MIGRATION IN THE PRESENCE OF A RUGGED INTERFACE WITH HIGH VELOCITY CONTRAST*

In the western coal‐mining area of Ruhrkohle AG, reflection seismic prospecting for the Carboniferous coal measures is severely impaired by structures with halokinetic features. These structures make the interface between Mesozoic and Paleozoic layers, i.e., the top of Zechstein in general, very rugged. Unfortunately the velocity contrast at this interface is very high in that area, the ratio of velocities being 1.5 to 2.0. Therefore, migration and stacking become a problem.

Three types of migration are presented:

• 1  (f, x)‐time‐migration with vertical time‐to‐depth conversion as a second step.
• 2  Kirchhoff migration down to a level determined approximately by the highest points of the top of Paleozoics, i.e., 0.35 s, and Kirchhoff‐downward continuation for all times exceeding 0.35 s. Intermediate static corrections for these latter times with subsequent (f, k)‐time‐migration and final vertical time‐to‐depth conversion.
• 3  Direct depth migration in the (f, x)‐domain using three interval velocities.

In all cases an intermediate picking of the velocity interfaces is necessary. In case 2 this occurs at an earlier stage of the process than in case 1, and in case 3 at a still earlier stage.

The results of the second and third migration procedures are superior to those of the first. Possibilities for misinterpretation of faults are reduced considerably when the second or third migration procedure is applied.