WHAT is the worldâs most valuable natural commodity? Gold? Platinum? Plutonium? Not even close. Diamonds. One large, flawless white diamond weighing just a few grams can be worth millions of dollars. Red diamonds are usually quite small, but because they are so rareâ only twenty or so have ever been foundâ they are also extremely valuable. Edwin van Leeuwen, a mathematician by training, plans to find plenty more of both.
Not that van Leeuwen would be overawed by such a diamond if he ever found one. He describes the best ones, like the Koh-i-Noor diamond in the late Queen Motherâs crown, as âquite prettyâ. Van Leeuwen is turned on by the pursuit, not the prize.
For ten years now, heâs been searching out technologies that his Melbourne-based multinational company BHP Billiton can use to hunt down oil and minerals such as diamonds. His most successful project is Falcon, named for a bird that surveys the terrain from on high, then swoops down on its prey with merciless precision. The Falcon project has spawned the geophysicistâs ultimate fantasy: a detector that can spot the minute changes in the Earthâs gravitational field generated by hidden reserves of valuable ores or the rock formations associated with diamonds from an aircraft barrelling along at 185 kilometres per hour and 90 metres above the ground.
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That might not sound so impressive until you stop to think that gravity is a force measured by detecting acceleration, and that an aircraft is accelerating and decelerating every which way as it bounces through the air. âTypically the effect of turbulence is 100 million times greater than the signal from an ore body. Thatâs the nightmare youâre trying to solve,â says van Leeuwen.
But solve it they have. Using what was until a few years ago top-secret cold war technology, the Falcon team has built a device so sensitive it could spot the gravitational pull of a two-year-old if one toddled by a few metres away. Called the airborne gravity gradiometer or AGG, it promises to make prospecting for minerals faster and cheaper than ever before and could just help to change the face of the diamond trade.
Diamonds form deep within the Earth where extreme heat and pressure forces carbon atoms into orderly tetrahedrons. They can only be brought to the surface by the Earthâs most powerful volcanic eruptions.
Once, lone prospectors trudged from river to river, panning for diamonds. When they discovered the gems, theyâd move upstream to locate the diamond pipeâ a tube of ancient volcanic magma, usually bluish kimberlite: the pudding to the diamond sultanas. The richest diamond pipes in South Africa, Canada and Russia can be up to a kilometre or more across, reach kilometres into the Earthâs crust, and yield several tonnes of diamonds each year.
Nowadays prospectors are more likely to be found racing around in land cruisers, but they still discover diamond pipes with an almost uncanny sixth sense, combined with painstaking searches for âindicatorâ minerals such as garnets, the precious stone associated with kimberlite. Multinational minerals companies work on a far grander scale. They prefer to employ teams of scientists and technicians with high-tech equipment to survey vast tracts of land for the geological signatures of diamond pipes.
Kimberlite often has different magnetic and conducting characteristics to the surrounding rocks, so the big companies rapidly survey remote areas for these properties using detectors carried in aircraft. But magnetic storms and salty water can interfere with these measurements. So in practice, the surveys sometimes need to be backed up with ground-based measurements that pick up small variations in the Earthâs gravitational pull caused by different density rocksâ kimberlite is usually around 10 per cent less dense than surrounding rock. Gravity surveys are also used to detect ore bodies of minerals such as silver, zinc and lead, which tend to be almost twice as dense as the surrounding rock. And in the past, gravity surveys have even been used to find oil and gas. âNo other signals can get in the way or screen out gravity. Itâs a beautiful signal,â says physicist Ken McCracken of Jellore Technologies in High Range near Sydney, who acted as a consultant on the early days of the Falcon project.
Sensitive to movement
The simplest way to measure gravity is with a gravimeter, basically an extremely sensitive weighing device comprising a mass hanging on a spring. The stronger the gravity field, the further the spring extends. But gravimeters are supersensitive to movement, so it takes time to set up the equipment and record a single reading, making surveys expensive. Using a gravimeter on a plane is just about possible, but only if you want to measure the difference between points kilometres apart â not a good way to find mineral deposits.
Falconâs mission was to solve that problem by building a gravity measuring device that could be flown in a light aircraft and still be sensitive to a gravitational signature of about 30 EĂśtvĂśs units, equivalent to one millionth of the gravitational pull of the Earth itself. From the beginning, the plan was to use a gravity gradiometer, a device that measures not absolute gravity but the gravity gradient between two points (see âGravity rocksâ). To the Falcon team that seemed the only way to get the necessary sensitivity from an aircraft.
