Time-lapse ground-penetrating-radar ͑GPR͒ surveys exploit signal-amplitude changes to monitor saline tracers in fractures and to identify groundwater flow paths. However, the relationships between GPR signal amplitude, phase, and frequency with fracture aperture and fluid electrical conductivity are not well understood. We used analytical modeling, numerical simulations, and field experiments of multifrequency GPR to investigate these relationships for a millimeter-scale-aperture fracture saturated with water of varying salinity. We found that the response of lower-frequency radar signals detects changes in fluid salinity better than the response of higher-frequency signals. Increasing fluid electrical conductivity decreases low-frequency GPR signal wavelength, which improves its thin-layer resolution capability. We concluded that lower signal frequencies, such as 50 MHz, and saline tracers of up to 1 S/m conductivity are preferable when using GPR to monitor flow in fractured rock. Furthermore, we found that GPR amplitude and phase responses are detectable in the field and predictable by EM theory and modeling; therefore, they can be related to fracture aperture and fluid salinity for hydrologic investigations of fractured-rock flow and transport properties.
[1] High-resolution reflection seismic data from Jakobshavn Isbrae, Greenland, reveal complex fabric development. Abundant englacial reflectivity occurs for approximately half the thickness of the ice (the lower half), and disruption of the englacial reflectors occurs in the lower 10-15% of the ice-thickness. These depths correspond to the higher impurity-content, and more easily deformed, ice from the Younger Dryas and Last Glacial Maximum to Stage-3. We conclude that the reflectivity results from contrasting seismic velocities due to changes in the crystal orientation fabric of the ice, and suggest that these fabric changes are caused by variations in impurity loading and subsequent deformation history. These findings emphasize the difference between ice-divide and ice-stream crystal orientation fabrics and have implications for predictive ice sheet modeling. Citation:
Abstract. Fractured aquifers present a number of problems when attempting to characterize flow on the well scale (less than 100 m). Standard hydraulic testing methods are expensive because of the need for installation of monitoring wells. Geophysical methods may suffer from a lack of resolution and nonunique solutions to data interpretation. We used ground-penetrating radar (GPR) surveying during a pumping test in a well-characterized, fractured, carbonate aquifer to monitor the response of a permeable subhorizontal fracture plane. We observed radar signal amplitude and waveform variations along a fracture reflector and correlated the radar signal response to changes in the water saturation of the fracture. Combining hydraulic measurements with GPR data and electromagnetic modeling, we identified an asymmetric fracture drainage pattern, provided accurate spatial information about the saturation of the fracture, and detected the presence of hydraulic boundaries. This study demonstrates that GPR surveying can be used successfully for real-time monitoring of pumping tests in fractured carbonate aquifers.
The point velocity probe (PVP) is an instrument capable of measuring ground water velocity in situ at the centimeter scale. It is based on detecting an electrically conductive tracer transported by ground water around the perimeter of the cylindrical probe. PVPs are easily constructed from inexpensive materials and can be deployed as a single sensor or in multilevel arrays. A multilevel array of these instruments, consisting of four PVPs stacked vertically on each of five stands, was installed as a fence within a sheet‐pile alleyway at the Canadian Forces Base Borden test site in Ontario, Canada. The data from the fence revealed notable velocity variations both spatially and temporally. Ground water velocity data of these kinds are likely to be valuable for permeable reactive barrier design and assessment, regulatory compliance assessments, and a variety of research level investigations concerned with local flow phenomena.
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