The Challenge of Multiples -- Demultiple Techniques on Data from the Gulf of Suez

By

 Chris Koeninger¹, Sayed Khalil¹, Ian Moore¹, Antoun Salama¹, Ian Threadgold², Norman Allegar²

(1) WesternGeco, Cairo, Egypt (2) BP-GUPCO, Cairo, Egypt

 One of the main problems in interpreting Gulf of Suez seismic data is the high level of multiple contamination. Surface related and interbed multiples, with varying periodicities and complex interference patterns, make it a very difficult task to identify primaries. This is compounded by the highly attenuative nature of the Zeit formation which constrains interpretation of pre-Zeit events to frequencies less than 25 Hz.

In 2001, BP-GUPCO acquired several 2D seismic lines in the Gulf of Suez, recording very near offsets to better satisfy surface-related multiple elimination (SRME) software requirements. The datasets were made available to interested contractors and universities to test their demultiple algorithms.

The methods tested involved the separability of multiples and primaries (Hi-res Radon), the periodicity of multiples (Tau-P Decon) and the modeling of multiples (SRME). Combinations of these methods are investigated and special emphasis is put on SRME (both surface and interbed related). Analysis of well and VSP data demonstrate Zeit formation effects on the seismic wavelet and aid in designing deconvolution operators. The data and algorithms are analyzed for their frequency-dependent response, their effectiveness on surface-related and interbed multiples and their dependence on factors like water depth and acquired near offset distance.

Utilizing the assumption of reciprocity, lines shot in several parts were merged pre-SRME providing improved SRME results in the overlap zones. It is expected that extending this to full 3D regularization will further increase SRME effectiveness.

The results of these tests should help in the design of acquisition programs optimized to attenuate multiples.