Introduction

Surface-related multiple elimination (SRME) uses the recorded seismic data to predict and iteratively subtract the multiple series Verschuur et al. (1992). 2D SRME can deal with all kinds of 2D multiples, provided enough data are recorded given the offset limitations of the survey line. Diffracted multiples from scatterers with a cross-line component cannot be predicted by 2D SRME but in principle can be predicted by 3D SRME as long as the acquisition is dense enough in both in-line and cross-line directions. With standard marine streamer acquisition, the sampling in the cross-line direction is too coarse and diffracted multiples need to be removed by other methods Hargreaves et al. (2003) or the data need to be interpolated and extrapolated to a dense, large aperture grid Biersteker (2001); Nekut (1998); van Dedem and Verschuur (1998). In general, multiples may not have their moveout apex at zero offset on a CMP gather. Peg-leg multiples ``split'' into independent events when reflectors dip. These events look similar to diffracted multiples and may similarly hamper standard Radon demultiple and velocity analysis. Hargreaves et al. 2003 proposed a shifted hyperbola approach to attenuate split or diffracted multiples in CMP gathers. This approach, however, relies on the moveout of the multiples to be well approximated by hyperbolas in data space, which is problematic in complex media. A similar apex-shifted Radon transform was proposed by Trad 2002 for data interpolation.

In most situations in which diffracted multiples are a serious problem, the wave propagation is rather complex, for example for multiples diffracted off the edge of salt bodies. Thus, the moveout of primaries and multiples tend to be very complex, making the application of data-space moveout-based methods for the removal of multiples difficult. In ADCIGs, however, since the complexity of the wavefield has already been taken into account by prestack migration (to the extent that the presence of the multiples allows an accurate enough estimation of the migration velocity field), the residual moveout of multiples is generally smoother and better behaved Sava and Guitton (2003).

In this paper we focus on attenuating diffracted multiples in ADCIGs by redefining the tangent-squared Radon transform of Biondi and Symes 2003 to add an extra dimension to account for the shift in the apexes of the moveout curves of the diffracted multiples. We show with a 2D seismic line from the Gulf of Mexico that our approach is effective in attenuating both, the specularly-reflected and the diffracted multiples. In contrast, ignoring the apex shift results in poor attenuation of the diffracted multiples.

The real impact of our method for attenuating diffracted multiples is likely to be in 3D rather than in 2D, though the results that we show in this paper are limited to 2D. Biondi and Tisserant 2003 have presented a method for computing 3D ADCIGs from full 3D prestack migration. These 3D ADCIGs are functions of both the aperture angle and the reflection azimuth. Simple ray tracing modeling shows that out-of-plane multiples map into events with shifted apexes (like the 2D diffracted multiples) and different reflection azimuth than the primaries. Attenuation of these multiples from 3D ADCIGs can be accomplished with a methodology similar to one we present in this paper.