Apparent apertures from ground penetrating radar data and their relation to heterogeneous aperture fields

Abstract

Considering fractures with heterogeneous aperture distributions, we explore the reliability of constant-aperture estimates derived from ground penetrating radar (GPR) reflection data. We generate geostatistical fracture aperture realizations that are characterized by the same mean-aperture and variance, but different Hurst exponents and cutoff lengths. For each of the 16 classes of heterogeneity considered, we generate 1000 fracture realizations from which we compute GPR reflection data using our recent effective-dipole forward model. We then use each (noise-contaminated) dataset individually to invert for a single 'apparent' aperture, i.e., we assume that the fracture aperture is homogeneous. We find that the inferred 'apparent' apertures are only reliable when fracture heterogeneity is non-fractal (the Hurst exponent is close to 1) and the scale of the dominant aperture heterogeneities is larger than the first Fresnel zone. These results are a direct consequence of the non-linear character of the thinbed reflection coefficients. As fracture heterogeneity is ubiquitous and often fractal, 2 A. Shakas, N. Linde our results suggest that robust field-based inference of fracture aperture can only be achieved by accounting for the non-linear response of fracture heterogeneity on GPR data.

Analysis and Modeling of GPR Signals to Detect Cavities: Case Studies in Morocco

“…The experimental data obtained by GPR were analyzed and interpreted as an image of a vertical slice of the basement. The image was received by the concatenation of the A-scan and Bscan data recorded by GPR along the measured area [15]. Objects that fled the basement were marked by the presence of the hyperbole in the B-scan ( Fig.…”

Computing Localized Breakthrough Curves and Velocities of Saline Tracer from Ground Penetrating Radar Monitoring Experiments in Fractured Rock

“…It depends on the interplay of antenna frequency, tracer conductivities and fracture apertures, as also seen in Figure 2b. Further, it depends on the validity of the thin-bed equations, which will hold true only to a certain extent in reality (e.g., [27]). However, it should be noted that while the quasi-linear dependency will likely not stay true for all applications, the relation between RMS(E m − E r ) and the fracture conductivity will stay monotonically increasing in most cases.…”

“…Shakas et al [26] used time-lapse difference reflection imaging with borehole GPR to monitor saline tracer migration in push-pull experiments and were able to calculate GPR breakthrough curves by analyzing trace differences for different antenna positions. Furthermore, they successfully inferred apparent fracture apertures from these experiments [27]. Shakas et al [28] could invert for the flow path of tracer on the fracture scale, revealing strong channeling effects during the tracer propagation.…”

Permeability Enhancement From a Hydraulic Stimulation Imaged With Ground Penetrating Radar

“…The high‐frequency GPR waves offer a high spatial resolution, and the governing material properties (dielectric permitivity and electrical conductivity) can be linked directly to quantities of interest, such as temperature and the presence of water. The GPR reflection response has been often analyzed using a homogeneous model for a fracture by exploiting analytical solutions (Deparis & Garambois, 2009; Tsoflias & Hoch, 2006), but GPR reflections carry also information on aperture variations along a fracture down to sub‐wavelength resolution (Shakas & Linde, 2017). In fact, apertures that are several orders of magnitude smaller than the source dominant wavelength are detectable (Dorn et al., 2012; Markovaara‐Koivisto et al., 2014; Shakas & Linde, 2017; Tsoflias & Hoch, 2006).…”

“…The GPR reflection response has been often analyzed using a homogeneous model for a fracture by exploiting analytical solutions (Deparis & Garambois, 2009; Tsoflias & Hoch, 2006), but GPR reflections carry also information on aperture variations along a fracture down to sub‐wavelength resolution (Shakas & Linde, 2017). In fact, apertures that are several orders of magnitude smaller than the source dominant wavelength are detectable (Dorn et al., 2012; Markovaara‐Koivisto et al., 2014; Shakas & Linde, 2017; Tsoflias & Hoch, 2006). In time‐lapse mode, GPR has been used to infer processes such as fluid flow and transport of saline tracers (Dorn et al., 2011; Shakas et al., 2016; Tsoflias et al., 2015) or fracture opening caused by pumping (Tsoflias et al., 2001).…”