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Australian Government - Department of Foreign Affairs and Trade

Advancing the interests of Australia and Australians internationally

Australian Government - Department of Foreign Affairs and Trade

Advancing the interests of Australia and Australians internationally

Australian Safeguards and Non-Proliferation Office

Annual Report 2000-2001

Infrasound: The CTBTS International Monitoring System

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.

Department of Foreign Affairs and Trade