Distributed Acoustic Sensing : A New Frontier For Continuous And Real-time Ocean Monitoring

       The sixth report published in March 2023, from the IPCC 2023, lists several alarming findings about the ocean (Lee et al., 2023).  The rise in sea level has accelerated and is now three times faster than it was during the period 1901-1971. The increase in ocean levels and the multiplication of energetic oceanic events represent a real problem for the management of coastal infrastructure. The proximity of human activities near the seaside makes these areas particularly vulnerable to risks. The submersion risks are seriously considered, given the damage that certain storms can have on the coast. Real-time measurement of the offshore wave field makes it possible to improve coastal submergence warning systems and predictions of submersion. Nevertheless, maintaining hydraulic measurement stations is still a challenge since the ocean is a hostile and vast environment that induces high installation costs. Consequently, a significant portion of our oceans remains unmonitored, making it difficult to find effective solutions for managing coastal infrastructures. On the other hand, ocean warming has been faster in the last century than in about 11,000 years (medium). For instance, marine heatwaves will increase in number and intensity, compromising many ecosystems. These marine heat waves can be detected on the surface with satellites. Still, their dynamic can greatly differ from the evolution of marine heat waves at depths where they remain poorly documented.\\
    
    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 is a new transforming approach for in-situ oceanographic measurements. For a recent DAS experiment performed on seafloor cables along the French Mediterranean coast, we show that it is possible to measure ocean swell fields up to depths of about 100 meters, and track water temperature variability from the coast to the bottom of the Mediterranean sea with mK sensitivity. Because DAS data are acquired at the speed of light from the land termination of the cable, the technology also enables the establishment of effective and rapid submergence warning systems, capable of anticipating the impact of storms or marine heat waves in real-time. Considering the vast network of submarine telecommunications cables and the ability of DAS to operate on fiber optic cables with live traffic, DAS could be easily and rapidly implemented across the globe.