People tend to think of seismic waves as just more than signals of tectonic events, such as an earthquake or lava moving beneath a volcano. But these vibrations are also our best way to get a clear picture of the internal structure of our planet. By watching the vibration paths shift as they encounter different materials, we can get an idea of where different rock layers meet, where rock melts, and more.
In some cases, we get this picture by waiting for a natural event to produce the seismic waves. In others, we become impatient and place explosive charges on or use a powerful audio device. Today the Václav Kuna en John Near Oregon State University describes another option: wait for a whale to float next. 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 finfish is not exactly the kind of thing you would normally describe as musical. It’s mostly around 20Hz, which sounds more like a series of clicks than a continuous sound, and the whales produce it in second-long bursts, separated by decades of seconds. But they are hard. A guidance on hearing risks puts danger at any level above 80 decibels and the loudest concerts are that it is around 120 decibels. The song of a fin whale can be about 190 decibels, and it usually takes hours.
As it turns out, the frequency of whale calls is within the range of a bunch of underwater seismographs that researchers have placed on the ocean floor west of the Oregon coast. These seismographs are signals 100 times per second, so they can easily record the song of a finfish.
And in fact, the equipment picked up the songs. By focusing on the calls themselves, Kuna en Nábėlek could detect the whales while singing.
The songs they recorded usually lasted for hours, during which the whales traveled between 4 and 10 kilometers per hour through the area. This meant that the journey filled with songs ranged between 15 and 40 kilometers. During these voyages, each individual whale produced 200 to 500 individual bursts that could be picked up by the seismographs.
But the whale’s songs don’t just reach the seismographs directly. Some of the sound waves come there after bouncing off the ocean’s surface or floor – and some bounce more than once between the two. Others hit the ocean floor and reflect the various layers of material beneath it. Everyone reaches the instrument at different times within the 40-second window between each sonic element of the song.
Kuna en Nábelek
To reconstruct the exact details of what signals originate, and when they are complicated, to soften them. But earth scientists have a lot of experience with these kinds of things. Waves following certain paths need enough physical space between the whale and the seismometer to undergo all the reflections involved. Thus, certain elements are cut off when the whale gets closer than 12 kilometers, and another set is cut out at 4 kilometers.
By combining these details, Kuna en Nábėlek was is able to determine the thickness of the sediment layer, a layer formed by lava flows beneath it, and more robust volcanic rock beneath it. The seismometers were even sensitive enough to register differences in the amount of sediment, which was between 400 and 650 meters thick.
Overall, the resolution was not as good as you would get with human-activated sounds from the water surface. But critically, you do not need an actual boat above the seismograph range to get data. This is not a big limitation for a particular study, but there are probably many more underwater sensors than people intentionally making seismic waves to pick up. Fin whales have a worldwide variety, so there will probably be a few.
There are areas with intricate topography – meaning there are a lot of bumps on the ocean floor – where reading a whale song would probably not 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 pot visions are likely to do the trick.
Science, 2021. DOI: 10.1126 / science.abf3962 (On DOIs).