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Interpretive imaging of seismic data

Paradigm Geophysical, Woking, England

Corresponding author: Ö. Yilmaz, oz@anatoliangeo.com

Editor's note: Öz Yilmaz is presently employed by Anatolian Geophysical in Istanbul, Turkey. Fugen Zhou is presently with Schlumberger Geco-Prakla in Houston, Texas.

The first 20% of the full text of this article appears below.

An interval velocity field for depth migration derived from Dix conversion of an rms velocity field often is judged to be inaccurate and, consequently, can require an excessive number of iterations to attain a final velocity-depth model with acceptable accuracy. The most likely reason for failing to derive an accurate interval velocity field from Dix conversion is that the input rms velocity field usually is based on velocities estimated from unmigrated data. For instance, a common approach to derive an rms velocity field is to take a set of stacking or DMO velocity functions picked at analysis locations, edit for any outliers, and interpolate them with some lateral and vertical smoothing. The key to an accurate interval velocity field from Dix conversion, however, is to estimate the rms velocity field from prestack time-migrated data—not from unmigrated data. By doing a better initial estimate, you minimize the number of iterations required to derive a final velocity-depth model suitable for accurate imaging in depth. Implicit to the model-building strategy advocated in this paper is doing prestack time migration for earth modeling prior to prestack depth migration for earth imaging in depth.

Time migration of seismic data requires an rms velocity field associated with events in their migrated positions. This means that you need to migrate the data to estimate the rms velocity field, which is what you need to migrate the data. This paper offers an interpretive approach to circumvent this circuitous argument. It also extends the analysis to depth migration using an interval velocity field derived from the rms velocity field associated with the time-migrated data. The model building and imaging strategy described here is extended and modified for 3-D seismic data (Yilmaz, 2001).

The method for deriving a time-migration velocity field—the rms velocity field associated with events . . . [Full Text of this Article]