International audienceThe in situ characterization of transport processes in fractured media is particularly challenging dueto the considerable spatial uncertainty on tracer pathways and dominant controlling processes, suchas dispersion, channeling, trapping, matrix diffusion, ambient and density driven flows. Weattempted to reduce this uncertainty by coupling push-pull tracer experiments with single-holeground penetrating radar (GPR) time-lapse imaging. The experiments involved different injectionfractures, chaser volumes and resting times, and were performed at the fractured rock research siteof Ploemeur in France (H+ network, hplus.ore.fr/en). For the GPR acquisitions we used both fixedand moving antenna setups in a borehole that was isolated with a flexible liner. During the fixedantennaexperiment, time-varying GPR reflections allowed us to track the spatial and temporaldynamics of the tracer during the push-pull experiment. During the moving antenna experiments,we clearly imaged the dominant fractures in which tracer transport took place, fractures in whichthe tracer was trapped for longer time periods and the spatial extent of the tracer distribution (up to8 meters) at different times. This demonstrated the existence of strongly channelized flow in thefirst few meters and radial flow at greater distances. By varying the resting time of a givenexperiment, we identified regions affected by density-driven and ambient flow. These experimentsopen up new perspectives for coupled hydrogeophysical inversion aimed at understanding transportphenomena in fractured rock formations

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The characterization of flow and transport processes in fractured rock is challenging because they cannot be observed directly and hydrologic tests can only provide sparse and local data. Time-lapse ground penetrating radar (GPR) can be a valuable tool to monitor such processes in the subsurface, but it requires highly reproducible data. As part of a tracer injection experiment at the Grimsel Test Site (GTS) in Switzerland, borehole reflection GPR data were acquired in a time-lapse survey to monitor saline tracer flow through a fracture network in crystalline rock. Because the reflections from the tracer in the sub-mm fractures appear extremely weak, a differencing approach has been necessary to identify the tracer signal. Furthermore, several processing steps and corrections had to be applied to meet the reproducibility requirements. These steps include (1) single-trace preprocessing, (2) temporal trace alignment, (3) correction of sampling rate fluctuations, (4) spatial trace alignment, (5) spike removal, and (6) postprocessing procedures applied to the difference images. This allowed successful tracer propagation monitoring with a clear signal that revealed two separate tracer flow paths. The GPR results are confirmed by conductivity meters that were placed in boreholes in the GTS. If sufficient data processing is applied, GPR is shown to be capable of resolving tracer flow through sub-mm aperture fractures by difference reflection imaging even in challenging surroundings where many reflectors are present.

please cite as: Shakas, A., and N. Linde (2015), Effective modeling of ground penetrating radar in fractured media using analytic solutions for propagation, thin-bed interaction and dipolar scattering. ABSTRACTWe propose a new approach to model ground penetrating radar signals that propagate through a homogeneous and isotropic medium, and are scattered at thin planar fractures of arbitrary dip, azimuth, thickness and material filling. We use analytical expressions for the Maxwell equations in a homogeneous space to describe the propagation of the signal in the rock matrix, and account for frequency-dependent dispersion and attenuation through the empirical Jonscher formulation.We discretize fractures into elements that are linearly polarized by the incoming electric field that arrives from the source to each element, locally, as a plane wave. To model the effective source wavelet we use a generalized Gamma distribution to define the antenna dipole moment.We combine microscopic and macroscopic Maxwell's equations to derive an analytic expression for the response of each element, which describe the full electric dipole radiation patterns along with effective reflection coefficients of thin layers. Our results compare favorably with finitedifference time-domain modeling in the case of constant electrical parameters of the rock-matrix and fracture filling. Compared with traditional finite-difference time-domain modeling, the proposed approach is faster and more flexible in terms of fracture orientations. A comparison with published laboratory results suggests that the modeling approach can reproduce the main characteristics of the reflected wavelet.

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.