Use whale songs to imagine under the ocean floor

People tend to think of seismic waves as little more than signals of tectonic events, like an earthquake or lava moving under a volcano. But these vibrations are also our best way to get a clear picture of the internal structure of our planet. By looking at how the trajectories of vibrations move as they meet different materials, we can get a picture of where different layers of rock meet, where rock merges, and more.

In some cases, this image is obtained while waiting for a natural event to produce the seismic waves. In others, we get impatient and trigger explosive charges or use a loud sound device. Today, Václav Kuna and John Oregon State University Nábėlek describe yet another option: wait for a whale to float. Using the songs of passing fin whales, the researchers were able to reconstruct the upper layers of the seabed off the coast of Oregon.

Quite a song

The song of a fin whale is not exactly the sort of thing you would typically call musical. It’s usually in the 20Hz zone, which feels more like a series of clicks than a continuous sound, and whales produce it in bursts of one second, separated by tens of seconds. But they are noisy. A hearing risk guide places danger at anything above 80 decibels, and the loudest concerts reach around 120 decibels. A fin whale’s song can be in the order of 190 decibels and usually lasts for hours.

It turns out that the frequency of whale calls is within the range of a group of underwater seismographs the researchers had placed on the ocean floor west of the Oregon coast. These seismographs sample signals 100 times per second, so they can easily pick up a fin whale’s song.

And in fact, the equipment had picked up the songs. By focusing on the calls themselves, Kuna and Nábėlek could follow the whales singing.

The songs they recorded were usually hours long, with the whales moving through the area at speeds between 4 and 10 kilometers per hour. This meant that the song-filled trips ranged from 15 to 40 kilometers. During these voyages, each whale produced 200 to 500 individual bursts that could be picked up by the seismographs.

But whale songs don’t just reach seismographs by a direct route. Some sound waves get there after bouncing off the ocean surface or floor – and some of them bounce between the two more than once. Others touch the ocean floor and then bounce off the different layers of material below. All of them reach the instrument at different times within the 40-second window between each individual sound element in the song.

Rebuild the exact details of what signals emerge and when is complicated, to put it mildly. But earth scientists have a lot of experience with this stuff. The waves that take certain paths require sufficient physical space between the whale and the seismometer to undergo all the reflections involved. Thus certain elements are cut when the whale approaches more than 12 kilometers, and another set is cut at 4 kilometers.

Putting these details together, Kuna and Nábėlek were able to determine the thickness of the sediment layer, a layer formed by lava flows below and a more robust volcanic rock below. The seismometers were even sensitive enough to record differences in the amount of sediment, which ranged from 400 to 650 meters thick, that had accumulated.

Overall, the resolution was not as good as that obtained with human-triggered sound sources on the water surface. Most importantly, you don’t need a real boat on top of the seismograph array to get data. That’s not a huge limitation for a specific study, but there are probably a lot more underwater sensors than there are people who intentionally produce seismic waves for them to pick up. And fin whales have a global range, so there will likely be a few.

There are areas with complicated topography – meaning a lot of bumps on the ocean floor – where reading whale songs probably wouldn’t be so effective. But the researchers behind this work believe they can probably increase the resolution by using a different whale with higher frequency song. They specifically suggest that sperm whales would probably do the trick.

Science, 2021. DOI: 10.1126 / science.abf3962 (About DOIs).