Introduction to migration
Migration moves dipping reflections to their true subsurface positions and collapses diffractions, thus increasing spatial resolution and yielding a seismic image of the subsurface. Figure 4.0-1 shows a CMP-stacked section before and after migration. The stacked section indicates the presence of a salt dome flanked by gently dipping strata. Figure 4.0-1 also shows a sketch of two prominent features — the diffraction hyperbola D that originates at the tip of the salt dome, and the reflection B off the flank of the salt dome. After migration, note that the diffraction has collapsed to its apex P and the dipping event has moved to a subsurface location A, which is at or near the salt dome flank. In contrast, reflections associated with the gently dipping strata have moved little after migration.
Figure 4.0-2 is an example with a different type of structural feature. The stack contains a zone of near-horizontal reflections down to 1 s. After migration, these events are virtually unchanged. Note the prominent unconformity that represents an ancient erosional surface just below 1 s. On the stacked section, the unconformity appears complex, while on the migrated section, it becomes interpretable. The bowties on the stacked section are untied and turned into synclines on the migrated section. The deeper event in the neighborhood of 3 s is the multiple associated with the unconformity above. When treated as a primary and migrated with the primary velocity, it is overmigrated.
Figure 4.0-1 A CMP stack (a) before, (b) after migration; (c) sketch of a prominent diffraction D and a dipping event before (B) and after (A) migration. Migration moves the dipping event B to its assumed true subsurface position A and collapses the diffraction D to its apex P. The dotted line indicates the boundary of a salt dome.
Figure 4.0-3a shows a stacked section that contains fault-plane reflections conflicting with the shallow gently-dipping reflections. Note the accurate positioning of the fault planes and delineation of the fault blocks on the migrated section in Figure 4.0-3b. From the three examples shown in Figures 4.0-1, 4.0-2, and 4.0-3, note that migration moves dipping events in the updip direction and collapses diffractions, thus enabling us to delineate faults while retaining horizontal events in their original positions.
The goal of migration is to make the stacked section appear similar to the geologic cross-section in depth along a seismic traverse. The migrated section, however, commonly is displayed in time. One reason for this is that velocity estimation based on seismic and other data always is limited in accuracy. Therefore, depth conversion is not completely accurate. Another reason is that interpreters prefer to evaluate the validity of migrated sections by comparing them to the unmigrated data. Therefore, it is preferable to have both sections displayed in time. The migration process that produces a migrated time section is called time migration. Time migration, the main theme of migration, is appropriate as long as lateral velocity variations are mild to moderate.
When the lateral velocity gradients are significant, time migration does not produce the true subsurface image. Instead, we need to use depth migration, the output of which is a depth section. Consider the data from an area with intense salt tectonics in Figure 4.0-4. Time migration has produced an acceptable image of the region above the salt. However, note the crossing of events that is a manifestation of overmigration of the reflection associated with the base-salt boundary (denoted by B in Figure 4.0-4b). The improper migration is the result of inadequate treatment by the time migration of the effects of severe raypath bendings at the top-salt boundary caused by the strong velocity contrast between the salt layer and the overlying rocks.
Figure 4.0-5 A 2-D CMP stack (a) represents a 2-D cross-section of a 3-D wavefield. Thus, it can contain energy from outside the plane of the 2-D line traverse. A 2-D migration (b) is inadequate when this kind of energy is present on the 2-D CMP-stacked section. (c) Clear imaging of the salt structure requires both 3-D data collection and 3-D migration (3-D poststack migration). (Data courtesy Nederlandse Aardolie Maatschappij B.V.)
Complex structures associated with salt diapirism, overthrust tectonics and irregular water-bottom topography usually are three dimensional (3-D) in character. A stacked section really is the seismic response of a 3-D subsurface on a two-dimensional (2-D) plane of profile. Therefore, 2-D migration is not completely valid for 3-D data from areas with complex 3-D structures. Figure 4.0-5a is an inline stacked section from a land 3-D survey. Figure 4.0-5b is a 2-D migration of this section, while Figure 4.0-5c is the same section after 3-D migration of the entire 3-D survey data. In particular, note the significant difference in the imaging of the top of salt T and base of salt B. In 2-D migration, we assume that the stacked section does not contain any energy that comes from outside the plane of recording (sideswipe). Three-dimensional imaging of the subsurface is discussed in 3-D poststack migration.
- Exploding reflectors
- Migration strategies
- Migration algorithms
- Migration parameters
- Aspects of input data
- Migration velocities
- Taner and Koehler, 1977, Taner, M.T. and Koehler, F., 1977, Wave-equation migration: Technical brochure, Seiscom-Delta, Inc.