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by Dan Zlotnikov on June 21, 2013 expertise
A versatile tool, airborne gravity gradiometry is changing the face of mineral exploration.
A relative newcomer to the resource exploration world, gravity gradiometry is already having a major impact. Its potential for producing high-quality data has caused many explorers to take notice and wonder how they might best make use of the technology.
Just like the commonplace airborne gravity survey, an airborne gravity gradiometry survey responds to the gravitational pull of large masses on an accelerometer. Here’s how it works: Fly over a particularly dense object, such as a rich underground ore deposit, and you register a spike. Traditional gravimeters measure the force exerted on them from one direction only, usually straight down. If a survey does not fly directly over an anomaly but slightly to one side, the odds it will detect that anomaly decrease sharply. Gravity gradiometers, on the other hand, measure forces from the sides as well, greatly improving the ability to detect objects.
Dan DiFrancesco, Business Development Manager for Gravity at Lockheed Martin, U.S.A., explains that the technology originated with a US military project in the 1970s: a navigation tool for Trident nuclear submarines. When the Cold War ended, the project was declassified and gradiometers became available for commercial use.
There are two gradiometer designs in use today, says DiFrancesco. One is the Full-Tensor Gradiometer (FTG), which is the original instrument, developed by Bell Aerospace and now being made by Lockheed Martin following its acquisition of that company in 1996. The other is the FALCON®, a partial-tensor instrument jointly developed by Lockheed Martin and BHP Billiton which was designed to aid in the latter’s search for diamond-bearing kimberlite pipes – small targets not easily found with a traditional airborne scalar gravity survey.
DiFrancesco notes that FALCON® measures two tensors with high fidelity whereas FTG measures all six with a lower fidelity. Though the two instruments arrive at the solution by different means, they produce very similar results.
In addition to the three commercial providers, a number of new airborne gravity gradiometry solutions are under development, but are not yet commercially available.
The Zeppelin airship used by Bell Geospace for a gravity gradiometry survey commissioned by De Beers. The resulting data allowed De Beers to find anomalies less than 100 metres across.
“In oil and gas exploration, gravity gradiometry has proved valuable for sub-basalt and sub-salt definitions, pinnacle reefs, and dykes."
- Dan DiFrancesco
Chris van Galder, AGG technical advisor for Fugro Airborne Surveys, says BHP was so pleased with the results produced by the FALCON® that it fast-tracked completion of a second instrument. Fugro, which took control of BHP’s gradiometers in 2008, has gone a step further: in 2012 it began offering commercial HeliFALCON® surveys, the helicopter platform allowing it to fly slower, take more measurements per kilometre, and produce higher-resolution data.
Airborne gradiometry data isn’t cheap. Alan Reid, a potential field geophysics expert who has served as client consultant, puts the price of a large airborne gravity gradiometry survey at roughly $200 per line-kilometre (though current market conditions have seen this figure decrease slightly). The high price reflects the high cost of the instrument itself.
“You spend several million dollars to buy one, work out how much you can charge per kilometre, and soon you decide you want to eat up a lot of kilometres,” Reid says.
Flying slower means booking more time on a rare machine (only 11 gradiometers exist, notes DiFrancesco), and that in turn means higher costs per kilometre.
Still, when the budget permits, gravity gradiometry delivers. Bell Geospace CEO and President Scott Hammond recalls a survey De Beers commissioned because it needed extremely high-resolution data to find small kimberlite pipes. Bell used a Zeppelin airship and the resulting data allowed De Beers to find anomalies less than 100 metres across. However, practical constraints intervened: The Zeppelin was caught in a dust devil and both it and the gradiometer suffered significant damage. Therefore, Hammond doesn’t expect to see widespread use of Zeppelins for surveys any time soon.
DiFrancesco believes more robust and resilient designs could prevent such problems. “There are hybrid airships coming on-stream that I think will make a difference.”
Anything and everything
A versatile tool, gravity gradiometry can locate anything with a density gradient.
