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  • Scientists used distributed acoustic sensing to detect low-frequency waves generated by a blue whale's movement near subsea cables in Norway.
  • The technique allows researchers to track whales that are silent or deep underwater, addressing limitations of traditional hydrophones and suction tags.
  • This method leverages existing global telecommunications infrastructure, potentially expanding the scale and duration of marine biological monitoring.

A network of fiber-optic cables buried beneath the ocean floor is proving capable of detecting the presence of whales even when they are not making sound. Researchers off the coast of Norway’s Svalbard archipelago successfully identified a blue whale by measuring the microscopic stretching of glass fibers caused by the animal’s movement through water. This development marks a significant shift in marine monitoring, moving beyond reliance on acoustic signals to include physical displacement data.

The detection occurred nearly one thousand feet below the surface. As the massive whale moved, it displaced water, creating low-frequency waves that radiated outward and rippled toward the seabed. These waves stretched the thin glass cable buried beneath the sediment almost imperceptibly. Onshore equipment fired laser pulses down the cable, and as light traveled through the fiber, microscopic imperfections scattered the light in response to the physical strain. This process, known as distributed acoustic sensing, converted the physical deformation of the cable into measurable data.

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Previously, scientists relied on hydrophones, which are underwater microphones fixed to the seabed or suspended from vessels, to locate whales by their calls. While effective for vocalizing animals, this method misses creatures that are silent or deep diving. In 2020, a team of Norwegian researchers discovered that subsea cables could also detect whale calls, publishing findings in 2022. The recent study builds on this work by demonstrating that the same infrastructure can identify whales based solely on their motion, even in complete silence.

The ability to distinguish between different types of disturbances was crucial for validating the technique. When submarines or large ships pass over cables, they similarly stretch the fiber-optic glass due to water displacement. However, researchers could confirm these were vessels using Automatic Identification System tracking data. With the whale, there was no such external tracking. The team observed the animal vocalizing on the surface before it dove. Once submerged and silent, the sensors continued to register a disturbance consistent with the low-frequency signals previously associated with moving ships.

Martin Landrø, a geophysicist at the Norwegian University of Science and Technology and lead author of the study, noted that the signal detected after the dive had nothing to do with vocalization. Instead, it was attributed directly to the swimming motion of the whale. By correlating the initial surface vocalization with the subsequent underwater disturbance, researchers could confidently trace the signal back to the animal. This allowed them to effectively see the whale near the cable despite its lack of sound.

The physics behind this detection relies on principles established over a century ago. The team utilized the Rayleigh equation, originally developed in 1917 to study how tiny bubbles collapse in water. Although blue whales and air bubbles differ vastly in size, the fundamental physics of pressure waves remains applicable. By synthesizing the whale’s low-frequency disturbance using this equation, scientists were able to interpret the hydrodynamic pressure and velocity fields captured by the distributed acoustic sensing data.

This approach offers a potential solution to longstanding challenges in studying deep-diving behavior. Currently, researchers often use suction tags placed on animals at the surface to monitor their subsea activities. These devices can fall off prematurely and have limited battery life, restricting the duration of observation. Fiber-optic cables, by contrast, are permanent fixtures that do not require physical attachment to the animal. This could enable long-term monitoring of seasonal or daily behavioral changes without disturbing the subjects.

Shima Abadi, a mechanical engineer at the University of Washington’s School of Oceanography who was not involved in the study, described the research as an important first step in evaluating the feasibility of this approach. While blue whales are large enough to displace significant water, studying pressure waves from movement is technically challenging due to their low frequencies. The success with a diving blue whale provides a clear path for applying similar techniques to other species, such as gray whales that feed on the ocean bottom.

The global web of subsea cables lacing across oceans presents an unprecedented opportunity for marine science. By repurposing telecommunications infrastructure for biological monitoring, researchers can access areas previously invisible to them. This method could unlock insights into what whales do during deep dives and how they interact with their environment over extended periods. As the technology matures, it may become a standard tool for conservationists seeking to protect vulnerable marine populations.

Future applications will likely focus on refining the ability to distinguish between different species and activities based on subtle variations in water displacement. The integration of this data with existing acoustic monitoring could provide a more comprehensive picture of ocean life. For now, the demonstration that silent whales can be detected via fiber-optic cables represents a breakthrough in non-invasive marine observation, expanding the reach of scientific inquiry into the depths.

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Go to the original reporting

  • Smithsonian Magazine↗The Global Web of Subsea Cables That Connects the World Can Now Detect Whales—Even When They're Silent