Storm detection in the North sea with a subsea telecommunication cable and State of Polarization sensing

Fibre-optic sensing (FOS) techniques have gained notoriety for geoscientific and industrial applications over the past years. While most research tends to focus on spatially distributed techniques (e.g. Distributed Acoustic/Vibration Sensing), here we investigate the use of State of Polarization (SoP) sensing for ocean observation. SoP is a technique that samples spatially-integrated measurements remotely, i.e. the net change in polarization of light between the ends of an optical fibre link up to hundreds of kilometers long, which can be affected by mechanical strains on one or more sections of the fibre.

We present statistical analyses of an 11-month data set of a continuous SoP recording on a 150 km-long subsea telecommunication cable that crosses the southwestern Norwegian trench between Egersund (Norway) and a shallow (<100 m) FPSO (Floating production storage and offloading) platform on the Eigersunds bank. Our system measures net variations of the S1 Stokes parameter of polarized laser beam injected into the fibre. We observe North sea storms represented in SoP measurements as prominent anomalies with well-defined temporal and spectral features that are consistent with surface gravity wave anomalies. These events are confirmed by comparison with both, hourly ocean wave analysis numerical model time series (0.05° grid resolution) and simultaneous oceanographic measurements at locations adjacent to the cable.

Preliminary results show linear regression coefficients of determination of up to nearly 70% between rms SoP values and modeled significant wave height anomalies above 1 m at frequencies between 0.035-0.56 Hz. Remarkably, variations in the correlation statistics are found as a function of wave propagation direction, which could potentially allow for the discrimination of storms with variable azimuthal properties for a given cable link interrogated with SoP.

We highlight the value of these measurements for in-situ sea state monitoring. Although SoP systems effectively average-out all measurable signals along the entire sensed link, require access to two cable ends (or looped fibres) and are comparatively less sensitive than distributed FOS techniques, they also offer advantages over the latter as well as over established oceanographic sensors due to its relative low cost, sensor simplicity, large coverage, inherent remote data transmission, and relaxed data management requirements.