** Next:** Splitting diffraction and lens
** Up:** WAVE-EXTRAPOLATION EQUATIONS
** Previous:** Lateral velocity variation

You may wonder where the two velocities *v*(*x*,*z*) and came from.
The first arises in the wave equation,
and it must be *x*-variable if the model is *x*-variable.
The second arises in a mathematical transformation,
namely, equation (15),
so it is purely a matter of definition.
Experience shows that complications will abound if we try to make
the defined velocity depend on *x*.
But it is nice if the two velocities can be equal
so the term containing their difference drops out of the analysis.
Thus ordinarily, is chosen
to be some kind of horizontal average of *v* (*x*,*z*).

** Next:** Splitting diffraction and lens
** Up:** WAVE-EXTRAPOLATION EQUATIONS
** Previous:** Lateral velocity variation
Stanford Exploration Project

10/31/1997