The newest tool in the geophysical kit may have been born a long time ago in a galaxy far, far away.
Researchers at Canada’s University of British Columbia are using cosmic ray muon measurements to reveal high-density underground deposits.
Doug Bryman is a professor of physics and astronomy at UBC, where he holds the J.B. Warren Chair. Bryman said muon geotomography uses underground sensors to detect muons as they penetrate the earth.
“The cosmic ray muons are coming from many different angles. They get attenuated as they pass through higher density material – fewer cosmic rays will penetrate,” Bryman said. “Underground sensors look up at the earth above and using a series of measurements, we can construct a three-dimensional image. It’s very much like CT scanning in the hospital.”
High-energy protons originating from distant cosmological sources produce unstable elementary particles, pions, in the upper atmosphere. These particles rapidly decay to muons, which are heavy cousins of the ordinary electron. The high-energy cosmic ray muons can penetrate the atmosphere and, with energies in the trillions of electron volts, some of these muons can reach several kilometers below the surface. Since the intensity of the muons falls exponentially with depth, underground flux measurements can reveal dense deposits.
His interest in the possible exploration applications began about five years ago after colleagues in Japan used the method to study volcanic magma chambers.
“Then a mining person contacted me to see if there was any way to use the technique to locate underground deposits, and I began to work on that,” he said.
“It appeared quite suitable for revealing deposits that have a higher density than the surrounding rock,” including massive sulfides and uranium deposits, he said.
Bryman said the technique was demonstrated successfully in field tests conducted in an existing mine in British Columbia. Sensors placed in different locations were used to image a volcanic massive sulfide deposit. After that, two more surveys were commissioned by major mining companies at other mines, he said.
“We’re developing borehole instruments that essentially can go anywhere. That’s a couple of years down the line,” he said.
Bryman said the method can help avoid expensive hit-or-miss core drilling.
“In the hospital, if you have a choice between a CT scan or expensive surgery, the choice is clear,” he said.
Muon tomography hasn’t been applied yet in searching for oil.
“In those situations, the density contrast may not be as great, but the potential is still there,” he said.
One possible application would be in the area of carbon sequestration, he said.
The UBC group has received support from TRIUMF (Canada’s National Laboratory for Particle and Nuclear Physics), its spinoff Advanced Applied Physics Solutions (a Canadian Centre of Excellence for Commercialization of Research) and the Geological Survey of Canada.
Two companies – Near Star, a zinc company, and Tech Resources have been participants in the work since 2011, Bryman said.
“We definitely see commercial applications and we’re seeking opportunities,” he said.
“It’s the only new technique introduced into the tool chest in a long time,” he said.