Non-linear infrasound signal distortion and yield estimation from stratospheric and thermospheric arrivals

Abstract

The propagation of sound through gases and fluids is intrinsically non-linear. The degree of non-linearity increases as the density of the propagation medium decreases. As a consequence, signals traveling through the upper atmosphere undergo severe non-linear distortion. This distortion takes two forms: waveform steepening and pulse stretching. These nonlinear effects are numerically investigated using non-linear ray theory. On one hand, waveform steepening is generally associated with stratospheric arrivals with sufficiently large amplitude for which the decreased density of the atmosphere causes shock fronts to form. On the other hand, pulse stretching is generally associated with thermospheric arrivals where severe attenuation prevents significant shock formation but severe non-linearity causes significant pulse stretching. Since non-linear effects increase with increasing signal amplitude, it is possible to use non-linear distortion to estimate signal source strength. Uncertainties in propagation path and in thermospheric attenuation will limit the accuracy of this approach. Using the non-linear propagation model, the fundamental limits of this approach are also investigated. Comparisons with available data will be made.