High-resolution 3D geological models are crucial for underground development projects and corresponding numerical simulations with applications in e.g., tunneling, hydrocarbon exploration, geothermal exploitation and mining. Most geological models are based on sparse geological data sampled pointwise or along lines (e.g., boreholes), leading to oversimplified model geometries. In the framework of a hydraulic stimulation experiment in crystalline rock at the Grimsel Test Site, we collected geological data in 15 boreholes using a variety of methods to characterize a decameter-scale rock volume. The experiment aims to identify and understand relevant thermo-hydro-mechanical-seismic coupled rock mass responses during high-pressure fluid injections. Prior to fluid injections, we characterized the rock mass using geological, hydraulic and geophysical prospecting. The combination of methods allowed for compilation of a deterministic 3D geological analog that includes five shear zones, fracture density information and fracture locations. The model may serve as a decameter-scale analog of crystalline basement rocks, which are often targeted for enhanced geothermal systems. In this contribution, we summarize the geological data and the resulting geological interpretation.

New in situ measurements to constrain the range, distribution, and spatial (meter-scale) variations of permeability in shallow crustal fault zones are reported based on systematic downhole tests at 0.5-km depth in crystalline rock. Single and cross-hole hydraulic packer tests were performed at a new dedicated test facility hosted in the Grimsel Test Site, in the Swiss Alps, following the technical instrumentation and isolation of discrete fault zones accessed by an array of boreholes. Single-hole test results are presented in this paper, while cross-hole experiments are reported in the companion paper. Our results reveal a sharp spatial falloff in permeability, from 10 -13 to 10 -21 m 2 , with off-fault distances of 1-5 m and characterized by a power-law relation with fracture density. Fractures linking subparallel faults were detected as high-permeability discrete spots several meters away from off-fault damage. Due to the narrow (centimeter-wide) thickness of fault cores, the hydraulic tests presented in this study do not characterize the permeability of fault core materials. The transmissivity of single fractures spans six orders of magnitude (10 -12 to 10 -6 m 2 /s) and is systematically higher in damage zones. In situ stresses appear to have a minor effect on natural, present-day fracture transmissivity at the borehole scale. We suggest that the geometrical and topological properties of fracture systems instead tend to control the permeability of the shallow crustal faults studied.Characterizing the structural properties of exhumed fault is key to understand their geometrical complexity at depth, and field studies have described how brittle damage around faults develops both along-strike and off-fault (i.e., off-plane) as fractures in damage zones organize themselves in response to off-fault stresses. Consequently, their spatial arrangement is not random (Kim et al., 2004;Peacock et al., 2017). Previous

Fluid pressure within the Earth's crust is a key driver for triggering natural and human-induced seismicity. Measuring fluid pressure evolution would be highly beneficial for understanding the underlying driving mechanisms and supporting seismic hazard assessment. Here we show that seismic velocities monitored on the 20-m scale respond directly to changes in fluid pressure. Our data show that volumetric strain resulting from effective stress changes is sensed by seismic velocity, while shear dislocation is not. We are able to calibrate seismic velocity evolution against fluid pressure and strain with in situ measurements during a decameter-scale fluid injection experiment in crystalline rock. Thus, our 4-D seismic tomograms enable tracking of fluid pressure and strain evolution. Our findings demonstrate a strong potential toward monitoring transient fluid pressure variations and stress changes for well-instrumented field sites and could be extended to monitoring hydraulic stimulations in deep reservoirs. Plain Language Summary The pressure of fluids in the subsurface is generally a function of depth as well as the regional geological history. Changes to the subsurface fluid pressure-be it natural or human induced-disturb the stress field and are known to drive volcanic eruptions, as well as to trigger earthquakes. For example, pressure increase by fluid injection for hydraulic stimulation and wastewater disposal has been linked to earthquake activity. Unfortunately, pressure measurements need direct access through boreholes, so that pressure data are only available for few locations. A method for estimating the spatial distribution of fluid pressure remotely would thus be highly beneficial. From measurements in a 20-m-scale experiment in granite, we find that fluid pressure propagation can be predicted from observed seismic velocity variations, based on a strong correlation between observed changes in seismic velocities and fluid pressure measured within the rock. As seismic velocities can be readily measured on the reservoir scale, our results demonstrate a strong potential of seismic velocity monitoring for remotely estimating fluid pressure changes in deep reservoirs, along faults, or in volcanic systems. The estimated pressure and stress changes could be an important input to real-time risk analysis of fault reactivation and volcanic eruptions.

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