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Using Dual Azimuth Acquisition to Illuminate

Beneath Salt Domes

Fiona Dewey 1

Julia Bernard 2

1 Wintershall Noordzee BV2 Now with Wintershall Erdoelwerke A.G.

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Agenda

The Problem Acquisition Time Processing Depth Migration ResultsConclusions Acknowledgements

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The ProblemPoor Imaging Below Salt Structures

W E

??

1990 Acquisition with (1999) PrSDM processing

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The Problem

Many gas discoveries close to salt features.

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The Solution

New AcquisitionDetermine best acquisition direction for

illuminationOptimize streamer length

Improved Processing TechniquesImproved noise suppressionPrestack Depth migration taking

anisotropy into account

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Dip vs Strike Acquisition

Industry rules:

Dip Shooting

Better imaging below complex overburden

Strike shooting

Better at imaging salt flanks

Multiple energy more coherent

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Illumination Study

Study Objectives:

Map illumination for different acquisition directions

Investigate effect of increasing cable length

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Illumination Study

Conclusions:

Illumination patchy under salt ridge

N-S acquisition provides additional illumination under salt flanks

Optimal cable length 5000-6000 m

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Acquisition Parameters Comparison1990 Acquisition

2003 Acquisition

Streamer length 3000m 5100m

Navigation Syledis DGPS

Configuration 2 Vessels4 Sources4 Streamers

1 Vessel2 Sources6 Streamers

Near Trace offset 117m 110m

Fold of Coverage 20 68

CDP trace spacing 75m 37.5m

CDP spacing In line Cross line

12.5m25m

6.25m25m

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2003 Acquisition Program

Main acquisition direction W-E (556 km²)

Two additional swaths N-S (196 +185 km²)

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Hide

Vessel Position Tracks

Shooting pattern optimised to reduce waiting on tides and maximise feather matching

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Conclusions

Illumination differences seen on sides and under salt structures

Away from salt illumination differences are small

In less complex areas summation of PrSTM data sets gives improved signal to noise

Depth migration moves steep dips to similar locations

Anisotropy parameters defined for each orientation

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Summary

Orthogonal acquisition combined with long streamers has illuminated previously hidden parts of the subsurface

Challenges still remain in processing to define one unique velocity field that will account for the differences in anisotropy of the two acquisition directions

Does the imaging improvements justify the extra cost ??

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Working Together

Contractor

Illumination study Concept systems

Acquisition PGS

Time processing Veritas DGC

Depth migration WesternGeco

Acquisition and processing QC

Monarch Technical Services

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Acknowledgement

Wintershall Noordzee BV

Energie Beheer Nederland BV

Petro-Canada