Plastic sheets similar to those used for packing tape, garbage bags, some string, a white shoebox-size box and a little charcoal dust are more than odds and ends. These are the supplies needed by Danny Bowman, a Sandia National Laboratories geophysicist, for building a solar-powered hot air balloon in order to detect infrasound.
Infrasound is sound of extremely low frequencies, below 20 hertz, which is considered to be lower than what humans can hear. African elephants generate infrasound for long-distance communication at almost 15 hertz. For comparison, a bumblebee’s buzz is usually 150 hertz and humans hear in the range of 20 to 20,000 hertz.
In July 2017, a fleet of five solar-powered balloons reached a height of 13 to 15 miles, considered to be twice as high as commercial jets, and discovered the infrasound from a test explosion. This experiment received financial support from Sandia’s Laboratory Directed Research and Development program. The results were presented by Bowman at the American Geophysical Union conference in December. These results will soon be published.
Infrasound is considered to be important as it is one of the verification technologies the U.S. and the international community employ for monitoring explosions, including those brought about by nuclear tests. Infrasound is traditionally detected by ground-based sensor arrays, which do not cover the open ocean and can further be muddled by several other noises, such as the wind. Bowman said air conditioners are also considered to be a common source of infrasound noise.
“The stratosphere is much less noisy so you can detect events of interest to science and national security from greater distances,” said Bowman. The stratosphere is the atmospheric layer from about 5 miles to 31 miles above the ground.
Inexpensive hot air balloons fly all day
It takes three hours for Bowman and fellow geophysicist Sarah Albert to make a solar-powered hot air balloon, and this balloon uses almost $50 worth of materials, excluding the reusable infrasound sensor or GPS tracker. The charcoal dust enables heating up the air inside the balloon, offering lift, without needing helium gas, a nonrenewable resource.
It is also possible to launch the balloons on partly cloudy days, said Albert. They remain up in the stratosphere all day and then come down after the setting of the sun. This “guaranteed termination mechanism” is both a pro and con, said Bowman.
It is a fool-proof way to bring down the balloons, the data they have collected and the sensors. On the other hand, longer flights would be beneficial. During the Arctic summer, the balloons will be able to fly for weeks, but the team also is working on those balloons that can stay aloft at night.
For future experiments, Bowman is concentrating on a balloon design along with an insulator on the top surface of the balloon and absorber place at the bottom, such that it absorbs heat from the Earth in order to allow it to keep flying at night.
Multiple sensors determine location
The most significant aspect of this experiment is that the five balloons developed a 3D array of sensors, said Albert. One sensor is capable of hearing a sound, but cannot provide any information related to location. 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 enables animals to define the source of a sound.
Five microphones in an array, as in ground-based sensor arrays or in this experiment, provide the same information — the direction from which the sound wave generates. Researchers synchronize the information from multiple arrays in order to triangulate the source of the sound.
Calculating where the sound wave is generating from can be challenging when each sensor in the array moves relative to each other and the source, explained Bowman. A number of computational algorithms assume stationary sensors, so the team thus had to adapt them to include GPS information.
Future use in treaty monitoring and solar system exploration
Bowman has suggested flying balloon-borne infrasound sensors to be a part of the next series of the National Nuclear Security Administration’s Source Physics Experiment project. This project develops new and enhanced, physics-based approaches suitable for monitoring underground nuclear explosions.
Besides potential treaty monitoring and national security applications, Bowman and Albert expect to fly hot air balloons in non-terrestrial experiments.
Bowman is assisting a NASA Jet Propulsion Laboratory project for exploring the likelihood of employing balloon-borne infrasound sensors on Venus in order to listen for Venus-quakes. In mass, Venus is similar to Earth, but it is geologically extremely different with no apparent plate tectonics.
Another possibility the team is focusing on exploring is flying infrasound sensors on Jupiter. Jupiter is considered to be a gas giant with open scientific questions regarding 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 obtained from Bowman’s previous research test flying separate infrasound sensors on balloons were published in Geophysical Research Letters and very 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.”