Identifying fractures in the subsurface is crucial for many geomechanical and hydrogeological applications. Here, we assess the ability of the Ground Penetrating Radar (GPR) method to image open fractures with sub-mm apertures in the context of future deep disposal of radioactive waste. GPR experiments were conducted in a tunnel located 410 m below sea level within the Äspö Hard Rock Laboratory (Sweden) using 3-D surface-based acquisitions (3.4 m × 19 m) with 160 MHz, 450 MHz and 750 MHz antennas. The nature of 17 identified GPR reflections was analyzed by means of three new boreholes (BH1-BH3; 9–9.5 m deep). Out of 21 injection and outflow tests in packed-off 1-m sections, only five provided responses above the detection threshold with the maximum transmissivity reaching 7.0 × 10−10 m2/s. Most GPR reflections are situated in these permeable regions and their characteristics agree well with core and Optical Televiewer data. A 3-D statistical fracture model deduced from fracture traces on neighboring tunnel walls show that the GPR data mainly identify fractures with dips between 0 and 25°. Since the GPR data are mostly sensitive to open fractures, we deduce that the surface GPR method can identify 80% of open sub-horizontal fractures. We also find that the scaling of GPR fractures in the range of 1–10 m2 agrees well with the statistical model distribution indicating that fracture lengths are preserved by the GPR imaging (no measurement bias). Our results suggests that surface-GPR carries the resolution needed to identify the most permeable sub-horizontal fractures even in very low-permeability formations, thereby, suggesting that surface-GPR could play an important role in geotechnical workflows, for instance, for industrial-scale siting of waste canisters below tunnel floors in nuclear waste repositories.
The AGU Outstanding Student Presentation Award, assessing Master’s and PhD students for their research in the geosciences, awards the top 2-5% of presenters in each Section. ENIGMA Justine Molron is one of them in 2019.
Event: AGU Fall Meeting 2019, San Francisco (USA) Poster by Justine Molron, Niklas Linde, Ludovic Baron, Jan-Olof Selroos, Caroline Darcel, Philippe Davy
The identification of (open) fractures in the subsurface is critical for evaluating potential routes for contaminant transport from deep disposal sites. Ground Penetrating Radar (GPR) is suitable for this task and its detection capacity depends on fracture characteristics (orientation, aperture and size) and on the dielectrical and electrical contrast between the fluid or material filling the fractures and the surrounding bedrock.
A GPR experiment was performed in the Äspö Hard Rock Laboratory (Sweden) in a tunnel located 410 m below the sea level with a length of 20 m long, a width of 4 m and a height of 4.5 m. The geological formations are fractured granite, diorite and granodiorite with negligible matrix permeability and very low transmissive fractures (10E-9 to 10E-10 m2/s for most permeable zones).
We used 160 MHz, 450 MHz and 750 MHz antennas, pulled along the clean and flat tunnel floor along parallel lines separated by 0.10 m for 160 MHz and by 0.05 m for 450 and 750 MHz antennas. This measurement set-up and antenna choices allow for a 3D identification of fractures from GPR diffractions and reflections, with different image resolutions and investigation depths reaching 10m, 8m and 5m for 160, 450 and 750 MHz, respectively.
Based on the data, we identify 15 reflections that could correspond to larger fractures with dimensions of 2 to 5 m. We compare the GPR-inferred fractures with the corelogging of three 9.5 m deep boreholes that were drilled after the GPR campaign. The strong GPR reflections in the borehole area largely correspond to the depth and orientation to the fractures identified by the Optical Televiewer (OPTV) data.
Additionally, pumping and injection tests in each borehole showed that the GPR-inferred fractures are situated in the most permeable regions. The occurrence of GPR fractures was then compared with a statistical description of the fracture network built from the intersection of boreholes and 2D trace maps from tunnel walls. Given the average size of the GPR-inferred fractures, we demonstrate that they are overall consistent with the expected fracture density below the tunnel.