"Seismic factories" tighten spacing, produce denser data sets

Field recovery monitoring taking hold

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Seismic at the speed of thought is a dream in every interpreter's mind, and only possible when a global "earth model" is built in some distant future. But what the industry lacks in access speed, it makes up for in acquisition technology, data density, database accessibility, processing, and communications.

Better data collection, denser data sets, faster processing, higher processing intensity, and lower $/process-cycle are the waves of the present. The most visible component of this technology wave is the newly built fleet of high-capacity seismic vessels.

Built for three-dimensional (3D) seismic survey collection, these vessels far outstrip their early two-dimensional (2D) forebears in size, capacity, and productivity. In every sense of the word, these new vessels are true ocean-going "seismic factories" that shoot, gather, process, store, and distribute new data sets in mass quantities.

They carry longer streamers to image deeper and image better. Longer streamers permit undershooting of salt and volcanic subsurface layers. This also creates better estimates of formerly hidden strata.

Streamer elements are improving as well, with more sensitive hydro phones and smaller streamer diameters. Control nets, either imbedded in the streamer or attached to them for depth control, continue to improve.

The changes are incremental, but stunning in their effect. The 1970's-era 2D data profiles set next to 3D extracted lines, pale in comparison.

Data density

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Advanced technologies like CGG's micro-seismic technique extends the benefits of 3D seismic directly into oil field production and monitoring.
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The improved imaging is a direct result of the shift from the 2D to the 3D seismic technique. Higher fold values gathered with 3D seismic permit improved signal to noise processing. Low energy signals are then boosted to image formerly missing detail. Scattered or diffused energy can be migrated back to its source to improve imaging below faults and salt tongues. Longer streamers capture more converted-wave energy, which improves the overall quality of the data.

Closer hydrophone-group spacings provide a more detailed image of the areas covered. There has been a steady movement to smaller and smaller bin sizes. The move from 50 meters by 50 meters spacing to 12 meters by 25 meters spacing captures more of the subsurface detail that leads to better well placement and better field production.

Of great advantage is the move by the major seismic contractors to underwrite the collection of regional 3D grids. This follows the pattern of an earlier era of building 2D data grids in active basins. By providing a consistent data set across a basin, regional questions of geology, oil-sourcing and productive trend relationships can be unraveled.

Processing power

Seismic contractors continue to be one of the major drivers of computer technology. They will use all the processing cycles the electronics industry can produce. The more densely packed, the better they like it. As PC's became more reliable and rugged, they went to the field. First they were used for quality control, then for preprocessing of 2D data sets.

As new larger vessels replaced the older space-limited vessels, mainframe-power Unix systems were installed for onboard processing. These multi-processor systems now have the power to handle all but the most iterative-intensive tasks. They can produce final sections for use onboard by operator personnel or by shore-based interpreters.

Denser data sets also provide grist for the processor's mill. With richer data, new algorithms create a feedback loop of better imaging, which leads to better interpretations and new ways to view the data sets. Processes, which were limited to select areas or small "windows" of data, can now be applied to larger areas to explore seismic linkages across basins or regions.

Communications

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The revolution in high-capacity vessels now makes 3D seismic available worldwide (Image courtesy of Veritas DGC).
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Having that kind of processing capacity is of limited use unless interpreters have ready access to the data, both raw and processed. To overcome this problem, satellite communications and high-speed data transfer protocols were created.

Vessels actively collecting data can record, store, and process raw data, which is then streamed over satellite networks to either major land-based processing centers or to operator's earth stations, then to the project team for review. The ability to forward new data quickly is a major time-saver (up to two months per project) and productivity enhancer.

The capability to communicate is not just one-way. Modern communications means that the project team can monitor operations from shore, yet make necessary changes to the survey plan if required.

They can add additional passes over areas of particular interest or cover missed areas from other directions to fill poor data areas. In the past, the contractor or company-man on board the vessel made these data-versus-money decisions. Now, the authority to make survey changes is closer to the onshore authority, which is especially important in multi-client or joint venture situations.

Data libraries

Old data is still valuable. In some parts of the world, it is the only data available, therefore invaluable. The industry is early in the process of creating very large linked data set libraries.

These libraries are currently based in either individual companies or on a regional basis by consortia. This makes possible the building of regional interpretations through the integration of many smaller survey sets when cross-licensing of data is permitted.

The success of the current 3D paradigm is encouraging the expansion of the technology into new territories. The most important of these is the move into oilfield monitoring. Both bottom-fixed streamers and in-hole hydro/geophone arrays are being used to create time-lapsed or four-dimensional (4D) seismic of individual reservoirs.

Proper use of these developing 3D techniques will give operators a more discrete value for their in-place reserves. This in turn creates efficiencies in the financial life of companies permitting them to take on the marginal project that would have had to wait.

Moving to thought-speed

Whatever seismic data interpreters can envision, can become reality. The only limitations are time and money. The seismic industry is a major driver of productivity and efficiency in the oil industry.

Through standardization, dense data sets, and improved communications, better data sets are being produced. Better data sets mean better interpretations, better well placement, and ultimately better hydrocarbon production.

Interpreters are more productive, and are testing more ideas than ever before. Perhaps in our lifetime, we will be able to "fly" through the world's earth model seamlessly, at the speed of thought.

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