“In oil and gas exploration, gravity gradiometry has proved valuable for sub-basalt and sub-salt definitions, pinnacle reefs, and dykes,” says DiFrancesco. “It has also been used in mining applications, to find iron oxide-copper-gold type deposits, banded iron, and kimberlites, as well as porphyry copper and placer gold-type activity. There have been a lot of case studies showing the value of this technology.”
The Fugro HeliFALCON system ready for deployment. The HeliFALCON system is credited with the discovery of new kimberlites in the Ekati diamond field.
Data visualization comparing fixed-wing versus helicopter survey data. Courtesy of Fugro Airborne Surveys.
But the gradiometer’s sensitivity can be a curse as well as a boon. As van Galder notes, “the good thing about a gradiometer is that it can detect anything; the bad thing is it detects everything.” This means geologic “noise” (van Galder points out that real-world rocks don’t have perfect, uniform densities), not to mention the noise produced by the plane’s acceleration.
Reid says the “astonishingly crisp” results produced by airborne gravity gradiometry are all the more impressive given that “the accelerometers are being run at their absolute limit. “You’re putting these accelerometers into something that's being buffeted around. What the pilots are doing is just a shade short of impossible. Flying sensitive accelerometers in a light aircraft is an incredibly difficult thing to do.”
Hammond puts the sensitivity into context:
“We're looking at one part in 10^11th in the force of gravity. It's such a small signal that it's hard to see. When you're trying to do that on an airplane that's flying through all kinds of turbulence, the noise is many orders of magnitude higher than the signal you're looking for. Getting rid of that noise is critical.”
Data processing algorithms are just one part of the puzzle. Bell has been using turbine-retrofitted Second World War DC-3 planes, which Hammond says are large, slow, and able to handle much more turbulence.
Marco Antonio Braga is exploration manager for iron at Vale, which has flown some 100,000 line-km of airborne gravity gradiometry over the past 10 years. He says it’s important to know how factors on the ground affect the quality of data. “We fly surveys at 5 a.m., stop at 8 a.m., then come back at 4 to 5 p.m. and fly again,” he says, adding that if the conditions aren’t right (e.g., strong winds or heat), they won’t fly since they know the data won’t be good enough.
Education and experience
An ARKeX aircraft on survey in Kenya. The Full Tensor Gravity Gradiometry technology utilised by ARKeX has helped oil and gas exploration companies accelerate their search for oil in East Africa.
Aerial pictures taken from an ARKeX aircraft over Greenland.
Neil Dyer, chief technology officer for ARKeX, says experience and specialization are equally important. ARKeX specializes in hydrocarbons, which enables it to focus on its customers’ end-goals,
“We pay a lot of attention to the design of our ratio between the spacing of the survey lines, the tie lines, and the processes we use to level the data and minimize the filtering that needs to be applied in the post-processing sequences,” says Dyer.
DiFrancesco points to customer education as a crucial component of successful gravity gradiometry use: “I strongly believe that the end-users – the oil and gas and mining companies – need a deep understanding of what the capability provides.”
Dyer agrees. He notes that just a few years ago, ARKeX was rejecting more than half of all inquiries they received after feasibility modelling indicated gravity gradiometry was not suitable in certain cases. ARKeX and its peers have been trying to educate potential customers about what gravity gradiometry can and cannot do, and these efforts are beginning to bear fruit. “There are now repeat customers for these services,” notes DiFrancesco, “and more and more people are using this technology in conjunction with, or in advance of, seismic surveys. In short, there’s more acceptance now.”
Right tool for the job
Still, despite the promise it holds, gravity gradiometry should not be seen as a cure-all. As Dyer points out, “there is perhaps a feeling that gravity gradient will solve all problems. Rather, the technology should be seen as expanding the proportion of situations in which a gravitational measurement is applicable.”
“We're still looking for density contrasts; if there isn't a structure with a density contrast, we're not going to help,” says Dyer. “That’s where we like to get involved, in identifying that situation before a lot of money is spent.”
Technical papers on gravity gradiometry, and other resources, are available on Gradiometry.com.