Ambient signals analysis and cable coupling characterisation from a DAS experiment offshore South Brittany

As fibre-sensing measurements on submarine fibre optic cables become more widely used in geophysical studies, new challenges arise that demand a deeper understanding of the collected data. In particular, characterisation of cable coupling to the seafloor as well as the response of local sediment under the cables is needed for a better quantification of external physical phenomena by fibre-sensing measurements.FiberSCOPE is a research project aiming to implement an intelligent seabed monitoring system for studies in seismology, oceanography and the positioning of acoustic manmade sources (ships, AUVs, etc.) using existing submarine fiber-optic cables. One of the main objectives of the project is to define tools for remote evaluation of fibre optic cable coupling with the seabed using both Brillouin Optical Time Domain Reflectometry (BOTDR) and Distributed Acoustic Sensing (DAS) measurements of ambient noise.Within the project’s framework, passive and active seismic experiments were performed during March-April 2025 offshore south Brittany. The experiment included acquiring DAS measurements on the electro-optic cable connecting mainland France to Groix island, combined with the deployment of 10 seismic nodes near the cable. Preliminary results show that although ocean waves dominate the DAS signals, ocean wave induced microseisms events can be extracted as they fluctuate over the 18 days’ of the passive acquisition. Interestingly, despite the short distance covered by the offshore portion of the cable, spatial variations of those events are also observed and seem consistent between cable and nodes measurements. Finally, both ocean waves and microseism signals are used to further quantify the cable coupling with the seafloor and cable response connected to changes in seafloor structure.