Use balloons to hear distant explosions

Balloon-mounted sensors can detect more visible sound waves from eruptions, volcanic eruptions, and earthquakes than commonly used ground-based sensors.

Written by Elizabeth Nadine, Ph.D., University of Alaska Fairbanks

Quote: Nadine, E, 2022, Airborne Acoustic Detection Limits Test, Temblor, http://doi.org/10.32858/temblor.253

Our planet is constantly making noise. From man-made eruptions to the constant movement of the oceans, these noises travel as waves in all directions. It turns out that altitude balloons — those twinkling silvery orbs that probe the atmosphere — can pick up sound waves from this noise. Emerging research presented today at the annual meeting of the American Seismological Society shows how far waves can be detected in the air. The results herald a distant survey of land and ocean activity on our planet, as well as on other planets with inhospitable surfaces.

The infrared sensor moves under the balloon. Credit: Beth Dzenitis

All sound waves are pressure waves, which result in vibrations when the pressure front moves through a solid, liquid, or gaseous substance. Sound waves are a special type of sound wave that people can hear; It lies in the range of 20-20000 hertz (Hz) because that is the number of waves that pass a fixed point per second. But there are a lot of signals transmitted below this frequency. Low-frequency sound waves, known as infrasound, have proven crucial for studying Earth surface phenomena from afar. Volcanoes, earthquakes, and even ocean waves emit ultrasound waves that are recorded using ground-based sensors.

Although still in the development stage, balloon-mounted sensors are proving to be more sensitive to explosions than ground-based sensors. Once the sound wave leaves the site of the explosion, it is distorted by the terrain it encounters along its path. There is no such burden in the air. Balloons passively float on the wind rather than being driven by a motor, reducing background noise. “We can record objects at a farther away and more clearly,” says Daniel Bowman, chief scientist at Sandia National Laboratories and lead author of the study. “This allows us to listen to things we might normally miss, such as earthquakes and the series of experiments in this study.”

Balloon carrying two ultrasonic sensors. Credit: Daniel Bowman

To test the sensor’s capabilities, Bowman and his colleagues flew a balloon over a series of subterranean chemical explosions at the Nevada National Security Site. At 1.25 miles (21 kilometers) above and 35 miles (56 kilometers) from the test site, the balloon’s record was dim. However, the waveform it recorded matches the signal recorded at a station on Earth only a third of a mile (500 metres) away. And on a sensor 28 miles (45 kilometers) away on Earth, the signal was absent.

says David Fee, a research professor at the Geophysical Institute at the University of Alaska Fairbanks who specializes in ultrasound detection.

Explore other worlds

Bowman says he’s excited about the capabilities of balloon-based detectors. “The body of work we’re building has implications for detecting volcanic and seismic activity on other worlds.” The surface of Venus is so hot that previous landings only lasted a few hours. Although the properties of the atmospheres of different planets differ, Bowman says that this test gives him great optimism about their capabilities.

“Every time we fly, we feel like we’re seeing something new,” Bowman says.

The field of airborne acoustic wave detection is still in its infancy; It was taken back in 2014 after a 50-year hiatus. Balloons currently have limited flight time, but Bowman says he’s seeing a new trend in flights lasting months or years, absorbing a wealth of information that can help us monitor all surface motions, from ash clouds to ocean waves.

references

Bowman, D.C. (2021). Airborne ultrasound causes scattering. Geophysical Research Letters, 48 ​​(23), e2021GL096326.

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