Whale songs suffuse the ocean depths with waves of sound that can travel thousands of miles. Now, scientists have figured out they can use the whales’ underwater serenades to study the seafloor, reports Robin George Andrews for the New York Times.
To study the Earth’s crust beneath the oceans, scientists use the way vibrations travel through the layers of sediment and rock to decipher details about their composition and structure. But crust-rattling vibrations like that don't come around every day. Traditionally, researchers have had to wait for jolts of tectonic activity to generate seismic vibrations, but undersea earthquakes aren’t always reliable collaborators. Alternately, scientists have resorted to making their own vibrations by blasting air guns from ships at sea, a technique that is also used by the oil and gas industry to search for deposits of fossil fuels. Air guns produce powerful seismic waves that generate high resolution images of the sea floor, but using them is expensive and the harsh noise can harm sea life sensitive to sound.
The new study, published last week in the journal Science, detected the calls of fin whales, one of the loudest creatures in the sea, via 54 ocean-bottom seismometers that were poised to detect undersea quakes. Unexpectedly, the researchers found that the fin whales’ vocalizations were powerful enough to reverberate through the Earth’s crust.
“The calls travel through the water and penetrate into the ground,” Václav Kuna, a seismologist at the Czech Academy of Sciences and co-author of the research, tells Karina Shah of New Scientist. “They then bounce off the layers within the oceanic crust and come back to the surface where we record them.”
Between 2012 and 2013, four of the seismometers stationed in the northeast Pacific Ocean recorded six fin whale songs ranging from 2.5 to almost 5 hours in length. The whale chatter translated to seismic waves powerful enough to allow Kuna and his colleagues to peer 8,200 feet below the ocean bottom, according to the Times. Fin whale calls can reach 189 decibels, reports Carolyn Gramling for Science News, which is nearly equal to the maritime din of a container ship.
These recordings suggest whale songs could be used as a way to estimate the varying thickness and geology of the Earth’s crust without waiting for tectonic activity or motoring noisy air guns out to sea.
“Air guns produce noise pollution in the ocean. It’s very expensive and it is not environmentally friendly,” Kuna tells New Scientist.
In the region the whale songs were picked up, the calls revealed an upper sedimentary layer ranging from around 1,300 to 2,100 feet thick sitting on top of a rocky layer of basalt more than a mile thick that was in turn undergirded by a type of oceanic rock called gabbro.
Unfortunately, air guns are still tops in terms of the geologic resolution they provide, with whale songs producing weaker seismic waves. “It’s never going to replace air guns,” Kuna tells the Times. “But it is a complement. And it’s free.”
Speaking with Sofia Moutinho of Science, Kuna says he’s hoping other researchers can apply the technique for other types of studies. “This study was a proof of a concept,” he tells Science. “I’m putting it out there for other people to find more uses for this.”