Migration

Figure  shows the migrated image after prestack depth migration. In this example I have concentrated on imaging only the sediments and the top of the salt, which already turns out to be a complicated task.

For the migration I have used a velocity function that changes linearly with depth, with the exception of a constant-velocity layer at the top (Figure ). The model is constant in the lateral direction. In reality, the velocity is rapidly changing with surface position, and the migration has done a poor job in imaging the subsurface. This is apparent from several features in the stack. First, coherency in the stack deteriorates below surface positions ranging from about 2 to 3.5 km, which is caused by a low velocity channel at the top of the section. Second, the frequencies in the stack are lower than in prestack migrated data (see Figures  and ): velocity effects cause large residual moveout that smears events when they are stacked. Finally, the top of the salt layer does not stack in into the image, but does show up in the prestack migrated constant-offset sections (at a depth of about 1.25 km on the left part of the section, and about 1.5 km on the right in Figures  and ). The same applies to the fault plane reflections. Although the velocity model is far from correct, the result suffices for the purpose of analyzing the migrated data.

The migration method is a Kirchhoff method, where I calculate Green's functions using finite-difference calculations (Van Trier, 1989b). A Kirchhoff method allows partial imaging of the subsurface, which is useful if only major events in the data need to be migrated (see later section). Although the stacked result stays the same, the prestack migrated data can be organized in different ways, such as migrated shot profiles or migrated constant-offset sections. The advantage of constant-offset migration is that these sections resemble the geology, and are individually interpretable. This is an important feature as will become clear later.

Examples of partial stacks of migrated constant-offset sections are shown for two different offsets in Figures  and . These sections display more detail than the stacked image. In particular the salt top is apparent in the section, although its exact shape at the peak of the intrusion is not distinguishable. The migrated data also reveal the dipping part of the salt top on the right, which is not visible in the stacked section (Figure ). Not enough depths are included in the migration to image the salt bottom on the right. Finally, note the events below the low velocity channel on the left; these events have almost completely disappeared in the stacked image.

Figures  and display slices through the prestack migrated data at various surface locations. If the correct migration-velocity would have been used, all the events in these panels would have been flat. Most of the events have been overcorrected, meaning that the migration velocity is too low. At the outer parts of the section the residual moveout is reasonably well-behaved, but in the middle part, above the salt dome, the residual moveout suggests considerable lateral velocity variations.