Remote assessment of cable coupling along an electro-optic cable near Groix island

Coupling between fibre optic cables and the seafloor directly influences signals observed by distributed fibre sensing. In addition, variation in coupling over time provides information on the dynamic nature of the environment surrounding the cable as well as local geohazards (e.g. landslides, coastal erosion, etc).  The assessment of cable coupling in marine environments is particularly challenging given the difficulty in performing physical inspection of cables and therefore requires the development of remote techniques. In this context, as part of the FiberSCOPE project, a passive and active offshore seismic experiment was performed during March-April 2025 in south Brittany using an electro-optic cable that connects mainland France to Groix island. The primary aim of this experiment is to use ambient noise and seismic shots to assess cable coupling remotely. Ten GPR300 Sercel nodes were deployed near the cable using divers, after which an airgun was used to perform 5 seismic profiles: 3 parallel and 2 perpendicular to the cable with over 342 shots. Following the active survey, data continued to be recorded, this time passively, over a period of 18 days using both the GPR nodes and cable.

Overall ocean waves dominate the DAS signals during the passive acquisition but ocean wave induced microseisms events can also be extracted as they fluctuate over the whole recording duration. 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. Analysis of the active survey data indicates a thin sedimentary layer, of variable thickness, overlaying the bedrock. A discussion on the use of passive and active signals for cable coupling in this setting will be provided.