Sea bed diffraction and impact on 4d seimsic data eage2010-baard osdal_notes

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Sea bed diffraction and impact on 4D seismic data-observations from synthetic modeling and field data by B. Osdal, Statoil and S. Grude and M. Landrø, NTNU

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Sea bed diffraction and impact on 4d seimsic data eage2010-baard osdal_notes

  1. 1. Sea bed diffraction and impact on 4D seismic data – observations from synthetic modeling and field data B.Osdal, Statoil, S.Grude and M.Landrø, NTNU Classification: Internal
  2. 2. Objective • Better understand the influence of sea bed diffractions on water layer correction in 4D processing – Corrections for water layer changes very important in 4D processing – Diffracted multiples degrade 4D quality Classification: Internal
  3. 3. Motivation 4D difference 4D difference Migrated stack No water layer correction With water layer correction 1 km a) Classification: Internal
  4. 4. Sea bed diffractions and impact on time shift correction • Data examples – 2D FD modelling of seabed diffractions – Field data from Norne 2006 and 2008 Classification: Internal
  5. 5. 2D modelling of point diffractions seabed Model Ice scour: 5-15m wide and 0-15m deep Modelling NMO corrected CMPs Classification: Internal
  6. 6. 2D modelling of point diffractions Observations: • Diffractions can not be flattened using standard primary reflection NMO correction • Asymmetrical amplitude strength  asymmetrical ice scour shape is likely Real Modelling NMO corrected CMPs Classification: Internal
  7. 7. Acquisition mis-position and seabed time shift  Acquisition mis-position - 0.2 0m 5m 10m - 0.4 Timeshift (ms) - 0.6 - 0.8 - 1.0 - 200 - 100 0 100 200 - 200 - 100 0 100 200 - 200 - 100 0 100 200 Distance from apex (m) seabed Model Classification: Internal Ice scour: 5-15m wide and 0-15m deep
  8. 8. Acquisition mis-position and seabed time shift  Acquisition mis-position 0m 5m 10m - 0.4 Timeshift (ms) - 0.6 - 0.8 - 1.0 - 200 - 100 0 100 200 - 200 - 100 0 100 200 - 200 - 100 0 100 200 Distance from apex (m) Mis-positioning creates non-negligible time shift error Classification: Internal
  9. 9. Approximations for time shift error caused by tuning Harmonic input signal , V1, V2 - base and monitor sea velocity  - Amplitude ratio between seabed and diffraction h – water depth x – distance from diffraction apex a= mis-positioning between base and monitor Valid for small offsets, where the two signal are inside same lobe Classification: Internal
  10. 10. Approximations for time shift error caused by tuning -0.2 0m 5m 10m Timeshift (ms) - 0.6 - 1.0 approximation - 1.4 -80 -40 0 40 80 -80 -40 0 40 80 -80 -40 0 40 80 Distance from apex (m) Approximation ok up to ca. 40m from apex point Classification: Internal
  11. 11. 2D FD modelling and field data 0 1km 380 (m) 385 Seabed Norne 390 Field data Norne Classification: Internal
  12. 12. Seabed time and time shifts Time Timeshift after tidal correction 1 swath ms ms 1km Classification: Internal
  13. 13. Seabed time shifts Timeshift after tidal correction 1 swath ms Time shift related to:  Variation of sea velocity Mean = -1ms (lateral and with calendar time)  DVp mis-positioning  Geometrical = ~ 2.5 m/s  Error in tidal correction  Structural dip and complexity of Classification: Internal seabed
  14. 14. Seabed time shifts Timeshift after tidal correction 1 swath ms Time shift related to:  Variation of sea velocity (lateral and with calendar time)  Geometrical mis-positioning  Error in tidal correction  Structural dip and complexity of Classification: Internal seabed
  15. 15. Seabed time shifts Timeshift after tidal correction 1 swath ms Mean = -1ms  DVp = ~ 2.5 m/s Classification: Internal
  16. 16. Velocity profiles 2006 and 2008 Calculated means 2006 Calculated means 2008 280706: 1488 m/s 080608. 1484 m/s 310706: 1487 m/s 170608: 1486 m/s 030806: 1486 m/s 090806. 1487 m/s 140806: 1487 m/s 180806: 1487 m/s -”Observed” velocity change ~2-3 m/s - variation within each survey 2006 2008 Based on measured temperatures and salinities Classification: Internal
