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 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.