The shallowwater shelf can be a tough place to work for a seismic contractor. Regardless of the depth of the drilling target, obstructions at the surface make getting around a bit of a challenge. Two technology developments are coming at this problem from very different vantage points, but both have the potential to add a new tool to the toolbox.

At one end of the spectrum, the Bureau of Economic Geology (BEG) at the University of Texas-Austin has teamed with the US Department of Energy and WesternGeco to find ways to image ultradeep gas targets in the Gulf of Mexico's Deep Shelf play using long-offset multicomponent ocean bottom cable (OBC) data. With prospects like ExxonMobil's Blackbeard targeting structures some 30,000 ft (9,150 m) (almost 6.21 miles or 10 km) beneath the seafloor, it's important to find ways to get seismic images of those deep targets. The best way to do that is with long offsets, "offset" being defined as the distance between the source and the receivers. Typically the length of the offset needs to be roughly equal to the depth of the target. In deep water long seismic streamers can handle this situation easily; in congested areas closer to the shore this method is virtually impossible.

OBC technology offers a viable alternative, according to Bob Hardage, a senior research scientist at BEG. "When you lay the cables on the seafloor, you can almost literally lay them right between the legs of a platform," Hardage said. "Then the only thing you have to maneuver is the shooting boat. It allows a tremendous increase in flexibility."

WesternGeco already has a large dataset over areas of interest on the Deep Shelf acquired using long-offset OBC equipment. The goal of the 3-year research program (see box) is to take advantage of newer computer technology to find new processing algorithms to better image these deep targets. Even with the first-generation processing that WesternGeco has done, Hardage said his research group has discovered that the compressional (P) waves acquired with 6.21 miles (10 km) offsets often image to depths that approach 60,000 ft (18,000 m). He added that the converted shear (SV) waves acquired with these offsets do not image as deeply as the P waves, but usable SV reflection signals have been found in numerous places that extend to depths of about 43,000 ft (13,000 m), well below the current deep drilling objective of 30,000 ft.

What's left to accomplish is to develop new algorithms that will hopefully provide a better representation of the actual configuration of the subsurface. "The first-generation processing that was used for these data, particularly for the converted shear mode, assumed isotropic propagation medium, but the earth simply is not isotropic," Hardage explained. "That assumption made it easier to formulate some of the algorithms and codes in order to process the data."

Today's advanced computer technology is giving processors the luxury to introduce into the processing algorithms parameters that allow the propagation medium to be anisotropic, a more accurate representation of the subsurface geology. "When you do that in the right way, the imaging fidelity of the data improves significantly," he said.

If the research results in better imaging and processing techniques, WesternGeco will be the most immediate beneficiary, but Hardage said his group is obligated to publish the results because of the government funding.

Slimming down

Not all new marine technology has to be ground-breaking to make a difference. A company called Geometrics has introduced GeoEel, a slim and environmentally friendly seismic streamer that can be used for engineering or geophysical work in places where traditional streamers might be risky.

The 24-bit digital seismic streamer has a wide bandwidth (up to 8 KHz) but is only 1.5 in. in width. It can be configured with up to 240 channels and multiple streamers, and it sends data by Ethernet to any industry-standard PC. The streamers are filled with an almost inert silicon oil.

These specifications offer a host of benefits. According to Rob Huggins, senior vice president for Geometrics, contractors using these streamers in congested shallowwater conditions don't have the same liability issues that they would have with standard marine streamers.

"If you cut a traditional streamer, which is filled with hydrocarbon liquid, on the Outer Continental Shelf of the United States, you can be fined as much as [US] $10,000," Huggins said. "The silicon oil is of great attractiveness to our clients, particularly those who work in shallowwater or transition zones."

The size of the streamers is also appealing. Response time in getting streamers to a job site is reduced because they can be shipped by a regular commercial air flight rather than having to be sea-freighted. They also pose less of a safety hazard since they're easy to deploy and don't take up as much space on smaller craft. The inert oil inside is non-toxic and non-flammable, so a spill onboard would not result in fire or injury to personnel.

The narrow design makes the system ultra-quiet, making full use of its circuitry. In-water digitization eliminates ground loops and shipboard electrical noise, resulting in quick, clean installations. Up to 48 channels can be deployed by hand on small vessels.

While they're not going to find targets like Blackbeard, the GeoEel streamers can image shallower targets down to about 6,500 ft (2,000 m).

This is Geometrics' first digital streamer, and Huggins said that though major contractors have been using digital streamers for several years, this is the only small-diameter digital streamer that offers such high resolution. The bandwidth allows the use of sources like sparks and boomers in addition to more standard airguns.

Geometrics has been building electronic geophysical instruments and systems since 1968. Huggins said that about 60% of its business is oil and gas; the other 40% is academic and research. The company has been making analog marine systems for about 15 years. Its first GeoEel customer was the Geological Survey of Japan. California State University-Long Beach also has purchased a system. Field tests have also been conducted with Fugro and with Mission Exploration, and Huggins said the reports "are very positive." ä

For more information, visit www.geometrics.com.

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