## Acoustic axes in weak triclinic anisotropy

**Vaclav Vavrycuk**
### Abstract

Acoustic axes can exist even under an infinitesimally weak anisotropy, and occur when slowness surfaces
of the S1 and S2 waves touch or intersect. The maximum number of isolated acoustic axes in weak triclinic
anisotropy is 16 as in strong triclinic anisotropy. The directions of acoustic axes are calculated by solving
two coupled polynomial equations in two variables. The order of the equations is six under strong anisotropy
and reduces to five under weak anisotropy. The weak anisotropy approximation is particularly useful,
when inverting for anisotropy from the directions of acoustic axes. The minimum number of acoustic axes,
which can be inverted for weak anisotropy, is seven, and at the most 13 combinations of elastic parameters
can be retrieved. Numerical tests have shown that the inversion is sufficiently accurate only for very weak
anisotropy with strength less than 5%, and provided that the acoustic axes used in the inversion are determined
with an accuracy better than 0.3°.

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In: Seismic Waves in Complex 3-D Structures, Report 15,
pp. 235-265, Dep. Geophys., Charles Univ., Prague, 2005.

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