Ground-penetrating-radar response to fracture-fluid salinity: Why lower frequencies are favorable for resolving salinity changes

Abstract

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.

Reflection waveform inversion of ground-penetrating radar data for characterizing thin and ultrathin layers of nonaqueous phase liquid contaminants in stratified media

“…Thus, we caution that this inversion algorithm, while performing robustly for ε r and d, is not likely to retrieve σ reliably for thin layers of these types of contaminants. Continued work to retrieve thinlayer σ should include testing on data at lower frequencies, e.g., <100 MHz, as per Tsoflias and Becker (2008).…”

“…Reliably estimating σ and its uncertainty may only be possible when layer σ > 10 −1.5 S∕m. However, the limit for σ estimation most likely also depends on f 0 as well as the layer thickness and overburden thickness (Tsoflias and Becker, 2008). Here, we have tested σ sensitivity using an ultrathin layer model, and it may be possible that sensitivity will increase with increasing layer thickness.…”

Tools and Techniques: Ground-Penetrating Radar

“…The measured GPR data were in agreement to analytic solutions of plane-wave oblique-incidence transmission through layered media. Tsoflias and Becker (2008) used time-lapse multifrequency GPR to investigate the influence of fracture-fluid salinity and show that for increasing fluid electrical conductivity, lower-frequency signals exhibit greater amplitude and phase-delay changes compared with high-frequency systems.…”

Coupling ground penetrating radar and fluid flow modeling for oilfield monitoring applications

“…As the contrast between the dielectric constant of water and the other soil and rock components is large, the distribution of water in the subsurface strongly affects GPR wave propagation. As a result, fluid saturation changes can be monitored by time-lapse GPR measurements; successful studies for water monitoring can be found in e.g., Tsoflias et al (2001), Talley et al (2005), Day-Lewis et al (2006), Deiana et al (2008), Tsoflias and Becker (2008), and Kuroda et al (2009) and for steam monitoring in Gregoire et al (2006). Although the mentioned literature is mainly devoted to shallow subsurface environments, knowledge of the fluid distribution and flow is highly desired at greater depths.…”