Hawaii’s Infrasound Laboratory
The Infrasound Laboratory, Hawaii Institute of Geophysics and Planetology (University of Hawaii) operates and maintains IS59, or KONA, a four-element infrasound array that is part of the International Monitoring System (IMS) of the Comprehensive Nuclear Test Ban Treaty (CTBT). Because of its unique location on the Big Island of Hawaii, the KONA array has exceptionally low ambient noise levels and acoustic detection thresholds.
“The difference between the present monitoring system and previous systems is that we have an unprecedented quality of recordings and density of global coverage,” says Dr. Milton Garces, director of the Infrasound Laboratory. “We can observe phenomena with much finer spatial resolution. At Kona, we routinely detect aircraft, surf breaking and local earthquakes.”
Infrasounds have frequencies below the 20 cycles per second threshold of the human ear. Because they have large wavelengths, they wrap around mountains, bounce against earth and sea, and are turned by strong winds, propagating for hundreds of thousands of kilometers.
The IMS is designed to detect nuclear blasts, according to Garces. He and his colleagues haven’t seen these events with the IMS. However, they’ve seen meteors exploding with the equivalent energy of several kilotons of TNT, comparable to the Hiroshima blast in Japan in 1945.
Sensitive microphones in North America detected low-frequency sound associated with the hypersonic re-entry and disintegration of the Columbia shuttle disaster on February 1, 2003. The infrasonic observations helped eliminate hypothesis suggesting possible impacts from meteorites and lightning, and support NASA’s working hypothesis that the Columbia was destroyed by structural failure. Comparison of infrasonic data from previous re-entries also suggests that the Columbia’s aerodynamics were stable until the final tragic moments.
Garces was part of a task team formed by the DoD to assist with analyzing and interpreting the wealth of infrasonic data associated with the shuttle disaster, as well as other re-entries with a similar orbital inclination. “The complete approach path from California to Texas was recorded acoustically, and clear warnings of imminent disaster were not evident from the complex sounds recorded by a network of acoustic stations in North America,” Garces says.
According to Garces, this suggests that the thermal instability and possible shedding of small fragments of the shuttle did not significantly alter the flight dynamics during the re-entry, and thus its catastrophic structural failure probably occurred rapidly.
Had a meteor or lightning discharge created significant damage to the shuttle, Garces and his colleagues would have observed associated infrasonic signatures. They looked carefully at signals that may have corresponded to the offshore approach, and did not find anything substantial.
Forensic studies of such energetic large-scale processes have only recently been made possible by a global network of infrasound stations that monitor atmospheric sound. Coverage over the United States is provided by the North American Infrasound Network. Stations in Hawaii, Alaska, California and Canada are part of the IMS, and stations in Nevada, New Mexico, Wyoming, Utah and Texas are operated by the Department of Energy.
All infrasound stations are composed of multiple sensors that permit an estimate of the arrival and propagation speed of a signal across the array.
Infrasound technology has many other potential applications. As in the 2001 Arequipa earthquake in Chile, strong ground displacements sometimes shake mountains and generate infrasound, according to Garces. Civil disaster management agencies can benefit from the IMS, and specifically the infrasound component’s capability for landslide, avalanche and tsunami detection. If sources are close to the ground, as for volcanoes or earthquakes, infrasound and seismology make a good marriage. For some volcanoes, the IMS may be used to forecast and monitor eruptions.
“There are several types of studies that can be furthered with the use of infrasound,” says Dr. Henry Bass, director of the National Center for Physical Acoustics (NCPA), University of Mississippi. “Members of our team are interested in volcanoes. During the evolution of the eruption and disturbance, there are rumblings, some of which take place at low frequencies. We bring another tool to study volcanoes. This might be of value in providing an early warning of ash release from a volcano. Aircraft were almost lost due to ash release.”
Scientists at Los Alamos are looking at the distribution of bolides, where they are likely to hit, and the probability of a populated area being hit by a bolide, according to Bass. These are important issues if Bass and his colleagues want to look at ways of destroying them before they become a threat. Infrasound provides an effective and relatively inexpensive tool for monitoring bolides. Without infrasound, some bolide impacts may go undetected.
Data from the global IMS infrasound system can be used to study the properties of bolides in detail. Infrasound data that provides timely detection of catastrophic volcanic eruptions can be used to provide an early warning of such events and could assist with the organization of disaster relief, especially in remote areas.
Scientists at the University of Alaska study the Aurora Borealis, which enables them to map the magnetic fields in the upper atmosphere. The IMS Infrasound Stations provide a new data set for studying these issues. For the first time, scientists are listening to the sounds of the planet and the atmosphere.
This will significantly enhance our understanding of the dynamic structure of the stratosphere and lower thermosphere. Tomographic studies will contribute to both a global model for the average temperature and wind profiles, and also to detailed models for the short term variability of the upper atmospheric winds and temperature.
More information about the IMS can be found at http://www.isla.hawaii.edu.
