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.

Monitoring Deep Sea Currents With Seafloor Distributed Acoustic Sensing

“…Mata Flores et al. (2023) reported identical results, after tracking the amplitude of DAS‐recorded strain rate along an underwater fiber cable spanning 26 km. This similar amplitude of fiber strain could indicate that background noise recorded by DAS is mainly dominated by instrumental noise, regardless of the mechanical coupling between the cable and the seafloor.…”

“…Particularly, we propose the detection of suspended underwater fiber sections by monitoring the amplitude of strain rate recorded by DAS and its standard deviation along the entire cable, as suggested by Mata Flores et al. (2023). We notice that the amplitude of the fiber strain recorded by DAS along a cable segment undergoing VIV is usually one to two orders of magnitude larger than the ambient noise recorded by the same cable section outside periods of VIV, which may be used by threshold detectors to readily pinpoint hanging sections of seafloor fibers.…”

Monitoring ocean currents during the passage of Typhoon Muifa using optical-fiber distributed acoustic sensing

“…Subsequently, Williams et al 28 recovered spatiotemporal variations in current speed along the cable via DAS-based surface gravity wave interferometry. More recently, Mata Flores et al 31 measured the deep-sea current speed with DAS-recorded vortex-induced vibration of suspended cable segments. However, these DAS-based measurements are solely geared towards ocean current speeds and have not yet been able to estimate the current directions, and the potential of utilizing DAS measurements as dense arrays remains to be further evaluated.…”