Page 11 - January 2006

INFRASOUND
Henry E. Bass
Jamie L. Whitten National Center for Physical Acoustics, The University of Mississippi University, Mississippi 38677
Joydeep Bhattacharyya
BBN Technologies Arlington, Virginia 22209
Milton A. Garcés
Infrasound Laboratory, University of Hawai'i at Ma¯noa Kailua Kona, Hawaii 96740
Michael Hedlin
Scripps Institution of Oceanography, University of California, San Diego La Jolla, California 92093
John V. Olson
Geophysical Institute, University of Alaska Fairbanks, Alaska 99775
Robert L. Woodward
Science Applications International Corporation Vienna, Virginia 22182
Rebirth of infrasound
Anew global network of micro- barometers is breathing life into a once dormant branch of acoustics: the study of infrasound. The nascent study of sub-audible sounds is shedding new light on a great variety of man-made and natural phenomena in the atmosphere, including Mount Saint Helens, pic- tured on the front cover of this issue. The re-emergence of this field is due largely to the Comprehensive Nuclear- Test-Ban Treaty (CTBT), which was opened for signatures in New York City in September of 1996. The CTBT held the promise of limiting the spread of nuclear weapons. In the process, the treaty created the International Monitoring System (IMS). The International Monitoring System consists of seismic, hydroa- coustic, radionuclide, and sixty infra- sound stations spread over the globe to provide almost uniform coverage (Fig. 1). Never before has there been a global system designed to listen for acoustic waves in the atmosphere con- nected via modern communications to allow for real time monitoring and correlation of signals. The International Monitoring System sta- tions employ modern electronics that allow for real time digitization of sig- nals and archiving signals from the
Fig. 1. Map showing the existing locations (blue circles) and planned locations (clear circles) of the 60-station IMS infrasound network.
global constellation of stations at a single location.
Because the absorption of sound in the atmosphere decreases with fre- quency, the low-frequency infrasound band holds the promise of signal detec- tion over long distances. One of the earliest reports of infrasound propaga- tion over long distances was from the cataclysmic 1883 explosion of Krakatoa. Barometric records of the explosion observed throughout the US, Europe, Russia, and reports of cannon- like sounds in surrounding islands (as far as Diego Garcia and Rodrigues Islands) demonstrated for the first time the ability of low-frequency sound to
propagate for thousands of kilometers. Preliminary studies of data from the International Monitoring System reveal a large number of signals detectable because of the low noise characteristics of modern stations and atmospheric conditions that often allow infrasound to travel great distances with little loss in amplitude. Interpretation of these signals challenges even the most advanced propagation and atmospher- ic models.
Infrasound stations and data analysis
The heart of a modern infrasound station is the highly sensitive micro-
Infrasound 9