A New Approach To Efficient 4D Acquisition by Peter B. Sabetl, Leif Fenstad, Statoil and Stuart Darling (ION Concept Systems)

1. A New Approach To Efficient 4D
Acquisition
Peter B. Sabel, Leif Fenstad (Statoil)
Stuart Darling (ION Concept Systems)
1- 2010-04-12

2. Agenda
• How do we typically shoot marine time-lapse (4D) seismic?
• What is the new approach?
– What do we do differently during the planning phase?
– What do we do differently during the acquisition phase?
• Recommendations
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3. How do we typically shoot time-lapse (4D) seismic?
• We repeat 3D surveys
– Design to minimise differences between the acquisitions, thus
suppressing 4D noise and highlighting the wanted 4D effect
• Acquisition configuration can be controlled (to some extent)
– Choose same source & streamer depth, same source, same guns,
same cable separation, same vessel …
• Repeating positions of marine towed surveys is not so easy
– Feather!
We have made our lives unnecessarily difficult by accumulating problems
during each vintage with traditional 3D-thinking for 4D acquisition!
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4. The new approach: Planning phase
Analysis strategy for base line data
• How to re-position the source?
– Straight line versus dynamic line
• How to re-position the receivers?
– Active streamer steering, how many overlapping cables?
• New approach
– Coverage, “overkill coverage” and clean-up
– How much feather deviation we can tolerate?
• Robustness criterion: Feather Aperture
– All lines for the monitor survey will receive an associated, line
specific, feather aperture value
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5. 4D acquisition: Source repeatability
Scenario a) Preferable for 4D base line acquisition
Positioning error ΔP= baseline position – monitor position
Only small steering error (< 3 m)
Scenario b) Straight monitor source track on dynamic (post-plot) base survey
Positioning error ΔP= baseline position – monitor position
Depending on how much was steered on base survey matching error can
be significant (100m and more). Only small steering error (< 3 m)
Scenario c) Dynamic monitor source track on dynamic (post-plot) base survey
Matching error ΔP= baseline position – monitor position
No matching error (only if smoothing is applied), Bigger steering error (≈ 6 m)
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6. Traditional 4D acquisition: Receiver repeatability
0° 0° 0° 0° 0° 0° 0° 0°
Survey design is based upon zero feather achieving uniform coverage
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7. Traditional 4D acquisition: Receiver repeatability
0° -4° +5° 0° +1° 0° -5° 0°
In reality we’ll have varying feather, coverage holes and subsequent infill passes
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8. Traditional 4D acquisition: Receiver repeatability
0° -4° +5° 0° +1° 0° -5° 0°
Overlap and duplicate coverage exists within the baseline survey
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9. The new approach: Receiver repeatability
…adjust feather on the prime lines to reduce we assess the overlap... coverage.
Following baseline lines…. analysis overlap and improve the
…remove the infill coverage
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10. Over-coverage and undisciplined infill
Base 1st monitor 2nd monitor
= High quality baseline = Coverage issues
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11. Over-coverage and undisciplined infill
225 m nominal sail line distance
Source tracks for three vintages
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Base line 1st monitor 2nd monitor 11

12. New 4D monitor strategy
• Traditional 4D monitors target replication of ALL lines
– Process becomes increasingly inefficient with each vintage
• How can we maintain 4D repeatability and minimise the number of
acquisition passes?
– We must examine vintage sail lines for their unique contribution
– Look at ΔSrc & ΔRec versus expected dB difference in 4D signal
– Remove excess lines from the base line & previous monitor
– Attach a target feather to all lines in order to improve coverage
– Based on chosen vessel’s cable capacity calculate line specific
feather aperture value
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13. Removal of excess lines
Important elements when doing “line clean-up”
• Analyse vintage sail lines for their unique contribution
• Based on field specific acceptable ΔSrc & ΔRec criteria obsolete lines
can be removed without sacrificing coverage and repeatability
15 lines removed => 1.9M US$ saved
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14. Removal of excess lines
CMP FAR (3600m offset)
Pre removal
Post removal
Important elements when doing “line clean-up”
• Analyse vintage sail lines for their unique contribution
• Based on field specific acceptable ΔSrc & ΔRec criteria obsolete lines
can be removed without sacrificing coverage and repeatability
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15. The Feather Aperture concept during planning
• Overlap from adjacent lines.
• Unique coverage from central line.
• Calculate overlapping bins.
• Calculate feather aperture.
7-8 bins overlap • Want to get an ideal match but
aperture defines feather limits to
4-5 bins overlap avoid infill pass
4-5 unique bin columns
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16. 16
The Feather Aperture concept during acquisition
• Three lines
• Target feather adjusted
• Central line
• High overlap
4-5 bins overlap • large feather aperture
7-8 bins overlap
4-5 unique bin columns
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The Feather Aperture concept during acquisition
• Port line acquired
• High feather mismatch
• Central line
• New unique coverage zone
• Reduced overlap
4-5 bins overlap
1-2 bins overlap • Reduced feather aperture
10-11 unique bin columns
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The Feather Aperture concept during acquisition
• Overlapping bins change as lines
are acquired
• Feather apertures must be
recalculated dynamically
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19. 19
Feather matching & performance QC measures
• Line selection based on baseline feather matching does not always
yield the best results:
 Baseline feather match of 1.3 is out with the Feather Aperture
 Baseline feather match of 2.1 is within the Feather Aperture
• Feather prediction must now be designed to comply with the Feather
Aperture
Mean Feather Diff = 1.3 Mean Feather Diff = 2.1
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20. 20
Feather prediction and feather aperture
• Relation between feather aperture and feather prediction
– Periods of high confidence => approach lines with narrow feather aperture
– Periods of low confidence => approach lines with wide feather aperture
Prediction 1
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Prediction 2 Measured current

21. 21
Recommendations
• Exactly repeat previous acquisition is not the optimal 4D strategy
– Will lead to increasingly inefficient monitor surveys with each vintage
• Baseline needs analysis on how to efficiently repeat source & receiver positions
– Coverage, “overkill coverage” and perform clean-up
– ΔSrc & ΔRec vs. expected dB 4D signal
– How much feather deviation can we tolerate?
– Dependant on seismic vessel’s cable capacity
– Robustness measure: Feather Aperture
– Lines of the next monitor survey will receive a specific feather aperture value
• During acquisition the feather aperture concept helps with line prioritisation
– Maximising feather windows, resulting in higher efficiency
• Dynamic infield feather aperture adjustment
– Potentially new pre-plot in case coverage target was NOT achieved
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22. Thank you
A New Approach To Efficient 4D Acquisition
Peter B. Sabel, Leif Fenstad (Statoil) and Stuart Darling (ION Concept Systems)
22 - 2010-04-12

23. 2010-04-12