sea bed diffraction and impact on 4d seimsic data eage2010-baard osdal_notes
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Classification: Internal
Sea bed diffraction and impact on 4D seismic data –observations from synthetic
modeling and field dataB.Osdal, Statoil,
S.Grude and M.Landrø, NTNU
Classification: Internal
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
Motivation
a)
4D differenceNo water layer correction
1 km
4D differenceWith water layer correctionMigrated stack
Classification: Internal
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
2D modelling of point diffractions
Model seabed
Ice scour: 5-15m wide and 0-15m deep
Modelling
NMO corrected CMPs
Classification: Internal
2D modelling of point diffractions
Real
Modelling
Observations:• Diffractions can not be flattened using standard primary reflection NMO correction• Asymmetrical amplitude strength asymmetrical ice scour shape is likely
NMO corrected CMPs
Classification: Internal
Acquisition mis-position and seabed time shift
Acquisition mis-position- 0.2
Distance from apex (m)- 100 0- 200 200100
Tim
eshi
ft (m
s) - 0.4
- 0.6
- 0.8
- 1.0
5m0m 10m
- 100 0- 200 200100 - 100 0- 200 200100
Model seabed
Ice scour: 5-15m wide and 0-15m deep
Classification: Internal
Acquisition mis-position and seabed time shift
Acquisition mis-position
Mis-positioning creates non-negligible time shift error
Distance from apex (m)- 100 0- 200 200100
Tim
eshi
ft (m
s) - 0.4
- 0.6
- 0.8
- 1.0
5m0m 10m
- 100 0- 200 200100 - 100 0- 200 200100
Classification: Internal
Approximations for time shift error caused by tuning
V1, V2 - base and monitor sea velocity
,
- Amplitude ratio between seabed and diffraction
h – water depth
x – distance from diffraction apexa= mis-positioning between base and monitor
Valid for small offsets, where the two signal are inside same lobe
Harmonic input signal
Classification: Internal
Approximations for time shift error caused by tuning
Approximation ok up to ca. 40m from apex point
Distance from apex (m)0 80
Tim
eshi
ft (m
s)
-0.2
- 0.6
- 1.0
- 1.4
5m0m 10m
-40 40-80
approximation
0 80-40 40-80 0 80-40 40-80
Classification: Internal
2D FD modelling and field data0 1km
Seabed Norne
385
380(m)
390Field data Norne
Classification: Internal
Seabed time and time shifts
ms
Time Timeshift after tidal correction 1 swath
ms
1km
Classification: Internal
Seabed time shifts
ms
Mean = -1ms
DVp = ~ 2.5 m/s
Timeshift after tidal correction
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 seabed
1 swath
Classification: Internal
Seabed time shifts
ms
Timeshift after tidal correction
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 seabed
1 swath
Classification: Internal
Seabed time shifts
ms
Mean = -1ms
DVp = ~ 2.5 m/s
Timeshift after tidal correction 1 swath
Classification: Internal
Velocity profiles 2006 and 2008
2008
Calculated means 2006280706: 1488 m/s310706: 1487 m/s030806: 1486 m/s090806. 1487 m/s140806: 1487 m/s180806: 1487 m/s
Calculated means 2008080608. 1484 m/s170608: 1486 m/s
-”Observed” velocity change ~2-3 m/s- variation within each survey
Based on measured temperatures and salinities
2006
Classification: Internal
Seabed time shifts
ms
Timeshift after tidal correction
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 seabed
1 swath
Classification: Internal
Time shift related to complex sea bottom and geometrical mis-position
DCMP (m)Blue = Best lineRed = Worst line
Seafloor(m)100m
mean
Seabed time shift (ms)
Classification: Internal
Time shift related to complex sea bottom and geometrical mis-position
Seafloor(m)100m
mean
Seabed time shift (ms)
2D FD modelling(DVp=3m/s)
Classification: Internal
Time shift related to complex sea bottom and geometrical mis-position
mean
Seabed time shift (ms)
2D FD modelling(DVp=3m/s)
Observation Seabed time shift strongly dependent on geometrical repeatability and complexity of the seabed
Classification: Internal
Complex sea bottom and geometrical mis-positionSmoothing of time shift
Modelled data –Seabed time shift (ms) Too weak smoothing
Seabed (m)
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
Complex sea bottom and geometrical mis-positionSmoothing of time shift
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.
Field Data - Seabed time shift (ms)
100mBlue = Best lineRed = worst line
mean
Classification: Internal
Complex sea bottom and geometrical mis-positionSmoothing of time shift
ms
Timeshift after tidal correctionTimeshift correction after tidal correction Mean for each line 1 swath
MotivationDiffractions and diffracted multiples
Unmigrated
4D diff
Diffracted multiples a problem in 4D difficult to repeat in 4D difficult to remove in processing
Final 3D migration
1km
ms4D diff
4D diff
Classification: Internal
Diffractions and diffracted multiples FD modelling
• 2D model
seafloor
1km
1km
Reservoir
Point diffractions below reservoir
Classification: Internal Base
Water corr. applied Water corr. applied
Diffractions and diffracted multiples 4D data quality – Unmigrated data
No mis-positionDv=0m/s 3m/s
Mis-position 5mDv=0m/s 3m/s
Classification: Internal Base No mis-position
Dv=0m/s 3m/sMis-position 5m
Dv=0m/s 3m/s
Water corr. applied Water corr. applied
Diffractions and diffracted multiples 4D data quality – Migrated data
Classification: Internal Base No mis-position
Dv=0m/s 3m/sMis-position 5m
Dv=0m/s 3m/s
Water corr. applied Water corr. applied
Diffractions and diffracted multiples 4D data quality – Migrated data
For 4D data diffraction and diffracted multiples become non- repeatable and degrade 4D data quality due to
- acquisition mis-positioning- changes in sea velocity
Classification: Internal
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
Acknowledgments
• Statoil and their partners ENI Norge AS and Petorofor 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 Classification: Internal 2010-06-11Classification: Internal 2010-05-03Classification: Internal 2010-05-03
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