Layer-stripping reverse-time migration
Levander, Alan R.
Doctor of Philosophy
Reverse-time migration has proven to be successful for structures with steep dips and strong velocity contrasts. Applying this algorithm to a large scale seismic model requires significant computational expense, particularly if strong velocity contrasts are present in the model. Here I present a layer-stripping migration technique, in which I use the reverse-time method to migrate seismic sections through constant or smoothly varying velocity layers, one layer at a time. As part of the migration in a given layer, the bottom boundary of the layer is defined and a seismic section is collected along it. This new section serves as the boundary condition for migration in the next layer. This procedure is repeated layer by layer. The final migration result is composited from the individual layers images. The layer-stripping migration algorithm can be summarized as three steps: (1) model definition, (2) wavefield extrapolation and imaging, and (3) boundary determination. The migration scheme posed in this way is similar to datuming with an imaging condition. The advantages of the layer-stripping method are: (1) it preserves the benefits of the reverse-time method, i.e., it handles strong velocity contrasts between layers and steeply dipping structures; (2) it eliminates artificial interlayer multiples; (3) it reduces computational expense in high velocity layers; and (4) it allows interpretational constraint during image formation. The method has been implemented with both an explicit 4th-order time, 10th-order space, finite-difference approximation to the scalar wave equation, and an implicit 2nd-order time, 4th-order space finite-difference scheme applied to the linearly transformed wave equation (Li, 1986). The capability of post-stack layer-stripping reverse-time migration is illustrated on a synthetic CMP data and a CMP data from a survey over a faulted anticline in a fold and thrust belt. For pre-stack layer-stripping reverse-time migration, I present migrations of two synthetic data sets and a field data example from part of the marine seismic reflection profile RU-3, crossing the Hosgri fault offshore southern central California. The Hosgri fault appears as a northeast dipping high angle fault with a thrust component.