A new acquisition system places sensors on the seabed without the need for ocean-bottom cables.

The ocean is such an inhospitable place. Ever since oil companies have been looking for hydrocarbons beneath the seven seas, seismic contractors have been scrambling to find some way to send sound waves not only through thousands of feet of rock but through hundreds if not thousands of feet of saltwater as well.

Until now the two main methods of seismic acquisition in the marine environment have been streamers, which are towed behind a vessel near the surface of the water, and cables, which are laid on the seafloor or sometimes even buried beneath it (mostly for reservoir monitoring purposes). As with any set of vying technologies, each method has its pros and cons. Streamers are obviously a much easier way to obtain data, but the image quality is rarely comparable to ocean bottom cables (OBC), particularly when it comes to measuring shear waves, which don't travel through water yet which provide important information about lithology and fluid characteristics. It can also be difficult to tow streamers around existing platforms and other infrastructure. OBC is orders of magnitude more expensive to shoot because of the difficulty of placing miles of cable on the ocean floor. The cables also have been depth-restricted, unable to image reservoirs in ultradeep water.

Fairfield Industries is introducing the Z Pod, a new method of seismic acquisition that will certainly complement, and in some cases might replace, these traditional acquisition techniques. In deep water, Z pods are laid out in a grid by a remotely operated vehicle (ROV), much like a land crew might lay cables across a field. But there are no cables. Each pod is a self-contained sensor with a battery and an extremely accurate clock. Once the sensors are turned on, they stay on, recording sound waves sent down from a source boat on the surface. The batteries last about a month, after which the nodes are retrieved by the ROV.

While this method is probably not going to replace streamer acquisition for large-scale exploration, it can fill an important niche in the development stage, said Steve Mitchell, vice president and division manager of data acquisition for Fairfield. "If an oil company has a discovery in deep water, the asset team might not know if it's a 100 million barrel field or a 300 million barrel field," he said. "At that point the cost of a survey like this is minimal compared to the difference in topsides cost."

It can also be a useful tool for time-lapse surveys. While the Z pods are not intended to be permanently placed in the reservoir, they're easier to deploy over an existing field than OBC due to the umbilicals and other hardware on the seafloor. And issues with streamer repeatability are such that many operators prefer a system that can be placed in the same spot for each survey, well below the currents and wave action that affect streamers on the surface.

Depth-rated to 9,800 ft (3,000 m), the pods are distributed about 1,300 ft (400 m) apart, a much greater distance than typical receiver spacing on OBC. But Mitchell said the shot density on the surface is so thick that the result is a high-fold, wide-azimuth survey.

Fairfield estimates that a typical OCS block in the Gulf of Mexico will take approximately 5 1/2 weeks to complete.

So far five shallow-water tests have been completed, comparing vector fidelity with different sensors and comparing coupling between different package weights and designs and in different sea bottom conditions. This spring the crews will perform deepwater deployment and retrieval experiments, and a deepwater data acquisition pilot program will be carried out this summer. Mitchell expects the system to be commercially available in the first quarter of 2005.
For more information, visit www.fairfield.com.



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