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| CSIRO | SOLVE | Issue 6 Feb 06 | |
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ARTICLE
INDUSTRIAL PHYSICS:
A Little SQUID, a Lot of Ore By Bianca Nogrady
New magnetic sensors enable mining companies to ‘see‘ previously undetectable ore deposits that may be deep underground. It is barely the size of a fingernail but when it comes to finding millions of tonnes of undiscovered ore, a little SQUID goes a long way. SQUIDs – Superconducting Quantum Interference Devices – are extremely sensitive magnetic sensors that are allowing mining companies to locate ore deposits missed by conventional technology.
With conventional coil magnetic sensors, a loop of wire is laid over an area of about 20,000 square metres and a pulse of electrical current is transmitted through the wire to generate a vertical magnetic field down through the soil and rock. This creates eddy currents that decay at different rates as they pass through different materials. Magnetic sensors on the surface measure the rate of decay of those currents. By collecting data from points spread all over the area, prospectors can build up a subterranean ‘map’. However, Keith Leslie from CSIRO Industrial Physics explains that large and highly conductive deposits of minerals such as nickel sulphide, gold and silver create a problem for conventional coil magnetic sensors: “If it’s a big conductive ore body, the rate of decay can be of the order of tenths of seconds and the problem with previous sensors is that they work well up to hundredths of seconds. If you have something that’s decaying slowly they are not sensitive enough,” he says. Another problem for Australian mining is that much of the country is covered by highly conductive overburden that can confound coil systems. But neither of these factors are a problem for SQUIDs because they measure the magnetic field itself rather than the rate of decay. This means they can observe the magnetic field for a much longer time, and pick up the decayed responses that are characteristic of larger ore bodies and ones that are buried much deeper. The SQUID sensor can also discriminate between the early response from the conductive overburden and the response from any buried, highly conductive, massive ore-body. “Depth is important because most of the ore bodies found to date have been either on the surface or just below the surface, so we’re really developing a tool that explores much deeper,” Mr Leslie says. LandTEM has already proven its worth in Australia. In an earlier form, it helped clarify coil system data on the Cannington silver mine, and more recently has been tested over a $60 million nickel sulphide deposit in Western Australia that conventional systems had failed to locate. SQUIDs were first developed in the 1960s and have been used in medical imaging to detect the tiny magnetic fields in organs such as the brain and heart. For this application, the SQUID was immersed in liquid helium at a temperature of just four degrees Kelvin (–269.1ºC), which required a large amount of specialised equipment. Then CSIRO developed ‘high-temperature’ SQUIDs that could function in the warmer medium of liquid nitrogen at 77 degrees Kelvin (–196.1ºC). “This means that you don’t have to use the technology required for liquid helium – with liquid nitrogen you can just put it in a thermos flask.” This makes LandTEM very appealing to mining companies prospecting in difficult terrain. The technology has been a particular hit in Canada, where one company which uses it to explore the difficult terrain of northern Quebec says it has cut exploration costs by 30 per cent. In Australia, CSIRO has licensed the technology to Australian geophysical exploration company Outer-Rim Exploration Services, which is manufacturing it for sale and rental to other contractors and mining companies. Outer-Rim CEO Brett Rankin says his company has big plans for LandTEM. “We have two operating in Africa and two operating in Canada and one in Australia, and we would like to build three more next year,” he says. The technology has already caught the interest of mining companies such as Jubilee Mines, Image Resources and Mincor Resources. Mr Rankin says LandTEM has been particularly useful for the WA goldfields and nickel belt, and in imaging through salt lakes, but believes the best of LandTEM is still to come: “We really haven’t explored the full potential yet.” APPLICATION Sensitive magnetic sensors allow mining companies to locate ore deposits that conventional technology is unable to see For further information contact: |
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Using SQUID technology, CSIRO has developed a highly portable geophysical exploration tool called LandTEM, which is already helping unearth large deposits of nickel sulphide and silver worth hundreds of millions of dollars.
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