In leaving no stone unturned in the search for its next diamond mine, De Beers is using a Zeppelin airship to fly a combination of gravity and magnetic geophysical surveys across the Kalahari sands of Botswana.
Home to some of the world’s biggest and richest kimberlites, Botswana is still considered by many to be among the most prospective countries for further diamond discoveries. The problem is the Kalahari sands, which are often 30 to 100 metres deep, and mask and play havoc with traditional exploration techniques.
The Zeppelin system uses airborne gravity gradiometer (Air-FTG) provided by Bell GeoSpace and produces better resolution and cleaner data compared with other fixed-wing gravity systems. The Zeppelin’s low noise and cruising speed, combined with advanced gradiometry technology makes this a compelling tool for mineral exploration.
“The airship is the best airborne gravity system in the world,” said De Beers’ geophysicist David Hatch during a presentation at last year’s Prospectors and Developers Association of Canada convention in Toronto. “We are getting the same quality as ground gravity data. It’s a really exciting story about innovation.”
Rooted in its origins of founder Count Ferdinand von Zeppelin, the Zeppelin company launched a new airship in 1997, called the LZ NT. Today, three Zeppelin-NTs have been built and are flying, used mainly for tourist and commercial ventures. A fourth airship is scheduled for commercial operation in spring 2008.
Zeppelin is a name that invokes memories of another era — the dawn of aviation, when tens of thousands of passengers were carried on trans-Atlantic flights during the 1930s. A total of 119 Zeppelins were built between 1900 and 1938. Eventually the era of Zeppelins came to an end in the early 1940s. The Zeppelin reputation never fully recovered from the historic 1937 Hindenburg fire, when an airship burst into flames over Lakehurst, N.J., killing 35 people.
The Zeppelin company lived on, however. A subsidiary formed in 1915 specialized in developing lightweight engines, transmissions and gear boxes. Today, the ZF Group is a leading automotive supplier of driveline and chassis parts, with sales worth 9 billion euros annually.
The airborne gravity technology was a breakthrough several years ago. Although gravity gradiometry has been measured for more than a century, it was only done from stationary measurements on the ground, which meant slow surveys and limited coverage. Attempts to fly gravimeters initially met with limited success due to their high sensitivity to aircraft turbulence and their long spatial resolution.
Advances in the technology were developed for the U.S. military by Bell Aerospace (currently Lockheed Martin [lmt-n]) for use aboard Trident Class nuclear submarines. The U.S. government spent something like US$400 million developing the proprietary technology.
Bell GeoSpace took to the air with this new technology in 2002 aboard the fixed-wing Cessna Grand Caravan.
“The Air-FTG is able to compensate for most of our turbulence experienced by the aircraft and retain the high-frequency signal that is critical to prospect-level geophysical surveying,” states a 2003 case study.
Gravity surveys are designed to detect variations in the density of surface and sub-surface rocks. Kimberlites of sufficient size, with their relatively low density, especially pyroclastic or crater facies, can be detected using this technology in combination with magnetic and electromagnetic surveys. Likewise, density contrasts can be explained by other geological formations of low density, such as meta-sediments, lake-bottom sediments and deep depressions in bedrock covered by overburden, as well as sulphide mineralization.
“Kimberlites are difficult targets and the Grand Caravan system was not quite good enough,” Hatch explained. “The added noise created by a fixed-wing aircraft poses a challenge in an exploration scenario in picking out different size kimberlites buried at various depths. If a kimberlite has subtle density contrasts then you won’t see them.”
He added: “The cost of exploration is higher because of these false anomalies. You end up drilling a lot more anomalies that are not kimberlite.”
De Beers looked for a way to better stabilize the instrument platform and saw the new Zeppelin airship as a viable alternative to the Grand Caravan.
“There is a dramatic improvement in the background noise and the resolution of the system. We see a four times improvement in resolution over the fixed-wing system and that comes simply from the fact we’re flying at one-quarter of the speed, so we get four times the resolution,” said Hatch. “We also have three times lower system noise and we have lost all of the false anomalies.”