Back in the early 1990s, when the programme started, several teams of physicists around the world were chasing the same prize, so BHP Billiton sent McCracken and other members of the Falcon team on a fact-finding mission to contact anyone who would talk to them about what theyâd got.
âLots of stuff we wouldnât touch with a bargepole,â he says. But one piece of technology stood out. The US Navy had recently declassified a piece of cold war technology developed by a company that is now part of Lockheed Martinâ a gradiometer used by nuclear subs to negotiate the underwater mountain ranges without using sonar pings, which alert the enemy to your presence. (In his 1984 novel The Hunt for Red October, which is rumoured to have shocked the US military, Tom Clancy described a Soviet sub that carried a similar piece of equipment. Not that the Soviets ever had one, claims van Leeuwen, âI know, I went to talk to them.â)
Fiction notwithstanding, Lockheed Martin is cagey about revealing the fine details of the gravity gradiometer it built for the American subs. But the following details can be pieced together: the guts of the device are three discsâ each about 10 centimetres acrossâ mounted at right angles to one another. On each disc are two pairs of accelerometers (see Diagram). Each disc is designed to measure the gravity gradient along the x, y or z axis, and mounting the accelerometers in pairs helps cancel out linear accelerations as the sub speeds up or slows down. Spinning these discs helps prevent the subâs rotational acceleration, when it changes course for example, interfering with the readings, and also minimises errors resulting from minute changes in temperature. The whole instrument is mounted on precision gimbals that allow the sub to move around it, eliminating the worst of the rotational accelerations.
Still, a Trident sub creeping along the seabed is a far more benign environment for a gradiometer than a small plane zipping through the skies. âWhen we said we wanted to fly it in a light aircraft, everyone laughed,â recalls McCracken. âThe signal to noise ratio in a plane is one part in 100 million. One part in a ten thousand is tricky. One part in one million, and you think, âhang on mate.â One part in 100 million is frightening in the extreme.â
Cracking the problem
Nonetheless, the Falcon team managed to persuade engineers at Lockheed Martin to work with them. Over three years, the joint team of 40 engineers, mathematicians, geophysicists and computer scientists identified the systemâs limitations and came up with the technical fixes needed to make the gradiometer fit for the skies. Once again, some of those fixes remain secretâ for commercial reasonsâ but van Leeuwen is prepared to reveal the following: the gradiometer is flown on an inertially stabilised platform fitted with hydraulic dampers to help absorb the aircraftâs vibrations. It takes one week to calibrate the device, by sitting the plane on a moving stand that mimics the yaw, pitch and roll of flight. The impact those movements have on the gradiometer are used during âpost-mission compensationâ, when secret Falcon software converts data collected during flight into gravity maps. That software also integrates topographical information collected by laser during flight, so compensating for the gravitational effects of hills and valleys.
âThe software is our competitive edge,â van Leeuwen told me when I interviewed him in BHP Billitonâs global headquarters in Melbourne. âOnly six or seven of us have access to the source code, and this office is screened for bugs.â
But the biggest breakthrough came when the Falcon team was tipped off to take a closer look at a gadget designed to detect concealed nuclear warheads that Lockheed Martin had developed for the US government. The device, called an Arms Control Verification Gravity Gradiometer, is possibly the only portable gradiometer in the world, and researchers claim it can pick up the gravitational pull of a lump of plutonium. Like the submarine gradiometer, itâs based on pairs of accelerometers. When the Falcon team married it with the hardware and software designed for the sub, the result was an airborne gravity-detection device five or six times as sensitive as the one the team set out to develop. âWhen we started, we didnât think weâd be able to find diamond pipes,â says van Leeuwen. âNow diamond pipes are dead easy.â
And with that success Falcon seems to have wowed the mining research community. âItâs a great achievement,â says Bob Smith of Greenfields Geophysics in Melbourne, who once directed a competing project at Rio Tinto. âThe system is outstanding,â agrees geologist Ross Fardon of Ross Fardon & Associates, a mining consultancy in Brisbane.
