series of controlled field experiments spanning over a decade. We compare field observations to analytical models and FDTD numerical simulations of the phase response of GPR signals to saline tracers in fractures. We show that changes in the electrical conductivity of fracture fluid cause predictable and detectable phase changes in reflected and transmitted GPR signals through fractures. Lower frequency signals (MHz range) show greater sensitivity to fluid electrical conductivity changes, while being relatively insensitive to fracture aperture. Phase changes over time (time lapse) are used to image channeled flow through discrete fractures and to monitor tracer breakthrough. Although GPR is used extensively in near-surface groundwater investigations, deployment in boreholes can allow adaptation of the technology to monitor saline tracers in hydrocarbon and geothermal systems.