Up: Clapp: Speeding up wave
In theory it should also be possible to use
sparser sampling in midpoint at low frequencies.
Since this would only affect the relatively inexpensive
portion of the downward continuation, I would expect
at most only a speed up.
Another method is to do a better job choosing
appropriate velocities, thereby reducing the number of velocities
needed for accurate downward continuation. Clapp (2002)
discussed one method to accomplish this.
Stanford Exploration Project