Fiber-Optic Cables Can be Used to Track "Silent" Bow Wave Vibrations

warning this is cool as hell! I should really make this NSFW for nerds but this is by definition Safe For Whales so?...

“The main challenge in detecting these low frequency signals is that they decay very fast with distance. A big ship is easy to detect as it moves a lot of water, but a whale is much smaller, so that it moves a lot less water. This means that they need to dive into the water column to be detected,” said Robin Andre Rørstadbotnen, the first author of the paper, and a postdoc at the Centre for Geophysical Forecasting.

One of the many advantages to the fibre-optic cable network in the waters around Svalbard is that there are a number of ships, both small and large, that cruise these same waters.

And if you think about, the shape of a ship’s hull isn’t that different from the shape of a whale’s body.

But unlike whales, all ships are required to have what’s called an Automatic Identification System [seems unfair to me], or AIS. That identifies both the ship itself and where it is travelling, allowing researchers to track the ships as they sail along or across the fibre-optic cables.

The researchers could detect the actual sound – the acoustic signal – of the different ships in the data from the fibre-optic cables, of course. They could also detect how fast the ships were travelling.

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A pressure wave from water movement is much lower frequency than a whale vocalization. It’s also information that the researchers hadn’t looked at before.

“The big surprise was that this fiber could detect this,” Landrø said.

Suddenly they had a way to interpret the low-frequency data because they already had so much information about the different ships.

link to open access paper https://www.pnas.org/doi/10.1073/pnas.2603077123

DAS is a fiber-optic measurement technique that uses coherent Rayleigh backscattering to record strain along the fiber with high spatial and temporal resolution. The method is based on phase measurements of the Rayleigh backscattering to capture the strain field along a fiber at timescales controlled by the kilohertz laser pulse repetition rate (12). DAS has traditionally been used to study high-frequency signals (4, 13, 14). More recent work has demonstrated that it can also resolve significantly lower-frequency variations (15–17).

In the marine environment, low-frequency strain variations (0.01 to 0.1 Hz) may arise from a range of physical mechanisms, including hydrodynamic processes associated with moving objects. The displacement of water generated by moving whales and ships induces a moving hydrodynamic pressure field that extends through the water column, dynamically loading and unloading the seafloor. This varying pressure field causes the seafloor sediments to deform both vertically and laterally.

When the fiber is buried in sediments, the DAS system primarily records the resulting lateral deformation of the surrounding material. Hydrodynamic pressure fluctuations produced by ocean waves have already been extensively studied (18–21). However, for more detailed characterization of pressure signals, dedicated pressure sensors are often employed. For example, Stenvold et al. (22) demonstrated that high-precision seabed pressure sensors can detect subtle pressure variations, highlighting their potential for hydrocarbon reservoir monitoring. Related studies have also explored acoustic and hydrodynamic signals generated by moving objects. Hegna (23) investigated how acoustic wavefields generated by moving vessels, recorded by towed streamers or ocean-bottom sensors, can be used to image the subsurface. Another study by Werner and Landrø (24) investigated the hydrodynamic pressure field generated by a buoy moving through the water column, while Scarpa et al. (25) used pressure sensors to measure ship wakes in Venice, Italy, and assess their impact on the lagoon.

The pressure field associated with ship-generated Kelvin wakes typically contains frequencies between 0.1 and 0.4 Hz (26–28). Buisman and Thiem (29) investigate hydrodynamic signals from ships recorded with different DAS configurations in both very shallow and deeper water settings, down to 58 m. In very shallow water, they report low-frequency signals attributed to ship wakes, including contributions from bow and stern waves within the Kelvin wake system. In deeper water, they observe even lower-frequency signals, which they describe more generally as “ship-induced water waves,” without drawing a firm conclusion on whether these arise from the Kelvin wake or from other displacement-driven processes. In this study, we present observations of ship- and whale-generated signals from a DAS installation in Svalbard, where the recorded signatures fall within the same low-frequency band as those observed in the deeper-water setting of Buisman and Thiem. Building on these earlier observations, we introduce a theoretical framework that explains the origin of these signals.

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DAS is topical across many scientific fields, including geophysics and marine biology (4, 30, 31). It has been labeled a ‘game-changer’ across many disciplines, especially in remote sensing and ocean monitoring. There are different reasons for this. 1) The instrumentation is placed on land or on a platform; 2) No DAS sensor components are exposed to the harsh underwater environment; 3) DAS can use the existing telecommunication infrastructure; 4) The interrogator signal can reach up to 140 km (12). New steps have been taken to increase the DAS range to exceed 1,000 km (32, 33), opening new possibilities for monitoring ships and whales over vast areas.

Most studies on whales using DAS and PAM have so far focused on the mammals’ acoustic signatures. The hydrodynamic pressure field signatures are independent of acoustic pressure signatures and can track and monitor silent whales as long as they are sufficiently close to the fiber.