NORFOX: A Fibre-Sensing Testbed for Seismo-Acoustic Monitoring

The strength of Distributed Acoustic Sensing (DAS) for vibration monitoring is the use of existing (telecom) fibre infrastructure. However, these cable geometries are typically not optimal for event localisation and beam-forming methods. Also, deployment (and thus ground coupling) is outside the control of geoscientists. To overcome these issues we designed and constructed the NORFOX array (NORwegian Fibre Optic eXperimental array) as a dedicated fibre-optic sensing testbed. NORFOX thus allows to investigate the opportunities DAS offers for both (array) processing and system-level characterization. The array comprises five fibre arms (~18 km total length, ~3 km aperture) and is co-located with the NORES seismic- and infrasound-array, enabling direct benchmarking against conventional instrumentation.

A key objective of NORFOX is to exploit DAS as a dense, continuous array for beamforming. Treating the fibre as thousands of spatially distributed channels, we demonstrate that DAS can retrieve coherent wavefield properties such as slowness and back-azimuth. The array geometry is specifically designed to balance directional sensitivity, robustness, and optical budget, providing improved azimuthal coverage compared to linear deployments. The design of NORFOX can be considered as best case scenario, and we can also simulate the impact of sub-optimal designs by reducing the geometry to various combinations of the 5 arms.

NORFOX also serves as a platform to calibrate fibre sensitivity. The installation includes both standard telecom fibres and enhanced backscatter cables, either directly buried in the ground or deployed inside a protective plastic conduit. This allows direct comparison of sensitivity, attenuation, and noise performance. In addition, multiple interrogators operating at different wavelengths (e.g., within the C- and L-bands) are tested, highlighting trade-offs between optical loss, backscatter efficiency, and signal-to-noise ratio. These comparisons are critical for understanding how hardware choices influence DAS data quality and array performance.

At the same time, NORFOX exposes key challenges in DAS. Cable-to-ground coupling strongly controls signal fidelity and varies with soil conditions and installation, leading to spatially heterogeneous noise. This variation occurs over both short-term (intra-day) and long-term (seasonal) timescales, leading to spatially heterogeneous noise conditions. DAS measurements are inherently directional and single-component, complicating beamforming and requiring adapted processing strategies. NORFOX records continuously with 250 Hz, 4 m spacing, and 20m gauge length, equivalent of about 4400 channels, and generating 93 GB data each day. It is possible to choose a secondary data-stream with different settings, or perform parallel measurements with other interrogators on additional. We present our considerations of for data management, data reduction, edge computing, and (long-term) storage.

By combining controlled array design, mixed fibre infrastructure, and multi-interrogator testing, NORFOX provides a unique experimental platform to investigate the full DAS sensing from fibre and interrogator physics to processing and interpretation. We here highlight the potential of fibre-optic arrays as next-generation sensing systems, while directly addressing the instrumental and methodological challenges limiting broader adoption.