Infrasound is low frequency sound. It travels long distances and could therfore be used to monitor a variety of activities. Due to the low value of the speed of sound it is generlly not a warning system. Infrasound is generated by a few animals, phonomena in the nature and by some man made devices. Examples of the first ones are elephants and whales. Phenomena in the nature are earth quakes, thunder, meteorites, avalanches and man made devices and events that generate infrasound are: supersonic missiles and planes, nuclear testing, big fires, slamming doors and many other things.
It is really simple to measure infrasound provided that you have a good microphone. The one we have goes down in frequency to parts of 1 Hz. Various events produce signatures that could be used to identify the event. Spectrograms and wavelet transforms are used for this purpose.
This seminar gives a general introduction to infrasound giving a few examples on how it can or could be used.