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CGG's ocean bottom seismic surface multiple attenuation
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| Reflectivity model from migration of the shallow section, a) from the up-going waves, b) from the first order multiples in the down-going waves using mirror migration. |
(6/25/2006 - OGI: Vienna) Compagnie Generale de Geophysique (CGG) today revealed to the Annual EAGE Conference in Vienna it has developed a unique solution to ocean bottom seismic surface multiple attenuation using its proprietary 3D wavefield multiple modeling approach.
CGG said since convolutional SRME methods cannot usually be applied to ocean bottom seismic, it developed its innovative wavefield modeling.
"Seismic data recorded for deepwater exploration and development often suffers from high-amplitude surface-related multiple energy which can easily obscure the reservoir zones," the company said. "Attenuating this multiple energy is a critical issue, especially when the data is acquired for reservoir characterization or time-lapse studies."
Recent advances in the industry have seen 3D surface-related multiple elimination (SRME) methods set new standards for the attenuation of these problematic multiples. The convolutional approach for 3D SRME based on the work of Berkhout,Verschuur, and Biersteker, is now widely adopted. It proves relatively effective at removing the high-amplitude surface-related multiples from streamer data, especially when the streamer geometry allows a fine sampling of the recorded wavefield. This convolutional method cannot be applied on OBS data (cables or nodes) as its two main assumptions are not fulfilled. Firstly there is no co-location of sources and receivers for the surface-consistent convolutions, and secondly, with typically a few, sparse receivers, the wavefield is not well sampled.
CGG said this method generates the surface multiple model using wavefield extrapolation of shot or receiver gathers within a reflectivity model. With no limiting assumptions on source-receiver co-location and a model-based approach, the company said, the method is as equally valid for sparse ocean bottom data as it is for marine streamer data.
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Raw stack of the up-going waves with multiple energy clearly visible , b) Stack after application of 3D wavefield multiple modeling SRME. |
The suitability of the wavefield modeling SRME algorithm is only part of the OBS solution. This method still requires a reflectivity model, a shallow migrated section which includes the water-bottom, to perform the wavefield modeling. With a sparse distribution of receivers sitting at the water bottom, conventional migration of the upgoing wavefield is unable to provide a clear image of the water bottom. However, by using the hydrophone and geophone components we can derive the downgoing wavefield recorded at the node which includes water-bottom multiples and the direct arrival. Using a "mirror" migration scheme the information contained within the multiples can be used to generate the shallow reflectivity section required for our wavefield modeling. This is comparable and, in this case, superior to the water-bottom image from streamer data.
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The CGG Symphony. |
This unique combination of techniques which allows effective surface multiple attenuation for OBS data, using only the OBS data, has successfully been applied to a deepwater dataset with excellent results. The comparison illustrated here shows a stack section of data from five OBS nodes in which the multiple energy is clearly visible, before and after the application of SRME. While this example is for an area with a flat (but dipping) seafloor, the model-based method naturally handles structured seafloors and any kind of receiver geometry and bathymetry.
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