A Perspective on the Evolution of Processing Seismic Primaries and Multiples
Authors: Weglein Arthur B. 
Published: October 2008

Abstract
Seismic reflection events are catalogued as primary or multiple depending on whether the arriving energy at the receiver has experienced one or more upward reflections, respectively. We can trace the evolution of progress in seismic processing by following the physics used to describe what the wave experiences between source and receiver. More realism, flexibility, and completeness in the model when applied appropriately, has an associated improvement in prediction and reduction in risk. Hence, there is alignment between providing step-change improvement in seismic effectiveness and a central need of the petroleum industry. Typically, primaries are considered as the signal to be analyzed to extract useful subsurface information; whereas, multiples are viewed as a form of coherent noise to be removed from seismic data. That has been the dominant conceptual and practical viewpoint and we anticipate that it will remain the model for the foreseeable future. Fifteen years ago the methods for processing primaries were conceptually more advanced and realistic in comparison to the processing of multiples; now that situation is reversed. In the 1980?s multiples were treated as deriving from a 1D earth with a known 1D velocity model whereas primaries were treated as deriving from a multi-D earth with a required multi-D velocity model. Over the past decade something very different happened to the processing of primaries and multiples. Primaries kept on their original track with every step of greater realism and reliability placing higher demands on subsurface information. For example, the differences between asymptotic, higher order asymptotic, and wave theory migration while representing a progression towards higher capability also place a commensurate higher bar on the accuracy of the velocity model. That mainstream trend in the processing of primaries continues today.
In contrast, multiple removal methods developed over the past decade, based on the inverse scattering series and feedback techniques were able to move multiple removal methods from their earlier 1D world to a multi-D heterogeneous, elastic, and anisotropic earth while not only not requiring a concomitant increase in subsurface information but in fact totally dropping the need for any subsurface information whatsoever. That represents a sea-change in thinking and capability whose benefit has yet to be fully realized in practice. In addition, we will show how the inverse scattering series provides a unique and comprehensive vision for processing primaries and multiples within the same framework and with the same assumptions. The objective of current research is to extend to the processing of primaries the same capability that the inverse series earlier first provided for removing free-surface and internal multiples. That would allow accurate depth imaging and inversion of targets beneath a complex ill-defined overburden, e.g., salt, basalt, and karsted sediments, without the current need for an adequate velocity model. We will describe this seismic evolution and discuss practical prerequisites, open issues and plans.