Finite Difference Time Domain Modeling of Infrasound Propagation: Application to Shadow Zone Arrivals and Regional Propagation

摘要

The finite difference (FD) method yields solutions to discretized versions of the full acoustic wave equation for arbitrarily complex media. The method is reliable at all angles of propagation, including backscatter. This offers an advantage over other standard propagation methods in wide use, as it allows for accurate computation of acoustic energy levels in the case where significant scattering can occur near the source, such as may happen for an explosion near the surface, or underground. It also allows for the investigation of the penetration of infrasound energy into classical shadow zones, where ray theory predicts the upward refraction of sound. This fits in with nuclear monitoring goals, in that it allows for an improved understanding of the generation and propagation of infrasound energy from arbitrary sources, including underground and near-surface explosions. The effects of (1) wind, (2) attenuation, and (3) gravity on infrasound propagation are examined separately, using finite-difference time-domain methods. The method is applied in 2-D Cartesian coordinates. The method, including the effects of wind, is applied to realistic problems in infrasound propagation, including the penetration of infrasound energy into 'shadow' zones, as defined by ray theory. The effects of diffraction and topography are examined for sources near the surface. It is shown that the FD approach can be used to solve for sound intensities in complex models that may include high material contrasts and arbitrary topography.