When seismic data are used to image the subsurface, assumptions and calculations are made about the near-surface to overcome the uncertainty of the velocities of the low velocity layer. A near-surface velocity model is generated to calculate a time shift that is used to correct for velocity anomalies in the near-surface for time migration.
Reflection statics are calculated because often the lack of detailed near-surface information leads to inaccuracies. A normal moveout (NMO) velocity field is picked and applied to stack the data in preparation for the reflection statics calculations. NMO is a two-term equation based on the assumption that the moveout can be approximated by a hyperbola. However, the accuracy of this assumption is valid when the moveout on data is near-hyperbolic and deviates when the moveout is more complicated than the two-term equation. A few scenarios of non-hyperbolic moveout are when the topography isn’t flat, strong lateral heterogeneity of velocity is present, and when there are variations in the seismic weathering thickness and velocities.
Raytracing in depth migration has overcome many of the issues with the assumptions in time migration. Foothills datasets and other geologically complex environments compel us to look for ways to overcome these assumptions as they are violated. Using the depth migration velocity model we apply the zero-offset traveltimes as the moveout correction for reflection static calculations in depth imaging.
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