Dec '11/Jan '12


Where no man has gone before

By T. Hynes

A new research project on the horizon for CMIC

Muon tomography is a technology using high-energy muons from cosmic rays. The study of its potential application to mining and exploration is fairly new. Since 2005, Douglas Bryman, a professor in the Department of Physics and Astronomy at the University of British Columbia (UBC), has been investigating this application in exploring for high-density mineral deposits deep within the Earth.

CMIC is currently reviewing a proposal to further develop this technique for the mapping of potential mineralization. As part of its research program, CMIC’s Exploration Innovation Consortium is considering taking on a portion of this ongoing research project advocated by Bryman and Advanced Applied Physics Solutions Inc. (AAPS), a research group affiliated with TRIUMF, Canada’s National Laboratory for Particle and Nuclear Physics, located on UBC’s campus in Vancouver.

Muons 101

Although muon tomography sounds “sci-fi,” it is not. Muons are elementary particles similar to electrons, but heavier. The cosmic rays penetrating the Earth’s atmosphere produce a downpour of various elementary particles – including muons, which are capable of penetrating several kilometres into the Earth’s surface. Their energy diminishes through a process of ionization as they travel through the Earth. More specifically, the attenuation of the muons as they lose their energy is related to the total amount of material they pass through. Therefore, by monitoring the rate and trajectories of muons that pass through the Earth with an array of muon detectors placed at underground locations, the distribution of underground mineral formations can be explored.

Applicability to mining

Tomography using cosmic ray muons is a technique that can determine the density distribution of geological structures at depths of up to one kilometre. It can potentially be used to localize, in 3D, valuable mineral deposits such as massive sulphides and uranium ores. Such deposits with higher density than surrounding materials are detectable due to higher levels of cosmic ray muon attenuation. Indirect measurements demonstrating the potential of this technique are well-documented and have been published, although none have been directed at exploration.

Measurement and simulations have been performed to validate the basic principles of muon geophysical tomography, and a program demonstrating the utility of this technology in situ was initiated by AAPS. The development of greenfield exploration equipment and methods is currently in the planning stage and other applications of cosmic ray muon tomography are being explored.

Principles of cosmic ray muon tomography

The cosmic ray muon tomography exploration technique is similar in concept to that employed in medical and industrial tomographic imaging, such as computed tomography (CT) where differential absorption of X-rays along various lines of sight is used to construct image slices of a patient or object under study.

The figure illustrates the muon tomography concept in which muon sensors are placed in a subterranean mine and in one or more bore holes. The underground sensors determine the directions of cosmic ray muon trajectories. After a suitable observation period, the data set for each detector position shows the intensity distribution or number of events observed at various angles. The information obtained from a set of sensors at different locations may then be inverted to obtain a 3D density map of the area above the detectors.

The sensitivity of the density image depends on the depth of the survey region, the complexity of the geological environment, the density contrast of the structures being surveyed, and the period of data collection for a given array of sensors. The limiting spatial resolution obtainable for significant density variations where deposits may be located is expected to be, for example, less than a few metres at a distance of 100 metres from the sensors. The technology for constructing appropriate cosmic ray muon sensors exists, but customized designs are required for underground measurements. Sensors may be placed in existing mine sites and tunnels or in boreholes at various depths.

Stage of development

Bryman and AAPS have successfully pursued a proof-of-principle demonstration of muon geotomography at a mine in Strathcona Park in British Columbia. The well-known Myra Falls VMS deposit was imaged, and analysis and imaging techniques were carried out in conjunction with scientists at the Geological Survey of Canada and the University of Bern. Additional muon sensors are currently in development for further validation at advanced mine sites in regions of unknown mineralization.

In addition to an infrastructure of muon sensors, inversion techniques have been developed to generate 3D models of mineralization zones in collaboration with the Geological Survey of Canada and the UBC Geophysical Inversion Facility. CMIC is considering becoming involved in the portion of the project that relates to the development of borehole muon sensors that can be deployed down drill holes.

So, no, it is not science fiction, but only because the fiction of science has been overtaken by its real-world application.

Tom Hynes has worked in the uranium and base metals industries, and has been a provincial regulator and a federal government research manager. He is the executive director of the Canada Mining Innovation Council.

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