Depth migration is more accurate than time migration in that it images seismic data correctly in the presence of<br>lateral velocity changes. However, for mild lateral velocity gradients we often use time migration for reasons of<br>cost and stability; the time spent deriving an accurate velocity field and the increased sensitivity of depth<br>migration to the velocity field make depth migration more difficult to apply. For these reasons, depth migration<br>has often only been used when time migration is perceived to fail to image the data properly. Very often, this<br>means that depth migration is only used in particularly difficult and complex geological environments such as<br>overthrust belts and beneath salt diapirs. Unfortunately, in these environments, depth migration often does not<br>provide a clear image where none existed without depth migration because either the ray paths diverge causing<br>the subsurface not to be illuminated by the recording, or current model building techniques are inadequate for<br>such complicated cases. For example, many model updating techniques assume the starting model is either<br>close to the correct answer, slowly varying or that flat horizons exist.<br>Experience of imaging beneath the basalts of the Shetland-Faroes basin, the tabular salt of the Gulf of<br>Mexico, the shallow carbonates of N. W Australia and the chalk layer of the North Sea has shown that prestack<br>depth migration can be used to improve imaging substantially beneath these low relief but high velocity layers.<br>In the Shetland-Faroes basin, volcanic flows form a barrier to imaging of deeper sedimentary rocks. PSDM has<br>improved imaging both within and below the multiple lava flows. In the Gulf of Mexico, tabular salt causes<br>significant ray path bending and subsequent distortion of the images of the underlying geology. In the Browse<br>Basin, rugged seabed topography can cause imaging problems that can be addressed with prestack depth<br>migration. In many parts of the North Sea a chalk layer with gentle dips lies above the oil and gas bearing<br>targets. All of these geological environments are characterised by fast velocity layers (for example, the chalk is<br>typically twice the velocity of the overburden) and even low dips at the top and base of these layers cause<br>problems for imaging deeper targets. The ray path bending can either prevent an image being formed at all,<br>distortion of any image that is obtained or loss of resolution. PSDM has been found to be beneficial in all these<br>cases. Prestack depth migration should not be viewed as just a tool for extreme geological cases.<br>Post stack depth migration may provide better positioning and imaging than post stack time migration.<br>Therefore, in low relief areas, it is tempting to use post stack depth migration instead of prestack. However, the<br>ray path distortion caused by the low relief, fast velocity layers we are considering is significantly offset<br>dependent. This causes image distortion and loss of resolution that cannot be recovered stack. Also, model<br>building via post stack depth migration is subjective – the only test is whether the final image agrees with the<br>interpreter’s expectation. Prestack depth migration affords the qualitative test that if the velocity model is<br>correct then the primary events within depth migrated image gathers are flattened and will stack coherently.


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