Reconstruction of nearshore surface gravity waves from Distributed Acoustic Sensing data

Distributed Acoustic Sensing (DAS) technology is a new photonic method that can convert several tens of kilometer-long seafloor fiber-optic telecommunication cables into dense arrays of strain sensors. With such spatial and temporal resolution, DAS appears as a potential future alternative to in-situ oceanographic measurements. Nevertheless, the DAS measurement needs to be calibrated because several factors can induce modifications to the measurements such as the structure of the cable or the level of burial in the sediments. In addition, the mechanisms through which the external stresses are conveyed to the fiber are still not well understood. An initial strategy involves analyzing the response of the DAS data to surface gravity waves as these are continuously present and well-characterized signals, that can be readily measured at shallow depth. From December 2020 to January 2021, an in-situ campaign was performed which involved deploying a pressure sensor at a depth of 15 meters for nearly 2 months next to the 50 km-long LSPM (Laboratoire Sous-marin Provence Méditerranée) seafloor cable in the bay of Les Sablettes, South of France. In the frequency band of surface gravity waves, we identify a remarkable linear correlation between the energy of the pressure sensor and that of the colocated DAS sensor, for various sea conditions. This result implies that the significant wave height can be reconstructed from DAS data at all points along the telecom cable, from the coast down to the cut-off depth of linear wave theory (roughly 100 meters in that region). From the linear wave potential theory, we derive an analytical transfer function linking the cable deformation and wave kinematic parameters. Even though this transfer function provides a first quantification of the effects related to waves and the cable response, it does not allow to distinguish between the different components of the wave spectrum (long-period, infragravity, surface gravity, ultra-gravity waves...), nor acknowledge the potential contribution of temperature variations. Also, the coupling processes between water, cable, and sediment remain to be included in this transfer function. In 2023, along the same telecom cable, we deployed 6 instrumented stations along the LSPM cable for 3 months. In particular, we installed instruments to measure seafloor pressure and temperature, the velocity components in the water column, as well as variations in the depth of sediments relative to a reference. These measurements will enable us to refine the analysis of the processes involved in the DAS response.