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.