In-Situ Monitoring of Nearshore Ocean Currents with Distributed Acoustic Sensing on a Submarine Cable

Nearshore ocean currents exhibit intricate complexities due to interactions among tides, rivers, and coastlines, making accurate modeling challenging. High-resolution, continuous in situ observations are essential for understanding these dynamics. In this study, we employed a 25-km-long submarine dark fiberoptic cable with distributed acoustic sensing (DAS) technology to record continuous signals associated with ocean waves. This innovative approach allows for real-time, high-resolution monitoring of underwater dynamics, which traditional methods may not capture with such detail or over such extensive areas. The use of DAS technology in oceanography is relatively new but promising due to its ability to convert each segment of the optical fiber into a series of individual receivers. This setup enables the detection of acoustic and seismic activities along the cable's entire length. In our deployment, part of the fiber-optic cable was installed inland and part extended underwater across the continental shelf, providing a unique cross-section of environmental interactions. The recorded signals demonstrate clear transition patterns between land and ocean, aligning well with tidal signals, suggesting the potential of this method for studying coastal interfaces. Microseismic noise induced by ocean surface gravity waves (OSGWs) was clearly observed in the 0.08–0.4 Hz frequency band for the submarine fiber segments. Interestingly, this signal was absent in the land fiber segments, highlighting the method's sensitivity to aquaticspecific phenomena. This distinction is critical for isolating marine signals from terrestrial noise, thereby improving the accuracy of data interpretation. We derived current velocity and water depth from velocity dispersion using frequency–wave number analysis matched against theoretical ocean wave propagation equations. This analysis involved comparing our measured data with models that predict how wave characteristics such as speed and frequency change with water depth and current flow. The method proved effective, as our results aligned closely with nearby current measurements and observations from a nearby tide gauge. These correlations underscore the reliability of DAS technology in capturing and quantifying dynamic oceanographic processes. These findings are significant, as they validate the feasibility of using DASinstrumented cables for real-time monitoring of nearshore ocean currents. The implications for this are broad and highly beneficial for coastal management. Accurate, real-time data on nearshore currents can enhance maritime safety, improve pollution tracking, and offer valuable insights for coastal development planning. Furthermore, the ability to monitor changes in current patterns over time can contribute to climate change research, particularly in understanding how rising sea levels and changing weather patterns, for instance typhoon, affect coastal and submarine environments. Moreover, the successful integration of DAS technology with existing oceanographic measurement tools could revolutionize how researchers study marine environments. By providing a detailed, continuous picture of underwater dynamics, DAS-equipped cables could significantly expand our understanding of underexplored or inaccessible areas of the ocean. Future research could explore the integration of this technology with satellite and drone observations to develop a comprehensive, multi-layered view of oceanic and coastal phenomena, enhancing our ability to protect and manage these vital ecosystems more effectively.