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
.