This section describes datuming from an irregular datum
downward to a planar datum.
It uses a rectangular grid similar to that in the previous section,
but now the wave equation is
run backward in time from the recorded maximum time to time zero.
And the desired wavefields at the new planar datum are taken as the history
wavefields from the new datum grid points.
At time zero, the wavefield
left in the computational grid points between the old and the new datums
is the reflectivity image in this strip.
The downward datumed wavefields
at the plane datum z=0.1 are shown in Figure 6.
In comparison to the upward datumed wavefields,
there are no boundary reflections from the two sides; because,
by datuming closer to the scatterer, the wavefields focus downward
and the recording aperture increases so that
the wave incidence angle upon the two edges decreases.
Thus, the absorbing boundary conditions absorb more completely.

d2.down
Figure 6 The extrapolated wavefields
at a lower planar datum. The recording aperture increases.