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Quasi-isotropic approximation of ray theory for anisotropic media |
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| Authors: | Matthias Zillmer Boris Kashtan & Dirk Gajewski |
| Institution: | Institute of Geophysics, University of Hamburg, Bundesstrasse 55, 20146 Hamburg, Germany,;Institute for Physics, St. Petersburg State University, Ulyanovskaya 1, Petrodvorets, St. Petersburg 198904, Russia |
| Abstract: | Wave propagation in weakly anisotropic inhomogeneous media is studied by the quasi-isotropic approximation of ray theory. The approach is based on the ray-tracing and dynamic ray-tracing differential equations for an isotropic background medium. In addition, it requires the integration of a system of two complex coupled differential equations along the isotropic ray. The interference of the qS waves is described by traveltime and polarization corrections of interacting isotropic S waves. For qP waves the approach leads to a correction of the traveltime of the P wave in the isotropic background medium. Seismograms and particle-motion diagrams obtained from numerical computations are presented for models with different strengths of anisotropy. The equivalence of the quasi-isotropic approximation and the quasi-shear-wave coupling theory is demonstrated. The quasi-isotropic approximation allows for a consideration of the limit from weak anisotropy to isotropy, especially in the case of qS waves, where the usual ray theory for anisotropic media fails. |
| Keywords: | anisotropy perturbation methods ray theory |