Whale songs: A tool for analysing the Earth’s crust
Geophysics, Seismology, Marine biology
When Dr. Václav Kuna and Prof. John Nábělek of Oregon State University discovered that the songs of fin whales could be used to image the Earth’s crust, they united the wonderful worlds of geophysics and marine biology. Their discovery, published in the journal Science in February of 2021, was a serendipitous one. Kuna and Nábělek started their research monitoring earthquakes along the Blanco transform fault, about 100 kilometres off Oregon’s western coast. They strategically placed a network of 54 seismometers on the ocean floor, allowing them to collect seismic readings for one year. In addition to 8,000 small earthquakes, the team also detected strange sound waves they didn’t recognise right away. Kuna figured out that these waves were the echoing calls of fin whales: short, low-frequency chirps that repeat every 7-40 seconds, forming songs that can last for up to 10 hours.
Transform fault: A type of boundary where tectonic plates meet. The type of motion that takes place where plates meet defines what sort of boundary it is. Convergent plates slide towards each other, divergent plates slide apart, and transform boundaries grind against each other horizontally.
Seismometer: An instrument that responds to ground motions, like earthquakes or volcanic eruptions.
Fast facts on the fin whale (Balaenoptera physalus):
They are the second largest whale on Earth (after the blue whale), growing up to 23 metres in length and weighing up to 80 tonnes.
They are endangered, with an estimated population between 50,000-90,000.
They are migratory, and travel in one of two migratory streams in either the Northern or Southern hemisphere (and ne’er the twain shall meet)
They have some of the most powerful vocalisations of any ocean animal, reaching up to 189 decibels – as loud as the engine of an oil tanker!
‘Translating’ these vocalisations into useful geological information relies on reflection seismology. Normally, when surveyors use reflection seismology to image underwater structures (like when searching for oil traps), they use large ships equipped with air-guns. These air-guns go off just beneath the surface of the water, directing a powerful wave of energy down to the ocean floor. When the wave strikes an interface, part of the wave is reflected back up, while another part of it is refracted deeper into the next layer. The waves that are reflected to the surface can be picked up by a series of receivers: either surface-level ones (that trail behind the ship like water skiers) or the ocean-bottom ones used by Kuna. The waves recorded at the seismometers provide scientists with the information they need to map the structure of the oceanic crust and estimate the density and elasticity of each layer.
Kuna proposed that role of the air-gun in a reflection survey could be filled by a fin whale – and it worked. Just like the waves generated by air-guns, whale calls bounce between the ocean surface and floor. Four types of waves travelling from the fin whale to the ocean-bottom seismometers, penetrating the oceanic crust to varying degrees, were identified. The recordings from three seismometer stations allowed Kuna to image the ocean sediment, the basalt layer beneath it, and the even deeper gabbroic lower crust. The results matched up with what you would expect from surveying with an air-gun, confirming Kuna’s work as a proof-of-concept for using whale calls to gather data about Earth’s subsurface.
Reflection seismology: Method that uses reflected seismic waves to gather information about layers of the Earth’s subsurface, such as their thickness, density, and elasticity.
Proof-of-concept: Also known as a proof-of-principle, these are scientific publications that propose a new technique and show that it is feasible.
Kuna’s method does have its limitations. For one, there is a difference in frequency content between a fin whale’s calls and an air-gun’s blast. It is this frequency content that defines how deep a wave can penetrate (low frequencies travel deeper into material) as well as the resolution of the reflected signal (high frequencies give higher resolution). In a fin whale call, most of the sound wave’s energy sits at a low-frequency 20 Hz. This means that calls can travel deep into the Earth’s crust but do not return a high-resolution signal. With an airgun, the energy distribution is much broader: both low and high frequencies are represented, giving the method the double advantage of distance and resolution. Another drawback of using whale calls is a limit on the intensity of sound from the source. Fin whales and air-guns both generate 190-decibel soundwaves, but multiple air-guns can be used at the same time to amplify their energy output. Unfortunately, amassing an army of fin whales to do the same is just not an option right now (but we can dream).
Though fin whale songs might never completely replace active-source reflection seismology, Kuna and Nábělek’s work revealed a possibility that had never been considered. In situations where standard survey methods are unavailable – if they are deemed too expensive or invasive – whale songs could serve as a useful complement. The method also shows a lot of potential for improvement. Sperm whale calls, for example, have a broader frequency band than their fin whale cousins, and could be used to improve the resolution of crustal imaging.
“Neat! The drawing is really cool, DaVinci would envy it! It is so fun to see people inspired by our paper.”
— Dr. Vaclav Kuna, Oregon State University
Journal references
Václav M. Kuna, John L. Nábělek. Seismic crustal imaging using fin whale songs. Science, 2021; 371 (6530): 731 DOI: 10.1126/science.abf3962