Acoustic waves with very low frequencies are called infrasound. In fact, these waves are below the frequency band audible to the human ear, which typically ranges from 20 to 20,000 Hertz. Infrasound is produced by a variety of natural and man-made sources: exploding volcanoes, earthquakes, meteors, storms and auroras in the natural world; nuclear, mining and large chemical explosions, as well as aircraft and rocket launches in the man-made arena.
The first observation of naturally occurring infrasound that was ever recorded using instruments was in the aftermath of the 1883 eruption of the Krakatoa volcano in Indonesia. Infrasonic waves that circled the globe at least seven times shattered windows hundreds of miles away, and were recorded worldwide.Infrasonic waves cause minute changes in the atmospheric pressure which are measured by microbarometers. Infrasound has the ability to cover long distances with little dissipation, which is why infrasound monitoring is a useful technique for detecting atmospheric nuclear explosions.
The adoption of the CTBT had a revitalizing effect on infrasound research. The ban on atmospheric nuclear tests brought about by the Partial Test Ban Treaty (PTBT) in 1963 somewhat diminished the interest in infrasound science and technology. The construction of infrasound monitoring stations as part of the CTBTO’s global alarm system to monitor the earth for nuclear explosions has contributed to a revival of scientific interest in this technology.
Acoustic waves with very low frequencies are called infrasound. These waves are below the frequency band audible to the human ear.
Infrasound technology has considerable potential for civil and scientific applications, not least in disaster prevention or mitigation.
For information on what scientific and civil applications infrasound can be used for, please click here.
Infrasound monitoring is one of the four technologies used by the International Monitoring System (IMS) to verify compliance with the CTBT. Atmospheric and shallow underground nuclear explosions can generate infrasound waves that may be detected by the infrasound network.
The construction of infrasound monitoring stations as part of the CTBTO’s global alarm system to monitor the Earth for nuclear explosions has contributed to a revival of scientific interest in infrasound technology.
By focusing mainly on the detection of a suspected nuclear explosion in the atmosphere, infrasound technology helps to identify the location of the explosion. The ability to provide such information enhances the potential for a successful on-site inspection in case of atmospheric nuclear explosions.
In addition, since underground nuclear explosions also generate infrasound waves, the synergetic use of both the infrasound and the seismic technologies allows for better information gathering and analysis of possible underground tests.
Building the station
In line with other types of monitoring stations , the first step entails conducting a site survey. It is imperative to select locations with the lowest possible background noise level in order to improve the signal’s reception. Stations are therefore built well away from natural sources of acoustic noise like coastal and windy areas, and from man-made sources of acoustic noise like airports, major highways, industrial centres and other types of human settlements.
Infrasound stations are built at locations with the lowest possible background noise in order to improve the signal’s reception. Stations are built well away from natural and man-made sources of acoustic noise.
Although stations exist in a wide variety of environments ranging from equatorial rainforests to remote wind-swept islands and arctic ice shelves, an ideal site for deploying an infrasound station is within a dense forest, where it is protected from prevailing winds.
The next step is to prepare the site. A contractor is chosen through a selection process to design and build the station. Following installation, the infrasonic array, including the communications equipment and the power equipment, are all tested before the station is certified as meeting all of the CTBTO’s minimum requirements.
IMS stations must ensure that data received at the IDC are authentic. This is achieved through a special digital “signature” embedded in the data flow from each station. Tamper-detection devices are placed on enclosures for station equipment to discourage tampering with the hardware.
IMS stations must ensure that data received at the IDC are authentic. This is achieved through digital “signatures” in the data flow and by tamper-detection devices at the station.
Finally, once the station is certified, operation and maintenance agreements are concluded between the CTBTO and a station operator to ensure the continuous functioning of the station.
How the monitoring network works
All 60 infrasound stations in the IMS network employ infrasound array systems. Each array contains four or more array elements arranged in different geometric patterns, a meteorological station, a central processing facility and a communication system for the transmission of data.
All 60 infrasound stations in the IMS network employ infrasound array systems. Arrays are built to achieve a higher signal-to-noise ratio, i.e. a better identification of a signal against the surrounding noise.
A greater number of array elements results in a higher signal-to-noise ratio, which means that the signal can be identified better against the surrounding noise. Stations with a larger number of array elements are usually built in areas that are exposed to strong winds.
Each array element includes a wind noise reducing system made up of several pipe structures connecting inlet ports to a summing chamber or manifold. Infrasound waves arriving at the ports pass through the pipes to the manifold, which is connected to a microbarometer located in the centre of the element. The microbarometer measures changes in the air’s micropressure that are produced by infrasonic waves.
This set-up reduces wind-generated noise at certain frequencies that is considered a disturbance. By filtering out the majority of the wind-generated disturbances, the system can better receive the infrasound waves associated with the actual signals which are measured for monitoring purposes.
Research is being carried out to develop a new type of more efficient wind noise reducing system.
Research is being carried out at the Australian National University to develop a new type of wind noise reducing system that reduces noise levels by a factor of ten or more. Such a breakthrough might significantly increase the sensitivity of the infrasound network.
To read more about this research, read this Spectrum article (PDF).
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