Whale songs provide new way to survey the Earth's crust
A research team led by John Nabelek, a professor in Oregon State University’s College of Earth, Ocean, and Atmospheric Sciences, has shown that it may be possible to use the songs of fin whales to produce seismic images of the Earth's oceanic crust.
The fin whale is one of the largest creatures ever to have roamed the Earth. Second only to the blue whale in size, it can reach a length of up to 85 feet (25.9 m) and weigh up to 74 tonnes. However, one of the more remarkable characteristics of this cetacean is the long and loud songs the males make that are among the lowest-frequency sounds produced by any creature. These songs are so unusual that when they were first detected, they were thought to be from a secret Soviet sonar system.
Now, the OSU team has found that there may be a practical use for these songs – practical from a human point of view. The tracking of fin whales by their songs is a well-established practice, but the new proof-of-concept study shows that these songs can be picked up by ocean seismographs, which are passive sensors that gather data about the seafloor by comparing acoustic signals from the water with seismic vibrations from the ground.
The team found that the fin whale songs interact with the ocean floor, being reflected and refracted by the sediments and bedrock, and can be used to measure the thickness of these layers as well as providing other data.
The findings are based on a study of earthquakes recorded from 54 ocean-bottom seismometers placed along the Blanco transform fault, about 100 miles (160 km) off Cape Blanco on the Oregon Coast. The data showed that strong seismic signals corresponded to whales singing in the area.
As the whales sing, the signals are reflected by the ocean surface and bottom. When they encounter the latter, they interact with the sediments, the basalt layer beneath, and the gabbroic lower crust, which is formed by the cooling of magma deep inside the Earth. These reflected and refracted signals can be recorded on seismographs to form an image of the crust's structure. This is further enhanced by using three seismographs to triangulate the whales' location.
According to the team, using whale songs can not only help to better understand ocean floor earthquakes, but they are also easier to use because, unlike the airguns used to produce artificial signals, they are noninvasive and no government permits are needed. The next step will be to apply machine learning to automate the process.
"The data from the whale songs is useful but it doesn’t completely replace the standard methods," says Nabelek. "This method is useful for investigating the Earth’s oceanic crust where standard science survey methods are not available."
The research was published in Science.
Source: Oregon State University