Detecting silent whales using seabed fiber-optic cables.

Distributed acoustic sensing (DAS) has emerged as a powerful tool for passive whale monitoring, enabling both the detection of vocalizations and the simultaneous tracking of multiple individuals. However, a fundamental limitation of passive acoustic monitoring is that most methods rely on acoustic data, which is only available when whales vocalize. This clearly demonstrates the need for new sensing methods that can detect silent whales. In this paper, we detect hydrodynamic pressure and velocity fields in the low-frequency DAS data induced by a whale's motion and develop methods to analyze these signals. First, we use ships as proxies to demonstrate and calibrate the proposed method. Then, we show that a simple fluid mechanical model can be adapted to understand how whale swimming can be detected and analyzed using DAS. We detect multiple silent whales simultaneously, estimate their characteristics, and show that whale motion signals decay as one over distance cubed. Moreover, we demonstrate that we can observe hydrodynamic pressure and velocity signals from a cruise ship at 413 m water depth, and up to 550 m from the fiber cable. In comparison, the smaller blue whales can be observed when diving within 40 m of the fiber-optical cable. This sensing method enables an approach to monitoring one of the world's most endangered species.