The infrasound component of the International Monitoring
System (IMS) for the CTBTuses a technology that is more than one hundred years old, but
which is still young in its development. It is also a technology that offers benefits well beyond its
contribution to verification of the test ban.

The explosion of the Krakatoa volcano in 1883 generated a
shockwave that travelled several times around the earth, and each time
registered on barometers around the world. This, and signals from the Tunguska meteor in 1908, stimulated early
scientific interest in atmospheric propagation of very low frequency acoustic
waves referred to as infrasound. More
active research in infrasound monitoringbegan in the late 1940s with the start of nuclear weapon
testing. After this, global networks of
infrasound monitoring stations were established, including in Australia, to listen
for and locate atmospheric nuclear tests.

Acoustic waves are generated by an atmospheric nuclear
explosion across a range of frequencies. Audible sound from a nuclear test attenuates quickly with distance, but
has been reported at distances of up to 250 km. Waves in the range of 0.01 to 10 Hz propagate through the
atmosphere much more efficiently, and enable detection and location of a small
(1 kiloton) test at distances of up to several thousand kilometres. The design of a network of 60 infrasound
monitoring stations for the IMS is based around this capability.

Interest in the Cold War infrasound monitoring networks
declined with the availability of satellite based systems for monitoring the
visible flash and electromagnetic pulse from an atmospheric test. But noting that satellite based monitoring
is beyond the scope of the IMS as currently defined by the CTBT, infrasound
will play a key role in monitoring for an atmospheric test, and in some
circumstances could detect an event missed by satellite systems.

Consequently, the CTBT has reinvigorated research in
infrasound, and modern designs for monitoring stations are considerably more
capable than their Cold War predecessors. IMS infrasound stations have arrays with five to eight elements laid out
at spacings of one to three kilometres. A central microbarometer is attached to an array of pipes perhaps 50
metres in diameter, with mushroom shaped ports open to the atmosphere. This design senses an average pressure
change over the whole area of an element, discriminating between infrasonic
noise from local air movements and larger distant events.

The data from IMS infrasound stations, like those from
seismic and hydroacoustic sensors, have other practical benefits. Early detection of volcanic events (which
may throw ash to high altitudes and endanger aircraft) is one example. Another example of interest to air
travellers is the role of infrasound research in understanding solitary wave
phenomena that create clear-air turbulence.

Australia will host five of the 60 IMS infrasound
stations. One at Warramungain Northern Australia is nearly complete and should be operational
in 2002. Two more (in Western Australia
and Tasmania) should be installed during 2001-02.