Accurate characterization of reservoir rocks is crucial for fracture stimulations and time-lapse reservoir monitoring in enhanced geothermal systems. Campaign cross-borehole seismic data were collected as part of a comprehensive characterization suite of the first EGS Collab testbed prior to EGS Collab hydraulic stimulation experiments. The testbed was established in metamorphic, crystalline rock at the 4850 ft deep at the Sanford Underground Research Facility in Lead, South Dakota. Previous anisotropic traveltime tomography results revealed a heterogeneous velocity structure and strong anisotropy with features trending in a primarily northwest/southeast orientation. Based on these results, we employ an anisotropic elastic-waveform inversion method to further refine structures and rock properties within the first EGS Collab testbed. This method uses seismic waveform data, which in principle is capable of revealing higher-resolution and more accurate rock properties compared with traveltime tomography. We obtain a set of high-resolution P-/S-wave velocity and Thomsen anisotropic parameter models, and compare them with the traveltime tomography models. We focus on the portion of the testbed relevant to the hydraulic stimulation and flow experiments, which were performed subsequent to the campaign cross-borehole seismic characterization efforts. Furthermore, we will show that the inverted anisotropic medium parameter models can provide quantitative information on the fracture orientation and density in space, and can serve as important guidance for time-lapse active seismic monitoring during the hydraulic stimulation activities.

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