1. A cross-equalization processing flow for off-the-shelf 4-D seismic data James Rickett Stanford University David E. Lumley Chevron Petroleum Technology Company

2. A cross-equalization processing flow for off-the-shelf 4-D seismic data James Rickett Stanford University David E. Lumley Chevron Petroleum Technology Company

3. Summary Gulf of Mexico dataset Motivation Cross-equalization issues Processing flow Results & conclusions

5. 1979 survey

6. 1991 survey

7. Motivation Cross-equalization important for 4D study –Remove processing/acquisition differences –Remaining differences are due to production Post-stack vs pre-stack –Pre-stack data not easily available –Post-stack analysis quicker and cheaper

9. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

11. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

12. 34  difference in azimuth Bin-size: 41 x 41ft vs. 247 x 82ft 1979 1991

13. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

14. Before matched-filteringAfter matched-filtering 1979 1991

15. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

16. 19791991Envelope

17. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

19. Cross-equalization issues Geometry/binning Wavelet/spectra Gain functions Differential phase/statics Migration imaging/positioning

20. 1979 survey1991 survey

21. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

22. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

23. Survey co-alignment 1979 survey –Bandpass and gain correction –Resampled from 4 to 6ms 1991 survey –Remapped onto 1979 grid –Spatial anti-alias filter –Rotation with linear interpolation Common window and mask

24. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

25. 19791991difference After co-alignment...

26. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

27. Global corrections Matched-filter –Bandwidth and phase –Least squares –Bulk shift Amplitude scale

28. 19791991difference After co-alignment...

29. 19791991difference After global corrections...

30. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

31. Non-stationary matched filtering Separate filters for each trace Design window: –0.5 s to 2 s depth (reservoir at 3 s) –3 traces wide Short operator (23 points)

32. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

33. Amplitude balancing Corrects for –Different T.V. gain functions –Incorrect amplitudes from matched-filters Assume signal >> noise –Scale data based on R.M.S. energy

34. 19791991difference After global corrections...

35. 19791991difference After non-stationary filtering/gain correction...

36. Cross-equalization flow Survey co-alignment Global operators –Matched-filtering & gain correction Non-stationary operators –Matched-filtering & gain correction Warping

37. Different velocity fields –Effects positioning of imaged events –Need residual migration operator  v unknown –Need estimate operator from the data

38. Warping Algorithm –Calculate local 3-D cross-correlation functions –Pick maxima –Obtain smoothly varying warp function –Apply vector shifts by interpolation

39. Typical “warp function”... (magnified x2)

40. After non-stationary filtering/gain correction... 19791991difference

41. 19791991difference After warping...

42. After realignment to common grid After global corrections

43. After non-stationary corrections After warping

44. "Normalized difference energy" = RMS(difference)  [ RMS(79 survey) + RMS(91 survey) ]

45. After global corrections After non-stationary corrections After warping After co-alignment Normalized difference energy

46. Conclusions Cross-equalization processing flow important for 4-D seismic monitoring Global operators not sufficient Spatially-variable operators required –Balance non-stationarity vs. degrees of freedom Warping –Residual map migration to correct for event mispositioning (e.g. due to migration velocity)

47. Conclusions Reduced non-reservoir differences using physical processing operators Enhanced reservoir differences –Now ready for 4-D interpretation

48. Acknowledgements CPTC’s 4-D team in La Habra Chevron for providing the data Sponsors of Stanford Exploration Project