Intuitively, you would assume that deep sea bottom is quiet, dead zone, devoid of activity. However, the reality is vastly different. When we drop seismometers to record what’s happening there, we pick up a lot of fascinating things. For the Seamstress project, we are putting seismometers on the bottom floor at more than 1000 m water depth along the west-Svalbard margin in the Fram Strait. We expected to listen and record earthquakes that can in turn give us information where tectonic plates press against each other, where they diverge and where slide in parallel. Since some of our observations took place in methane-seeping sites, we were hoping to record gas bubbling through the cracks in the seafloor, ultimately ending in hundreds-of-meter-long flares in the water column. And we got all of that. But we also recorded some other noise sources, trivial really. Only the biggest mammals that live in the world currently – the magnificent whales.
It is a tricky business to separate useful signal from the noise. However great and fascinating these creatures are (especially to biologists), they do not bring any information about subsurface that help geoscientists advance their understanding of underwater processes. Their very regular calls are often difficult to distinguish from, let’s say, signals that we interpret to be gas bubbling from the bottom of the sea.
One of the ideas to separate different sources is good, old Fourier transform. By assuming that what we recorded as one signal is really a combination of various frequencies, it is mathematically possible to separate all of them and visualize in the image called spectrum. We can then look at the portion of recording that we have doubts about and see what really it is consisting of.
On this image you can see what we think are separated fin and blue whale calls. While there is a great deal of studies related to recording their songs, each individual can have its own characteristic frequency range, making determination of the species difficult. It is a little easier to recognize fin whales, their regular song is usually a down-swept repeating signal in the frequency of 16-40 Hz that is dominating this image. But if you look closely at the 20 Hz, you can also see repeating, pulsating source, much weaker than the rest of the picture. There is a big chance that this is a call of blue whale, the largest mammal that ever existed.
It is fascinating to observe these calls, recognize different species and even track them using the direction of signal. When they are present in majority of your 10-month-long experiment designed to search for earthquakes and seepage however, they can become a bit of nuisance as well.
Text and illustration by Przemyslaw Domel