  The concept of residual migration to zero offset is introduced for the case
that a prestack migration and a subsequent residual moveout analysis have been
performed for a seismic survey and a new depth model has not yet been
determined. Travel times of reflected events for individual traces are
determined for diffraction points situated along horizons of analysis. These
events are downward continued into the model used for the migration of the
survey. The NIP wave theorem can then be applied to events exhibiting small
subsurface offsets in order to determine residual radii of curvature to be used
in the updating of seismic velocity model by seismic stripping. Alternatively,
it is suggested to construct an aplanat and hence to determine a focus point of
the reflected event without knowledge of the true velocity model. The distance
of the estimated point of focus from the observed zero offset response of the
migrated reflector serves as an indication of the focusing of the considered
event at the particular subsurface offset and can be used as a measure for the
necessity of a subsequent remigration of the survey with a new velocity model.
A summation over the first Fresnel zone at the migrated zero offset position is
suggested as an alternative to conventional stacking leading to improvement of
the signal to noise ratio. Applications to model computations and to a seismic
survey over an overthrust structure show promising results regarding the
applicability of the suggested method.
