In practice, migration of seismic data requires decision making with regards to:
- an appropriate migration strategy,
- a migration algorithm compatible with the strategy,
- appropriate parameters for the algorithm,
- issues concerning the input data, and
- migration velocities.
Migration strategies include:
The spectrum of migration strategies extend from 2-D poststack time migration to 3-D prestack depth migration. Depending on the nature of the subsurface geology, any other in-between combination can be selected. In practice, 2-D/3-D poststack time migration is used most often for a good reason — it is the least sensitive to velocity errors, and it often yields results acceptable for a reliable interpretation. Table 4-1 is an overview of different migration strategies applied to different types of seismic data (2-D, 3-D, stacked, and unstacked).
Choice of an appropriate migration strategy requires input from the interpreter as to the structural geology and stratigraphy in an area. Dipping events on a stacked section call for time migration. Conflicting dips with different stacking velocities is one case in which a conventional stacked section differs from a zero-offset section. Thus, strictly speaking, poststack migration which assumes that the stacked section is equivalent to a zero-offset section is not valid to handle the case of conflicting dips. Instead, one needs to do prestack time migration.
|dipping events||time migration|
|conflicting dips with different stacking velocities||prestack migration|
|3-D behavior of fault planes and salt flanks||3-D migration|
|strong lateral velocity variations associated with complex overburden structures||depth migration|
|complex nonhyperbolic moveout||prestack migration|
|3-D structures||3-D migration|
Conflicting dips often are associated with salt flanks and fault planes, which can have 3-D characteristics. This then requires 3-D prestack time migration. In prestack time migration, we shall discuss a practical alternative to 2-D prestack time migration strategies. The alternative sequence includes the application of normal-moveout (NMO) correction using velocities appropriate for flat events followed by 2-D dip moveout correction (DMO) to correct for the dip and source-receiver azimuth effects on stacking velocities. As a result, conflicting dips are preserved during stacking, and thus, imaging can be deferred until after stacking using 2-D poststack time migration strategies. This series of processing steps is largely equivalent to 2-D prestack time migration and results often are comparable. The same workflow also is applicable to 3-D prestack time migration (3-D prestack time migration).
Accurate imaging of targets beneath complex structures with strong lateral velocity variations requires depth migration. Aside from the problem of conflicting dips with different stacking velocities, strong lateral velocity variations associated with complex overburden structures usually cause conventional stacking based on the hyperbolic moveout assumption to fail. Therefore, a case of complex overburden structures calls for depth migration before stacking the data.
Furthermore, complex overburden structures, encountered in areas with salt tectonics, overthrust tectonics and irregular water-bottom topographies can often exhibit 3-D characteristics. Thus, imaging such structures may require 3-D prestack depth migration.
Field surveys are designed such that line orientations are, as much as possible, along the dominant strike and dip directions, so as to minimize 3-D effects. Under these circumstances, the 2-D assumption for migration can be acceptable. However, if the subsurface has a truly 3-D geometry, without a dominant dip or strike direction, then it is imperative to do 3-D migration of 3-D data. In such cases, 2-D migration (whether poststack or prestack, time or depth) can lead to potential problems in interpretation.
A practical alternative to 2-D prestack depth migration can be a prestack layer replacement to correct for the complex nonhyperbolic moveout followed by time migration after stack. This, however, is applicable to situations involving a single overburden layer, such as irregular water-bottom topography for it to be reasonably practical.
- Exploding reflectors
- Migration algorithms
- Migration parameters
- Aspects of input data
- Migration velocities