Geoscience Reference
In-Depth Information
Fig. 2.34
Examination of CMP gathers during an iteration of pre-stack depth migration. Pre-stack
depth migration is required in areas of complex lateral velocity variation. In such areas the standard
normal moveout equation is insufficient to align events owing to the different paths that the waves
take through the subsurface. In order to apply pre-stack depth migration one needs to supply the
correct velocity-depth model. This is generally not known and so is built in an iterative fashion
during the pre-stack migration. The correct velocity model is the one where the migrated gathers
are flat across all offsets. Here we see gathers during a pre-stack migration iteration. Some of the
events are flat, indicating that the correct model has been determined. Others are still dipping and
the velocity model needs updating to flatten them also.
time migration is the preferred solution. Since full pre-stack time migration is still
relatively expensive a cheaper, faster approximation has been used. As computer power
continues to increase, these approximations are being replaced by the full pre-stack
migration solution. Steps 15 and 16 are an application of Dip Moveout (DMO), so
called because it removes the effect of dip on stacking velocities and trace positions. As
part of the DMO process, one also needs to apply an approximate NMO correction. At
this stage it is usual to have picked velocities approximately on a sparse grid, say every
1 km in all directions. This is generally sufficient since detailed velocity analysis for
final stacking (maximum analysis separation of 0.5 km) is performed after DMO and
constant velocity migration in step 19. There is an excellent article on the benefits of
One of the benefits of DMO is that it takes a constant offset section and transforms
