The way that the songs of fin whales echo back from the seabed could become a useful tool for scientists studying the sediment and rock that make up Earth's crust, according to new research carried out in the northeast Pacific Ocean.
These songs are some of the strongest and most far-reaching vocalisations in the ocean, and with tens of thousands of fin whales spread out across the world, the calls could help add to existing scans or even fill in gaps where it's more difficult or even dangerous to marine life to use the conventional air gun approach.
Seismometers on the ocean floor can be used to pick up the echos and fluctuations of the fin whale calls, revealing the thickness of the crust underneath as well as other seismic information that's helpful to scientific research.
"People in the past have used whale calls to track whales and study whale behaviour," says geophysicist John Nabelek from Oregon State University.
"We thought maybe we can study the Earth using those calls. After each whale call, if you look closely at the seismometer data, there is a response from the Earth."
Nabelek and his colleagues were studying earthquakes near the Blanco Fracture Zone off the coast of Oregon, using a network of 54 seismometers, when they noticed strong signals on the instruments that corresponded to the presence of whales in the area.
Further analysis of six calls revealed that the subaqueous songs – which can be as loud as large ships, and can last for 10 hours or more – were travelling through the ground beneath the ocean as seismic waves, before being reflected and refracted by ocean sediment, the basalt layer below, and the gabbroic lower crust even lower down.
The make-up of these bouncing signals can inform estimates of the ocean crust structure, the researchers determined: the readings calculated from whale signals matched other scientific observations of the area.
"This method expands the use of data that is already being collected," says Nabelek. "It also shows that animal vocalisations are useful not just for understanding the animals, but also understanding their environment."
More research will be required to get this method ready for full deployment – the fin whale songs don't offer such a high resolution scan as other seismic survey methods like the air gun, for example, so they won't be completely replacing conventional technology.
What they could potentially do, however, is give scientists a more detailed look at ocean crust and the seismic faults that lead to earthquakes and tsunamis. The researchers suggest that songs from other whales, like the sperm whale, could also be used.
Further down the line, machine learning is one option for making some of this analysis automated – and using natural whale calls is certainly a less invasive and simpler method than other approaches when it comes to finding out what's going on underneath the ocean floor.
"The paper serves as a proof of concept that could provide new avenues for using data from whale calls in research," says Nabelek. "What we discovered is that whale calls may serve as a complement to traditional passive seismic research methods."
The research has been published in Science.