Looking back, says van Leeuwen, one of the stickiest part of the operation was persuading the US government to grant export licences for the two key pieces of technology. Fortunately, at the end of the cold war, there was a brief window of opportunity when optimism prevailed and the exports were approved.
The Falcon project may have been more successful than anyone dared to predict, but thereâs room for improvement. The technology needs to be cheaper, says Fardon, and the Falcon team must up the sensitivity of their aerial gradiometer if it is to detect thin ore bodies as it flies over them. This is quite a big deal, says Fardon. âThere are countless very important ore bodies of less than 50 metres thick,â he says. âBut itâs [Falcon] or nothing at moment.â In fact, Van Leeuwen refuses to be drawn on just how sensitive the Falcon gradiometer is, claiming that knowledge is part of BHP Billitonâs commercial advantage. But, he insists, âitâs a lot better than our competitors are guessingâ.
And everyone, including van Leeuwen, Fardon, and McCracken, agrees that the next big thing in mining technology will be a means of fusing gravity gradiometry data with other information such as magnetism, conductivity, radioisotopic and infrared signatures to give one detailed exploration map. âAny one signal by itself, even gravity, is not enough,â says McCracken. âYou ultimately have to drill a hole and see how rich it is. So when the various signals agree, itâs a high priority.â
Meanwhile, everyoneâs waiting to see when Falcon will start paying its way. To date, itâs cost BHP Billiton $18 million. Since Falcon started commercial flights in October 1999, it has detected gravity anomalies suggesting diamonds in Canada, Australia, South Africa and Botswana. Some 10 per cent of those anomalies have been drilled, although so far none has turned up enough diamonds to be commercially viable. Van Leeuwen doesnât seem too worried. He points out that the exclusivity deal with Lockheed Martin lasts for up to 10 years, and that the crucial software is entirely Falconâs own.
Whatâs more, that edge-of-your-seat wait for a major find has done nothing to dampen enthusiasm for the technology. â[Falcon] has the capacity to increase the availability of diamonds for the whole world. Thatâs wonderful,â says geologist and diamond expert Garry Holloway, who runs Precious Metals, a jewellery business in Melbourne. Itâs no surprise, then, that De Beers is helping fund a competing aerial gravity gradiometer being developed by the CSIRO, the government research agency in Australia.
However, diamonds are not the only mineral that BHP Billiton has set its sights on. And there may even be uses for the technology beyond mineral prospecting. Falcon is being used to search for oil in the Bass Strait between Tasmania and the mainland, iron ore in Australia and the US, silver, lead and zinc in Australia, and copper ore in Chile.
In the future, van Leeuwen and his team plan to collaborate with a consortium of companies and universities to examine the San Andreas Fault. And last year, scientists from the National Institute of Standards and Technology in the US approached the Falcon team to discuss whether its gravity gradiometry could be used to find vaults under the collapsed World Trade Center. The team also hopes to discover whether the technology could be used as a water-divining tool to detect underground aquifers in an increasingly water starved world.
Not that diamond hunting has lost any of its pull. âIn ten years,â says van Leeuwen. âWe hope to have found everything.â
Gravity rocks
Physicists have realised that the best way to spot small changes in gravity from a moving vehicle is to measure gravity gradients â how gravity changes with distance (see âTreasure huntingâ). While the 3D gravitational field can be described by three components acting at right angles to each otherâ Gx, Gy and Gzâ gravity gradients are described by nine components: Gxx, Gxy, Gxz, Gyx, Gyy, Gyz, Gzx Gzy and Gzz. To work out Gzzâ the critical vertical component of gravity which can reveal what lies beneath the groundâ you must measure at least two of these nine components, so gravity gradiometers are more sophisticated than gravimeters. However since these devices work by measuring different components of gravity, not simply the vertical component, this makes it easier to take into account the accelerations and rotations of flight.FIG-mg23615801.jpg
One of the first gravity gradiometers relied on a pair of masses fixed at different heights at the end of a horizontal beam suspended on a wire (see âDoing the twistâ). The angle the beam swings through is a measure of one of the components of the gravity gradient. Rotate the beam 90° and you can measure a second component.FIG-mg23615802.jpg
Today some of the most sensitive instruments measure gravity gradients from the way metal bars suspended in magnetic fields move. Motion due to changes in the gravity field alters the magnetic field. This is detected via ultra-sensitive superconducting quantum interference devices dubbed SQUIDs.