Listening to the Heartbeat of the World
If a tree falls near the University of Alaska, Buck Wilson hears it. His microphones, planted in the forest, record volcanic eruptions from around the world and meteors hurling into the atmosphere. In 1980, he detected a nuclear bomb all the way from China.
Wilson, a professor emeritus at the Geophysical Institute, studies sounds too low for the human ear. He and John Olson, a professor of physics at the Geophysical Institute, recently gave me a tour of a microphone array they use to catch inaudible signals from volcanoes, the aurora, winds over distant mountains, and manmade disturbances in the air.
Through willows and birch, we walked on a wooded trail near the University of Alaska in Fairbanks. There, strung out like spokes on a bicycle rim, were plastic tubes connected at the hub by a sensitive microphone enclosed in a plywood shelter. The system is one of three types of microphone arrays at the site designed to catch infrasound waves like a spider catches flies.
Infrasound waves are sound waves with a frequency below 20 Hertz, which is about the lowest frequency detectable by the human ear. Like waves from a rock thrown in a lake, infrasonic signals move around the planet in concentric circles. They are big, slow, and long lasting. The infrasound wave from the nuclear explosion in China took more than six hours to get to Fairbanks. The wave registered again 37 hours later as it completed another lap around Earth.
Detection of above-ground nuclear blasts is a common use of infrasound arrays, 60 of which will eventually be set up around the world as part of the Comprehensive Nuclear Test Ban Treaty. World leaders who signed the treaty agreed to ban the testing of nuclear bombs in the atmosphere. When three infrasound microphones are set up in a triangular array, each about one kilometer from the other, researchers can detect where and when a bomb has exploded. Bombs exploded underground or below the sea can be monitored by methods other than infrasound arrays.
Wilson and Olson aren't in the business of busting treaty violators or providing feedback to bomb builders. Their purpose in setting up microphone systems near Fairbanks is to see what combinations of equipment work best. They use pipes made of metal, rigid plastic, and permeable water hose to capture tiny changes in air pressure that can signify disturbances in the atmosphere around the planet. From those tubes, Wilson and Olson can detect signals as variable as a moose walking in the woods or winter storms churning in the Gulf of Alaska.
The signals from ocean storms, called microbaroms, are particularly troubling to the Department of Defense. Microbaroms befuddle electronic equipment because their frequency is close to that of nuclear explosions. Microbaroms are so prevalent in winter that a graduate student working with Wilson and Olson was able to track the movement of a storm in the Gulf of Alaska from Fairbanks.
Right now, Wilson and Olson are cataloging infrasound waves from natural events, such as erupting volcanoes or whirling auroras that stir up the atmosphere. The information they gather will help others filter out natural sounds when searching for the signals from nuclear blasts. In the future, Wilson and Olson hope to set up two groups of microphone arrays in Antarctica. Until then, they'll continue listening to global vibrations from a little patch of woods near Fairbanks, where they hope no trees fall on their microphones.
