This document discusses how seabed diffractions and changes in sea velocity can impact 4D seismic data quality. Through synthetic modeling and field data examples, it is shown that seabed diffractions cannot be flattened using standard processing and create asymmetric time shifts. These diffractions and diffracted multiples become non-repeatable between surveys due to acquisition mis-positioning and water velocity changes, degrading the 4D signal. Proper time shift smoothing is needed to correct for water velocity variations and seabed complexity while avoiding removal of systematic shifts from geometric errors.

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. 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. Motivation
4D difference 4D difference
Migrated stack
No water layer correction With water layer correction
1 km
a)
Classification: Internal

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. 2D modelling of point diffractions
seabed
Model
Ice scour: 5-15m wide and 0-15m deep
Modelling
NMO corrected CMPs
Classification: Internal

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. 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. 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. 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. 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. 2D FD modelling and field data
0 1km
380
(m)
385
Seabed Norne
390
Field data Norne
Classification: Internal

12. Seabed time and time shifts
Time Timeshift after tidal correction 1 swath
ms
ms
1km
Classification: Internal

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. 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. Seabed time shifts
Timeshift after tidal correction 1 swath
ms
Mean = -1ms
 DVp = ~ 2.5 m/s
Classification: Internal

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. 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. 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. 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. 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. 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. 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. 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. 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. Diffractions and diffracted multiples
FD modelling
• 2D model
seafloor
1km
1km
Reservoir
Point diffractions below reservoir
Classification: Internal

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. 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. 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. 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. 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. Classification: Internal
Classification: Internal
2010-05-03
2010-06-11Classification: Internal 2010-05-03