Subsea Fiber-Optic Cables Detect Silent Whales via Water Displacement

Norwegian scientists have demonstrated that the global network of subsea fiber-optic telecommunication cables can detect whales even when the animals are completely silent. The team, led by geophysicist Martin Landrø at the Norwegian University of Science and Technology, used a 160-mile cable connecting two settlements in the Svalbard archipelago to observe a blue whale diving near the seabed.

The method relies on distributed acoustic sensing (DAS), which fires laser pulses down the fiber and measures microscopic backscatter caused by tiny stretches in the glass. When a large object moves through the water, it displaces fluid and generates low-frequency pressure waves that reach the buried cable, stretching it imperceptibly.

In a 2022 study, the same researchers showed that cables could pick up whale vocalizations. The new finding extends that capability: the whale was first heard calling at the surface, then tracked by its hydrodynamic signature alone as it dove. The diving animal produced a disturbance pattern similar to those previously recorded from passing ships and submarines.

To confirm the source, the team applied the Rayleigh equation, a physics formula from 1917 originally developed to model collapsing bubbles. Despite the vast size difference, the same mathematics describes the low-frequency waves radiated by the whale's body moving through water.

The detection range remains limited. The study reports observing a cruise ship at up to 550 meters from the cable, but the blue whale was only detectable within about 40 meters. Blue whales are the largest animals on Earth; smaller species may produce signals too weak to capture at current sensitivities.

Access to cables is another constraint. Most fibers are owned by telecommunications or data companies, and researchers typically depend on unused "dark fibers" granted for scientific use. Commercial rates for bandwidth would make such research unaffordable, and geopolitical sensitivities near strategic areas can restrict data sharing.

Independent experts call the work an important first step. Shima Abadi, a mechanical engineer at the University of Washington, notes the approach could eventually provide an underwater observing capability analogous to satellite imagery. William Wilcock, a marine geophysicist also at UW, suggests it may help study bottom-feeding species like gray whales that are difficult to monitor with surface tags.

The technique could improve estimates of whale population density and behavior at depth, information that is increasingly sought by shipping, military, and industrial operators planning offshore activities. Researchers emphasize that questions remain about maximum detection depths, species-specific thresholds, and long-term access to cable infrastructure.