Schlumberger

Time-shift Decomposition

The removal of timing differences between time-lapse surveys

We frequently observe time shifts between the surveys that comprise a time-lapse (4D) seismic dataset. The timing differences may be consistent, caused, for example, by different acquisition reference time definitions, or they may vary from line to line or shot to shot, perhaps as a result of variable source timing. The observed timing differences are frequently the sum of timing errors that exist on each of the surveys that are being compared. Inaccurate navigation data, common with older surveys, may result in consistent errors in the source-to-detector distances that cause time- and offset-variant time shifts between surveys. These offset errors often occur simultaneously with timing errors. It is critical that timing errors are corrected without affecting those time differences between surveys that are caused by changes in the subsurface.

Consistent timing differences are addressed by Global Matching. We have implemented a series of workflows known as time-shift decomposition, which can correct variable survey timing differences without removing time-lapse signal.

Time-shift decomposition uses estimates of the relative time shifts between surveys, or absolute timing measurements (for example, the time to a prominent horizon) made on each survey. Decomposition techniques analogous to surface-consistent decomposition allow estimation of absolute timing errors for each survey, provided that the errors are consistent over a given attribute. Offset and timing errors can be solved for simultaneously if we can assume that one survey is accurate.

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Correct Variable Survey Timing Differences

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