Iterative depth migration

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Seismic Data Analysis
Series Investigations in Geophysics
Author Öz Yilmaz
ISBN ISBN 978-1-56080-094-1
Store SEG Online Store

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.

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.

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