Successful processing in overthrust areas has relied on heavy interaction between interpreters and processors (Burke and Knapp, 1995; Tilander and Mitchel, 1995). In recent years several advances have been made, not the least of which has been improved acquisition technology. Increased shothole depth, shot charge, and fold go a long way towards increasing data quality. Data processing has generally consisted of early stage filtering and noise suppression followed by iterations of static shift and preliminary velocity estimation. The final output datum is usually set above the highest topography, but during processing, data are often corrected to a local floating datum which is CMP consistent rather than to a final output datum. This is done to minimize nonhyperbolic distortions caused by static shift. When the assumption of vertical raypaths is invalid, CMP velocity analysis will suffer. In this case, it is better to apply wave-equation datuming rather than large static shifts.
Because of the nature of overthrust data, it is often difficult to pick velocity from CMP semblance analysis; therefore, velocity is often estimated from constant velocity stacks and post-stack migration. When constant velocity stacks and migration are used, the data are shifted to the final output datum. This shift is done primarily so that an interpreter's insight can be incorporated in the velocity estimation. As pointed out by Tilander and Mitchel, stacking to a floating datum in the presence of significant topography can introduce artificial structure. This hampers the interpreter's ability to use geological intuition in picking horizons and defining dominant structural style. The added benefit of shifting data to the final output datum is that fast and efficient algorithms can be applied to the estimation of migration velocity. If static shift is used to transform the data to the output datum and the assumption of vertical raypaths is not valid, the data will be distorted and the accuracy of the migration result will be compromised. If prestack wave-equation datuming is used instead of static shift, the event trajectory will not be distorted and the migrated image will be superior.
Refraction analysis is often used to determine low spatial frequency statics and to compensate for the weathering layer. The refraction statics solution is often used with a replacement velocity to correct data to either a floating or final datum. While this works well in regions of mild topography and well behaved weathering layer, the refraction model often does not match the geologic reality of mountainous terrain (Bevc, 1991). Although useful near-surface information may be gleaned from refraction analysis, it is more accurate to apply wave-equation datuming to the recorded data rather than to apply static shifts based on refraction analysis for the final datum correction.
For extremely poor signal-to-noise data, wave-equation datuming may not be appropriate because it is likely to generate strong artifacts. In such situations, judicious use of datum statics and residual statics is more likely to be successful. When there is significant noise in the data, which does not support the wave equation, it may be safer to rely on statics and stacking to improve the image rather than try to ``force'' a solution with wave-equation datuming. However, when the data quality is adequate, the incorporation of wave-equation datuming into the processing flow can offer significant benefits.