Page 530 - Acquisition and Processing of Marine Seismic Data

11.7 PRESTACK MIGRATION 521

FIG. 11.31 Schematic illustration of 2D poststack migration over an inclined reflector in the subsurface (bottom) and its
stack section (top). The reflection at time t(x) at trace number 7 may be originated from any reflector segment tangent to a
semicircle with a radius of Vt/2 (red dashed curve), centered at zero-offset location X. To obtain the true subsurface location
of the dipping reflector, the tangents of all the semicircles calculated for the entire zero-offset traces are obtained.

any subsurface reflector tangent to this semicircle major disadvantages of poststack migration is
can produce this reflection, since all possible con- that it removes relative amplitude information
tributions along the semicircle arrive at time t(x) existing in the prestack data, which prevents
to the trace number 7. To positively locate the the application of prestack amplitude analysis,
reflections of a dipping event in the subsurface, such as amplitude versus offset (AVO) studies
we can apply similar semicircle analysis to all on the migrated data.
of the traces in the data (grey dashed curves)
using the arrival times of that specific reflection
event at each specific trace. The final reflector 11.7 PRESTACK MIGRATION
can be obtained from a common tangent to all
of these semicircles (Fig. 11.31). Although it produces acceptable results for
Although it does not produce a perfectly clear most cases, the poststack migration procedure
and exactly accurate image of the subsurface, is only correct for horizontally layered subsur-
it is still used by processors, since it is computa- face geology, where there is no lateral velocity
tionally efficient and hence economical as com- variation. However, in the case of dipping
pared to the prestack migration methods. The reflectors, the stacking velocity also depends
poststack migration assumes that the input on the inclination angle of the reflector (Levin,
data is zero offset, and it yields reasonable 1971). The poststack migration also assumes that
results if the data has small (poststack time the stack section is identical with the zero-offset
migration) or moderate (poststack depth migra- section, which is not valid in particular for com-
tion) structural dips, and if there are only plex subsurface settings where reflections with
smooth lateral velocity variations. One of the conflicting dips exist in the data. In such cases,