  17. 17. Seabed time shifts Timeshift after tidal correction 1 swath ms Time shift related to:  Variation of sea velocity (lateral and with calendar time)  Geometrical mis-positioning  Error in tidal correction  Structural dip and complexity of Classification: Internal seabed
  18. 18. Time shift related to complex sea bottom and geometrical mis-position Seabed time shift (ms) mean Blue = Best line Red = Worst line DCMP (m) 100m Seafloor(m) Classification: Internal
  19. 19. Time shift related to complex sea bottom and geometrical mis-position Seabed time shift (ms) mean 2D FD modelling (DVp=3m/s) 100m Seafloor(m) Classification: Internal
  20. 20. Time shift related to complex sea bottom and geometrical mis-position Seabed time shift (ms) mean 2D FD modelling (DVp=3m/s) Observation  Seabed time shift strongly dependent on geometrical Classification: Internal repeatability and complexity of the seabed
  21. 21. Complex sea bottom and geometrical mis-position Smoothing of time shift Seabed (m) Modelled data – Too weak smoothing Seabed time shift (ms)  Smoothing line by line remove much of the disturbance from complex sea bottom - assume that the seabed irregularities do not create systematic time shift error and strong smoothing is needed Classification: Internal
  22. 22. Complex sea bottom and geometrical mis-position Smoothing of time shift Field Data - Seabed time shift (ms) Blue = Best line Red = worst line 100m mean  Systematic time shift related to geometrical mis-positioning occurs - Should not correct for this since this is handled in the 3D migration. Strong smoothing or one average time shift value for each swath is therefore preferred. Classification: Internal
  23. 23. Complex sea bottom and geometrical mis-position Smoothing of time shift Timeshift correction after tidal correction Timeshift after tidal correction Mean for each line 1 swath ms Classification: Internal
  24. 24. Motivation Diffractions and diffracted multiples Unmigrated Final 3D migration 1km ms 4D diff 4D diff 4D diff Diffracted multiples a problem in 4D  difficult to repeat in 4D  difficult to remove in processing
  25. 25. Diffractions and diffracted multiples FD modelling • 2D model seafloor 1km 1km Reservoir Point diffractions below reservoir Classification: Internal
  26. 26. Diffractions and diffracted multiples 4D data quality – Unmigrated data Water corr. applied Water corr. applied Base No mis-position Mis-position 5m Classification: Internal Dv=0m/s 3m/s Dv=0m/s 3m/s
  27. 27. Diffractions and diffracted multiples 4D data quality – Migrated data Water corr. applied Water corr. applied Base No mis-position Mis-position 5m Classification: Internal Dv=0m/s 3m/s Dv=0m/s 3m/s
  28. 28. Diffractions and diffracted multiples 4D data quality – Migrated data Water corr. applied Water corr. applied For 4D data diffraction and diffracted multiples become non- repeatable and degrade 4D data quality due to - acquisition mis-positioning - changes in sea velocity Base No mis-position Mis-position 5m Classification: Internal Dv=0m/s 3m/s Dv=0m/s 3m/s
  29. 29. Conclusions  Seabed time shifts strongly depend on changes in water velocity, geometrical repeatability and complexity of the seabed  Applying constant or strong seabed time shift smoothing line by line will correct for changes in water velocity, error in tidal correction and complexity of sea bottom. For 4D data diffraction and diffracted multiples are non-repeatable due to - acquisition mis-positioning - changes in sea velocity Classification: Internal
  30. 30. Acknowledgments • Statoil and their partners ENI Norge AS and Petoro for permission to use their data • NFR for financial support to the ROSE project at NTNU • Børge Arntsen and Jon A. Haugen and for help and support with the FD modelling and Hossein Mehdi Zadeh for Promax help Classification: Internal
  31. 31. Classification: Internal Classification: Internal 2010-05-03 2010-06-11Classification: Internal 2010-05-03

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