Multi-scale hydraulic and petrophysical characterization of a heterogeneous fault zone in the Gotthard massif's crystalline basement

Abstract. Accurately characterizing fault zones in crystalline basement rocks is essential for understanding fluid migration in the Earth's crust and how this influences fault stability and seismicity. While it is known that fault zones exhibit strong heterogeneity in structure and hydraulic properties, quantifying these variations across scales remains a challenge. The study presented investigates a deeply buried fault zone intersected by two inclined boreholes within a high overburden underground research laboratory (URL). As part of the FEAR (Fault Activation and Earthquake Rupture) project, this work provides key hydraulic and structural constraints needed to select and prepare experimental injection sites. These findings pose a necessary foundation for developing controlled fluid injection experiments and emphasize the importance of understanding scale-related effects during multi-scale observations. Through a combination of field-scale hydraulic testing, geophysical logging, and petrophysical analyses of core samples, we evaluate permeability, porosity, wave velocities, and fracture characteristics across multiple structural facies and on varying scales. The study finds that permeability varies over several orders of magnitude, largely controlled by the presence and connectivity of open fractures. Comparisons between lab and field data reveal pronounced scale effects, with lab tests underestimating the in-situ permeability due to the exclusion of large fractures and structural discontinuities. The fault zone shows a combination of localized and distributed flow behaviours, with no evidence of a continuous low-permeability fault core.