Identifying multiples on seismic sections

Abstract

One of the most serious difficulties faced by geophysicists when interpreting seismic sections from areas like the Northwest Shelf of Australia is the problem of distinguishing multiples from primaries. Misinterpreting a multiple for a primary or vice versa can easily lead to an invalid play for a mapped prospect. Current techniques for identifying multiples on migrated sections are based on periodicity or dip and tend to be speculative and ambiguous. Conflicting dips can not always be resolved by associating one event with a multiple from higher in the section. Identifying an event as part of a reverberation is difficult, especially if processes like predictive deconvolution have been applied pre-stack to suppress periodicities. Correlation with synthetics can also be used when well data are available, but even then the interpreter is rarely able to identify multiple events with certainty. The root of the problem is that current seismic processing techniques for multiple suppression are not fully effective. To demonstrate this, percentage differences in wavelet amplitudes between stack sections produced with and without processes like f-k demultiple are shown. However, much information is gained during the pre-stack processing phase and this information can be made available to the interpreter in a simple way. Multiples and primaries can often be separated with less ambiguity on CMP gathers (during velocity analysis) than they can be after stack. A colour overlay technique is proposed which labels events (both primaries and multiples) on the section by velocity. This labelling is derived during velocity analysis from the deviation of semblance peaks from those corresponding to the primary velocity function. These event labels depend on the choice of primary velocity function and the pre-processing leading up to velocity analysis. The parameters are investigated for key processes used for velocity analysis, such as f-k demultiple, and it is shown how parameter selection in velocity pre-processing affects the interpreter's final decision as to what is a primary and what is a multiple event. The application of these techniques to data from the Timor Sea is presented as a case study. A seismic section with a colour-coded overlay is used to demonstrate how an interpreter may incorporate this extra information to decide on two opposing geologic models for a line.