Surface multiple attenuation and sub-salt imaging

Abstract

Surface multiple attenuation (SMA) is a prestack inversion of a surface-recorded, 2-D wavefield that removes all orders of all surface multiples present within the wavefield. In addition, the process determines the average acquisition wavelet embedded in the wavefield. SMA requires no assumptions or modelling regarding the positions, shapes, or reflection coefficients of the multiple-causing reflectors. Instead, SMA relies on the internal physical consistency between primary and multiple events that must exist in any properly recorded marine data set. A general SMA inversion equation can be derived in a simple fashion from just two assumptions: (1) the recorded wavefield is a superposition of primary events, 1st-order surface multiples, 2nd-order surface multiples . . . and Nth-order surface multiples; and (2) a recursive relationship can be found that allows Nth-order surface multiples to be predicted given the primary wavefield and the surface multiples of order N-1. A specific inversion equation is obtained depending on the nature of the wavefield and whatever relationship is used to satisfy assumption (2). Thus, for a 1-D Earth, SMA inversion is accomplished by deconvolution, while for a 2-D Earth, one can use the Kirchhoff integral or the 2-D scalar wave equation in assumption (2). The Kirchhoff integral approach, described in this paper, formulates SMA in the f-x domain, in which the inversion is accomplished simply by inverting a matrix. Because SMA handles all surface multiples, it is an ideal method of removing surface multiples generated by the top and bottom of a salt-injection feature. An example line from the Gulf of Mexico has shown that multiple attentuation via SMA can greatly improve the final migrated image obtained below a salt layer.