Modelling and Removing WAZ OBC Interbed Multiples

Abstract

Interbed multiples, caused by ringing between strong reflectors deeper within the section, have been historically difficult to remove. Conventional separation methods, based on multiple periodicity or residual moveout, will typically fail. As an example, interbed multiples from limestone packages can actually have a faster apparent velocity than the real primaries. Different techniques have to be used in these situations. Two different interbed demultiple methods have been tried successfully on a recent 3D Ocean Bottom Cable (OBC) seismic survey (Tridacna survey) over the North Scott Reef offshore Australia. In these data, strong events in the upper section do produce interbed multiples below, -and with very little normal moveout discrimination. This paper will present a new (and old) development to resolve our imaging problem. Solution 1: Old method adapted to this: a horizon-based pattern-recognition algorithm is used pre-stack on migrated data. A robust algorithm. Assumptions, limitations, results, where-when will be discussed. Solution 2: New method with new methodology and tools: 3D Interbed removal by (deterministic) wavefield modelling. Adapting a 3D SRME (surface multiple) approach to interbeds. Originally this uses one-way WE to model interbeds between the reflectors situated under and over a horizon. We now run this many times over many windows to make all possible multiples, and adaptively subtract in 3D all models at once. We hit upon a new constraint, -the amplitude spectra, to stabilize even more. Runs per-migration. Successful results (may be a first here) -requiring no interpretation. The new algorithms and ideas will be described/shared. Discussion results, limitations, etc.