Changes

While migration algorithms are capable of accurately imaging reflections from steep interfaces, shortcomings in CMP stacking lead to destruction of such reflections before conventional ''poststack migration'' is applied. Two alternatives to poststack migration of CMP stacked data preserve reflections from steep interfaces. Migration can be applied to the unstacked data (so-called ''prestack migration'') so that the data need not be reduced to an approximation to zero offset before migration. The improvement in imaging of steep reflectors by this approach, however, is bought at the price of a great increase in the amount of computation required for the migration.

While migration algorithms are capable of accurately imaging reflections from steep interfaces, shortcomings in CMP stacking lead to destruction of such reflections before conventional ''poststack migration'' is applied. Two alternatives to poststack migration of CMP stacked data preserve reflections from steep interfaces. Migration can be applied to the unstacked data (so-called ''prestack migration'') so that the data need not be reduced to an approximation to zero offset before migration. The improvement in imaging of steep reflectors by this approach, however, is bought at the price of a great increase in the amount of computation required for the migration.

A cost-effective and accurate alternative to full prestack migration is to apply poststack migration to data that have had the added step of ''dip moveout'' (DMO) applied after normal moveout (NMO) correction, but before the data are stacked. DMO, a form of ''partial'' prestack migration, completes the process that NMO only imperfectly accomplishes—it converts data recorded with separated sources and receivers to a close approximation to zero-offset data, preserving reflections from both gently dipping and steep reflectors. [[:file:seismic-migration_fig5.png|Figure 5]] shows the improvement in imaging of the steep flank of a salt dome achieved by poststack migration when applied to DMO-processed data.

A cost-effective and accurate alternative to full prestack migration is to apply poststack migration to data that have had the added step of ''[[dip]] moveout'' (DMO) applied after normal moveout (NMO) correction, but before the data are stacked. DMO, a form of ''partial'' prestack migration, completes the process that NMO only imperfectly accomplishes—it converts data recorded with separated sources and receivers to a close approximation to zero-offset data, preserving reflections from both gently dipping and steep reflectors. [[:file:seismic-migration_fig5.png|Figure 5]] shows the improvement in imaging of the steep flank of a salt dome achieved by poststack migration when applied to DMO-processed data.

The additional accuracy of either DMO or prestack migration over that of conventional poststack migration demands special care in the field acquisition of seismic data. Too coarse a spatial sampling, that is, too large a geophone group interval, may preclude high resolution imaging of steep reflectors by any migration method.

The additional accuracy of either DMO or prestack migration over that of conventional poststack migration demands special care in the field acquisition of seismic data. Too coarse a spatial sampling, that is, too large a geophone group interval, may preclude high resolution imaging of steep reflectors by any migration method.