The wave equation for elastic medium is a vector equation. It belongs to a class of equations of the type:
where is a vector and is a matrix operator:
- a matrix.
If we develop the same considerations as with the scalar case, we shall obtain instead of equation (19) the vector equation
with a matrix
It is obvious that characteristics satisfy the equation
Let us check the Lame's equation (isotropic elasticity):
It is easy to derive the correspondent system of characteristic equations. The first line is:
The matrix of the equation system may be written in the following form:
Let us prove that the vector is an eigenvector of :
where , so
Let us consider a vector :
So we may satisfy the equation (19) in both cases:
This means that in isotropic elastic media, there are only two types of body waves which move with velocities vp and vs, respectively. In the general case of anisotropic elastic media, there are three families of characteristics and correspondently three body waves: quasi-P, quasi-SV, and quasi-SH. In Maxwell's equations of electrodynamics, there is only one characteristic which propagates with the velocity of light.
We may say that geometrical seismics (as well as geometrical optics and geometrical acoustics) describes the propagation of discontinuities. We have studied here only the pure geometrical aspect of wave propagation, but it will be shown that our results are also true for the amplitudes of waves.