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Thursday 26 August 2021

Whale geologists

The voice of the fin whale penetrates the earth’s crust beneath the sea floor – a whale born ultrasound.
The Earth’s layered structure. (1) inner core; (2) outer core; (3) lower mantle; (4) upper mantle; (5) lithosphere; (6) crust. Wikimedia commons.

By pure coincidence, it seems to have the right amplitude and frequency to easily penetrate the rocks for hundreds of metres in depth, reflecting and refracting where the rock changes its density, or there is a a crustal interface – a change between one type of rock and another, between a rock and a sediment, or just a big crack and an air or liquid gap. At these points, abrupt changes in structure and density, some of the sound bounces back, giving us information about how deep it went and how dense the stuff it bounced off (because of how much comes back).

Eventually, at very great depths, there is too little sound left, and the picture from there downwards is darkness.

Whilst seismic investigations are normally carried out using air guns on board ships, scientists think that fin whale song may allow them to passively observe the structure of underlying rock beneath the sea floor, without polluting the marine environment with noise.

Of course, this doesn’t mean scientists have a lot of choice over where they investigate, where the fin whales lives and how they travel – or, indeed, when they sing. They may not give us a complete picture of the ocean crust, but they can certainly give us something. I envisage a scientific future of seismologists trying to temp whales into new territories, and serenading them into song. Could it work?

why don't all references have links?
V. M. Kuna and J. L. Nábělek. Seismic crustal imaging using fin whale songs. Science. Vol. 371, February 12, 2021, p. 731. doi: 10.1126/science.abf3962.

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