ABSTRACT
The quasi-isotropic approximation (QIA) equations enable matching of the geometrical acoustics of 3D inhomogeneous isotropic media, as elaborated by M. L. Levin and S. M. Rytov, with the Courant-Lax method of independent normal waves. Consider acoustic waves propagating in a perfectly elastic and isotropic, but pre-deformed medium. It is implied that this preliminary deformation substantially exceeds the amplitude of the propagating waves and thus that the deformed medium is similar to an anisotropic medium. Perturbation theory in terms of the components of the anisotropy tensor not only simplifies calculations, but, more important, makes a description of normal wave interaction effects possible by invoking the QIA approach.
