3-D Simulations of Seismo-acoustic Coupling over Topography

Abstract:

Experimental and observational data have shown that a propagating acoustic wave can generate a variety of elastic waves as it impinges upon the earth. Theoretical treatments have thoroughly examined reflections and refractions at a planar interface, but nonplanar interfaces, particularly topography, can have considerable impact on local infrasonic propagation and have not been addressed in coupling scenarios. Due to the rarefied nature of the atmosphere, calculated estimates of acoustic-seismic energy transfer are on the order of a few percent and are frequency-dependent. Numerical approaches to this problem are somewhat limited in the presence of topography, and it is typically assumed that the ground surface is a rigid interface. SPECFEM3D, a spectral finite element code developed in the seismological community, can numerically simulate the coupling between acoustic and elastic waves over meshed topography. With the addition of particle motion analysis, this allows us to investigate the effect of the angle of incidence and subsurface structure on acoustic to elastic coupling. Initial results show significant deviation at steep incidence angles from the commonly assumed expressions for transfer coefficients, which has implications for propagation with multiple surface interactions and larger propagation distances. We make quantitative comparisons with a commonly-used finite difference code that treats topography as a rigid interface. We also present acoustic analogs for a variety of moment tensor sources and show that the general diagonal moment tensor in a fluid medium can be related to a classical acoustic quadrupole source. When the moment tensor is isotropic, this expression collapses to an exact relationship between the seismic moment and a monopole acoustic source.

Approved for public release; Distribution is unlimited.
Funding for this effort was provided by the US Defense Threat Reduction Agency (DTRA)

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