* Departement de Sismologie, Institut de Physique du Globe de Paris, 4 Place Jussieu, 75252 Paris Cedex 05, France; e-mail: email@example.com
Perturbation theory is a useful tool for approximate study of wave properties in weakly anisotropic media. Especially its first-order approximation yields simple and transparent formulae only slightly more complicated than those for isotropic media. In this contribution we study effects of the first-order approximation on the position of acoustical axes in weakly anisotropic media. We show that longitudinal directions in the first-order approximation are identical with actual ones. In singular directions, however, the first-order formulae provide singular directions, which may deviate from the exact ones or which may even provide false singular directions.
First- and higher-order perturbations, acoustical axes, weak anisotropy
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