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Star: Shallow Time Anomaly Replacement
The presence of spatially varying low velocity thin layers in the near surface can account for the surface consistent terms in the description of reflection times. If the same low velocity material occurs at a greater depth in the section then its effect on reflections will vary with reflector depth. The dynamic characteristics of such a “buried static” feature is discussed and a subsurface consistent, time-variant static description is given.
The description that is obtained of this moveout phenomenon is asymtotically static, which is to say, as the buried LVT approaches the surface the description is identical to surface consistent statics. Thus, conventional statics’ modelling is a special case of STAR corrections. Furthermore, like statics, the fundamental Information required is the vertical transmission time through the anomaly. However, this simplified theory makes no attempt to account for the effects of wave front “healing” which will modify predictions based on ray considerations only. This aspect is investigated using wave equation modelling.
The effect on the reflection time curve of a reflector, due to the presence of buried stream channels, shallow gas “pockets” and erosional (or dissolved) surfaces with low velocity fill, can be modelled as a low velocity thin layer (LVT). The distortion of RMS velocity measurements and of the structure picture represented by the stack section, due to such buried static anomalies, could be substantially reduced.
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