They are so loud that they penetrate underground and are reflected there: the calls of fin whales can be used to explore the structures of the oceanic crust, researchers report. In order to provide information about the subsurface to seismic measuring instruments on the sea floor, sonic cannons have been used up to now, which can damage marine animals through their intense thunderclaps. The use of whale calls cannot completely replace this technology, but in many places it can offer a non-invasive addition to traditional seismic research methods, say the scientists.
What are the characteristics of the earth’s crust in the oceanic subsurface? Insights into depth are important for researching and predicting earthquakes, among other things. Up to now, powerful guns have been deployed to explore the geological structures: Sonic cannons send intense pulses from ships into the depths, which rush through the water column and then penetrate the sea floor. There they are reflected by the different layers and rock formations and broken down in a characteristic way. These echoes are then picked up by seismic instruments located on the ocean floor. The characteristics of this echo then enable researchers to draw conclusions about the structures in the subsurface.
Literally piercing calls
Oceanic seismology is also the specialty of Václav Kuna and John Nábelek from Oregon State University in Corvallis. Initially, their research project actually had nothing to do with whales: the two scientists carried out seismological studies on a fault on the sea floor off Cape Blanco on the Oregon coast. A network of 54 measuring stations on the sea floor served them for the seismic investigation. As Kuna and Nábelek report, these devices repeatedly detected strong signals that were not caused by geological processes.
As it turned out, the vibrations were caused by the calls of fin whales roaming the area. It is known that these marine mammals, which weigh up to 70 tons, make sounds that are among the most intense noises in the underwater world. Due to the low-frequency and extremely loud sounds, the fin whales, which are widespread in the world’s oceans, can communicate with each other over hundreds of kilometers. “So far, the calls have only been used to locate the whales and investigate their behavior. We now came up with the idea that their strong sound waves could perhaps also be used to study the earth’s crust, ”says Nábelek.
Whale calls instead of the thunder of cannons
The evaluations of the seismometer data showed that the devices actually not only detected the direct calls of the whales, but also the fine echoes they generated underground. “If you look closely at the seismometer data, it becomes clear that after every whale call there is an answer from the earth’s crust,” says Nábelek. Even if weaker, the calls of the whales ultimately produce the same effect as the sonic cannons that seismologists use. A part of the energy of the whale calls is therefore transferred to the sea floor. The seismic waves travel through the oceanic crust, where they are reflected and refracted by the sediment, the basalt layer underneath and other structures, the scientists explain.
As part of their study, Kuna and Nábelek were able to specifically document that it is actually possible to gain insights into the underground using the whale call echoes. They analyzed six fin whale songs and their effects recorded by seismometers off the coast of Oregon. As the scientists report, the data made it possible to precisely determine the location of the respective whale and to use its reputation: By analyzing the echoes, the researchers gained geological insights into the earth’s crust under the seismometers. As they explain, such information can, among other things, contribute to an understanding of the physical processes involved in earthquakes.
The researchers emphasize, however, that the whale calls do not reach the resolutions and depths that are possible through the use of sonic cannons. “The use of whale calls cannot completely replace the standard methods,” says Nábelek. “What we were able to show, however, is that whale calls can serve as a supplement to traditional seismic research methods”. The advantage is obvious: Due to the widespread distribution of the fin whales, their calls can be heard in many places, they are free, in contrast to the sonic cannons, and do not damage the marine ecosystems. “This method is useful for studying the oceanic crust of the earth where standard methods are not available or problematic,” says Nábelek.
There may also be opportunities to further refine the informative value of the data using the whale call method: “Future research could include the use of machine learning to automate the process of identifying whale calls and developing images of the ocean floor,” says Nábelek in conclusion.
Source: Oregon State University, Article: Science, doi: 10.1126 / science.abf3962