Time-lapse, or 4D, seismic technology is a tool to monitor underground change for oil field maximum recovery or other purpose especially associated to human being activities. Repeatability is a key issue for time-lapse seismic and geometry repeatability is a fundamental element to essentially affect the repeatability. There were many practices to improve repeatability during data processing, but geometry repeatability needs to be acquired during monitor data acquisition phase. Theory and practice shows that geometry repeatability determined in acquisition cannot be thoroughly improved in processing. Thus, geometry repeatability analysis is important. Multi-trace geometry repeatability rises from practical situation. Multi-trace geometry repeatability is important for overall geometry repeatability evaluation. The difference or repeatability of time-lapse seismic data is evaluated by normalized RMS difference (NRMS). The definition of multi-trace repeatability can be derived from NRMS for time-lapse seismic/4D seismic data, showing that multi-trace repeatability is weighted RMS of all single traces. Noting that the uncertainty of the match between monitor data and baseline data and that probable data size difference between monitor and baseline, the repeatability of the best baseline-based match with imaginary data for mismatch was employed. Derived from previous researches, the linear model of relationship between seismic data repeatability and geometry repeatability was established, and the weighted RMS geometry repeatability of the best baseline-based match with extrapolation for mismatch was obtained as the equivalent of the repeatability of the best baseline-based match with imaginary data for mismatch. The weighting coefficient is determined on the basis of NMO and its stretch. Application study was also conducted based on real data to demonstrate that the new geometry repeatability can be utilized to valuate geometry repeatability during 4D monitor seismic data acquisition. The application study showed that one display of the multi-trace geometry repeatability upon best baseline-based match with extrapolation for mismatch can indicate repeatability and effect of fold of coverage simultaneously. The calculation in application was simplified for speed improvement, which is not yet the main point in this article.
