SEISMIC ACQUISITION From targeting submarines to acquiring accurate 3D seismic

Vertical cable seismic launched by PGS-Texaco alliance

In the rapidly occurring geophysical revolution that's transforming the petroleum exploration industry these days, numerous new techniques are being introduced that purport to lower the cost of acquisition, make it easier, faster, more accurate, or a combination of all of these. One such innovation that actually appears to achieve all these goals is the new vertical cable seismic acquisition technology developed by Paul Krail and Dwight Sukup, geophysicists at Texaco's exploration and production technology division in Houston.

The difference between conventional seismic surveys and those employing the new vertical cable technique is, as its name implies, in the way in which the hydrophones are deployed. Conven-tional surveys have the hydrophones attached to long streamers that are pulled through the water horizontally behind the seismic vessel, which also pulls the energy sources, usually high pressure air guns.

A vertical cable survey, on the other hand, has its hydrophones attached to cables that are suspended vertically between strong buoys on the sea surface and heavy anchors on the sea floor in specific acquisition designs around the target subsurface structure, regardless of the degree of seabed irregularity. The source boat fires its high pressure air guns above and around the cables in an even larger pattern.

When shot, the vertical cables are then redeployed to another location and another swath of data is acquired.

Texaco has developed a proprietary data processing program specifically for use with data acquired with the new vertical cable method, that economically and efficiently produces the 3D information with higher resolution than that acquired through conventional 3D methods. Furthermore, the data is actually acquired in a 3D sense rather than the conventional series of 2D lines. Thus more complete information is produced and more accurate analysis is possible.

Ronald J. Robinson, general manager of Texaco's Exploration & Production Technology Department, says, "Vertical cable seismic is a significant step forward in our ability to more accurately image the subsurface. The integration of data acquisition and data processing provides true three dimensional information faster and cheaper than before. In so doing, it expands opportunities for evaluation of exploration and producing properties."

To commercialize the new vertical cable seismic technology, Texaco has entered into an alliance with Petroleum Geo-Services (PGS), with whom it shares the patent, which will also hold exclusive license to the Texaco-developed vertical-cable-acquired data processing technology.

Record of successes

PGS has been working with Texaco on the development and utilization of the vertical cable technology for quite some time. An early experimental implementation led to the definition of Texaco's Gulf of Mexico Gemini prospect, recently announced as a major deepwater discovery, but the first production scale utilization of vertical cable seismic acquisition was on Texaco's Strathspey Field in the UK North Sea. (Its final processed dataset will be available during the next quarter of the year.)

PGS will begin another vertical cable seismic survey in April on Texaco's recently announced Fuji discovery in the deepwater Gulf of Mexico.

The Gemini Field survey, conducted in 1992, was of a Gulf of Mexico 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 didn't appear large enough to generate the return needed to drill a well. But after a 35 sq mile vertical cable survey was done, it was obvious that the deepwater images, especially below the salt, were superior to the conventional data, and indicated additional objectives that showed Gemini's reserves justified the wildcat Texaco and Chevron drilled last year which has shown substantial hydrocarbon reservoirs below a 2,900 ft tabular salt formation.

For the Strathspey survey, Texaco and PGS shot a 20 sq mile 3D VCS survey over the Strathspey Field in the North Sea. Water depth at the survey site was about 750 ft, and horizontal spacing between vertical cables was 750 meters. The shoot took place when the weather conditions were worsening and seas were too high to record with conventionally towed streamers, but operations continued without problems until the weather forced the source boat off course. All the while, the data were quite good.

The deepwater Fuji prospect, in 4,243 ft of water on Green Canyon Block 506, is a joint Texaco Shell Offshore project.

Long-term project

As with so many revolutionary developments, vertical cable seismic acquisition technology was discovered by the subsalt imaging task force at Texaco's exploration & production technology development unit almost by accident. In the late 1980s, while looking for better seismic techniques for imaging structures below salt formations, the team happened upon anti-submarine warfare techniques that appeared to solve their problem. They were studying vertical seismic profiling, VSP variations, and needed a way to conduct VSP surveys without drilling a borehole. A researcher found similar sensor arrays being used by the US Navy for anti-submarine warfare.

Bob Tatham, Texaco's manager of geophysical research, says that the team essentially "ended up taking some off-the-shelf technology that had been used mostly against submarines and reconfiguring it."

Texaco first used transmitters on the recording buoys to send data back to the seismic vessel, but later started recording the data directly in the buoy and retrieving it with chase boats. By collecting the vertical-cable-acquired seismic data at fixed sites within a 3D grid and returning it to be merged with navigational data, the need for preliminary processing in the field was eliminated, because each position is pre-sorted into a separate tape cassette.

Tatham points out that 3D VCS data achieved are a marked improvement from that derived from conventional acquisitions, in that the seismic signal is received by the recording buoys from any direction, whereas conventional shoots have the source vessel towing the hydrophone streamers, forcing the signal to pass the hydrophones from only one direction, requiring the survey to incorporate signal redundancy with a series of 2D profiles. He says, "By shooting from all directions, you acquire true 3D data. In a conventional shoot, even though you develop a 3D image, you don't have the redundancies you want for forming images in all azimuths."

The lengths of the vertical acquisition cable varies with water depth, thus spacing of the hydrophones on the cable varies as well, but the deeper the water, the wider the spacing.

The seismic wave reflects off the seabed in deeper water, then passes through the hydrophone array en route to the surface, where it is reflected back down to the array again. As a consequence, the data can be processed to double the number of data points in the survey by incorporating the signal's mirror image from the water surface.

"It's just a lot more efficient," says Tatham, "especially in deeper water, where the ocean is quieter, and because the hydrophones are not moving through the water, there isn't any interference from towing noise."

The vertical cable seismic data acquisition technique has numerous advantages in addition to these, including providing for greater maneuverability for the source boat and a wider weather window for operations, making it ideal for such areas as the North Sea, off West Africa, and in the storm zone of Southeast Asia. Tatham maintains, however, than although vertical cable technology offers some of the same advantages of bottom cable acquisition, it actually compliments it rather than competes with it, since bottom cable acquisition is limited in its effective maximum water depth of operation, whereas vertical cable is most effective in deeper water.

Copyright 1996 Offshore. All Rights Reserved.