Evaluating dark-fiber DAS noise interferometry for geothermal site investigation: a comparative study with a co-located nodal arrays

Distributed Acoustic Sensing (DAS) enables unused telecommunication fiber optic cables, commonly referred to as dark fibers, to operate as dense arrays of virtual seismic sensors. These systems are capable of recording both natural and anthropogenic seismic waves, providing opportunities for high-resolution, non-invasive subsurface investigations. This technology offers a cost-effective and scalable alternative to conventional seismic networks, particularly in areas where dense sensor deployment would otherwise be impractical. However, the use of existing telecommunication infrastructure for seismic experiments introduces several challenges, including heterogeneous cable installations, variable cable coupling conditions, and the directional sensitivity of DAS to the angle of incidence of seismic waves. As a result, DAS ambient-noise interferometry is strongly affected by these factors, which can influence the reliability of results.

Within the framework of the RUBADO project, we investigate the influence of cable geometries, noise-source distributions, source directions on DAS passive seismic interferometry by comparing DAS recordings with co-located conventional seismic sensors. The study aims to identify potential biases and improve the reliability of DAS-based monitoring and imaging of deep geothermal reservoirs. Experiments are being conducted at several spatial scales using the Karlsruhe Institute of Technology fiber infrastructure combined with nodal seismic measurements. By integrating dense DAS measurements with traditional seismic observations, we aim to better understand the recorded wavefield properties and improve the robustness of interferometric results. Preliminary results demonstrate the potential of DAS, using dark fiber, for large-scale geothermal monitoring and imaging across the wider Upper Rhine Graben region