2010-04-12 2010-04-12 2010-04-12 2010-04-12 2010-04-12 2010-04-12 2010-04-12 2010-04-12 1 - 2010-04-12 A New Approach To Efficient 4D Acquisition Peter B. Sabel, Leif Fenstad (Statoil) Stuart Darling (ION Concept Systems)
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2010-04-122010-04-122010-04-122010-04-122010-04-122010-04-122010-04-122010-04-121 - 2010-04-12
A New Approach To Efficient 4D Acquisition
Peter B. Sabel, Leif Fenstad (Statoil) Stuart Darling (ION Concept Systems)
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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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• 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!
How do we typically shoot time-lapse (4D) seismic?
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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
The new approach: Planning phase
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Positioning error ΔP= baseline position – monitor positionOnly small steering error (< 3 m)
Scenario a) Preferable for 4D base line acquisition
4D acquisition: Source repeatability
Positioning error ΔP= baseline position – monitor positionDepending on how much was steered on base survey matching error can be significant (100m and more). Only small steering error (< 3 m)
Scenario b) Straight monitor source track on dynamic (post-plot) base survey
Scenario c) Dynamic monitor source track on dynamic (post-plot) base survey
Matching error ΔP= baseline position – monitor positionNo matching error (only if smoothing is applied), Bigger steering error (≈ 6 m)
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0° 0° 0° 0° 0° 0° 0° 0°
Survey design is based upon zero feather achieving uniform coverage
Traditional 4D acquisition: Receiver repeatability
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0° -4° +5° 0° +1°
In reality we’ll have varying feather, coverage holes and subsequent infill passes
0° -5° 0°
Traditional 4D acquisition: Receiver repeatability
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0° -4° +5° 0° +1° 0° -5° 0°
Overlap and duplicate coverage exists within the baseline survey
Traditional 4D acquisition: Receiver repeatability
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…adjust feather on the prime lines to reduce overlap and improve the coverage.…remove the infill lines….Following baseline coverage analysis we assess the overlap...
The new approach: Receiver repeatability
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Base 1st monitor 2nd monitor
= High quality baseline = Coverage issues
Over-coverage and undisciplined infill
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Source tracks for three vintages
Over-coverage and undisciplined infill
Base line 1st monitor 2nd monitor
225 m nominal sail line distance
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• 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
New 4D monitor strategy
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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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Removal of excess linesCMP 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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4-5 bins overlap
7-8 bins overlap
4-5 unique bin columns
The Feather Aperture concept during planning
• Overlap from adjacent lines.
• Unique coverage from central line.
• Calculate overlapping bins.
• Calculate feather aperture.
• Want to get an ideal match but aperture defines feather limits to avoid infill pass
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7-8 bins overlap
4-5 bins overlap
• Three lines
• Target feather adjusted
• Central line
• High overlap
• large feather aperture
4-5 unique bin columns
The Feather Aperture concept during acquisition
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1-2 bins overlap
10-11 unique bin columns
4-5 bins overlap
• Port line acquired
• High feather mismatch
• Central line
• New unique coverage zone
• Reduced overlap
• Reduced feather aperture
The Feather Aperture concept during acquisition
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• Overlapping bins change as lines are acquired
• Feather apertures must be recalculated dynamically
The Feather Aperture concept during acquisition
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Mean Feather Diff = 2.1
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
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• 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
20Feather prediction and feather aperture
Prediction 1 Prediction 2 Measured current
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• 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
21Recommendations
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A New Approach To Efficient 4D Acquisition
Peter B. Sabel, Leif Fenstad (Statoil) and Stuart Darling (ION Concept Systems)
Thank you
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