Texaco Inc. and Petroleum Geo-Services AS (PGS), Houston, have joined forces to commercialize a new method to acquire 3D marine seismic data.
The venture will offer vertical cable seismic (VCS) data acquisition, a 3D technique developed by Texaco within the past decade at its exploration and production technology center in Houston.
Texaco said VCS yields higher resolution 3D data cheaper and faster than conventional 3D marine data acquisition techniques. In addition, data are acquired in a truly 3D fashion, reducing processing requirements and allowing more complete analyses of surveys.
The biggest difference between a conventional 3D marine seismic survey and a 3D VCS shoot is the way in which hydrophones are deployed. (80814 bytes) Rather than towing long streamers of hydrophones horizontally behind a seismic vessel, crews for a VCS survey attach the hydrophones to cables anchored to the seabed. The cables are arrayed in a predetermined pattern and held vertical by a buoy.
Field personnel place hydrophones on the vertical cable at constant depths below the waters surface, regardless of the degree of seabed irregularity. A source boat then maneuvers through the cable array, generating a seismic signal by firing air guns at specified time intervals.
Once adequate data are collected with a given cable array, crews collect the cables, redeploy them in a new configuration, and acquire another subset of data.
Texaco has developed a data processing computer program for the VCS technique and has licensed the program to PGS as part of the companies venture.
Developing VCS technology
Texaco began working on VCS concepts in the late 1980s as part of a program aimed at developing better seismic methods for imaging structures beneath salt formations. The technology evolved from the work of a task force Texaco set up to solve subsalt seismic problems.
The task force was studying vertical seismic profiling (VSP) variations when it began looking for a way to conduct VSP surveys without drilling a borehole. One logical way of achieving the desired verticality was to anchor a cable outfitted with an array of hydrophones to the seabed.
A researcher in Texacos Exploration and Production Technology Development (EPTD) unit found that some manufacturers of marine seismic instruments were providing similar sensor arrays to the U.S. Navy for use in antisubmarine warfare. The subsalt imaging task force took it from there.
Basically, we ended up taking some off the shelf technology that had been used mostly for antisubmarine warfare and configuring it into a vertical cable, said Bob Tatham, EPTD manager of geophysical research. The only significant development was the recording buoy to record the data.
Texaco at first installed a transmitter on the recording buoys to send data back to the recording vessel.
As it evolved, we started recording the data directly in the recording buoy and retrieving it with a chase boat, Tatham said.
Chase boats return the VCS data from each recording buoy to the source boat, where it is merged with navigation data. Gathering VCS data at fixed sites within a 3D grid eliminates the need for some preliminary processing in the field.
Each position already is sorted into a separate tape cassette, Tatham said.
True 3D data
Tatham said 3D VCS data are better than conventional 3D marine data for several reasons.
For one thing, recording buoys receive the seismic signal from any direction desired. In a conventional 3D marine shoot, in which the source boat also tows hydrophone streamers, the signal passes the hydrophones from only one direction. As a result, surveyors must achieve signal redundancy with a series of 2D profiles.
With VCS technology, you get true 3D data because youre shooting from all directions, Tatham said. With conventional 3D marine techniques, even though you develop a 3D image, you dont have the redundancies youd like for forming images in all azimuths.
In addition, because VCS hydrophones are at rest, they dont have interference from towing noise. The ocean is just quieter at depth, Tatham said.
Hydrophone spacing on the vertical cable varies with water depth because the lengths of cables vary. The deeper the water in which the survey is to occur, the farther apart the cables can be spaced.
Thats where some of the efficiency comes in, Tatham said.
In deeper water, the seismic wave reflects from the seabed, passes through the hydrophone array, goes all the way to the surface, and reflects back down to the array. Texaco and PGS can process the signals mirror image from the waters surface to double the number of data points in the survey.
You only get that efficiency in deeper water, Tatham said.
The VCS data acquisition method also is more efficient and flexible because the source boat doesnt tow the long, horizontal hydrophone streamers. Maneuverability is much better because the air gun array towed by the VCS source boat is very similar to that used in a streamer marine survey.
VCS field tests
The promise of the VCS technique was underscored by a 1992 test in the Gulf of Mexico at Texacos Gemini subsalt prospect in 3,393 ft of water on Mississippi Canyon Block 292, about 90 miles southeast of New Orleans.
Conventional 3D seismic data on Gemini showed some prospective zones, but they didnt appear large enough to generate the return needed to drill a well.
Texaco conducted a 35 sq mile VCS survey of the area. Cable spacing on the VCS shoot was greater than was desirable, and shallow water data, as a consequence, didnt format well.
But the deepwater images, especially below the salt, did, Tatham said. In fact, they were superior to the conventional 3D data.
The VCS data showed additional objectives, and it was those intervals that indicated Geminis reserves might be adequate to justify drilling a wildcat. Earlier this year, Texaco and partner Chevron said they were evaluating apparently substantial hydrocarbon reservoirs at Gemini below a 2,900 ft tabular salt formation.
Tatham said Texaco and Chevron likely wouldnt have attempted the 17,976 ft Gemini wildcat without VCS data.
Texaco and partner Shell Offshore Inc. plan another VCS shoot in the gulf late this year or early next on their Fuji deepwater prospect. Fuji is in 4,243 ft of water on Green Canyon Block 506.
Texaco and PGS also have conducted a 3D VCS survey over a 20 sq mile area in Strathspey field in the North Sea. Water depth at the survey site was about 750 ft, and horizontal spacing between vertical cables was 750 m.
VCS advantages emerging
What few VCS field tests have occurred are turning up indications that VCS technology will bring new flexibility to marine seismic data acquisition.
During the Strathspey survey, Texaco and PGS found the technique was superior to traditional 3D seismic acquisition methods.
We were shooting when the seas really were too high to record with a towed array, Tatham said. But we didnt have to shut down the operations until weather conditions deteriorated to the point that the source boat couldnt hold course, and the data still were pretty good.
Because of VCS technologys apparent effectiveness in bad weather, Tatham said, explorationists conceivably could add a month to either side of the weather window at higher latitudes.
We hope to see it applied off western Africa, as well.
While VCS has some of the same advantages provided by bottom cable seismic acquisition, Tatham said, VCS complements more than competes with the better known technique.
Bottom cable seismic has a maximum effective operating water depth, he said. VCS works best in deeper water.
So in that sense, it really is a complement in areas where you could do both. n
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