MULTIPLE ELIMINATION USING WAVEFIELD TRANSFORMATIONS OFF THE COAST OF WESTERN AUSTRALIA

Abstract

There are two principle reasons why water bottom multiples off the coast of Western Australia can be very difficult to attenuate:A strongly reflective sea floor (often caused by shallow carbonates) gives multiples large amplitudes compared with the primary events they overlay.A widely occurring velocity inversion, beneath the carbonates, causes multiples and primaries to have similar moveouts.A range of processes are commercially available to attenuate multiples, including FK Demultiple, Radon Demultiple, and Predictive Deconvolution. These methods can be very successful under the right conditions. Two dimensional autoconvolution methods, although very promising, still have drawbacks and are extremely computationally expensive.Two new wavefield transformations, Multiple MoveOut (MMO) and IsoStretch Radial Trace (ISR), have been developed to precondition data prior to the removal of surface related multiples by existing techniques. These form the basis of a new multiple attenuating procedure.MMO shifts the data so that the simple water bottom multiples become periodic with the primary event. Water bottom pegleg multiples become approximately periodic.ISR interpolates oblique traces of constant stretch which also approximately map constant angles of incidence on the sea floor. The water bottom primary and multiple events form stationary time series after ISR. They are then amenable to removal by Event Prediction (one dimensional autoconvolution) or Predictive Deconvolution.The results of the new procedure are demonstrated on field data from off-shore Western Australia. It is shown to be more effective at removing both simple and pegleg water bottom multiples than traditional techniques. Finally, it is not computer intensive and does not require velocity analysis prior to its application (besides estimate of water velocity).