The two phase shift schemes are suitable for use with a constant
velocity model or a model which varies only in the direction in which
the data is extrapolated. For rectangular coordinates this is a
*v*(*z*) model, and for polar coordinates it is a *v*(*r*) model. When the
velocity model is not of a suitable form, a more complicated algorithm
must be used.

In many cases a *v*(*z*) model
is geologically reasonable, but this is rarely true of a *v*(*r*) model.
One disadvantage of working in polar coordinates is that the simple phase
shift algorithm cannot be used for a *v*(*z*) model.

11/17/1997