On-fault monitoring using Fibre-Optic Sensing: a multi-borehole network

As part of the Fault Activation and Earthquake Rupture (FEAR) experiment [1] at the Bedretto Underground Laboratory for Geosciences and Geoenergies (BULGG), a comprehensive monitoring network has been installed to observe fault activation and rupture processes during controlled, in-situ experiments. Within this framework, we planned and installed a dense fibre-optic sensing network that complements a wide range of geophysical, hydraulic, and geomechanical instrumentation. Our contribution focuses on providing continuous, high-resolution measurements of seismic and deformation signals with fibre-optic sensing techniques across and around the target fault zone.The fibre-optic network consists of 1 km of sensing fibre cemented six boreholes intersecting and surrounding a well-characterised target fault zone, enabling distributed acoustic, temperature, and strain sensing (DAS, DTS, and DSS). To ensure mechanical coupling and long-term monitoring, fibre-optic cables were cemented along the borehole walls and inside the tunnel floor. This combined borehole–tunnel geometry enables dense spatial coverage of the fault zone, allowing the observation of both localized fault slip and more distributed deformation.We present the design and installation strategy of the fibre-optic network, discuss coupling conditions in different installation environments, and evaluate data quality in relation to other co-located sensors at Bedretto, installed both in the tunnel and in boreholes. The resulting multi-borehole fibre-optic array is deployed directly on and around the target fault, forming one of the most densely and closely monitored fault zones instrumented to date. Integrated within a vast multi-sensor observatory, this network provides an exceptional in-situ experimental setting to observe seismicity and related processes at close range. [1] Meier, Men-Andrin, et al. Activating a natural fault zone in the Swiss Alps, Seismica, 2024 (doi: 10.26443/seismica.v5i1.2065).