Iterative depth migration
Historically, depth migration has been used in an iterative manner to obtain an earth image in depth from CMP-stacked data. When performed iteratively, depth migration is done using an initial velocity-depth model and the result is interpreted for the layer boundaries included in the model. The velocity-depth model then is modified accordingly and depth migration is performed once more. The process is continued until convergence is achieved.
Figure 8.2-10 Depth migrations of the zero-offset, CMP-stacked, and prestack data associated with the salt diapir model using an incorrect velocity-depth model.
Convergence means that what is input to depth migration as the velocity-depth model matches with the velocity-depth model inferred from the output from depth migration. With iterative depth migration, we know that we have achieved convergence when differences between the velocity-depth models from two consecutive iterations are minimal. We shall demonstrate that, by way of convergence, the final velocity-depth model from iterative depth migration, albeit not guaranteed to be accurate, can be made at least consistent with the input data. Consistency means that the modeled zero-offset traveltimes match with the observed reflection traveltimes on the stacked data associated with the layer boundaries included in the velocity-depth model. Convergence and consistency are the two necessary, but not sufficient, conditions for an earth model to be certified as a valid, geologically plausable solution from seismic inversion. For a velocity-depth model to be valid, a further requirement is that it also needs to be consistent with prestack data; thus, the strategic requirement for doing prestack depth migration.
In this section, we shall examine iterative depth migration for three data types — zero-offset, CMP stack, and prestack. We shall consider six different initial velocity-depth models, each having errors in layer velocities and/or reflector geometries. Convergence rates and the end results for each starting model will be evaluated and some practical conclusions about iterative depth migration will be drawn from these experiments.
- 2-D poststack depth migration
- Image rays and lateral velocity variations
- Time versus depth migration
- Iteration with zero-offset data
- Iteration with CMP-stacked data
- Iteration with prestack data
- Iteration in practice