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A field assessment of the ability of Ground Penetrating Radar to detect fractures in very low permeable crystalline rock Justine MOLRON 1, 4 , Niklas LINDE 2 , Ludovic BARON 2 , Jan-Olof SELROOS 3 , Caroline DARCEL 1 and Philippe DAVY 4 1 Itasca Consultants SAS, Ecully, France ([email protected]); 2 Université de Lausanne, UNIL, Lausanne, Switzerland; 3 Svensk Kärnbränslehantering AB, SKB, Solna, Sweden, 4 Géosciences Rennes, OSUR, CNRS, Université de Rennes 1, Rennes, France 2. Assessment of GPR results 3. Conclusions 1. Methodology • Manual choice of reflections: 17 sub- horizontal + 3 sub-vertical fracture planes • Three 9.5 m deep boreholes drilled after choice of reflections (BH1, BH2, BH3) • 9 “GPR fractures” crossing the boreholes A. Comparison with borehole data B. Comparison with statistical model B.1 3D statistical model • Estimation of the GPR detection capacity (%): GPR fracture density/3D statistical density • Estimation of GPR fracture density distribution and comparison with statistical model • Fracture area between 1-10 m 2 • Fracture dip between 0-25° • 2D data: 3513 fracture traces with orientation • 3D model: derived from 2D traces by stereological rules: ➢ Fracture size density distribution (Piggott, 1997) ➢ Fracture orientation distribution (Mauldon and Mauldon, 1997) This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska- Curie Grant Agreement No 722028 Sensing capacity of surface-based GPR to image fractures with sub-mm apertures GPR reflection nature: • Open fractures • 5/9 are situated in transmissive zones (above the measurement threshold) Bibliography Ericsson, L., Vidstrand, P., Christiansson, R., & Morosini, M. (2018). Comparison between blasting and wire sawing regarding hydraulic properties of the excavated damaged zone in a tunnel–Experiences from crystalline rock at the Ӓspӧ Hard Rock Laboratory, Sweden. Paper presented at the 52nd US Rock Mechanics/Geomechanics Symposium. Mauldon, M., & Mauldon, J. G. (1997). Fracture sampling on a cylinder: From scanlines to boreholes and tunnels. Rock Mechanics and Rock Engineering, 30(3), 129-144. Molron, J., Linde, N., Baron, L., Selroos, J-O., Darcel, C. and Davy, P. (submitted). Which fractures are imaged with Ground Penetrating Radar? Results from an experiment in the Äspö Hardrock Laboratory, Sweden. Engineering Geology. Piggott, A. R. (1997). Fractal relations for the diameter and trace length of disc-shaped fractures. Journal of Geophysical Research, 102(18), 121-125. doi: https://doi.org/10.1029/97JB01202 A. Tunnel and GPR experiment design B. GPR fracture model and borehole siting Study area • Location: Tunnel (410 m depth) at the Äspö Hard Rock Laboratory, Sweden • Prior knowledge: 42 2-m deep boreholes • EDZ*: 0.5 m of floor sawn off to remove blasting-induced fractures • Transmissivity: 2.2-7 10 -10 m 2 .s -1 GPR* experiment design • Methodology: Surface-based acquisition with parallel profiles covering 3.4 m x 19 m • Frequencies: 160, 450 and 750 MHz • Data sampling: 0.023 m • Profile spacing: 0.10 m (160 MHz) and 0.05 m (450 and 750 MHz) • Profile number: 34 (160 MHz) and 69 (450 MHz) • Depth investigation: 10 m (160 MHz), 8 m (450 MHz) and 5 m (750 MHz) GPR detectability in terms of fracture sizes and orientations (Molron et al., submitted): • 5.5 % of detection: open + sealed fractures, dip 0-90°, surface area 1-10 m 2 • 42 % of detection: open + sealed fractures, dip 0-25°, surface area 1-10 m 2 • 80% of detection: open fractures, dip 0-25°, surface area 1-10 m 2 * EDZ: Excavated Damage Zone; *GPR: Ground Penetrating Radar B.2 GPR detection ability * Tadpole plots show fracture localizations (circle) as a function of depth (y-axis) and dip (x-axis). The segments represent the dip direction with the North at the top. Ericsson et al. (2018). Photo: Rickard Enér; Illustration: Jan Rojmar ➢ Fracture detection ability of the GPR in terms of fracture orientations and sizes ➢ Nature of the GPR reflections Next experiment: Time-lapse GPR imaging during tracer tests
