Infrasound monitoring in the Netherlands

The Royal Netherlands Meteorological Institute (KNMI) monitors infrasound in the Netherlands with a series of
microbarometer arrays. Infrasound is inaudible sound; its lower frequency cut-off is limited by the thickness of
the atmosphere. The KNMI has developed a microbarometer capable of measuring infrasound between 500
seconds and 40 Hz. Wind noise reduction is of major concern in infrasound measurements. Arrays are deployed
to increase the signal-to-noise ratio (snr) by averaging out the incoherent wind signals. Wind noise is also
reduced at each array element, the microbarometer, by applying eg. porous hoses or pipe arrays. Doing so, the
wind noise is further reduced at each element by sampling the atmosphere over an area rather than at one point.
The direction of arrival and apparent sound speed can also be determined by the array to characterize the
signals. Multiple arrays are used for localization of the source. KNMI operates four infrasound arrays varying in
aperture between 35 and 1500 meters while 6 to 16 elements are used. Sources of infrasound are: explosions,
severe weather, meteors, sonic booms, sea waves, volcanoes, nuclear tests, .. Coherent infrasound of unknown
origin is also detected at the arrays. The motivation for measuring infrasound is twofold. Firstly, as Seismology
Division one has to discriminate between sources causing vibrations from the solid earth, earthquakes, and from
the atmosphere, eg. sonic booms. Secondly, infrasound is applied to verify the Comprehensive Nuclear-Test-
Ban Treaty (CTBT). Worldwide 60 arrays are currently constructed to monitor the atmosphere for clandestine
nuclear tests. Infrasound can travel over large distances with minimal damping due to its low frequency
contents.
This paper describes the technique of infrasound measurements and the use of array processing techniques, like
beamforming, and detection algorithms. These subjects will be exemplified with infrasound from a sub sonic air
plane.