GGI no longer classified

‘New’ Gravity Can Give 2-D a 3-D Look

American Association of Petroleum Geologists (AAPG)

Do you need to evaluate a subsurface area but lack adequate seismic data?

If so, a new technology being tested in a Gulf Coast transition zone locale may soon become your favorite must-have tool.

The other good news is you won’t have to bother with the challenges and expense of acquiring new seismic data until you have de-risked and high graded areas of potential interest.

In essence, you can kind of have 3-D without having 3-D.

The test project is being implemented by IONGEO and ARKeX, and it entails the integration of multiple geophysical measurements using both seismic data and gravity gradiometry (GG). The project was close to being wrapped up in February, with the final report scheduled to be complete by the end of March.

This multi-disciplinary approach to evaluation is designed to provide enhanced geologic understanding in areas that are either too difficult or too costly to acquire new 3-D seismic data, according to Peter Nuttall, director of geophysics integrated services at IONGEO.

Many areas of interest to operators, including a number of Gulf Coast locales, lack 3-D data. Instead, they may have legacy 2-D data, which are not optimized for today’s exploration objectives.

Acquiring new 2-D is not the answer as the data still are two-dimensional, and removing the uncertainties associated with these data requires 3-D information.

“Airborne gravity gradiometry can solve some of these issues by providing a 3-D measurement of the earth’s gravitational field,” Nuttall said. “These data act as an intelligent geologic interpolator among existing 2-D seismic data, plus update and validate the velocity field and aid seismic processing.”

Getting Critical Information

Gravity gradiometry imaging (GGI) was derived from a classified military technology. The initial 3-D full tensor gradiometer trial for commercial use took place in the Gulf of Mexico in 1994. Since that time, several systems have been deployed, and ARKeX operates three of these for airborne and marine use.

The technique has considerable advantages over conventional gravity.

Duncan Bate
Duncan Bate

“While a conventional gravity survey records a single component of the three-component gravitational force, usually in the vertical plane, full tensor gravity gradiometry measures the derivative of all three components in all three directions,” said Duncan Bate, senior sales geophysicist at ARKeX.

“The method measures the variation of each component of the gravitational force – vertical and horizontal – in the vertical direction and in two horizontal directions,” Bate said. “It also provides higher resolution and increased bandwidth.”

“The 3-D nature of the gravity gradiometer measurements means that the integration with 2-D seismic data can provide critical information leading to a much improved 3-D interpretation,” Bate added.

In the test project area in the Gulf Coast transition zone, there’s a gap between existing, vintage onshore data and newly acquired offshore data. The 3-D data collected by ARKeX using its FTG is being jointly integrated and interpreted with 2-D seismic data from ION’s GulfSPAN program to provide 3-D infill between GulfSPAN 2-D regional datasets.

Nuttall emphasized there are three key components to the project:

  • Two-D seismic data.
  • Good well coverage.
  • Three-D gradiometry.

“The combination of all those lets you calibrate seismic with gradiometry and identify features such as specific fault trends, as well as highs and lows in gravity that may be introduced by density variations at depth,” Nuttall said.

“It doesn’t matter if you have new or old seismic data,” he said. “Modern processing and imaging techniques employed by IONGEO can uplift legacy seismic data, and the GulfSPAN data provide new insight into the Gulf Coast region.

“Gradiometry allows you to integrate the components into a 3-D image, so now you have a much better understanding of where the shallow salt is, the deep salt is, where the fault fracture zones are,” Nuttall said. “Then you can high grade areas that look interesting.

“Using existing seismic data and new gradiometry data,” he said, “can make this affordable.”

Tough Targets

The pilot study is all about areas that are difficult and expensive to access, according to Nuttall. The study’s specific challenges of transition zone, onshore and offshore provided a combination of what the team wanted to investigate.

Because seismic data can be exceptionally expensive to acquire, particularly in the onshore Gulf Coast, the integration of GG and seismic has the potential to add value in areas other than the transition zone. Piecing together the fabric from onshore to offshore in the transition zone, however, can have exceptional impact for explorationists going after plays there, where the original seismic data may be as much as 20 years old.

Meanwhile, the trial project team is pleased with the outcome of the program.

“The initial plan for how the project should go in an ideal situation has been achieved at this stage,” Bate said. “So everyone is happy from that point of view.

“I think everyone has been encouraged,” he continued, “and we hope to do more similar projects both in the Gulf of Mexico and the rest of the world.”

Adding a bit of intrigue, the new high-profile McMoRan Exploration Davy Jones discovery that’s creating such a buzz is in the area of the GG survey.

“We can see what we believe is a significant influence on the gradiometry in the area,” Nuttall said. “The McMoRan well shows up as a gravity feature because you have salt down there that’s creating a structure,” he said.

“The well is below the canopy salt but above the Louann salt.”

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