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correlation of source and reservoir rocks in adjacent Norwegian and Greenland margins gave promising indications of hydrocarbons. A similar geological structure in the Icelandic
prospects could be reasonably assumed, stimulating interest in the area. The company reprocessed over 5000 km of 2D seismic data that had been shot in Icelandic and Norwegian waters. By using pre-stack time migration (PSTM)
and surface related multiple elimination (SRME), for example, the images could be sharpened up. Geoscientists overseeing the reprocessing decided that these techniques, with the addition of radon combined with bespoke techniques developed in house would achieve better multiple attenuation and improved the imaging of sub-basalt reflectors. As a result, the team working in the Woking office near London was able to offer an offshore Iceland survey with much more clearly defined Mesozoic geology.
Complex velocity models
In May, a recent 28 000km Gulf of Mexico seismic survey was processed using wave equation migration (WEM) techniques to reveal sub-salt geology. WEM takes into account the multiple arrivals of seismic waves in the subsurface, overcoming the limitations of standard Kirchhoff PSDM which considers only the first arrival of seismic waves. Compared with normal Kirchhoff migrations, the WEM method increases the focusing of seismic images in areas with complex velocity models and excels in imaging below geology that is traditionally difficult to penetrate with seismic signals, such as salt formations. This dataset clearly illustrates the nature of seismic data processing in practice. There are no ‘silver bullets’ so it’s not so much a case of ‘the best method’ but ‘the right method’. In this case, both the Kirchhoff and WEM processed datasets held value to the petroleum geoscientist, because they helped to reveal different geological features. However, the workflow was also quite different so
it’s not a ‘push-button’ exercise. For WEM, the input consisted of shot gathers.
Using both techniques, the seismic data processing team has been able to produce a dataset that helped the parties involved to learn important information about the sub-salt structure of Eastern Gulf of Mexico. Impressed with the results from this survey, the team in the Houston office also tested the WEM technique on the company’s Big Wave Phase 1 multi client project, also in the Eastern Gulf of Mexico. The extra work paid off and the team concluded that it showed clear potential to improve the imaging of the Florida escarpment and the base of salt definition.
The main deep (>400m water depth) offshore
exploration efforts offshore India have concentrated on the east coast, where a number of large discoveries have recently been made in the Krishna Godvari and other basins. Attention turned to the deep offshore area of
the West Coast of India as the Directorate General of Hydrocarbons (DGH) NELP rounds started. Here, outside the petroliferous (shallow water) province of the Mumbai High, exploration has been frontier in nature with very few wells drilled to date. Imaging created using seismic techniques such as radon demultiple, PSTM, and PSDM applied to a 12 000 km regional 2D seismic survey collected for the DGH in 2002 upgraded the petroleum potential of the area – showing plays both in the Tertiary and in the deeper Mesozoic section where a potential petroleum province can now be recognised below the Deccan basalts.
Technology challenges
As a pure play seismic services provider, the company is now conducting research into new techniques and approaches that it feels can make a very valuable contribution the improvement of seismic datasets. Its top priorities include data regularisation, a ‘hybrid’ velocity model building technique for PSDM, integration of seismic with gravity and magnetic data for velocity model constraints and full waveform inversion. Many of the above techniques hold great potential for improving the imaging of the increasingly deep geological structures that oil exploration companies believe could hydrocarbon potential.
Improving data quality
Data regularisation is very important in seismic processing algorithms, as the application of these assume that the seismic data is regularly sampled. However, seismic data recorded in the field is usually sparsely sampled in source-receiver offset and azimuth. This can introduce serious artefacts to the seismic image in the form of processing-operator induced aliasing effects. The Woking office is now working on full wavefield techniques and methods to overcome operator- induced aliasing and other artefacts to resolve this problem.
Noise attenuation is another practical consideration for newly acquired and older datasets. Again, it is not a case of ‘one size fits all’ and research is being done into the use of wavelet transforms, especially for ground roll attenuation and has plans to apply it to other types of noise as well. Going forward, more work is planned on characterising waveforms that are reflected back from the subsurface.
Building velocity models
Today, linear seismic processing methods are common, but a clear need has been indentified to
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