GEOPHYSICS Data compression breakthrough makes real-time seismic evaluation a reality

A Chevron subsidiary, Chevron Petroleum Technology Company, has launched a software system for seismic data compression that its developers believe will revolutionize seismic acquisition and facilitate getting new oil and gas fields onstream sooner. The system, an algorithm to be applied primarily to 3D datasets, is to be employed on board seismic vessels during a shoot and as the data is being acquired in order to compress the data for transmission by satellite to an onshore processing center. This to facilitate rapid processing and interpretation in real-time while the vessel is still on prospect in the event questionable or interesting areas need reshooting.

Developed by John Villasenor of the University of California at Los Angeles and a Chevron team headed by Paul Donoho and Ray Ergas, the new technique makes timely evaluation and decision-making possible and is expected to significantly reduce the cycle-time in the exploration phase of a field.

The software program is capable of compressing 3D marine seismic datasets to less than one percent of their original size, permitting their transmission by satellite without disturbing the integrity or quality of the data.

Ninian test

Chevron successfully tested the new seismic data compression technology last summer on its Ninian Field in the UK sector of the North Sea. Ninian was chosen because a multi-streamer, 3D survey was already planned to further delineate the field's reservoir. Compagnie Generale de Geophysique (CGG) was chosen to carry out the seismic acquisition, using the M/V Mistral seismic vessel, which had been outfitted with the CPTC-developed seismic data compression software kit that runs on most standard computer architectures.

During the field test, the data was preprocessed by the contractor on board as it was acquired, then the preprocessed dataset was read into the Chevron workstation and compressed from 4.5 gigabytes (raw data of 18 gigabytes per boat preprocessed to 4.5 gigabytes) to 80 megabytes. Thereupon, it was transmitted via satellite link to Brechin, Scotland for relay on to Chevron's London processing center, where it was decompressed and processed in parallel with the original data recorded on shipboard tape. The original data required nearly 500 3480 tapes, whereas the compressed data, reduced by a ratio of 60 to 1, filled just one high capacity tape.

When compared, the compressed and uncompressed datasets were virtually identical. As a consequence of this successful trial, Chevron plans to use the technology in support of several seismic surveys this year, including its those on its West of Shetlands prospects.

Economy

The tremendous surge in the use of 3D seismic data has brought with it escalating costs and lengthened turnaround time between acquisition and processing - sometimes as much as a year, creating tremendous difficulty in managing large datasets. Usually, upon acquisition, seismic data is recorded on magnetic tape and the tapes - in considerable volume - are then transported to a processing center when the vessel completes the shoot and returns to port, frequently six or more weeks after the first data was acquired. Until now, no better than 20 to 1 compression ratios have been achieved, but Chevron's seismic data technology is able to reduce a terabyte-sized (trillion bytes) dataset to a manageable 10 gigabytes or 10 billion bytes. And, in its trials last summer, averaged a 30-hour turnaround time from data acquisition to decompression.

"Considering that industry conducts 200-300 seismic surveys each year, the technology offers extraordinary opportunities for cost savings and improved time efficiency," said Robert Heming, general manager of CPTC strategic research.

The acquisition of seismic data for oil and gas exploration is now a $2 billion a year business and growing. The impact of Chevron's data compression technology could be far-reaching - in seismic data collection, processing, analysis, and storage. And research is continuing. Chevron hopes to see it used to keep entire prestacked surveys on-line in disk storage, to store data compressed and kept in RAM for high speed access, and to reduce tape volume to save storage and regeneration costs. So far, Chevron reports that 12 companies have expressed their interest in the compression technology.