Subsurface characterization of crystalline rocks at the Einstein Telescope candidate site (Italy): Insights from seismic tomography, geoelectrical and morphostructural analyses

The Einstein Telescope (ET) will be the first European underground observatory of gravitational waves. The observatory's interferometric detectors will be housed in a large underground infrastructure,which necessitates a stable and quiet geological context. We present the results of a geognostic campaign conducted for the Italian candidate site in Sardinia, during which two ∼270 m-deep boreholes were drilled in granites and orthogneiss at two sites that are possible locations of the ET infrastructure. We acquired high-resolution, dense seismic and electrical resistivity tomography (ERT) profiles to complement borehole data, constraining the thickness of the weathered layer and characterizing the rock properties in terms of intact versus fractured zones down to depths of 100–240 m. At depths >50 m, we observed high P-wave velocity (Vp ∼ 5000–5500 m/s, while very high Vp (∼6000 m/s) paired with very high resistivity (ρ > 1000 Ωm) was found at depths of 150–200 m, suggesting unfractured or weakly fractured rocks consistent with borehole logs and literature data on geophysical surveys on crystalline rocks. We recognized a couple of sub-vertical low-Vp (∼4250–4500 m/s) and low-resistivity anomalies (ρ < 500 Ωm), up to ∼15–35 m-wide, suggesting the occurrence of fracture zones with groundwater, matching the intersection with fault zones mapped at the surface. Comparison with co-located resistivity sections, downhole seismic surveys, well logs, and field-based structural and morphostructural analyses allowed us to attribute these anomalies to fault zones ∼0.3–0.5 km-long that belong to an immature fault network with shallow water circulation. This methodological approach highlights the utility of tomographic techniques combined with structural investigations and represents a guideline that can be applied in similar contexts characterized by poorly fractured crystalline rocks.