Modelling waveforms of infrasound arrivals from impulsive sources using weakly non-linear ray theory

Abstract

Typical infrasound propagation paths extend into the middle and upper atmosphere before turning back to Earth near the stratopause or lower thermosphere. The modelling of infrasound propagation is complicated by several factors. Propagation of sound in the atmosphere becomes increasingly non-linear as the mean density of the atmosphere decreases. As a consequence, infrasound propagation, which can follow paths high into the atmosphere before turning back to earth, is intrinsically non-linear. Further, attenuation of sound also increases as the mean density of the atmosphere decreases. Linear propagation modelling predicts severe attenuation along thermospheric paths, in contradiction with observation. Finally, the currently available atmospheric specifications necessarily involve temporal and spatial interpolations which can lead to the omission of atmospheric fluctuations to which infrasound propagation is sensitive. In this paper, existing methods for modelling weakly non-linear propagation are extended to account for the moving inhomogeneous medium and are then used to study the modelling of waveforms produced by impulsive events. It is found that there is a substantial interplay between signal attenuation and non-linear distortion. Waveform steepening and shocking in the middle and upper atmosphere associated with higher harmonic generation is moderated by attenuation while attenuation in the upper atmosphere is moderated by period lengthening associated with low frequency generation. The modelling of observed data is then considered. Using traveltime data, corrections to the atmospheric specifications are obtained. The predicted waveform evolution, modelled using the developed non-linear propagation model and the corrected atmospheric specifications, matches the observations well.