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 amplitudedependency 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 2390 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.

Identification of two vibration regimes of underwater fibre optic cables by distributed acoustic sensing

“…We propose that LF and HF oscillations can be used to assess the mechanical coupling between offshore fibre cables and the seafloor. LF oscillations require hanging cable segments, as documented by Mata Flores et al (2022). We interpret here that HF oscillations require two pinning points that prevent longitudinal motion of the cable and trap compressional cable waves along a finite span length.…”

“…We observe LF oscillations that extend coherently between pairs of points along the MEUST, NESTOR and HCMR cables. As previously reported by Mata Flores et al (2022), LF oscillations can last from hours to days and they occur along hanging sections of underwater fibre cables exposed to Vortex-Induced Vibrations (VIV) driven by deep ocean currents. We occasionally observe harmonics of these oscillations (Supplementary Figure 5 ).…”

“…LF and HF oscillations result from independent mechanisms. We showed here that HF oscillations are triggered by seismic waves from earthquakes, while Mata Flores et al (2022) showed that LF oscillations are driven by ocean currents. We found here that the frequency content of LF and HF oscillations is well separated and their occurrence is not correlated, which suggests distinct sources for each vibration regime.…”

“…To decrease the deployment cost, offshore cables are often buried only on the first kilometers near the coast. Hence, most of the cables are exposed to the ocean dynamics and the irregularities of the bathymetry, which may lead to cable segments mechanically decoupled from the seafloor or even cable sections hanging in the water column (Mata Flores et al 2022). A significant correlation between irregular bathymetry and unfavorable DAS measurements has been noted by Lior et al (2021), for seismology applications of seafloor DAS.…”

Monitoring Shelf Sea Dynamics with Ocean-Bottom Distributed Acoustic Sensing

“…The Distributed Acoustic Sensing (DAS) technology uses an interrogator unit (IU) to probe fiber-optic cables and measure the vibrations (i.e., strain or strain rate) of the Earth over tens of kilometers with a high spatio-temporal resolution (every ∼1-50 m at ∼100-1000 Hz depending on the experimental setting, Hartog, 2017). Over the past few years, DAS experiments have probed underwater telecommunication cables and recorded a variety of physical signals including near-coast microseisms (Guerin et al, 2022;Xiao et al, 2022;Spica et al, 2020;Viens, Perton, et al, 2022), local, regional and teleseismic earthquakes (Lior et al, 2021;Shinohara et al, 2019;Spica et al, 2022;Viens, Bonilla, et al, 2022), T-phases and other acoustic waves (Rivet et al, 2021;Ugalde et al, 2021;Spica et al, 2022), and ocean surface gravity waves (OSGWs) and deep-ocean water mixing processes (Mata Flores et al, 2022;Ide et al, 2021;Lindsey et al, 2019;Sladen et al, 2019;Williams et al, 2019Williams et al, , 2022. Most of these datasets are relatively short, spanning from a few days to a few weeks, and therefore do not capture the ocean's seasonal dynamics.…”