An array of listening devices deployed by researchers at Scripps Institution of Oceanography is one of the first stations in an important new global network that will detect signals from events as diverse as secret nuclear weapons tests, volcanic eruptions, and hurricanes in early formation.
The Scripps array consists of eight microbarometers spread across two kilometres at the Cecil and Ida Green Pinon Flat Observatory, located in the mountains south of Palm Springs.
Each device is equipped with a noise reduction system that filters unwanted energy from atmospheric turbulence and increases sensitivity to signals at the ‘infrasonic’ scale that fall below the 20 hertz level of human hearing. The array records signals that are too faint, and vary too slowly, to be detected by humans.
The array is one of the first in a planned network of 60 that will play a vital role in efforts to monitor the globe for clandestine nuclear testing blasts.
The infrasonic network tracks the atmosphere as part of a network that combines infrasonic signal tracking with seismic stations that pick up signals from the solid earth, hydroacoustic stations that monitor energy in the oceans, and a radionuclide network that checks the air for radioactive particles.
Michael Hedlin, associate researcher at the Cecil H. and Ida M. Green Institute of Geophysics and Planetary Physics at Scripps, said an infrasonic network is capable of providing data not only from nuclear blasts, but from a variety of natural phenomena that may become useful in scientific research.
This was the case on April 23, when the large meteor crashed into the atmosphere over the Pacific Ocean several hundred miles west of Baja California.
‘If this rock had come into the atmosphere at a slightly different time, it might have exploded not over the Pacific, but over a large metropolitan area,’ said Hedlin. ‘With this global listening network we can develop much better statistics on large meteors and get a better idea of how often these massive objects enter the atmosphere.’
Large explosions send part of their acoustic energy into the audible range, but those signals dissipate rapidly.
They also emit large amounts of energy into the infrasonic range in signals that decay slowly across vast distances.
In addition to meteors, chemical explosions, supersonic aircraft, tornadoes, landslides, earthquakes, and volcanoes generate infrasonic energy.
‘Our colleagues in Japan have learned that minor volcanic eruptions of magma or gas might be missed seismically but produce strong acoustic signals,’ said Hedlin. ‘Seismic and infrasound data taken together give a much fuller account of activity inside the volcano that might be indicative of an impending, significant eruption.’