Geological science is constantly on the lookout for new techniques to augment the tried-and-true methods of electromagnetic, gravity and seismic imaging. This is especially true when it comes to exploration, as imprecise findings can lead to wastefully expensive borehole drilling.
A group at the University of British Columbia, TRIUMF (Canada’s National Laboratory for Particle and Nuclear Physics) and its spinoff, Advanced Applied Physics Solutions (a Canadian Centre of Excellence for Commercialization and Research), is working to identify more accurate, less expensive exploration techniques. Researchers from these institutions are developing a technique of geophysical tomography using cosmic rays. This is considered promising in part because the basic physics is well understood and most elements of the technique have already been demonstrated.
The science behind it
High-energy protons originating from distant cosmological sources produce unstable elementary particles — pions (or pi mesons) — in the upper atmosphere. These particles rapidly decay to muons. The high-energy muons, a heavy type of ordinary electron, can penetrate the atmosphere. With energies in the trillions of electron volts, some of these muons can reach deep into the earth, and some can even reach kilometres below the earth’s surface. Since the intensity of the muons falls exponentially with depth, they can easily reveal dense deposits such as massive sulphides, which cause a significant reduction in the muon flux. Cosmic ray muons show up on standard types of charged particle sensors (e.g. scintillators), which produce flashes of light when penetrated by charged particles and gas proportional chambers in which residual ionization is detected.
Although the idea itself is not new, its applications are. Tomography based on X-ray attenuation is common in medical imaging such as CT scanning. With appropriate underground sensors, cosmic ray muon tomography can also be a non-destructive method of imaging dense objects in the earth. Like gamma rays travelling through human tissue (a familiar phenomenon), high-energy muons travel in relatively straight lines. However, unlike X-rays, cosmic ray muons are able to penetrate very dense matter.
Muons are routinely observed and tracked using instruments found in high-energy physics experiments around the world in labs like TRIUMF (Vancouver) and CERN (Geneva). Muon detectors are accurate enough to precisely locate high-density regions with dimensions as small as one metre at a distance of 100 metres from the detectors.
Physicists have built numerous underground labs, such as SNOLAB in Sudbury, with detectors observing cosmic ray muons deep within the earth. For geophysical exploration, detectors can be placed in an underground mine drift or in boreholes to sense the presence and determine the direction of the cosmic muons, which penetrate to those depths.
Advance Applied Physics Solutions is currently in the process of demonstrating the utility of this technology at an existing mine in British Columbia.
Douglas Bryman holds the J.B. Warren Chair and is a professor in the Department of Physics and Astronomy at the University of British Columbia. He is currently involved in research projects in fundamental particle physics, medical imaging and geophysical tomography.