High-Resolution Velocity Model-Building for Pre- Stack Depth Migration in the Nordsj?n Area, Norwegian North Sea

摘要

Gridded tomography is generally perceived as being very good at resolving velocities down to fine detail, but when it comes to resolving very sharp contrasts, such as at top chalk or top salt levels, it breaks down, giving way to the hybrid approach. In this, gridded tomography is confined to certain velocity domains and another velocity estimation method, such as flooding or vertical update, is used in the problematic domains. We show in this presentation that the gridded approach works throughout the entire section in the Nordsj?n Area. This part of the Norwegian North Sea is characterized by shallow heterogeneities and a thin chalk layer at intermediate depths. The chalk layer is relatively thin (I.e. less than 200m thickness) but well constrained by top and bottom reflections. This allows us to capture the chalk velocities accurately, except in a small area where the chalk thins out below our model-building resolution. As a result of this approach, a 3D PreSDM volume of a far superior imaging resolution than the vintage data set was produced. Examples from the model-building sequence and the final result are shown.