Title: Sensing whales, storms, ships and earthquakes using an Arctic fibre- optic cable

Abstract

Our oceans are critical to the health of our planet and its inhabitants. Increasing pressures on our marine environment are triggering an urgent need for continuous and comprehensive monitoring of the oceans and stressors, including anthropogenic activity. Current ocean observational systems are expensive and have limited temporal and spatial coverage. However, there exists a dense network of Fibre-Optic (FO) telecommunication cables, covering

Eavesdropping at the Speed of Light: Distributed Acoustic Sensing of Baleen Whales in the Arctic

“…The interrogator injects linear frequencymodulated optical pulses which are backscattered by anomalies in the fiber (Waagaard et al, 2021). These defects are displaced at the nanometer scale under the influence of acoustic pressure waves [but also seismic, oceanographic (Landrø et al, 2021)], introducing delays in the backscattered pulse (Figure 1B) (Nishiguchi, 2016). The interrogator calculates the timedifferentiated phase change of the backscattered response from consecutive sweeps at regularly spaced intervals along the fiber, further named channels (Figure 1C).…”

“…Normal signal strength decay along the fiber is 0.2 dB/km, such that the returned signal strength from 100 km is ≃−40 dB with respect to 1 km. A gauge length of 8.16 m was chosen as a compromise between maximizing the signal-to-noise ratio for the 4.08 m channel spacing (Dean et al, 2017), giving sufficiently large bandwidth for different applications (Landrø et al, 2021) and, optimizing the near-real-time data transmission rate. Data was streamed from Svalbard to NTNU (Trondheim, Norway) in near-real-time via Uninett's research network, using the 1 Gbit/s network interface of the interrogator.…”

“…The method is described in § 4.4. It was previously ground-truthed using acoustic signals received from a cargo ship and corresponding Automatic identification system (AIS) positioning, crossing the Svalbard DAS array at 86.6 km (Landrø et al, 2021). The localization, tracking and beamforming method is illustrated in Figure 5, using a long (more than 10 min) recording with noticeable movement of a blue whale.…”

“…Such information could be obtained using an active source, e.g., during DAS calibration experiments. Without this information, our take was to use the acoustic signal from a ship and its AIS positioning (Landrø et al, 2021). However, ship signals are continuous while whale calls are transient, requiring a different initial processing step.…”

“…The threshold is chosen to differentiate between signal and noise windows. A similar localization and tracking method was applied to signals received from a ship on the Svalbard DAS array and its positioning system (Automatic identification system; AIS) was used to ground-truth this localization method (Landrø et al, 2021).…”