Thanks to free-flying albatrosses with special sensors on the back, geophysicists from TU Delft and KNMI measured the entire spectrum of infrared above sea level. This enables KNMI to monitor compliance with the Nuclear Non-Proliferation Treaty, which prohibits nuclear testing anywhere on Earth. Such tests produce infrasound, but the oceans also complicate the measurements. The study, a major international collaboration with biologists and statisticians, was completed by the end of August Published by In Geophysical Research Letters.

Infrasound is inaudible to the human ear and has a frequency of less than 20 Hz (20 vibrations per second). Olivier Dennden, a geophysicist at KNMI, one of the researchers, says that such low noise occurs naturally in the ocean as it moves up and down from the surface of the ocean. “The air above the surface of the water moves with it, which means there is a lot of air movement. This vibration has a low frequency but can echo over long distances.

Nuclear tests

Infrasound is also generated by nuclear experiments such as volcanic eruptions and earthquakes. Measuring this noise is an important way to monitor these events. Infrasound of the ocean is an obstacle in this regard. Den Dunn: “A volcanic eruption produces a relatively small infrared signal.

To filter out this ocean sound, you need to know exactly how it sounds. The ocean produces two types of infrasound. A signal emerges from the collision of surface waves, which does not carry that far and cannot be measured over large distances. Another type of infrasound is the echo of waves across the water column, which vibrates the ocean like a large speaker. This sound can travel long distances and be detected on land through the measuring stations used by KNMI to examine the Nuclear Non-Proliferation Treaty. The first transient infrasound signal has not been measured before.

Thanks to the Albatrosses now. During the breeding season, large seabirds sometimes fly continuously over the ocean for weeks in search of food. In this way, they can cover a speed of 10,000 km / h. To investigate these flights and measure infrared over the ocean, KNMI developed a sensor the size of a credit card that can be attached to dorsal springs.

Sound recording

The sensors allowed the researchers to make infrasound measurements at an altitude of five meters above sea level in the South Seas. Together, the sensor-mounted albatrosses covered a distance of 42,184 km. After returning, the sensors read.

Using the measurement data, the researchers mapped the entire soundtrack of the ocean. In this way, nuclear tests, earthquakes and volcanic eruptions can be detected and more reliably detected. But albatross measurements may also contribute to animal knowledge. The bird has unprecedented navigation capabilities: even after flying thousands of kilometers over the ocean, the animal returns home in a straight line. How he does it is still unknown.

One possibility is that Albatross is now using measured infrasound. “It’s not a rule of thumb, but it’s a very popular theory,” said David Lenting, a zoologist and aviation technologist at the University of Groningen who was not involved in the research. “It is not yet known whether these birds can hear the infrasound.”