Tariq A Alkhalifah1
(1) KACST, Riyadh, Saudi Arabia
Imaging the pre-salt reflections for data acquired from the coastal region of the Red Sea is a task that requires prestack migration velocity analysis. Conventional poststack time processing methods lacks the lateral-inhomogeneity capability needed for such a problem. Prestack migration velocity analysis in the vertical time domain reduces the velocity-depth ambiguity usually hampering the performance of prestack depth-migration velocity analysis. In prestack TAU migration velocity analysis, we keep the interval velocity model and the output images in time. This allows us to avoid placing reflectors at erroneous depths during the velocity analysis process, and, thus, avoid inaccurately altering the shape of the velocity model, which, in turn, slows down its convergence to the true model. Using a one-dimensional velocity update scheme, the prestack TAU migration velocity analysis produces good images of data from the Midyan region of the Red Sea. For the first seismic line from this region, only three prestack TAU migration velocity analysis iterations were required to focus pre-salt reflections in time. However, the other line, which crosses the first line, is slightly more complicated, and thus, required five iterations to approach the final, reasonably focused, time image. These results compared favorably with images obtained for the same two lines using the Common-focus-point imaging technique, developed recently at Delft University. After mapping both images to depth using the final velocity models, the placement of reflectors in the two 2-D lines were consistent at their crossing point. Some errors occurred due to the influence of out-of-plane reflections on 2-D imaging. However, such errors are identifiable and are generally small.