Seismic singing

The songs of fin whales can be used to survey the ocean crust, international researchers have just discovered.

With a population of about 100,000, fin whales (Balaenoptera physalus) inhabit the oceans from pole to pole. Their calls are one of the strongest animal vocalisations known, reaching up to 189 decibels – similar to the noise produced by large ships – and carrying across hundreds of kilometres.

Now, a new study in Science explains that these powerful calls can be used for seismic imaging.

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Research highlights

• Fin whale calls can penetrate the ocean floor as a seismic wave and reflect back off its various layers
• The return signal gives scientists insight into the structure of the ocean crust
• This natural seismic imaging technique is fairly low resolution, but could complement traditional surveying
• The calls of other animals, like sperm whales, could provide a higher resolution picture

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The discovery was made coincidentally by seismologist Václav Kuna, from the Czech Academy of Sciences, who was studying earthquakes in the Pacific Ocean off the coast of Oregon, US.

“Accidentally, I found fin whale call recordings in our data,” Kuna says – not just waterborne signals but also some that had been reflected and refracted from the oceanic crust.

When the call hits the ocean floor, part of its energy is transmitted into the ground as a seismic wave, which travels into the crust and bounces off the various layers. By picking up the return signal, Kuna explains, they can “estimate the thickness and seismic velocities of these layers”.

This discovery made him wonder if fin whale songs could complement seismic surveys.

Currently, seismic ocean surveys use high-energy airgun signals to send shockwaves down to the seabed from a ship. These penetrate the Earth’s crust and reflect back in ways that allow scientists to determine the structure and composition of the layers beneath the surface – including potential deposits of oil and gas.

The airguns generate explosive, high-decibel pulses that are among the loudest human-made sounds in the ocean, but – as scientists have warned for decades – as the marine landscape gets louder, wildlife is being negatively affected.

Kuna’s research raises the possibility that natural sounds could also be used. However, the novel method tested does provide a lower-resolution picture of the ocean crust than airgun surveys, because fin whale calls only cover low frequencies within a narrow band.

But in their paper, the team note that “higher-pitch whale vocalisations with a broader frequency band, such as those of sperm whales, could be used for high-resolution studies of the ocean floor sediments”.

Still, fin whale calls could be useful. This method could be used to improve current methods to locate earthquakes, as well as to improve our understanding of the shallow ocean crust and ocean sediments, providing important knowledge for disciplines such as geology and climatology.

This study also reveals that animal vocalisations carry more information than previously thought. Kuna notes that they “are useful not only for studying the animals themselves but also for investigating the environment that they inhabit”.

Hopefully, this research will encourage other scientists to look seriously at the applications of marine animal vocalisations.

“It is better to utilise sounds already available in the ocean than add more human-made noises that may harm ocean wildlife,” Kuna concludes.