Pre-stack depth migration experience in less complicated geological environments

Abstract

Depth migration differs from time migration in that it images seismic data correctly in the presence of lateral velocity changes. However, for mild lateral velocity gradients we often use time migration for reasons of cost and stability. The time spent deriving an accurate velocity field and the increased sensitivity of depth migration to the velocity field make depth migration more difficult to apply. For these reasons, depth migration has often only been used when time migration is perceived to fail to image the data properly. Very often, this means that depth migration is only used in particularly difficult and complex geological environments. Unfortunately, in these environments, depth migration often does not provide a clear image where none existed without depth migration because either the ray paths diverge causing the subsurface not to be illuminated by the recording, or current model building techniques are inadequate for such complicated cases. For example, many model updating techniques assume the starting model is either close to the correct answer, slowly varying or that flat horizons exist. Experience in the Browse Basin and the North Sea has shown that prestack depth migration can be used to improve imaging substantially in comparatively simple geological environments. In the Browse Basin, rugged seabed topography can cause imaging problems that can be addressed with prestack depth migration. It is worth noting that in this context no well information or geological model from an interpreter was necessary; building the model became almost entirely a velocity picking exercise. In many parts of the North Sea a chalk layer with gentle dips lies above the oil and gas bearing targets. The chalk has an interval velocity that is typically twice that of the overburden and the underlying strata. Consequently, the gentle dips at the top and bottom of the chalk are sufficient to cause image distortion at the reservoir level that can only be corrected using prestack depth migration. Consequently, prestack depth migration can be viewed as a tool to obtain better images in regions of either steep dip and moderate velocity changes between layers or moderate dip and high velocity changes between layers. It is not just a tool for extreme geological cases.