GEOPHYSICS Longer cable multi-streamer acquisitions key to greater accuracy in 3D seismic data

Average 3D cable length has grown from the North Sea 4 km to 6-8 km in the Gulf of Mexico. [Courtesy Western Geophysical]

Depth imaging below high-velocity salt mass using long cable multi-streamers from two vessels in tandem.

Ever deeper water, deeper geological targets, and complex frontier stratigraphy present far greater challenges to accurate acquisition of 3D seismic data than do normal depths and geology. The technological cutting edge that is reducing these obstacles to accurate surveys to a manageable size is proving to be the towing of longer cables on multi-streamer programs.

Most petroleum reservoirs are at least 1000 meters beneath the seafloor (3,280 ft), and many are at far greater depths. These deeper geological targets, particularly those in a number of frontier regions, may require streamer lengths between 4,000 and 6,000 meters and even more, rather than the standard lengths up to 3,600 meters. This despite the difficulty of deployment and positioning, especially when four to eight streamers of greater length are towed over large areas in order to achieve cost-effectiveness. The longer the streamers are, however, the greater the challenge in deploying them and maintaining their positions.

Furthermore, typical pay zone depths are overlaid by rock layers that lead to what are called earth losses. Heavy faulting, unconformities, and sudden lithology changes cause these losses and obscure standard seismic signals. When the depositional environment has these characteristics, it attenuates higher frequencies. The recovered bandwidth is expected to be between eight and 60-70 Hz.

Longer offsets

"Different geologic environments dictate streamer length," said Ron Chambers, geophysical advisor to the president of Western Geophysical. "In the North Sea, where they have the short streamer, they also have a rather high velocity area, and usually the noise patterns that come through the water layer actually intersect with the primary or reflection energy rather short into the cable, so you don't really have a very long cable you can use without having this interference from the direct arrivals and the refracted arrivals.

"As you get into the lower velocity of the Gulf of Mexico, especially in the very deep water, where you have the very low velocity of the water layer, these refracted arrivals come in much longer offsets, so you are able to take advantage of longer, much longer offsets.

"The Gulf of Mexico is an anomaly relative to the rest of the world, however. The predominant streamer length in the rest of the world is approximately 4,000 meters, but in the Gulf of Mexico people are still out there shooting with 4,500 to 4,800 meters. All of Western Geophysical's Gulf crews are shooting with at least six km cables, and our new spec survey, the Ultra Survey in the Mississippi Canyon, Atwater area, which we're just getting ready to do, is going to be with 8,000-meter streamer lengths."

"There are many reasons why you'd want to take advantage of longer offsets," said Chambers. "One is for the attenuation of multiples, a problem we suffer in all marine environments." (A multiple is seismic energy that has been reflected more than once, and persists through the arrival of desired signals, obscuring structural and stratigraphic details. The effect is something like hearing an announcement over a public address system in a large empty hall: the multiple echoes make it difficult to comprehend what is being said.)

"What happens," Chambers said, "is that when you put your shot off, it goes down and bounces off the water bottom, comes up and bounces off the water surface, then goes back down just like another shot, just like an echo or repeat, but these things are usually within a rather long period. Also, you get inter-bed multiples when you're in the bedding itself. It's the same kind of phenomena, in that the shot will go down, hit the bottom of the bed, go back up to the top of another one, then bounce back.

"The methodology typically used to suppress multiples in our processing algorithms, is to use differential move-out, because the travel times are different for the primary information and the extra bounces. The longer the offsets, the more travel time difference, so you can make use of that information to attenuate your multiples."

3D subsalt AVO horizon map showing possible channel system below the salt. [Courtesy Diamond Geoscience Research Corp.]

AVO

Another primary reasons for using longer cables is to obtain considerably more information on AVO (amplitude with offset variation) characteristics. This is particularly important for acquisitions occurring in deepwater, where, it appears, the longer the streamers are, the better. (In fact, Shell Oil, which has embarked on a major deepwater development program, has put out feelers to see if anyone can do a multi-streamer 3D seismic shoot in the Gulf of Mexico with 10,000-12,000 meter streamers.)

Chris Ross, reservoir group manager with PGS Tensor, says the longer offsets give you considerably more information on your AVO characteristics. "Historical trends in AVO, at least in the Gulf of Mexico, indicate we are looking deeper, in that the anomalies that we go after are deeper. In order to keep the technological status quo that has been used with AVO in the past to help reduce risk and identify prospects, we need to record longer and longer offsets, because you really need to have sufficient angular aperture in order to get the stronger AVO measurements."

