Infrasound propagation in realistic atmosphere using nonlinear ray theory

Abstract

Using ray theory, long range propagation of infrasound through the atmosphere is modeled in the framework of the Comprehensive Nuclear-Test-Ban Treaty. In atmospheric propagation, the high frequency hypothesis is based on the assumption that space and time scales of atmospheric properties (temperature, wind, density) are much larger than acoustic wave scales. An operational 3D nonlinear ray tracing code is developed to compute the temporal pressure signature at receivers. The global pressure signature at the receiver is the sum of eigenray contributions that link the source to the receiver. They are obtained by solving a generalized Burgers' equation along each eigenray taking into account nonlinear effects, shear and bulk viscosity absorption and molecular vibrational relaxation mechanisms. This equation is solved using a Fourier Galerkin spectral scheme. Specific developments are performed to pass through caustics and take into account ground reflection. The propagation of infrasound emitted by a motionless point source in a realistic atmosphere will illustrate the analysis. To quantify the validity limits of our approach, we investigate effects of the wind, atmospheric absorption, nonlinearities, refraction and scattering by small atmospheric scales on observed phase kinds, their travel time and their waveform. To estimate the nonlinearity effects relative to the linear dissipative effects we evaluate the Gol'dberg number. We note that nonlinear mechanisms are important to model the evolution of infrasonic waveform signatures. The 'N' and the 'U' measured waveform shape of, respectively, thermospheric and stratospheric paths are associated with nonlinear mechanisms. Nonlinearities are weak but the development of nonlinear models is necessary in order to characterize the source yield. Comparisons will be made with results available in the literature.