Wired Science

How the UN uses Infrasound to Monitor Nuclear Testing

Milton Garces is a busy man. When the director of the Infrasound Library at the University of Hawaii is not busy monitoring volcanoes, he's using infrasound to listen for something at least as important: the sounds of nuclear explosions. In 1996, as part of the Comprehensive Nuclear Test Ban Treaty, the United Nations decided to build the International Monitoring System—a global network of scientific stations including 60 infrasound stations—to listen for nuclear explosions and unannounced nuclear tests. Garces is part of the global team working to build these infrasound stations around the world and to provide topnotch software and technology to the UN authority that analyzes the data in real time, searching for hints of suspicious activity.

Infrasound is ideal for tracking atmospheric explosions for several reasons. When someone makes a noise—say, by dropping a glass—air is displaced, producing waves that travel in all directions. When the waves reach a person standing nearby, they are perceived as sound. Generally, the more air that is displaced by an event, the lower the pitch is that can be heard. Infrasound, which comprises sounds so low as to be inaudible to the human ear, is generally produced by events that move large volumes of air in the atmosphere—such as nuclear explosions.

And just as the bass notes from your neighbor's stereo are easier to hear than the treble, the low-grumblings of infrasound travel farther and clearer than sounds at higher pitches. Garces explains that this is because with low sounds, the air is being moved more slowly than with higher sounds, so there is less of a chance for the waves to dissipate into heat. "You keep [the energy] in the sound rather than turning it into heat," he says. As a result, very low sounds travel far and can be detected by microphones even thousands of miles away.

But infrasound is only good at tracking explosions in the atmosphere, so in order to listen for activity below the ground or underwater, other forms of data are necessary, too. In addition to infrasound, the UN's International Monitoring System relies on seismic data, which tracks events occurring deep underground; hydroacoustic data, which monitors underwater events; and radionucleotide data, which checks for radioactive elements released into the atmosphere. Together, the four types of data, which will be collected at 337 monitoring facilities worldwide, provide detailed information about suspicious nuclear activity occurring anywhere on or inside the globe.

The Global Infrasound Network being built by Garces and his colleagues as part of the UN's International Monitoring System will eventually consist of 60 infrasound stations, each containing arrays of at least four microphones. By comparing the time at which a particular signal reaches each of the arrays, the scientists can locate the origin of the noise. "If we have a big explosion or big event, we point a whole bunch of our arrays in that general direction and say, this is where it happened, " Garces explains.

The difficult part lies in identifying an infrasound signal and where it is coming from. "Part of the problem is that infrasound travels so far," explains Henry Bass, a physicist at the University of Mississippi who is also involved in the Global Infrasound Network. "We could be looking at a very, very strong event 10,000 miles away, or a very, very weak event a few kilometers away." The scientists use pattern recognition software to decipher what's happening, much like the way our brains can tell the difference between the sound of a motorcycle and that of a crying baby—but the technology is not yet foolproof, so scientists are continually working to improve it.

The responsibility for correctly identifying a potential nuclear explosion doesn't, however, rest on the shoulders of scientists like Garces and Bass. Real-time monitoring and analysis is instead left to the central UN monitoring authority in Vienna. Still, academic scientists have access to the data and can use it to look into events more closely after the fact. "We spend our time and make more of a forensic analysis," Garces says. For example, when the Columbia space shuttle exploded upon re-entry in 2003, "We were able to monitor that, and I think that we were able to provide the investigation team with valuable data that could help direct the searches for debris," Bass explains.

More than half of the infrasound stations have been completed. The stations that are left to be built, however, are the difficult ones—either for geographical reasons, in that their locations are difficult to reach, or due to political or environmental concerns. For example, the station planned for Midway Island in the North Pacific Ocean has just been named as a bird sanctuary, and the Fish & Wildlife Department is concerned that an infrasound detector could disrupt the animals, Garces explains. But assuming such problems can be solved, the network should be completed within the next three or four years.

Garces and Bass are excited to use the network to not only keep the world safe, but also to advance scientific exploration. By providing a way to listen to Earth's "hidden" sounds, infrasound gives scientists a new tool for probing the planet's secrets. "We really have just begun to scrape the tip of this iceberg," says Bass. "We don't know what we're going to uncover once we begin to go down deeper into it, but it looks very, very promising."