A new generation of airborne gradiometry measurements offers explorationists a fresh tool, particularly in areas of complex geology and rough terrain. Gravity gradiometry is a geophysical technique that measures derivatives of the earth's gravitational field. It looks at each of the three components of gravity, in all three directions.

"Gravity gradiometry is a step change in resolution over conventional gravity surveys," says Phill Houghton, a cofounder and vice president of Arkex Ltd., a U.K.-based firm that offers the service. "What we are really measuring is the rate of change of density within the earth."

The airborne technique has been commercially available from Arkex for just two years; originally it was developed as a military tool for the U.S. Navy's Trident missile program. Indeed, the instruments Arkex currently deploys were manufactured by Lockheed-Martin. "It's a tried and tested technology, albeit for military applications."

Now Arkex has imported the technique to the oil and gas business. The company married its expertise in potential fields to state-of-the-art equipment, and it has developed proprietary workflows and algorithms to process and interpret the data. Additionally, it adapted the equipment to an airborne platform, ideal for onshore exploration in remote and difficult areas.

Arkex's method opens up geologic trends and provides details on subsurface images that previously weren't available from the air. In the past, conventional gravity was largely confined to regional reconnaissance, because images were not of sufficient quality for interbasin studies.

That's changed with the new tools. "Our instrument is orders of magnitude more sensitive than existing conventional equipment, with considerably increased signal bandwidth and much higher signal-to-noise ratio." The result is an ability to see detailed structure at depth.

The depth of investigation depends on how large the bodies are and the density contrast between host and target rocks. That's why gravity gradiometry works well in salt basins and in structural provinces, because salt, structures and faults have significant differences in density distribution.

Situations that seismic struggles with-such as near-vertical faults and complex structures-can be resolved with gravity gradiometry. Furthermore, it's a tool that can be used in areas where seismic acquisition is prohibitively expensive. "Anything that is airborne clearly has appeal in such trends as the foothills of British Columbia and the Rocky Mountain region," Houghton says.

The technique has applications in several levels of work, from frontier exploration to lead identification to prospect evaluation. Clients use it to drive leasing decisions, to help locate 2-D and 3-D seismic programs and to confirm prospects.

Obviously, the resolution of the data depends on the geologic setting. The best results are obtained when high-resolution gravity information is integrated with existing well control and seismic. "A multidisciplinary approach is what truly delivers exploration benefit."

At present, Arkex has two instruments deployed, both in North America, and a third on order. Soon it will begin acquiring a survey in West Africa. And there's more innovation to come: "We are now developing and manufacturing our own system, which will be the next generation of the technology. It's a superconducting instrument, which will give us all the wins of low-temperature, low-noise instrumentation."

Cost-wise, the technique falls between conventional gravity surveys and seismic data. A ballpark estimate is that a gravity gradiometry survey will cost about a tenth as much as a seismic survey.

Arkex advises potential clients to run feasibility models on their areas of interest to determine if the technique can deliver needed information. "We sit down with a company's structural geologist to get an idea of the geologic target, then we produce a feasibility model that uses basic parameters," says Houghton.

On the geologic targets Arkex has looked at, more than 40% are too subtle for the current technology. "The modeling is a service we provide so clients can get a better idea of what the technology can deliver and where it can be used."

For efficiency, the minimum survey size is about 150 square miles, plus the survey aperture has to be sufficient to image the geology at the target depth. Arkex also collects magnetic gradiometry, Lidar (light detection and ranging) and digital video data at the same time as the gravity gradients. (The package of services is marketed under the term BlueQube.) It believes in acquiring as much geophysical information as possible to help unravel the geology.

"Business is good. Our biggest problem is convincing clients to release case studies, as clients often wish to keep results to themselves," he says. "What we can say is the majority of customers we have flown for have all come back for repeat surveys."

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