Monitoring Deep Sea Currents With Seafloor Distributed Acoustic Sensing

Abstract

Underwater fiber optic cables commonly traverse a variety of seafloor conditions, which leads to an uneven mechanical coupling between the cable and the ocean bottom. On rough seafloor bathymetry, some cable portions might be suspended and thus susceptible to vortex‐induced vibrations (VIV) driven by deep ocean currents. Here, we examine the potential of distributed acoustic sensing (DAS) to monitor deep‐sea currents along suspended sections of underwater telecom fiber optic cables undergoing VIV. Oscillations of a seafloor fiber optic cable located in southern France are recorded by DAS along cable sections presumably hanging. Their characteristic frequencies are lower than 1 Hz, at different ocean depths, and have an amplitude‐dependency consistent with the driving mechanism being VIV. Based on a theoretical proportionality between current speed and VIV frequencies, we derive ocean current speed time series at 2,390 m depth from the vortex shedding frequencies recorded by DAS. The DAS‐derived current speed time series is in agreement with recordings by a current meter located 3.75 km away from the hanging cable section (similar dominant period, high correlation after time shift). The DAS‐derived current speed time series displays features, such as characteristic periods and spectral decay, associated with the generation of internal gravity waves and weak oceanic turbulence in the Mediterranean Sea. The results demonstrate the potential of DAS along hanging segments of fiber optic cables to monitor a wide range of oceanography processes, at depths barely studied with current instrumentation.Ocean current speed recordings are rare below 1,500 m depth due to the cost of instrumentation in deep ocean environments. In this study, we demonstrate that deep‐sea current speeds can be obtained using measurements of deformation along suspended sections of seafloor fiber optic cables by a technique called distributed acoustic sensing (DAS). As ocean currents hit hanging cable segments, they generate vortex that shed off from the cable at a frequency called “vortex shedding frequency.” We show field evidence that the resulting cable oscillations can be recorded by seafloor DAS. We infer ocean current speed time series from the vortex shedding frequencies recorded by DAS across a hanging section of an underwater fiber optic cable in southern France. The DAS‐derived time series is in agreement with the one recorded by a nearby current meter. Our results demonstrate the potential of DAS to monitor the generation of internal gravity waves in the Mediterranean Sea at depths barely studied with the current instrumentation, and suggest that the technique presented here to estimate seafloor current speeds may be used for investigating several processes in oceanography. Sections of seafloor fiber cables feature oscillations driven by ocean currents that are observable with distributed acoustic sensing (DAS) Cable oscillation frequencies provide ocean current speed time series through the Strouhal number relation of vortex‐induced vibrations Spectral analysis of DAS‐derived ocean current time series provides valuable information on oceanic internal gravity waves