People tend to see seismic waves as little more than signals of tectonic events, such as an earthquake or lava shifting under a volcano. But these vibrations are also our best way to get a clear picture of our planet’s internal structure. By watching how the paths of the vibrations shift as they encounter different materials, we can get a picture of where different rock layers meet, where rock is melted, and more.
In some cases, we get this picture by waiting for a natural event to produce the seismic waves. In others, we get impatient and release explosives or use a powerful device to make noise. Today, Václav Kuna and John Nábėlek of Oregon State University describe yet another option: waiting for a whale to float by. Using the songs of passing fin whales, the researchers were able to reconstruct the upper layers of the seafloor off the Oregon coast.
Better a song
The song of a fin whale is not exactly something you would normally describe as musical. It’s generally in the 20 Hz range, which sounds more like a series of clicks than a continuous sound, and the whales produce it in bursts of seconds separated by tens of seconds. But they are loud† A guideline on hearing risks places danger at any level above 80 decibels and the loudest concerts hit about 120 decibels. A fin whale’s song can be close to 190 decibels (although that’s in water, which emits sound differently from the air), and it usually goes on for hours.
It turns out that the frequency of whale calls is within the range of a number of underwater seismographs that researchers had placed on the ocean floor west of the Oregon coast. These seismographs search for signals 100 times per second, so they can easily pick up the song of a fin whale.
And in fact the equipment had picked up the songs. By focusing on the calls themselves, Kuna and Nábėlek could follow the whales as they sing.
The songs they recorded usually went on for hours, during which time the whales passed through the area at speeds of between 4 and 10 kilometers per hour. This meant that the song-filled rides ranged from 15 to 40 kilometers. During these voyages, each individual whale produced 200 to 500 individual eruptions that could be picked up by the seismographs.
But the whale’s songs don’t just reach the seismographs by a direct route. Some sound waves end up there after bouncing off the ocean’s surface or bottom — and some bounce between the two more than once. Others hit the ocean floor and then bounce off the various layers of material below. They all reach the instrument at different times within the 40 second period between each individual sound element of the song.
Reconstructing the exact details of which signals arise, and when, is complicated to say the least. But Earth scientists have a lot of experience with these kinds of things. Waves taking certain paths need enough physical space between the whale and the seismometer to undergo all the reflections involved. So certain elements are cut off when the whale gets closer than 12 kilometers, and another set is cut off at 4 kilometers.
By putting these details together, Kuna and Nábėlek were able to calculate the thickness of the sediment layer, a layer formed by lava flows below, and more robust volcanic rock below. The seismometers were even sensitive enough to register differences in the amount of sediment, ranging from 400 to 650 meters thick.
In general, the resolution was not as good as you would get with human-activated sound sources from the surface of the water. But most importantly, you don’t need a real boat above the seismograph matrix to get data. That’s not a huge limitation for a specific study, but there are probably many more underwater sensors than there are people deliberately creating seismic waves to pick up. And fin whales have a global range, so there are probably a few around.
There are areas of complicated topography — that is, lots of clumps on the ocean floor — where reading whale songs probably wouldn’t be as effective. But the researchers behind this work think they can probably increase the resolution by using another whale with a higher-frequency song. They specifically suggest that sperm whales would probably do the trick.
Science2021. DOI: 10.1126/science.abf3962 (About DOIs).