GPR imaging of shear zones in crystalline rock

GPR data were acquired at the Grimsel Test Site to improve the geological model prior to high-pressure water injections into shear zones. Data acquired in tunnels using shielded 160 MHz antennas are of exceptionally high quality and could image shear zones up to a distance of approximately 24 m from the tunnel. The interpretation is based on i) comparing modelled shear-zone arrivals to measured ones and ii) directly interpreting fully processed (i.e., migrated) data. Our results add significant detail to the geological model and agree with findings of an anisotropic seismic tunnel-to-tunnel traveltime inversion.