Balloons detect initial signs of airborne ‘sound channel’

Sound channels in the atmosphere similar to oceanic channels can be used to monitor volcanic eruptions and bomb explosions . About a kilometer below the surface, there is a channel that transmits whale calls and submarine noises to great distances. Since scientists discovered this sonar location and ranging (SOFAR) channel in the 1940s, they have suspected that a similar channel exists in the atmosphere. But other than a top-secret Cold War operation, few would bother to find it. Now by listening to distant rocket launches using solar-powered balloons, researchers say they’ve found signs of an airborne channel, although it’s not as simple or reliable as ocean SOFAR. If this claim is confirmed, atmospheric SOFAR could pave the way for a network of aerial receivers to help researchers detect long-range eruptions from volcanoes, bombs and other sources of infrasonic (sound waves below the frequency range of human hearing). “It would be useful to have detectors on it,” said William Wilcock, a marine seismologist at the University of Washington in Seattle. Although ground-based seismic sensors pick up most of the largest explosions on Earth, “certain areas of the planet are well-received.” coverage, but not in other areas.”

SOFAR channels in the ocean are bounded by a layer of lighter and warmer water above and a layer of cooler, denser water below. The sound waves are trapped within the channel, travel at their slowest speed at this depth, and are constantly bouncing off the surrounding water layer, like a guard guides a bowling ball forward. Researchers rely on SOFAR channels to detect earthquakes and volcanic eruptions on the ocean floor, and even measure the rise in ocean temperatures caused by global warming. Geophysicist Maurice Ewing began searching for a similar layer in the sky after discovering the SOFAR channel in 1944. At an altitude of 10 to 20 kilometers is the tropopause, which is the boundary between the troposphere, the lowest layer of the atmosphere (where the weather occurs), and the stratosphere. Like ocean SOFAR, the tropopause represents a cold region where sound waves should travel more slowly and farther. Ewing reasoned that acoustic waveguides in the atmosphere would allow the U.S. Air Force to listen in on the nuclear weapons tests detonated by the Soviet Union. He launched a top-secret experiment code-named “Project Mogul” to lift hot air balloons equipped with infrasound microphones into the air.

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