On April 23, 2001, scientists manning a network designed to detect covert nuclear
tests noticed something unusual a very loud sound coming
from above the Pacific Ocean. This global network, consisting of sensitive sound-recording
instruments, had picked up on a large meteor slamming into the atmosphere several
hundred miles west of Baja California, and exploding with a force comparable
to that of the nuclear bomb that was dropped on Hiroshima.
On Feb. 1, 2003, these same instruments heard something else the Space
Shuttle Columbia reentering Earths atmosphere on its tragic final mission.
NASA subsequently used those recordings to rule out potential causes of the
missions demise, including a bolide or missile impact.
The sensitive instruments that recorded the meteors entrance and the end
of the Columbia record a range of low-frequency sound that is inaudible to the
human ear called infrasound. Its sort of like infrared light, which
is the part of the electromagnetic spectrum that we cant see, in that
its the part of the sound spectrum that we cant hear, says
Michael Hedlin, a geophysicist and infrasound specialist at the University of
California, San Diego, and Scripps Institution of Oceanography.
The instruments detect atmospheric noise, such as storms, winds, volcanic eruptions,
ocean waves and airplane traffic, and they help scientists understand just
whats going on out there, Hedlin says. Right now, were
just listening to the world, he says, but soon, the researchers will begin
to more fully comprehend the interactions between solid earth, the oceans and
the atmosphere.
An explosive history
When the Krakatoa volcano erupted in Indonesia in 1883, the eruption was so
explosive that it sent low-frequency sound waves around the world several times.
Scientists noticed the sound waves through air pressure changes measured by
barometers. That was the first recorded instance of infrasound.
Around the turn of the 20th century, scientists recorded a giant meteor exploding
above Russia. During World War II, the instruments recorded aircraft movement
and munitions explosions. During the Cold War, scientists and the U.S. Department
of Defense used the instruments to monitor nuclear bomb tests in the atmosphere.
Then in the late 1960s, the application of infrasound for monitoring nuclear
tests around the world became limited by the development of reliable satellite
technology, which could see atmospheric nuclear explosions and the subsequent
onset of underground testing, which was monitored by seismic data. So
the field sort of went away for 30 years, says Milton Garces, a volcanologist,
oceanographer and infrasound specialist at the University of Hawaii. But
luckily, there were a few individuals who kept the knowledge alive, he
says. And then, in 1996, the world adopted the Comprehensive Test Ban Treaty,
which brought about a renaissance for infrasound, Garces says.
To enforce compliance with the treaty, the United Nations created the International
Monitoring System (IMS), a network of geophysical sensor stations located around
the world that monitor seismic signals, atmospheric radioactive material releases,
hydroacoustic signals and infrasound, Hedlin says (Geotimes, October
2002). Luckily, he says, so far they havent heard any nuclear tests
only a plethora of atmospheric sounds.
At work
The IMS
infrasound network will eventually contain about 60 stations somewhat evenly
distributed around the world, but it is only about one-third complete right
now, Garces says. We hope itll be all up and running in the next
couple of years, he says, but that might be a little optimistic.
Nevertheless, the scientists are already recording more information than they
can comprehend, he says.
Geophysicist Michael Hedlin of the Scripps Institution of Oceanography monitors
an international network of infrasound stations, like this one south of Palm
Springs, Calif. The sensitive instruments that measure low-frequency sound waves
resemble thermoses surrounded by wagon-wheel spokes of PVC piping. Photo by
Fred Greaves, Scripps Institution of Oceanography.
The infrasound instruments (microbarometers) work similarly to basic barometers
that meteorologists use to detect weather patterns through atmospheric pressures.
Resembling large thermos bottles, the microbarometers contain sensitive electronic
instruments that convert atmospheric pressure to an electric signal. The sensors
are connected to white wagon-wheel spokes of PVC piping that filter unimportant
sounds from important sounds, Garces says, and keep animals and debris
away.
