A new system enables contractors to identify coverage gaps during initial acquisition.

Thinking outside the box has led Concept Systems, based in Edinburgh, Scotland, to an unexpected breakthrough in seismic survey planning, acquisition and processing.
Until a couple of years ago, the company was best known in the exploration and production industry as the provider of navigation and positioning software used by most marine seismic contractors. One of its newer interests has been research into some of the associated geological and geophysical issues in seismic operations at sea and on land. Illuminator, launched in 2001, has been one of the first results of this diversification.
BP was one of the first companies to test Illuminator on its Machar field in the North Sea. Since then, several major oil companies and seismic contractors have used the tool. Illuminator appeals to the industry's basic instincts - dramatic improvements in efficiency and significant cost savings. At its simplest, Illuminator can be said to offer a novel way of ensuring optimal coverage for a seismic survey at the planning stage and during the actual acquisition. And there even is a knock-on benefit at the data processing stage. One example of the power of Illuminator would be its ability to identify gaps in seismic coverage during operations in sufficient time for the vessel to retrace its steps and infill those gaps. Previous technology would have taken too long, and the gaps would be left as costly and time-consuming headaches for a potentially unsatisfactory resolution at the processing stage.
When it addressed the geophysical accuracy of surveys, Concept found conventional survey design required the analysis of the coverage results to be undertaken in the common midpoint (CMP) domain. But this approach can be prone to error because it does not take into account any notion of geology in the prospective area. A ray-tracing technique generating common reflection point (CRP) coverage can overcome this problem, but it has not been routinely used on surveys because the analysis until now has taken too long to perform.
Illuminator allows the geophysicist to model different acquisition techniques and geometries to assess the "illumination" achieved by each and then optimize the survey design to the geology. Its principal application in conventional 3-D and 4-D survey design is to identify more targeted infill, which can be achieved in the field. Operationally, the technique allows results of ray tracing to be produced orders of magnitude faster than conventional systems. Illumination analysis of complex structures, revealing CRP fold maps, are obtained within minutes as opposed to days or weeks with conventional systems. In addition, for the seismic processor at a later stage, areas of poor illumination, such as missing near or far offsets, can be flagged in advance, and this gives the processor a head start in dealing effectively with the data.
Machar field project
In the illumination project for BP Aberdeen, the geological model of the Machar field was known to be dominated by a steep salt diapir that was about 40 sq miles (100 sq km) and consisted of seven layers (Figure 1). A 3-D survey had been carried out in 1989 with a single-source and dual-streamer configuration. The plan was to shoot a new 3-D survey, dual-source and six streamers, in April or May 2001. The lead time on this project was short; hence a fast turnaround of results was of prime importance to allow decisions to be made before the acquisition program took place.
The illumination analysis was designed to investigate any illumination differences between the two basic designs and optimize the data quality of the new 3-D survey. In summary, this required:
• simulation of the 1989 survey, generating CRP fold and CRP-CMP deviation maps;
• simulation of the 2001 survey, generating the same outputs;
• optimization of the 2001 acquisition; and
• rapid results to be provided via Concept's secure Web site service.
Concept processing
The processing could have been done either with templates or by using the original position (P190) data from the 1989 survey. To allow the analysis of purely the acquisition footprint, without including the effects of infield acquisitions issues (feathering or steering), it was decided to perform the processing using a template based on perfect acquisition for the 1989 and 2001 surveys.
Figure 2 shows the results after processing the two surveys, which was done in 2 days. Figure 3 shows the results overlaid on the model. The main conclusion drawn from this was that there was not a significant difference between the 1989 and 2001 surveys (except some near-offset striping with the 2001 survey due to the wider acquisition geometry). But these results led to a more interesting issue. The model offered very little coverage on the flanks in the crossline direction due to the steepness of the salt diapir (70°). Given that the 1989 survey also exhibited the same characteristics for 4-D processing, this was deemed sufficient. But in the interests of improving the new 3-D dataset, it was clear that acquiring additional prime lines in the orthogonal line direction would significantly improve the dataset.
Figure 4 shows an example of the coverage obtained on the diapir based on the original plan and the data including the orthogonal lines. Based on this illumination analysis, BP decided to acquire additional lines of prime acquisition to improve the dataset in these areas.
With this case study, the speed of service was important. The time from the initial inquiry to the start of acquisition was only a few weeks. The work also allowed BP to make informed decisions on the survey design, enabling its geophysicists to look at the 4-D issues and at optimizing the new 3-D dataset. They commented subsequently that "with the time scales on this job, turnaround was paramount. This, tied in with the modeling itself, has allowed us to improve the acquired dataset."
Other applications
Concept Systems' illumination technology has been applied on more than 20 projects. The company has performed several service projects specifically aimed at different design issues, including four-component ocean-bottom cable acquisition. With such a project, CMP binning is even more inaccurate than usual, with the conversion from compressional wave (Vp) to shear wave (Vs) velocities. Illuminator technology can have an impact here using different acquisition geometries or modeling the uncertainty in the Vp-to-Vs ratios.
The technique is also proving massively beneficial in processing. By the time the seismic processing stage is reached, the actual surface positions of source and receivers are known. The illumination analysis then is used to highlight potential problem areas. This helps focus efforts on where data exists rather than spending needless time trying to make something out of nothing. Angle of incidence modeling also can be beneficial in aiding the selection of processing parameters. In addition, during prestack depth migration (PSDM) projects, the structural information is extremely important. In many situations, especially in Gulf of Mexico salt structures, the shape, position and velocities are not well known. In such cases, the technique is proving an excellent tool for a quick check of the illumination provided by a particular choice of structure. In this way the effort applied during the PSDM can be significantly more focused and final results dramatically improved.
Acknowledgements
Some of the material in this article first appeared in the June 2001 issue of First Break, a publication of the European Association of Geoscientists and Engineers. Permission to publish data from BP Aberdeen is gratefully acknowledged.

Comments