Can cosmic rays discover ore bodies?
By Ernest Granson
In November of 2017, researchers confirmed the existence of a large, previously undiscovered void inside the 4,500-year-old Great Pyramid of Giza, Egypt. The researchers used several sets of muon detectors inside and outside the pyramid to determine that a cavity of some kind existed.
What is of interest to the resource exploration sectors is the technology behind this type of muon tomography, allowing mining companies to significantly improve their exploration techniques and enabling petroleum companies to identify potential carbon sequestration sites.
“While the concept of using cosmic ray muons in mineral exploration has been around for decades, it’s only through recent advances in detector technology, 3D inversion algorithms and computing power that have enabled us to make this a practical solution,” says CRM Geotomography Technologies Inc. CEO, Don Furseth. “Last year, $8 billion was spent on non-ferrous mineral exploration, so that tells you there is a huge global market, and that is why there is so much interest. And, while most of our revenue has been driven by that interest in mineral exploration, we’re still discovering where else the technology can be applied.”
The technology has already been used to map the structure of volcanoes and versions are being tested for use in the detection of nuclear cargo.
“For example, can we help with monitoring oil sands production in the SAGD process? Right now, those underground reserves aren’t visible, so can that process be improved by enabling its visualization? Well, we’re not the experts in those areas, so we’re interested in working with people who are,” says Furseth. “And we’d be happy to talk to experts in other industries where the ability to see through rock or thick structures, and apply 3D density imaging and monitoring, could have a big impact on their processes, operations and business.”
Cosmic ray muons (CRM) are elementary particles like electrons but much heavier, about 200 times heavier. They are created when cosmic rays strike the earth’s upper atmosphere and they travel near the speed of light to the earth’s surface where they can penetrate hundreds of metres below the surface, unless, that is, there’s a dense body in the way. If that happens, the muons lose energy proportional to the amount of matter they pass through.
Using this concept, CRM, based in Vancouver, B.C., has developed technology that measures the muon intensity or flow passing through the earth’s geological structures. That measurement can be integrated with other geophysical and geological data and assembled into a 3D model, enabling businesses such as mining exploration companies to detect ore deposits that, otherwise, would not be visible.
It was Physics Nobel Prize winner, Carl Anderson, who established the concept of energy loss by cosmic rays in the early 1930s. His “cloud chamber” experiments confirmed that when particles passed through a lead plate in the chamber, they would emerge from the other side at a lower energy than when they started. Professor Anderson and his graduate student, Seth Neddermeyer, went on to identify those previously unknown particles as muons.
One of the first practical applications of muon detection took place in the tunnel of a hydroelectric power station in Australia’s Snowy Mountains in 1954. Australian physicist, E. P. George used the technology to measure the overburden or thickness of the rock in the tunnel.
Advancements continued to be made in the technology and during the 1960s, another Physics Nobel Prize winner, Dr. Luis Alvarez, of the University of California at Berkley and the University of Chicago, used his own muon detector technology in an experiment to search for hidden spaces in Khafre’s pyramid which is located right beside the Great Pyramid. No new chambers were discovered but he was able to successfully demonstrate the technology.
Although these applications seem quite exotic, CRM’s muon detection technology is primarily being used for more practical applications such as mineral exploration. The potential for CRM’s muon detectors, especially with its next generation detectors, will be quite significant from the resource industry perspective.
One reason for that is because it’s becoming more difficult to discover mineral deposits, according to Furseth.
“The days of tripping over rocks and making a consequential mining discovery at the surface are passing,” says Furseth. “Because we’re now searching for hidden ore bodies, this technology can help take a lot of guesswork out of the exploration. Geology can be a complex thing. When geologists try to discover mineral deposits it’s like a detective game analyzing the structures, flows and patterns which can alter the density in a geological structure. Innovative technology can make that exploration more effective by making it smarter and more targeted.”
CRM’s muon detection technology originated in 2009 at Advanced Applied Physics Solutions (AAPS, now called TRIUMF Innovations) which is the commercialization arm of TRIUMF, Canada’s particle accelerator centre. TRIUMF is a consortium of Canadian universities and is situated on the University of B.C. campus.
In 2011, AAPS conducted its first controlled field test of the technology at Nyrstar’s Price volcanic-hosted massive sulfide (VHMS) ore at the Myra Falls mine on Vancouver Island. The Myra Falls location was chosen because it’s an existing mine and it’s relatively close to the surface with accessible tunnels to place muon sensors at locations below and adjacent to the ore. As well, previous drill core data is available for comparison.
The sensor, which is enclosed in a large metal box about 4’x 4’x 6’, was then placed at seven underground locations in an underground tunnel for exposures of about two weeks at each location. The research group concluded the resulting 3D density image of the deposit, produced by the geotomography by combining multiple views, corresponded well with the model derived from the drill core data.
Backed by the successful field trial, by 2013, the technology was spun off by the into CRM GeoTomography Technologies Inc. which began advancing and commercializing the technology. CRM now has ten advanced muon detectors, with some deployed in underground mines.
“These sensors are very robust,” says Furseth, who is an engineering physicist with experience in imaging related technologies. “We can place them down mine shafts and they aren’t affected even with blasting taking place nearby or water falling on top of them. We’re also working on the next generation detector which will be more compact and will have higher resolution. These new detectors will go into field trials this year. We are also working on a detector to place inside boreholes, making it a far more versatile imaging system.”
The mining sector could benefit significantly using muon geotomography. According to the paper, “Muon Geotomography – Bringing new physics to ore‐body imaging”, authored by the research group that conducted the Myra Falls field trial (D. Bryman, J. Bueno, K. Davis, V. Kaminski, Z. Liu, D. Oldenburg, M. Pilkington, and R. Sawyer) “three dimensional images from muon surveys may be used to guide drilling operations towards regions of high density contrast, thereby significantly reducing costs and environmental impact associated with locating and delineating ore bodies. The environmental benefits gained from improved imaging include reduced trenching, reduced ore dilution, and improved economic performance of mines. Reduced dilution increases ore grade, reduces the energy expanded in hauling, crushing and treating waste rock, and further minimizes the volume of tailings.”
In addition, says Furseth, by enabling mining companies to identify ore bodies that are still undiscovered near existing underground mines, the life of the mine can be extended, maintaining a minimal environmental footprint by avoiding the need for a new, potentially open pit, mine.
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