The instruments constantly measure ambient atmospheric sound waves and transmit
them to the International Data Center in Vienna, Austria, which keeps all data
related to the test ban treaty. The data then go to a data center in McLean,
Va., and also back to the lab that operates the station, where the scientists
do their own analysis in-house, Garces says.
Truthfully, most of the time we dont know what were listening
to, says Henry Bass, a physicist and infrasound specialist at the University
of Mississippi. We receive hundreds of thousands of signals each year
and we can only tell what a small fraction of them are, he says. But the
more the scientists listen, the better they get at differentiating one sound
from another and picking out the important information from the clutter.
Each station is kind of like your neighborhood, Garces says. You
live there for a year and slowly begin to understand whats going on, all
of the idiosyncracies of your neighbors. You have that chatty neighbor
you train yourself to cut out all the chatter and learn to recognize what is
important. Thats what we do with each station.
After three years of continuous observation, the scientists are now beginning
to see patterns and differentiate the sounds emanating from storm fronts, explosions,
ocean waves, volcanoes and jet airplanes. For example, Garces says, the
ocean is always singing in this deep bass voice. Now we know what its tune is,
so we can pick up on swells and storms.
The researchers have also noticed strong seasonal patterns, Hedlin says, which
has been really exciting. From the station nearest San Diego, for
example, the researchers hear a lot more signals from the west in the winter
and from the east in the summer, which is related to seasonal wind patterns.
In general, stations will pick up atmospheric sounds from hundreds to thousands
of miles away, Hedlin says, but those data are not well understood yet. The
scientists monitoring the transmissions can always tell the direction of the
recording, but the distance is harder to discern. Furthermore, depending on
winds and the strength of a signal, the stations can register waves from around
the world.
Ash warnings
Hedlin, Garces
and Bass work on a number of applications in concert, although each has their
own pet project. Hedlin pays close attention to storms in the Pacific. Garces
concentrates on infrasound emitted from the constantly erupting Hawaiian volcanoes
as well as sounds of the ocean. Bass focuses on using infrasound to create a
3-D image of the internal structures of the atmosphere (tomography). Together,
all three scientists are developing a project that uses infrasound to create
a volcanic ash warning system for airplanes.
To airplane pilots, ash escaping from erupting volcanoes, such as Mount St.
Helens (pictured here), can resemble normal clouds. Scientists can use infrasound
to divert planes around erupting volcanoes. Courtesy of USGS, Cascades Volcano
Observatory.
When volcanoes erupt, they frequently lift large plumes of ash and dust thousands
of feet into the atmosphere. To a pilot, the ash and dust can resemble regular
clouds; however, if a plane flies into that ash plume, the engines will fail.
Especially in volcanically active remote areas over which planes fly,
wed like to work out a system to warn pilots of eruptions to divert them
around the ash clouds, Bass says.
Seismic sensors note any rumbling in volcanoes. But rumbling does not necessarily
mean there is an ash release, Garces says. Currently, scientists work with airline
companies to divert planes around any rumbling volcano, but diversions can be
costly. And much of the time, those are false alarms, Bass says.
If infrasound is in place, the scientists think they can detect the atmospheric
disruption from the ash clouds and give the airlines precise rerouting information.
This project is in its infancy, Bass says, but the scientists hope to have sensors
in place near some active volcanoes to test their theory by the end of this
year. Right now, they are working with the U.S. and Canadian volcanic ash warning
centers to get the project off the ground. It is an exciting application, Garces
says, in which scientists can contribute to society.
All of these projects are fascinating, Garces concludes. We
are just at the christening stage of infrasound, where we were with seismic
technology 30 years ago. By working with the existing seismic, satellite and
other observation networks, this technology has endless possibilities for teaching
us about the world.
![]() |
Geotimes Home | AGI Home | Information Services | Geoscience Education | Public Policy | Programs | Publications | Careers ![]() |