The key equation is the dispersion relation, the equation that links
the temporal and spatial frequencies of an eigenfield in the Fourier
domain. The four variable parameters all have units of velocities squared.
*W*_{x} and *W*_{z} are the actual horizontal and vertical square
velocities that apply equally well to waves and rays, and *W*_{xnmo}
and *W*_{znmo} are the apparent square velocities as determined from move-out
measurements. For example, *W*_{xnmo} is the square NMO velocity that
we would use every day in surface-to-surface data processing. If the
*x* subscript seems unusual for a paraxial measurement about the
vertical, remember that in an elliptic world it is the horizontal
velocity that move-out measures.

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With minor modifications that are discussed in Muir & Dellinger (1985), there are precisely equivalent plane wave and partial differential equations.

1/13/1998