Identification of two vibration regimes of underwater fibre optic cables
by Distributed Acoustic Sensing
Abstract
Distributed Acoustic Sensing (DAS) enables data acquisition for
underwater Earth Science with unprecedented spatial resolution.
Submarine fibre optic cables traverse sea bottom features that can lead
to suspended or decoupled cable portions, and are exposed to the ocean
dynamics and to high rates of marine erosion or sediment deposition,
which may induce temporal variations of the cable’s mechanical coupling
to the ocean floor. Although these spatio-temporal fluctuations of the
mechanical coupling affect the quality of the data recorded by DAS, and
determine whether a cable section is useful or not for geophysical
purposes, the detection of unsuitable cable portions has not been
investigated in detail. Here, we report on DAS observations of two
distinct vibration regimes of seafloor fibre optic cables: a
high-frequency (> 2 Hz) regime we associate to cable
segments pinned between seafloor features, and a low-frequency
(< 1 Hz) regime we associate to suspended cable sections.
While the low-frequency oscillations are driven by deep ocean currents,
the high-frequency oscillations are triggered by the passage of
earthquake seismic waves. Using Proper Orthogonal Decomposition, we
demonstrate that high-frequency oscillations excite normal modes
comparable to those of a finite 1D wave propagation structure. We
further identify trapped waves propagating along cable portions
featuring high-frequency oscillations. Their wave speed is consistent
with that of longitudinal waves propagating across the steel armouring
of the cable. The DAS data on cable sections featuring such cable waves
are dominated by highly monochromatic noise. Our results suggest that
the spatio-temporal evolution of the mechanical coupling between fibre
optic cables exposed to the ocean dynamics and the seafloor can be
monitored through the combined analysis of the two vibration regimes
presented here, which provides a DAS-based method to identify underwater
cable sections unsuitable for the analysis of seismic waves.