Aspects of input data
When migrating seismic data, one needs to be concerned with various aspects of the input data set itself:
- line length or areal extent,
- signal-to-noise ratio, and
- spatial aliasing.
The line length must be sufficient to allow a steeply dipping event to migrate to its true subsurface location. It is a fatal error to record short profiles in areas with complex geology. Also, for 3-D migration, the surface areal extent of a 3-D survey is almost always larger than the target subsurface areal extent.
Random noise at late times on a stacked section, when migrated, potentially can be hazardous for shallower data. One may have to compromise on migration aperture for deep data to prevent this problem to occur.
Trace spacing must be small enough to prevent spatial aliasing of steep dips at high frequencies. Although this is not an issue for modern prestack data, a coarse shot-receiver spacing can degrade the fidelity of prestack migration severely. Old data and 3-D marine data in the crossline direction often are trace interpolated prior to poststack migration.
Figure 4.0-12 is a CMP stacked section before and after migration. From an interpretation viewpoint, the reliable part of this migrated section is confined to the upper central part. Lack of any reflection energy to the right of the dotted line does not mean that there is a structural discontinuity there. It only means that the reflections associated with the imbricate structure have been migrated in the updip direction from right to left. As a result, a zone with no reflectors to the right of the dotted line is left behind because of the truncation of the wavefield represented by the right-hand edge of the stacked section. The deeper part is useless, because the noise dominates the section.
Figure 4.0-12 A CMP stack (a) before, and (b) after migration. Lack of any event to the right of the dotted line on the migrated section is a result of the finite line length.
- Exploding reflectors
- Migration strategies
- Migration algorithms
- Migration parameters
- Migration velocities