Borehole GPR Fracture Imaging of a Crystalline Rock Aquifer, Stang-er-Brune, Brittany, France

The spatial organisation of fractures and their properties dictate the hydraulic behaviour of fractured rock aquifers. Identification and characterisation of the exact flow paths between the boreholes is an unresolved problem. In an extensively studied crystalline aquifer in Brittany (Stang-er-Brune), borehole logging and hydraulic testing have been used to characterise fractures intersecting boreholes. The results indicate that only a few well-connected fractures are important for describing the hydraulic behaviour. In an attempt to improve our understanding of potential flow paths at this site, we have acquired, processed and interpreted GPR single-hole and cross-hole data using 100 and 250 MHz antennas (down to 100 m depth). To separate the reflected signals from direct wave energy and source-generated noise, we performed a data processing scheme that included bandpass filtering, F-X deconvolution, eigenvector filtering, muting and depth migration. The final stacked and migrated GPR sections clearly image fracture zones and single fractures (dipping 30-90°, 2-14 m away from the boreholes) including large subvertical fractures that were not identified by borehole logging. Most of the fractures previously identified as being hydrologically important and dipping 30-90° could be recognised as moderate to prominent reflections.