Scientists from the Norwegian University of Science and Technology have unveiled a method for tracking silently moving underwater and surface objects by using existing fiber-optic cables to measure water displacement.
The scientists behind the novel detection approach, which was designed to detect whales even when they aren’t vocalizing, also demonstrated the method’s ability to track surface ships and potentially underwater vessels, such as submarines, that traditionally rely on silence for stealth.
Tracking Silently Moving Whales and Other Objects
In the research team’s study, the authors explain that it is already possible to record and reconstruct marine mammal vocalizations, including those of whales, using seabed fiber-optic cables. They note that the current method, called distributed acoustic sensing (DAS), allows scientists to passively monitor whale behavior, “enabling both the detection of vocalizations and the simultaneous tracking of multiple individuals.”
However, they add, although this data can prove valuable in tracking these increasingly endangered animals, “it has not been possible to track marine mammals when they are silent.”
“A fundamental limitation of passive acoustic monitoring is that most methods rely on acoustic data, which is only available when whales vocalize,” the study authors explain, adding that this limitation “clearly demonstrates the need for new sensing methods that can detect silent whales.”
Although the researchers propose several methods, including combining DAS data with satellite and observational data to paint a more complete picture, they also note a “second path” that involves exploring current datasets to discover new, valuable acoustic signatures even when the animals are silent.
Detecting Hydrodynamic Pressure Changes Caused by Ships and Animals
When exploring new ways to interpret DAS data collected by seabed fiber-optic cables, the team explained how low-frequency strain variations may arise from “a range of physical mechanisms.” These include hydrodynamic processes “associated with moving objects.”
For example, the authors explain, when a ship moves across the ocean surface, it pushes water ahead of it and draws water along in its wake. This motion through the water medium generates a strain on the surrounding water described as a hydrodynamic pressure field. According to the team, this motion-induced pressure field “extends downward through the water column.”
“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,” they explain.
Changes in this pressure field caused by moving objects cause sediments on the ocean floor to deform laterally and vertically. In areas of the seafloor with fiberoptic cables buried underneath, the DAS system records this “lateral deformation” of the sediments, which “translates into strain that can be observed in the DAS data.”
“This means that no acoustic signal is involved, and we can ‘listen’ to silent whales,” they write.
Tests Demonstrate Tracking Surface Ships and Submerged Whales
To calibrate their approach, the team used surface ships as “proxies” in a series of field experiments. According to their study, the four interrogator units (IU) were installed “during the DAS experiment on Svalbard in 2022.” Then, another unit was installed in Longyearbyen in March 2025. Although the study focuses on the newer IUs, the team notes that all the experimental data “was used to analyze how coupling affects the detectability of hydrodynamic pressure field signatures and how the signal characteristics change with increasing source-receiver distance.”
In those experiments, the team investigated four ships of different sizes. Notably, all ships were observed at a depth of 70 to 80 meters and were traveling at roughly the same speed. Critically, ship size, speed, and route data can be verified externally, providing a check on the study’s results.
“Ships are observed in the DAS data year-round, and the AIS is available to confirm which ship is sailing above the fiber and at what time,” they explain.
As expected, they found that the DAS signal amplitude “scales with the size of the ship.” For example, the smallest ship they observed barely registered above the background noise, making it the hardest to track. Conversely, the largest vessel tracked by the team, an icebreaker, left the clearest signal.
The researchers also demonstrated the ability to observe hydrodynamic pressure and velocity signals from a cruise ship at a water depth of 413 m, “and up to 550 m from the fiber cable.” However, when the team searched for whales, the range was much more limited.
“The smaller blue whales can be observed when diving within 40 m of the fiber-optical cable,” they explain.
During a dedicated test segment, the team monitored what they interpreted as several silently moving whales traversing through the detection zone. Although they were unable to independently confirm they were whales in the same way they could confirm the ships, the team notes that “multiple lines of analysis support the interpretation that whales are responsible for the observed signals.”
The team also notes that AIS data confirmed that no acoustic ship signatures were present “in the time window studied.” They add that this result “reduces the likelihood that the observed low-frequency signals originate from vessels.”
The Signal Amplitude Decreases with Increasing Distance From the Fiber
When discussing the implications of their findings, the team said their data shows that the observed low-frequency response captured by the fiber-optic cables “is consistent with translating hydrodynamic pressure fields generated by the displacement of water by moving ships and other large bodies such as whales.”
“This sensing method enables an approach to monitoring one of the world’s most endangered species,” they explain, adding that it is even more effective at detecting submerged objects than at detecting surface ships.
“A fully submerged object generates a more symmetric pressure field, while a surface-bound ship produces a truncated field due to the air–water interface, which limits the downward extent of the pressure field,” the study authors write.
When discussing the limitations of their innovative, silent, object-tracking approach, the team notes that it is only effective for tracking smaller ships and whales at close range.
“Kvitungen is the smallest ship investigated in this work and is barely observed above the noise floor for a water depth of ∼75 m,” they write. “Blue whales are even smaller and are unlikely to be detected at distances greater than approximately 30 to 40 m from the fiber.”
The team also notes that although larger ships generate stronger signals than smaller ones, “the signal amplitude decreases with increasing distance from the fiber.”
Other limitations involved data returns suggesting whales that were either “abnormally long or surprisingly small.” The team said this seemingly impossible result “underlines that our simple model needs further development to obtain more precise results.”
The study “Detecting silent whales using seabed fiber-optic cables” was published in the Proceedings of the National Academy of Sciences (PNAS).
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.
Login
