In the 15 continuation
equation , scaling the depth *z* is
indistinguishable from scaling the velocity.
Thus, downward continuation with the wrong velocity
is like downward continuation to the wrong depth.
Stephen M. Doherty [1975] used this idea
in a velocity-estimation scheme--see Figure 8.

Figure 8

The idea is to downward continue with a preliminary
velocity model and to display the zero-offset trace,
a function of *t*', at all travel-time depths .If the maximum amplitude occurs at ,then your preliminary model is good.
If the maximum is shifted, then
you have some analysis to do before you can say what velocity
should be used on the next iteration.

10/31/1997