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Recording infrasound on land is not particularly complicated; you can place sensors almost anywhere. This is not the case in the oceans of the Southern Hemisphere: the sensors can only be placed on mostly small, solitary islands, so the coverage is poor.
And, says Dan Ouden, in the open ocean, “the great chaos of the waves” makes a lot of unwanted noise. Part of this irritating infrasound occurs when waves interact on the surface of the sea. “The ocean starts to go up and down in rhythm,” says Day Ouden. The sea acts like a giant speaker, emitting energy into the atmosphere, which moves up and through the water to land, like an invisible tidal wave. The other ocean infrasound is less problematic, but more mysterious: the movement of the sea causes atmospheric vibrations that are emitted straight up. But these waves have proved so difficult to detect that their existence has long been an open question.
This collection of infrasonic waves, which are technically known as microbaromas, is called the “voice of the sea”. Most researchers want to silence it. “We’re trying to get rid of the microbar signal because we’re interested in explosions,” Iezi said.
Ideally, offshore infrasound detectors could not only fill a huge gap in the coating, but also document the microbaromas well enough to be effectively removed with filtering software. But where would you put these detectors? The boats would not work. “The problem with them is that they move up and down all the time,” says Lamb, “and that would confuse the record.” Balloons have been used to record infrasound on land, but their flights over the sea would be too unpredictable to be useful. (However, they would be useful for recording lightning, earthquakes, and volcanic eruptions on Venus, because the surface of the Earth’s evil twin is so hot that any instruments placed on Earth there would quickly melt. Or, at the very least, overheating.)
The open ocean is “an extremely challenging place to record sound,” says Bowman, “so challenging, in fact, that if you asked me before looking at this document, I would say it’s essentially impossible.”
As it turns out, Samantha Patrick, a seabird ecologist at the University of Liverpool, was curious about the ability of seabirds to move with the help of infrasound. After a conversation with Day Ouden and his colleagues focused on weather and geophysics, they came up with a surprising idea: Why not attach microbar detectors to birds? And not all birds: wandering albatrosses. Their wingspan, which can be 11 feet long, is longer than a tall person. This allows them to spend considerable time simply floating in air currents over open waters, something that saves energy as they embark on food-seeking journeys. Not only do they fly over huge stretches of isolated ocean, but they also don’t dive into the water, so all the sensors attached to them won’t get particularly wet.
In a short time, the researchers built insignificant infrasonic sensors and put them in bags – packages no heavier than a TV remote control. As fun as it is to imagine these bags being dragged the way a school kid carries a backpack, it would be unnecessarily complicated. Instead, the bags were simply taped to the backs of the bird’s assistants.
Last year, the team headed to the Croze Islands, small plots of land in French sub-Antarctica where wandering albatrosses like to nest. But how, please, make the albatrosses cooperate? With a very special kind of hug, obviously – one that prevents any potentially injurious cracking and biting. “They don’t really have predators – there are certainly no natural predators,” said Patrick, who helped the team with their research. “So you literally just approach him and then you put your hand on his bill and then you have to hug him because he’s so big. You hug it and lift it from the nest, then one holds it, and then the other sticks the tree with tape on its back.
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