From strain to pressure, evaluation of Distributed Acoustic Sensing’s response to fin whale calls

Distributed Acoustic Sensing (DAS), capable of detecting water-borne sources such as low-frequency baleen whales and ships, holds great promise in underwater acoustics. However, investigating its instrumental response across the virtual channels created along an instrumented fiber is essential to enable quantitative measurements. We estimate DAS’s response dependent on source-receiver configuration, incorporating gauge length impact and fiber-cable coupling. We estimate DAS received levels by framing the problem as a passive sonar equation, including the response and a conversion term between strain and acoustic pressure. Simulated received levels are compared to data from three distinct deployments, processed to isolate well-characterized and relatively stable fin whale 20 Hz calls. The datasets we used were collected from different seafloor telecommunication cables in the Northeast Pacific, the Mediterranean, and the North Atlantic with different interrogators. The similarity of the results underlines the strong influence of the grazing angle on the response. Our approach enables the identification of a conversion term applicable across sites, aligning closely with water compressibility. Additionally, we present a sensitivity analysis of selected simulation variables. This research marks a significant step in DAS characterization for marine monitoring and underscores current limitations due to instrument noise floors.