An ambitious regional 2-D multicomponent survey is one of the first attempts to bring shear (S) wave data to the masses.
I've written about multicomponent seismic technology many times within the pages of this magazine, usually in the mode of wondering when it's going to move from the subject of case studies to a technique that's widely used in exploration.
The hurdles have been many, particularly offshore. One of the main problems is the price. Multicomponent data, often called
4-C data, which incorporates S waves as well as compressional (P) waves, must be acquired through the use of ocean-bottom cables (OBC) because S waves don't travel through water. So immediately the cost of acquisition rises.
The result has been sort of a catch-22. Geophysical contractors are waiting for the market conditions to improve, and oil companies are waiting for the price to come down.
In the midst of this impasse, a new company called Aim Geophysical has teamed up with WesternGeco to undertake a large multiclient program in the Gulf of Mexico under an alliance called Multivision. And it's skirting the pesky cost issue by shooting the survey as a 2-D instead of 3-D program.
This might seem like a step backward rather than forward, but 2-D lends itself well to multiclient programs because it's less expensive to acquire than 3-D and thus can be used in a rank exploration type of manner, covering large regions and helping explorationists hone in on more specific areas of interest. The larger the area, the more potential customers. And Multivision is targeting a pretty large area - about 24,000 miles (38,616 km) of data over prime acreage in the Gulf of Mexico.
WesternGeco and Aim, under Multivision, will acquire, process and market the data. The overall project, which encompasses water depths from less than 40 ft to 3,000 ft (12 m to 915 m), is expected to take 3 years to complete.
Is the market ripe for this type of data? Aim co-founder David Wegner thinks so. "I've talked to numerous clients, and they showed an interest in having a regional long-offset shear wave grid," he said. "The industry at this point has a higher interest in 2-D 4-C from an economic standpoint than they do 3-D 4-C. The price is still too expensive for 3-D 4-C. By doing a 2-D grid, it allows this technology to fit into the group-shoot model."
Teaming with WesternGeco has its advantages. The company has helped push the limits of OBC technology from just a few hundred feet to more than 3,000 ft (915 m) in a relatively short period of time. And it has developed proprietary algorithms to process the shear wave data.
This is a crucially important factor when it comes to offering S-wave data in an economically viable fashion, said Bob Hardage of the University of Texas' Bureau of Economic Geology (BEG). Hardage has been preaching the gospel of multicomponent seismic for several years, and BEG is acting as one of the technical advisers to Aim.
Standard P-wave data is processed using common midpoint processing because the velocity of the wave field going down is the same as the wave field coming back up. In marine multicomponent, the P wave travels downward and is converted to an S wave at the reflection point. The converted S wave travels back up at a different velocity. This requires a new technique called common conversion point (CCP) processing.
"The common conversion point is always closer to the receiver than it is to the source," Hardage said. "As you move your reflecting interface shallower, the CCP point moves closer and closer to the receiver station. So it has somewhat of a parabolic curvature as you move shallower."
The processing of marine multicomponent data also must take statics into account. Static conversions are done in
land surveys to offset the elevation differences encountered laying geophones on the ground. Standard marine streamer surveys overcome this problem, but OBC surveys do not.
"You need to go to a high-tech processing shop to process this type of data," Hardage said.
He added that he's "just about now" becoming confident that certain industry leaders like WesternGeco have the know-how to process S-wave data.
The result, it is hoped, will be a dataset that provides a wealth of new information. In the deeper water it will provide S-wave information that, for the most part, has never been available. In the shallower water the S-wave information, which can help image through gas clouds as well as offer fluid and lithology characterization, will be useful for companies making exploitation decisions about older fields. It also will help characterize the deep structures that have loomed tantalizingly on older data but needed longer offsets to be imaged properly.
Already the survey has been amended to better meet customer interest. As the recent Gulf of Mexico lease sale indicated, many companies are returning to the Shelf to study deep gas targets, and the Multivision alliance is reshooting much of the central Shelf area with longer offsets of 32,810 ft (10,000 m) to image the deeper targets more accurately. Hardage said some of the clients are more interested in the OBC aspect of the data than the S-wave aspect since it's almost impossible to shoot a long-offset streamer survey in a heavily congested area. But he thinks the converted wave data will convert some of the doubters.
"They're not particularly interested in the horizontal component; they just want the good P-wave data with long offsets," he said. "But I think we'll make believers out of them.
"I'll make this prediction: People will throw rocks at single-component data in a few years. It's got to be totally multicomponent."

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