Equations that describe wave propagation in orthorhombic media are derived under the acoustic assumption. These include
a dispersion relation, which is a key equation for imaging, and a wave equation, which is important for modeling *P*-waves.
Though the Earth subsurface is always elastic in anisotropic media, these acoustic approximations
yield accurate kinematic and dynamic descriptions of *P*-wave propagation in elastic media. In addition, the acoustic equations
are simpler than the elastic ones, and thus, they are more efficient and easier to use.

8/21/1998