Listening to 'singing volcanoes'

Infrasound is detectable from many geophysical events

By Christine McGourty BBC science correspondent in Denver

Infrasound - low-frequency sound beyond the scope of the human ear - is providing scientists with a new way of detecting tornadoes, incoming asteroids and erupting volcanoes.

Some [volcanoes] are operatic. Others have no singing talent whatsoever,

Milton Garces, University of Hawaii

The researchers working in the field reviewed their progress at the American Association for the Advancement of Science (AAAS) annual meeting in Denver.

They have been the beneficiaries of a £20m global infrasound network that was originally intended to help verify the Comprehensive Nuclear Test Ban Treaty.

But atom bombs are not the only things the soon-to-be-completed 60 monitoring stations of the network will listen in on.

There are important spin-offs for scientists who want to use the technology to devise new ways of detecting potentially hazardous, large-scale natural events.

Growing network

Infrasound consists of sound waves in frequencies below about 20 Hertz - out of the range of what humans can pick up.

The waves have enormously long wavelengths, measured in kilometres, and are poorly absorbed. Infrasound waves triggered by a large asteroid impact high in the atmosphere, for example, can be detected travelling as many as five or six times around the globe before dissipating.

Douglas Christie, president of the technical secretariat of the Test Ban Treaty Secretariat in Vienna, Austria, said 15 infrasound monitoring stations were already fully operational and 16 more were under construction.

The aim is to establish stations uniformly across the globe - countries already hosting a "listening" station include Germany, Canada, Brazil, Paraguay, Australia, Madagascar, the USA, Greenland and Antarctica.

"The network will very reliably detect even very small nuclear explosions at any point on the globe," he said. "But we also detect infrasound from a large number of other geophysical events.

"We've detected a large number of exploding meteors - we can also monitor volcanic eruptions very well, even at great distances."

Civil aviation authorities have asked the treaty secretariat to provide data from the infrasound network to help them detect volcanic eruptions in remote areas and provide early warning to pilots of hazardous emissions.

Researchers in Hawaii are using infrasound to help them monitor the behaviour of active volcanoes.

'Singing volcanoes'

Milton Garces, of the University of Hawaii, said each volcano could be said to have its own "voice". "Some are operatic. Others have no singing talent whatsoever," he said.

"We're developing the ability to understand the language of volcanoes, and translate that into something we can use to forecast an eruption."

At the National Oceanic and Atmospheric Administration (Noaa) in Boulder, Colorado, scientists are building a prototype infrasound network to provide early warning of tornadoes.

It is being established in parts of Colorado, Nebraska, Wyoming and Oklahoma.

The infrasound detectors commonly used consist of ultra-sensitive "micro-barometers", picking up changes in pressure associated with the sound waves.

Mr Christie said they were "the most sensitive pressure devices ever made". As you go up in the atmosphere, the pressure goes down, he added.

"Our micro-barometers are so sensitive they can measure the pressure difference between the top and bottom of a single piece of paper."