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Static Corrections to Remove Elevation and Near-Surface Heterogeneities.

A major goal of reflection processing is to provide reflectivity images of the correct reflector geometry. This goal can be thwarted by the statics problem. The statics problem is defined to be static time shifts introduced into the traces by, e.g., near-surface velocity anomalies and/or topography. These time shifts distort the true geometry of deep reflectors.

As shown in Figure 1.13, an undulating topography can produce moveout delays in the CSG that can also be interpreted as undulations in the reflector, even if the reflector is flat. After an elevation statics correction (i.e., time shifts applied to traces) the data appear to have been collected on a flat datum plane. Figure 1.13 shows that static shifts can also be introduced by near-surface velocity anomalies which usually delay the traveltimes, resulting in reflections having a non-hyperbolic moveout curve.

Static shifts introduced by topographic variations fall under the class of field statics, and those due to near-surface lithological variations that occur within a cable length fall under the class of residual statics. Correcting for static shifts in the traces can make a significant difference in the quality of a migrated or stacked image. It is easy to determine elevation static corrections, but not so easy to find the residual static corrections. One means is to determine the near-surface velocity distribution by refraction tomography, which will be discussed later.

  
Figure 1.13: (LHS) Elevation static correction applied to CSG traces to correct data so that it appears to have been shot on a level datum plane. Still, there is a residual static correction which must be removed to correct for the low-velocity pod.
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Figure 1.14: CMG collected over the Oquirrh fault, Utah where the jitter in the reflections at depth is due to near-surface velocity variations. These jitters must be corrected prior to NMO correction and stack.
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next up previous contents
Next: Velocity Filter Data to Up: Basic Processing Steps Previous: Bandpass Filter Data to
Gerard Schuster
1998-07-29