Understanding 4D Repeatability Variograms

Recent discussions of 4D repeatability have focused primarily on the effect of acquisition differences prior to processing. We show that acquisition differences in areas with target reflector dip will always result in non-repeatability prior to regularization and that this is caused by differing reflection points. Processing elements such as 4d binning and regularization attempt to move traces to a common subsurface position (in a 4D sense) and to their bin center. They are designed to overcome differences in acquisition and cannot be overlooked in a full repeatability analysis. Since regularization and imaging destroy the association of traces and navigation attributes, a variogram analysis based on differences in source and receiver positions cannot be performed beyond these steps. Consequently it is difficult to predict final repeatability from a variogram analysis prior to regularization.