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Linear and Nonlinear Infrasound Propagation to 1000 km

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Technical Report,11 Jun 2013,30 Sep 2015

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University of California, San Diego La Jolla United States

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The Navier-Stokes equations have been solved using a finite-difference, time-domain FDTD approach for axi-symmetric environmental models, allowing three-dimensional acoustic propagation to be simulated using a two dimensional Cylindrical coordinate system. A method to stabilize the FDTD algorithm in a viscous medium at atmospheric densities characteristic of the lower thermosphere is described. The stabilization scheme slightly alters the governing equations, but results in quantifiable dispersion characteristics. It is shown that this method leaves sound speeds and attenuation unchanged at frequencies that are well resolved by the temporal sampling rate, but strongly attenuates higher frequencies. Numerical experiments are performed to assess the effect of source strength on the amplitudes and spectral content of signals recorded at ground level at a range of distances from the source. It is shown that the source amplitudes have a stronger effect on a signals dominant frequency than on its amplitude. Applying the stabilized code to infrasound propagation through realistic atmospheric profiles shows that nonlinear propagation alters the spectral content of low amplitude thermospheric signals, demonstrating that nonlinear effects are significant for all detectable thermospheric returns.

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