Elevation statics

• In land areas, corrections for elevation changes relative to processing datum are made assuming a laterally constant replacement velocity for the near surface. Errors in this assumption are handled through residual statics.
• In marine areas, the sea surface elevation and source and cable depth changes are normally negligible but in areas with high tidal range elevation corrections can be applied to minimize line mis-ties.

Refraction statics

• Statics calculated from laterally and vertically varying models of the near surface are applied. The models, based on several alternative algorithms, rely on pickable first-breaks from one or more high velocity refracting interfaces. In order of preference the algorithms available are:
  
  Tomographic arrival time inversion: The GLI-3D package from Hampson-Russell/Veritas is used
  
  Generalized reciprocal method: A less precise technique suitable only for split spread data
  
  Delay time technique: A least squares solution using estimated refractor velocity

Residual (reflection) statics

• Corrections are calculated statistically to compensate for the effects of any near surface variations having wavelengths less than the recording spreadlength and not fully comprehended by the elevation or refraction statics models.
• Windowed, horizon-centered cross correlations between individual traces in NMO corrected CMP gathers and a model stack trace from the same location are used to derive a residual static for each trace. These shifts are iteratively decomposed into shot, receiver and CDP (structure) consistent components.
• The most successful algorithms available to us are:
  
  Maximum power autostatics: Uses the objective function of maximum stack power to optimize the iterative decomposition of trace shifts. Can be run in 3D mode
  
  Correlation autostatics: Uses the Gauss-Seidel technique for decomposing trace shifts into shot, receiver, channel and residual horizon velocity components. Latter can be used as QC of picked velocities.