Researchers at the University of Texas in Austin are developing what they hope will be the next 3-D in the evolution of seismic technology -- 3-D vector wavefield seismic.
This multi-component technology provides more data than conventional 3-D, says Bob Hardage, senior research scientist at the University of Texas Bureau of Economic Geology.
The industry-sponsored research consortium has been working on the project for two years and is now conducting field recordings.
Although the bureau is just now developing case studies for the technology, Hardage speculates that information content will be significantly increased with multi-component data.
"You can get far more information about the rock and the fluid systems that are being imaged," he said.
Presently, the cost for this 3-D vector wavefield seismic is too steep to be made available to the industry, but developers expect the cost to fall over time, just as 3-D seismic did over the last 15 to 20 years.
"We see nine-component technology following the same course," Hardage said. "We'll keep working on it and the cost will come down."
The nine-component technology provides a more detailed picture of an underground reservoir's internal architecture, using multiple sources to send waves through the earth's crust and multiple receivers to collect the signals.
With this technology, three geophones are deployed at receiver stations to record vertical, inline horizontal, and cross-line horizontal movements. At source stations, it also deploys three vibrators -- a vertical vibrator, inline horizontal vibrator and cross-line horizontal vibrator.
"The reason for doing it this way is so we can capture the full elastic wavefield," Hardage said. "We get nine-component data."
Research, Work Continue
The conventional practice in the industry is to deploy geophones with only the vertical component at receiver stations and only a vibrator at the source station.
"Seismic wavefields consist of compressional components (P-wave components) and shear wave component (S-wave components)," Hardage explained, "but there's more than one S-wave, always at least two.
"Standard practice in the industry has recorded only the vertical component of only the compressional part of the wavefield," he added. "There's far more rock and fluid information in the shear wave component, so we want to capture the total wavefield."
Although the research group can detect the P-wave and S-wave images, Hardage said it will be some time before the process is perfected and costs come down.
Because the technology requires taking three different soundings instead of one, the cost is three times as high as 3-D.
"We want to be on the cutting edge of helping to get the industry there," he said. "We have a full-time seismic research crew that collects data and we have established a data processing center here. We're just beginning to get into the interpretation phase."
The group has received more interest from small independents who concentrate on onshore work while majors are more interested in offshore. Some 15 energy companies are funding the consortium at $45,000 per member each year.
For the last two years a seismic team from the bureau has been testing the system in a group of prospective oil fields where success at locating oil has been elusive using traditional 3-D seismic. The team is now analyzing the data and expects to recommend drilling sites by the end of this year.
To date, almost all seismic images on the subsurface have been only P-wave images. By developing technology that allows P- and S-wave images to be constructed, much more information about subsurface geology, stratigraphic relationships and pore-fluid properties becomes available.
Hardage said some software development also must be done to make interpretation more efficient.
The kind of seismic waves the new technology records does not travel well through fluids, which limits its effectiveness at finding offshore prospects -- unless sensors are placed on the sea floor.
Still, that practice is now becoming more common.
Hardage noted that P-waves do travel through fluids and they're strongly affected by changes in core fluids. If the data is processed, the P-wave will see the reflection and the S-wave won't.
"We'll have to pin it down and map it more accurately," he said.
While Hardage and his group focus exclusively on onshore technology, this new technology is just beginning to be implemented in the offshore world.
"In the marine environment, the rave is to do multi-component data recording," he said -- a difficult process, however, since shear waves do not propagate in water.
"To record there, you have to have sensors on the sea floor, and this is the new ocean-bottom cable technology," he said. "That is what the industry is developing so they can lay four component receivers on the sea floor."
One of the leading companies employing this technology offshore is Seitel Data Ltd., a Houston-based geophysical contractor and seismic database business. Seitel will begin shooting its first 3-D, four-component seismic survey in the Gulf of Mexico in mid-November, said company vice president David Wegner.
"Seitel has been involved in the Gulf for the last 21 months and has identified 500 blocks," Wegner said. The company will shoot multi-component surveys in eight areas.
"In the marine environment, you cannot create S-waves so you must rely on a technique called mode conversion," said Seitel geophysicist Nigel Paine. This technique uses the normal seismic source of water.
"The wave goes down into the earth like a P-wave and then gets converted back up through the earth as an S-wave," he continued. "The problem with S-waves is that they cannot be transmitted through fluids."
The company did a test survey in the Gulf in West Cameron Field, which he said was extremely successful.
Seitel also has been working in the North Sea for about four years in a joint venture with Schlumberger conducting 3-D, four-component surveys, he said.
"They have been extremely useful in areas of the North Sea where you have gas clouds," Paine said. "Normal seismic P-waves get absorbed by this gas, and in a normal survey, you get a dead zone or shadow and cannot see any reflections. But S-waves are unaffected so it doesn't matter to them if there's gas or fluid."
Hardage, of the Bureau of Economic Geology, is optimistic that the new multi-component seismic technology is the next technical revolution for the industry.
"There have been some notable evolutions that have occurred in seismic," he observed. "One goes back to the 1930s, when the industry changed from refraction to reflection imaging. Then in the 1960s it converted from analog to digital recording capability. That was tremendous."
Then came the 1970s and 1980s and the conversion from 2-D imaging to 3-D.
"Now it will be converting from single-component data to multiple component data," he said. "It's the new breakthrough."