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Unstacked migrated traces for AVO and velocity analysis

WellSeismic Computing Services, Newport Beach, California, U.S.

Corresponding author: johnf@wellseismic.com

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

There are a number of reasons why it would be nice to directly map reflections on a time trace to their image point in depth without all the trace mixing required by conventional migration. Such one-to-one mapping would preserve the shot and receiver coordinates of the trace, thereby allowing reflections to be modeled within the framework of the data.

Consider what must be done now for velocity analysis using reflection tomography or for AVO analysis on depth-migrated traces. The first step is to create common image gathers of traces over a range of offsets. This necessitates a series of common-offset migrations at selected image points (defined here as an x-y location on the migrated image) in which a large number of traces are mixed together and, consequently, the concept of a shot-receiver location for each migrated trace is lost. If there is structural dip or a lateral velocity gradient, this loss of trace geometry poses a problem for both AVO and reflection tomography because the next step is ray tracing. We have a reflection point and an offset, but to proceed with the ray tracing either the dip or a shot-receiver coordinate is required. Because the shot-receiver coordinate is lost in the migration process, the dip must be known. This is what is meant by going outside the framework of the data—a model of reflecting surfaces must be constructed by looking at preliminary migrations, and then the rays must be traced by some trial and error procedure until the incident and reflection angles are as they should be for a given offset.

The process can be made somewhat efficient, but still it takes time, can be tedious, and is subject to error if the modeled surfaces are seriously in error. For tomography, where iterations are often required, this procedure may . . . [Full Text of this Article]