This means the farther out you get, the longer the angle of incidence. Longer offsets correspond to larger angles of incidence, and "We can see stronger changes in reflection amplitude caused by gas at larger angles of incidence."

Ross says that, in general, there are areas of the world where velocities are less dependable, but "The bottom line is the longer the offsets, the deeper you can do AVO, and in the Gulf of Mexico and other tertiary basins, you can do this.

"Additionally, we find that you can get an AVO response if you have long enough offset data for gas reservoirs and light oil reservoirs that have either lower porosity or are in geologic settings that don't produce bright spots, or non-bright spot areas. It's in these types of areas where AVO can help you and to really see it, you need to have offsets that are actually larger than your depth (offset to depth ratios that are greater than one).

Gregg Parker, Marine Sales Manager at Geco-Prakla, says that Geco specifies the use of longer cables in their multi-streamer 3D shoots mainly in order to image deeper geological targets, secondly because of water depth, and also to undershoot salt structures.

"In the Gulf of Mexico," he says, "which is mainly a sedimentary basin, longer offsets are particularly useful, because they can handle the salt bodies, since the salt is overlain by sediment rather than carbonates, as is the case in the North Sea.

"When you work on the shelf in the Gulf of Mexico AVO is an extremely important issue, but when you step off the shelf and go into deeper water with correspondingly longer offsets, I doubt if you're going to be looking for an AVO response. More likely, its going to be a depth of target.

"All of Geco's surveys are now 6,000-meter surveys, and we are looking at the option of even increasing them. There are programs out there that even exceed 7,000 and 8,000 meters, and there has even been talk of longer offsets than that. Some companies have even discussed shooting in excess of 12,000 meters, probably by shooting a 6,000-meter near offset, then coming in and shooting a longer one. I haven't seen any like that shot yet, though."

3D depth slice from pore-pressure volume for pressure-transition mapping [Courtesy Diamond Geoscience Research Corp.]

Subsalt

Davis W. Ratcliff, president of Diamond Geoscience Research and executive vice president of Diamond Geophysical, says that the use of longer cables for 3D seismic acquisition of subsalt prospects is essential.

"We use the long offsets of 6,000 meters and greater to get the appropriate velocity field below salt. In addition, at Diamond, we're now doing some amplitude variation with offset work, AVO work, below salt, and the long offset data is critical for to that work. And one of the breakthroughs we've been able to make, using long offset data to develop 3D subsalt AVO horizon maps from full volume 3D prestack depth migrations, is the identification of meandering channel systems beneath 4,000 ft of salt.

"An equally important reason for using long offsets is for multiples suppression. The multiples that exist below salt and which are generated from the top and base of salt are a big problem, and long offset data help suppress these multiples and provide a clearer image of the primary reflections below the salt.

"Lastly, we have gained leading edge technology from the use of long offset data, since it enables us, after we use it to develop velocity models, which are accurate velocity models for imaging the subsalt geology, to take that information and develop pressure volumes. By combining the long offset data with our 3D pore-pressure technology, we've been able to map some of these pressure regimes below the salt body quite accurately, within a half-pound per gallon mud weight."

Cost-effective surveys

There are a number of valid reasons to tow longer cables in multi-streamer surveys, but cost-effectiveness isn't one of them. Most contractors admit they don't really provide a cost-saving benefit, since they're heavier and longer and it is more difficult to carry out an efficient survey with them. There are, as well, considerations that must be made for the hardware employed, the strain on boats, and the increased difficulty of processing and interpretation, since there are many more traces involved, with far more complex raypaths to consider and new algorithms to be applied. The savings (or expense) may come about in other ways related to the method of the acquisition itself rather than the simple geometry of the reflections.

Geco's Gregg Parker points out that "Feathering, separation, and deepwater currents are increasingly important issues when looking at the use of longer cables with multiple-streamer projects."

Many methods have been developed to minimize the problems associated with loss of data due to feathering, separation, and the capture of streamers by erratic currents. For the most part, however, only infill shooting - sometimes 10% or more of the project - can correct the loss.

Western Geophysical and Geco-Prakla employ longer cables on some of their 3D seismic acquisition projects, while PGS Exploration employs two vessels towing relatively shorter cables of 3,000 to 4,000-meter lengths in tandem to accomplish approximately the same length - as a combined long offset - as the single-boat acquisitions. Profits and losses occur not from the length of the cable, but from the ability of the contractor to achieve timely, dependable, high quality data - data upon which drilling either a dry hole or a discovery may well depend.

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