## Acoustic axes in weak triclinic anisotropy

**Vaclav Vavrycuk**
### Summary

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 6 under strong anisotropy and reduces
to 5 under weak anisotropy. The weak anisotropy approximation is particularly
useful, when calculating the acoustic axes under extremely weak anisotropy
with anisotropy strength less than 0.1 per cent because the equations
valid for strong anisotropy might become numerically unstable and their
modification, which stabilizes them, is complicated. The weak anisotropy
approximation can also find applications in inversions for anisotropy from
the directions of acoustic axes.

### Keywords

Elastic-wave theory, perturbation methods, polarization,
seismic anisotropy, seismic-wave progagation, shear-wave splitting.

### Whole paper

The reprint is available in
PDF (8010 kB !).

*Geophys. J. int.*, **163** (2005), 629-638.

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