摘要：Ground-penetrating radar (GPR) and seismic imaging haveproven to be important tools for the characterization of rock volumes. Bothmethods provide information about the physical rock mass properties andgeological structures away from boreholes or tunnel walls. Here, we presentthe results from a geophysical characterization campaign that was conductedas part of a decametre-scale hydraulic stimulation experiment in thecrystalline rock volume of the Grimsel Test Site (central Switzerland). Forthis characterization experiment, we used tunnel-based GPR reflectionimaging as well as seismic travel-time tomography to investigate the volumesbetween several tunnels and boreholes. The interpretation of the GPR datawith respect to geological structures is based on the unmigrated andmigrated images. For the tomographic analysis of the seismic first-arrivaltravel-time data, we inverted for an anisotropic velocity model described bythe Thomsen parameters v0, ϵ and δ to account for therock mass foliation. Subsequently, the GPR and seismic images wereinterpreted in combination with the geological model of the test volume andthe known in situ stress states. We found that the ductile shear zones areclearly imaged by GPR and show an increase in seismic anisotropy due to astronger foliation, while they are not visible in the p-wave (v0)velocity model. Regions of decreased seismic p-wave velocity, however,correlate with regions of high fracture density. For geophysicalcharacterization of potential deep geothermal reservoirs, our results implythat wireline-compatible borehole GPR should be considered for shear zonecharacterization, and that seismic anisotropy and velocity information aredesirable to acquire in order to gain information about ductile shear zonesand fracture density, respectively.
