The characteristic equation describes the
propagation of a discontinuity in space (*x*,*y*,*z*). Let us assume that equation
can be solved with respect to *t*: and . Then we get from
equation (20):

(21) |

That is exactly the Eikonal equation for a medium with elliptic anisotropy. In the case of isotropy, it follows that and equation (21) is reduced to

(22) |

Let us consider, for example, the linear acoustic equation

1/13/1998