Whale songs allow researchers to take ‘ultrasound’ of sea floor

Fin whales have one of the loudest animal calls in the ocean.

Jordi Chias/Minden Pictures

Scientists have studied whale calls for decades to learn more about the communication and migration of these oceanic giants. Now, they’ve found a new use for the vocalizations: creating an “ultrasound” of the ocean floor. The strategy, reported today in Science, could help reveal details about Earth’s crust and earthquakes without disturbing sensitive marine life.

“It is a simple and very creative idea,” says James Miller, an ocean engineer at the University of Rhode Island, Narragansett Bay, who has studied underwater acoustics for 37 years.

Until now, whale calls have been an annoyance to seismologists. The researchers place ball-shaped seismometers on the ocean floor to record earthquakes. But the instruments often capture the repetitive songs of whales, which the scientists must delete from their analysis. When Václav Kuna was a Ph.D. student tracking earthquakes at Oregon State University, however, he wondered whether the “trash” of detected fin whale vocalizations in his recordings could be “treasure” instead. Kuna found that, besides the sound waves directly emitted by the whales, the ocean-bottom seismometers also capture their echoes: waves that passed through the ground and bounce back off subsurface rock layers. Scientists use similar echoes—produced by humanmade instruments, such as air guns—to map the sea floor and look for oil and gas deposits. “We can use whales in the same way, and for free,” Kuna says.

For the new study, he focused on fin whale vocalizations, which are among the strongest animal calls in the ocean. They can travel up to 1600 kilometers and reach 189 decibels, as loud as the noises produced by large ships.

Kuna analyzed six fin whale songs—22 hours of recordings in total—captured by seismometers located 3000 meters deep in an area close to the Blanco fault off of Oregon’s coast. By looking at the whale’s song echoes, Kuna could assess how compact and thick different layers of Earth’s crust were as deep as 3 kilometers underneath the ocean floor, he and colleagues report today. The findings are similar to previous studies made in the same region using air guns to analyze the oceanic crust. However, the whale sounds provided less information than air guns, whose waves can penetrate up to 8 kilometers beneath the ground.

As such, Kuna says whale calls can’t completely substitute for air guns. But he believes the approach could be valuable in certain parts of the ocean—such as marine conservation areas—where air guns are outlawed because their loud noises disturb marine life.

Kuna says the approach could also be used to study earthquakes. Before deploying seismometers to study seismic waves, he notes, scientists need to measure the thickness of the sediments underneath to know how fast the waves travel there and more precisely locate the origin of the temblors. Because they are expensive, air guns are typically not used for this purpose. Instead, Kuna says, researchers usually work with estimates, which are less precise than the information they can now take from the whale songs.

“It’s fascinating that listening to these fine tunes, you can learn something about the environment,” says marine biologist Ana Širović at Texas A&M University, Galveston. However, she points out the technique is limited to the fin whale’s distribution, which is mostly in colder offshore waters. “In the Gulf of Mexico, for example, an area with a lot of seismic activity, there are no fin whales.”

But Kuna is already thinking beyond earthquakes. He hopes other researchers will pick up the idea and look for more ways to use whale songs. “This study was a proof of a concept,” he says. “I’m putting it out there for other people to find more uses for this.”

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