Near Real-Time In Situ Monitoring of Nearshore Ocean Currents Using Distributed Acoustic Sensing on Submarine Fiber-Optic Cable

In the nearshore area, ocean current display intricate complexities due to interactions among tide, river, and coastline, which makes accurate current modeling challenging. Continuous in situ observation with high spatial and temporal resolution helps to better understand the dynamics of these currents. In this study, we used a 10-km long submarine fiber-optic cable with distributed acoustic sensing technology to record seismic signals associated with ocean waves. The current velocity and water depth were obtained from the velocity dispersion using frequency-wave number analysis matched against theoretical ocean wave propagation equations. The results show remarkable agreement with observation of a nearby current meter, confirming the dominance of tidal currents as well as a small-scale residual current. The temporal variation of water depth is consistent with observation by a nearby tidal gauge. This study demonstrates the potential of using submarine fiber-optic cable for long-term, high-resolution, near real-time nearshore current monitoring. The ocean currents in the nearshore area are complicated due to coastlines, seabed topography, and other factors. The complexity makes it crucial to monitor high-resolution currents. However, it is challenging to observe them in situ with high temporal and spatial resolution over long periods using conventional methods. The novel distributed acoustic sensing technology can turn submarine fiber-optic cables into high-density vibration sensors. This allows us to sense nearshore ocean wave pressure loading and obtain the propagation speed of ocean waves. Ocean waves propagate faster in the direction of the current than in the opposite direction. Therefore, the current velocity can be obtained by measuring the difference of propagation speed of ocean waves in different directions along the fiber-optic cable. In this study, we used a 10-km long submarine fiber-optic cable in the Yellow River Delta to conduct submarine seismic observations. We obtained the current velocity with a temporal resolution of 5 min during a 25-day period. The reliability and strengths of our method were verified by comparison with other observations. This study demonstrates that integrating our approach with submarine fiber-optic cables can offer an innovative approach for high-resolution ocean current monitoring in the nearshore area. We obtained the velocity of landward and oceanward ocean waves from in situ data recorded by distributed acoustic sensing on a submarine fiber-optic cable We propose a near real-time method to monitor current and water depth for the asymmetry of ocean wave propagation The obtained tidal current agrees with a nearby observation and the residual current pattern reveals a small-scale eddy