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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Vertically oriented thin fractures are not always detected by conventional single‐polarization reflection profiling ground‐penetrating radar (GPR) techniques. We study the polarization properties of EM wavefields and suggest multipolarization acquisition surveying to detect the location and azimuth of vertically oriented fractures. We employ analytical solutions, 3D finite‐difference time‐domain modeling, and field measurements of multipolarization GPR data to investigate EM wave transmission through fractured geologic formations. For surface‐based multipolarization GPR measurements across vertical fractures, we observe a phase lead when the incident electric‐field component is oriented perpendicular to the plane of the fracture. This observation is consistent for nonmagnetic geologic environments and allows the determination of vertical fracture location and azimuth based on the presence of a phase difference and a phase lead relationship between varying polarization GPR data.

The spatial distribution of fracture/matrix heat exchange was measured while hot water was circulated through a single bedding plane fracture in a cold reservoir. Thermal breakthrough was recorded at the production well and Fiber-Optic Distributed Temperature Sensing (FODTS) monitored temperature in the rock matrix. Conservative tracer tests revealed that the reservoir fluid volume in two separate experiments were nearly identical. Thermal breakthrough measurements, however, revealed that reservoir fluid volume did not correlate to thermal performance because the two experiments encountered different heat transfer areas along the fracture. Ground Penetrating Radar imaging of subsurface tracer transport and FODTS corroborate these findings.

[1] Saline tracer transport experiments were performed to compare flux-averaged and resident concentration in a single subhorizontal fracture in sandstone bedrock. Tracer migration over a 14 m distance was monitored at an extraction well and imaged within the rock as it passed below a ground penetrating radar (GPR) positioned at the surface. Reflected radar amplitude was calibrated to tracer concentration by circulating saline fluid of known concentration through the fracture. Saline breakthrough curves measured at the well and within the rock were comparable but showed differences in both magnitude and shape. Transport differences were explored using flux-averaged and resident concentration first-passage-time models combined with streamline advective tracking. Application of the appropriate transport model to the two breakthrough curves produced identical estimates of dispersivity and similar estimates of effective fracture aperture. The tracer-derived fracture aperture also agreed reasonably well with hydraulic aperture derived from cross-hole pump tests. The availability of both flux and resident concentrations helped constrain the interpretation of the flow and transport behavior in the fracture. Flow appeared to be highly channelized with less than half the hydraulically swept area of the fracture contributing to efficient tracer transport.Citation: Becker, M. W., and G. P. Tsoflias (2010), Comparing flux-averaged and resident concentration in a fractured bedrock using ground penetrating radar, Water Resour. Res., 46, W09518,