The Zeppelin airship, which is almost as big as a football field, measures 75 metres long and 15 metres wide. It’s powered by three engines — two on the side and one on the rear — which are mounted on the airship’s solid frame.
“Of course, Zeppelins have a bit of a reputation for exploding, but they now use helium, which is inert so it’s not flammable,” Hatch stressed. “It’s probably one of the safest aircrafts in the air.”
Even though it floats, it’s heavy — over 8,000 kg, and the aerodynamic lift is minimal.
“It’s big. If a small wind or gust hits it, it doesn’t move,” Hatch said. “It’s like a big ship on the ocean; the little waves don’t knock it around.”
The airship can be flown in winds of up to 25 km per hour.
“If it changes its angle at the time, it tilts back a bit; it doesn’t accelerate up, it just keeps floating straight,” Hatch explained.
The Zeppelin was initially tested using a small accelerator, which measures the vibrations in the airship. It tested between 10 and 100 times quieter than a normal fixed-wing aircraft.
On the positive results, De Beers did a feasibility study that concluded that using gravity and magnetics in combination would improve kimberlite discovery rates, and using the instruments on the airship would increase resolution and decrease noise.
“There would also be a substantial cost benefit over ground gravity and existing airborne systems,” Hatch said.
De Beers began its first test trials in Botswana in November 2005. The first survey was flown over an area already covered by good ground and fixed-wing gravity data. Nearly 4,000 line-km of data was collected during a 5-week period.
“The ground gravity data showed a couple of quite subtle anomalies,” Hatch said. “Those same features are clearly visible (in the Zeppelin survey).”
The first 180 line-km from the survey were plotted against the first 180 line-km from the Grand Caravan survey. Between 30% and 40% of the data in Grand Caravan was rejected because the acceleration got too high, something that Hatch blamed on weather conditions. None of the Zeppelin data for this initial period was rejected because of acceleration.
Data quality
“The quality of data that we see over the test block is virtually identical to ground gravity,” Hatch said. “The improvement that we have is that we collect as much data in one day on the airship as forty-five ground teams. You generate a lot more data and cover a lot more ground.”
During the initial test survey, flights were limited to 5 hours and 20 minutes, allowing about 209 line-km of surveys to be completed each day. De Beers has improved that to 6-7 hours of flying time, collecting more than 300 line-km daily. The company expects to collect 60,000 line-km per year, the equivalent of 10,000 sq. km of ground, Hatch said.
During the summer in Botswana, surveys are flown at night to take advantage of lower winds and thermal activity, and that serves to increase the payload of the airship. The surveys are done about 80 metres off the ground and at an optimal speed of 55 km per hour, “so we get a good balance between resolution and productivity,” Hatch explained.
De Beers continued to use the Zeppelin to fly airborne gravity and magneti
c surveys across its holdings in Botswana through 2006. The company currently holds some 46,000 sq. km of ground in the country, either in its own right or in joint-venture partnerships, which includes one with Firestone Diamonds (FRDIF-O, FDI-L).
Firestone holds more than 17,000 sq. km of diamond exploration rights in and around the Orapa and Jwaneng mines. De Beers can boost its interest in these holdings to 61% by carrying Firestone through to completion of a bankable feasibility study. Firestone reports that De Beers started flying the Zeppelin surveys over the optioned Jwaneng ground in mid-2006.
Since optioning the Orapa and Mopipi ground from Firestone in June 2004, De Beers has drilled 51 geophysical targets resulting in the 2005 discovery of three new kimberlites — AK21, AK22 and AK23, 15 km south of the Orapa mine — and one new kimberlite in 2006, called BK53, which is 10 km north of the Lethlakane mine. The initial microdiamond results from the three discoveries in 2005 proved promising enough that 100-tonne mini-bulk samples were collected from each of the bodies. The results from the follow-up work, however, were disappointing and no more work is planned.
Initial microdiamond results from follow-up core drilling on the BK53 kimberlite discovery haven’t been released.
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