Balloon-borne infrasound sensor array detects explosions
It’s a fool-proof way to bring down the balloons, the sensors and the data they have collected. On the other hand, longer flights would be useful. During the Arctic summer, the balloons could fly for weeks, but the team also is working on balloons that can stay aloft at night.
For future experiments, Bowman is interested in a balloon design with an insulator on the top surface of the balloon and absorber on the bottom, so it absorbs heat from the Earth to allow it to keep flying at night.
Multiple sensors determine location
The most important aspect of this experiment is that the five balloons formed a 3-D array of sensors, said Albert. One sensor can hear a sound, but cannot provide any location information. Albert said, “My mom is deaf in one ear so it’s hard for her to tell where a sound is coming from.” Having two ears allows animals to determine the source of a sound.
Five microphones in an array, as in this experiment or ground-based sensor arrays, provide the same information — the direction from which the sound wave comes. Researchers coordinate the information from multiple arrays to triangulate the source of the sound.
Calculating where the sound wave is coming from can be a challenge when each sensor in the array is moving relative to each other and the source, said Bowman. A lot of computational algorithms assume stationary sensors, so the team needed to adapt them to include GPS information.
Future use in treaty monitoring and solar system exploration
Bowman has proposed flying balloon-borne infrasound sensors as a part of the next series of the National Nuclear Security Administration’s Source Physics Experimentproject. This project develops new and improved, physics-based approaches for monitoring underground nuclear explosions.
In addition to potential treaty monitoring and national security uses, Bowman and Albert hope to fly hot air balloons in non-terrestrial experiments.
Sandia notes that Bowman is assisting a NASA Jet Propulsion Laboratoryproject to explore the possibility of using balloon-borne infrasound sensors on Venus to listen for Venus-quakes. Venus is similar to Earth in mass, but is geologically very different with no apparent plate tectonics.
Another possibility the team is exploring is flying infrasound sensors on Jupiter. Jupiter is a gas giant with open scientific questions about its internal structure and geology that infrasound could help answer. “We’re still decades out from an actual mission,” said Bowman. “But I’m excited to see how far it will go.”
The results from Bowman’s prior research test flying individual infrasound sensors on balloons were published in Geophysical Research Lettersand more recently in Journal of Geophysical Research: Atmospheres.
Bowman said, “This is a really exciting new area of research. Balloon-borne infrasound sensors will never replace ground-based acoustic arrays, but I think it can augment them. And the most exciting thing is flying in the atmospheres of other planets and what we can learn from them.”
— Read more in Daniel C. Bowman and Jonathan M. Lees, “Infrasound in the middle stratosphere measured with a free-flying acoustic array,” Geophysical Research Letters42, no. 22 (28 November 2015) (DOI: 10.1002/2015GL066570)
