A field assessment of the ability of Ground Penetrating Radar to detect fractures in very low permeable crystalline rock
Résumé
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.