Using Dual Azimuth Acquisition to Illuminate Beneath Salt Domes Fiona Dewey 1 Julia Bernard 2 1 Wintershall Noordzee BV 2 Now with Wintershall Erdoelwerke A.G.

Agenda The Problem Acquisition Time Processing Depth Migration Results Conclusions Acknowledgements

The Problem Poor Imaging Below Salt Structures WE ? ? 1990 Acquisition with (1999) PrSDM processing

The Problem Many gas discoveries close to salt features.

The Solution New Acquisition Determine best acquisition direction for illumination Optimize streamer length Improved Processing Techniques Improved noise suppression Prestack Depth migration taking anisotropy into account

Dip vs Strike Acquisition Industry rules: Dip Shooting Better imaging below complex overburden Strike shooting Better at imaging salt flanks Multiple energy more coherent

Illumination Study Study Objectives: Map illumination for different acquisition directions Investigate effect of increasing cable length

Illumination Study Conclusions: Illumination patchy under salt ridge N-S acquisition provides additional illumination under salt flanks Optimal cable length m

Acquisition Parameters Comparison 1990 Acquisition 2003 Acquisition Streamer length3000m5100m NavigationSyledisDGPS Configuration2 Vessels 4 Sources 4 Streamers 1 Vessel 2 Sources 6 Streamers Near Trace offset117m110m Fold of Coverage2068 CDP trace spacing75m37.5m CDP spacing In line Cross line 12.5m 25m 6.25m25m

Acquisition Program Main acquisition direction W-E (556 km²) Two additional swaths N-S ( km²)

Hide Vessel Position Tracks Shooting pattern optimised to reduce waiting on tides and maximise feather matching

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

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 ??

Working Together Contractor Illumination studyConcept systems AcquisitionPGS Time processingVeritas DGC Depth migrationWesternGeco Acquisition and processing QC Monarch Technical Services

Acknowledgement Wintershall Noordzee BV Energie Beheer Nederland BV Petro-Canada