SEISMIC INTERPRETATION Ensign technique improves economics of multi-source acquisition

Conventional stack with f/k multiple attenuation. The same data processed with wave field reconstruction. The new generation of seismic vessels are capable of acquiring 3D data with up to 12 streamers, using a single source for a higher fold of coverage. Currently, most 3D surveys are still acquired using two or even three sources and four to six streamers. In these cases, the increase in the number of subsurface lines obtained with each pass of the vessel has to be balanced against the lower

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Conventional stack with f/k multiple attenuation.


The same data processed with wave field reconstruction.


The new generation of seismic vessels are capable of acquiring 3D data with up to 12 streamers, using a single source for a higher fold of coverage. Currently, most 3D surveys are still acquired using two or even three sources and four to six streamers. In these cases, the increase in the number of subsurface lines obtained with each pass of the vessel has to be balanced against the lower fold of coverage for each of those lines.

Since processing cost is only about one-tenth of acquisition cost, a method which achieves higher fold from a processing technique might allow explorationists to use up to 12 streamers and a dual source for more cost-effective and productive acquisition, i.e. up to 24 subsurface lines with each vessel pass.

Wavefield reconstruction is a new processing technique offered by Ensign Geophysics based in Addlestone, UK. It can be used on any 2D or 3D survey and addresses the issue of sparse shot sampling using a combination of deterministic spatial interpolation and a de-aliasing procedure.

According to Ensign's Terry Quinn, "After reconstruction, which occurs as an initial step in the processing, the data are consistent with a smaller shot interval and higher-fold shooting geometry. "Specifically, the de-aliasing extends the spatial bandwith accessible in processing, while the interpolation extends the spatial bandwith preserved in processing. After Wavefield Reconstruction, both the coherence energy and character of the data are similar to those for higher-fold acquisition using the reduced shot interval."

The objective of higher fold is to improve the performance of many pre-stack multi-channel processing techniques. In particular, both dip move-out correction and (common offset) pre-stack migration benefit from increased midpoint sampling at each offset, while F-K multiple attenuation and velocity analysis benefit from increased offset sampling within each midpoint. The finer sampling within common midpoint (CMP) gathers also improves coherent noise attenuation in stacking.

Serious consideration can be given to reducing the acquisition effort or increasing the volume of data from each vessel pass, knowing that the anticipated shortcomings in data quality can be retrieved in processing.

Last year, Texaco acquired a 3D survey of approximately 30,000 subsurface kilometers over Blocks 14/18 and 14/20 in the UK North Sea. Testing had satisfied Texaco that it could use a dual source for more cost-effective acquisition: the shooting configuration would obtain 30-fold of coverage, which could then be doubled to 60-fold in processing with the wavefield reconstruction technique, in order to achieve the desired data quality.

In the event, all the anticipated objectives were met: good data quality, faster acquisition of the data and a net cost saving of several million dollars.

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