Infrasound phenomenology, propagation, and detection

Abstract

Infrasound is generally understood to refer to acoustic disturbances in the atmosphere in frequency bands below the threshold of human hearing, but above the frequencies at which internal gravity waves propagate. The infrasonic band is nominally taken to be 0.05 Hz up to about 20 Hz. Infrasonic signals tend to be generated by large, violent events and propagate efficiently in sound ducts formed by wind jets and temperature gradients in the middle and upper atmosphere. Infrasonic signals can be detected at very large distances from the source, sometimes even globally. This tutorial will present an overview of the generation of infrasound by both natural and anthropomorphic sources and of the subsequent propagation and detection of infrasonic signals. Two particular sources will be discussed in detail: the signal generated by a large explosion and the so-called microbarom signal generated by colliding ocean waves. Signal propagation through the atmosphere will then be discussed. Available open source infrasound propagation packages and their use will be introduced, the significant atmospheric sound ducts will be identified, and the difficulties inherent in modeling propagation through a dynamic atmosphere will be emphasized. The current state and availability of atmospheric specification will be touched upon. Finally, the use of array processing for the extraction of infrasonic signals from the pressure fluctuations inherent to a turbulent atmosphere will be discussed.