Suppression of multiples from complex sea-floor by a wave- equation approach Dmitri Lokshtanov Norsk Hydro Research Centre, Bergen. Dmitri Lokshtanov Norsk Hydro Research Centre, Bergen.

Outline Main features of WE approach Suppression of water-layer multiples and peg-legs Prediction of water-layer multiples and peg-legs Conclusions

WE approach - t he main features Suppresses water-layer multiples and peg-legs. Requires approximate knowledge of the water-bottom geometry. The predicted multiples are split into three terms. Each term contains multiple events which require the same amplitude correction. All multiples of all orders are suppressed simultaneously in one consistent step (in one or a few time windows). The prediction of multiples is performed in the same domain as used for multiple suppression. Both multiple reflections and diffractions are predicted.

Subtraction of water-layer multiples - 1 Subtraction of water-layer multiples - 1

The operator P g transforms the primary reflection event recorded at receiver 1 into the multiple event recorded at receiver 2 (Wiggins, 1988; Berryhill & Kim, 1986).

Subtraction of water-layer multiples - 2

Two approaches for multiple prediction Simple ‘locally’ 1D sea-floor; arbitrary 2D structure below it. The procedure starts from the Radon transformed CMP gathers Complex sea-floor; arbitrary 2D structure below it. The procedure starts from the Radon transformed CS gathers.

 T. Shetland  T. Draupne  T. Brent Stack before multiple suppression Stack after WE multiple suppression

Constant P sections (angle at the surface is about 10º) Input After WE multiple suppression

Stack before multiple suppression (left) and after WE multiple suppression (right). The pink line shows the expected position of the first-order water-layer peg-leg from the Top Cretaceous (black line). The multiple period is about 140 msec.

Constant P sections (angle at the surface is about 8º) Input After WE multiple suppression Difference

Prediction of multiples from the receiver side for irregular sea-floor

Prediction of multiples from the source side for irregular sea-floor

Velocity model for FD modelling (with ProMax)

Input P-section (zero angle) Receiver-side prediction

Input P-section (zero angle) After prediction + subtraction

Input P-section (angle 20 degrees) Receiver-side prediction

Input P-section (20 degrees) Receiver-side prediction Source-side prediction

Input P-section (20 degrees) After prediction + subtraction

Input CS gather After prediction + subtraction

Recent papers on WE approach Jiao J., Leger P. and Stevens J, 2002, Enhancements to wave-equation multiple attenuation, 72 nd SEG Meeting, Expanded Abstracts. Hill R., Langan R., Nemeth T., Zhao M. and Bube K., 2002, Beam methods for predictive suppression of seismic multiples in deep water, 72 nd SEG Meeting, Expanded Abstracts. Hugonnet P., 2002, Partial surface related multiple elimination, 72 nd SEG Meeting, Expanded Abstracts.

WE approach – performs well if the main free-surface multiples are water-layer multiples and peg-legs and if the structural variations in the crossline direction are not severe Both multiple reflections and multiple diffractions are accounted for. All predicted multiples of all orders are suppressed simultaneously in one consistent step The prediction of multiples is performed in the same domain as used for multiple suppression Conclusions Conclusions

Acknowledgements Acknowledgements Many thanks to Norsk Hydro and CREWES for one year of freedom in a fantastic country – Canada ! Thanks to Norsk Hydro for permission to present the paper.