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

2010-04-122010-04-122010-04-122010-04-122010-04-122010-04-122010-04-122010-04-1222 - 2010-04-12

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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