February 2013

Deeper understanding

Innovations in geophysics and geochemistry extend the reach of mineral explorers

By Zoë Macintosh

Modern electronics permit gravity sensors the size of golf balls to fit down slender boreholes | Courtesy of Scintrex

The most accessible ore bodies, typically surface deposits, have already been found – the mines of tomorrow lie buried at depths and in terrains that have defied exploration. Advances and innovative applications of existing technology, however, are expanding the frontiers of discovery to help explorers more precisely locate economically significant ore bodies – both before and during drilling.

New geophysical sensors

Surveys that map the reflected energy of subsurface conductors blasted with electromagnetic (EM) waves are already helping miners explore much of Canada’s uncharted territory; the method was instrumental in their discovery of the Reed Lake nickel deposit in Manitoba. But two geophysical instruments that became commercially available in the last year will push visibility of subsurface ore bodies to an unprecedented depth: B-field coils and micro-scale gravity sensors.

A million times more sensitive than conventional magnetometers, B-field coils such as the ARMIT, developed by Abitibi ­Geophysics rival expensive superconducting quantum interference device (SQUID) in their sensitivity to conductive ore covered by deep overburden. The technology sprung from the medical industry before modifications for geophysical applications by James Macnae, a geophysics professor at the Royal Melbourne Institute of Technology (RMIT) University in Australia.

Magnetometers, or EM coil sensors, measure the electromagnetic flux across a loop of copper wires to detect very small magnetic fields in their vicinity. While other coil EM sensors used by Abitibi Geophysics have detected deposits at a depth of 450 metres, the next generation ARMIT coil extends the reach of EM surveys to 1,000 metres, according to Roman Wasylechko, a marketing manager for the company.

Last September, the company completed the first field survey using ARMIT technology that was commercialized in 2012. Results from the test site in Caber, Quebec, showed the sensor cajoled an “outstandingly clear” response from a bedrock deposit covered by conductive overburden, Wasylechko says.

The femtotesla sensitivity and high signal-to-noise envelope that ARMIT offers was formerly available only through the use of SQUID, which was developed by the Institute of Photonic Technology (IPhT) in Germany. Both “high” and “low” temperature versions of the SQUID require cryogenic cooling in a vacuum vessel in the interior of the apparatus. Unlike these, the ARMIT coil operates at ambient temperatures, says Wasylechko. A nano-engineered material in the coil’s hollow permits its internal currents and associated magnetic fields to flow for a long time without decaying, similar to how superconductors react to EM stimuli but without the need for zero resistance.

“If you can make these things cheap enough, in my view it will take over the electromagnetic part of mineral exploration,” says Dennis Woods of Discovery International Geophysics, a Canadian company that used high-temperature SQUID sensors to better define the celebrated Lalor deposit in the Snow Lake area – a high-tonnage formation found 1,200 metres underground. While B-field coils are less sensitive than the SQUID at very low frequencies, their lower price margin could enable their use in a distributed array of 50 or more to dramatically improve electromagnetic imaging underground, says Woods. Comparing the future application to current seismic surveys used for oil exploration, Woods adds: “It’s getting closer and closer to sonograms of a pregnant woman. [Mineral explorers] want images that are every bit that good.” Discovery International is currently developing its own commercial version of the B-field coil in collaboration with San Diego’s GroundMetrics.

In the gravity survey domain, a long-awaited tool already gives mineral explorers an advantage previously enjoyed by those in the petroleum sector. Gravilog by Scintrex, based in Concord, Ontario, is the first and only gravity sensor to fit inside slender boreholes specific to mineral exploration.

Gravity surveys measure the subtle changes in gravity caused by massive nearby structures. Their importance to exploration follows from their ability to pick out heavy metals like copper from low-density conductors, such as graphite, which look similar in electromagnetic readings. “If you start drilling all your conductors you are going to go broke fast,” says Chris Nind, CEO of Scintrex. The company developed the two-inch-diameter sensor with funding from CAMIRO, a mining consortium responsive to the needs of industry. In the past, borehole gravity meters were twice as large and were limited to the large holes used for petroleum exploration.

Since the tool can uniquely detect the mass of off-hole structures underground, the Gravilog offers explorers early estimates of ore body tonnage, which is “something of great value to mining companies,” says Nind. Because it detects excess mass within a few hundred metres of the hole, the instrument also reduces the number, and with that the cost, of drill holes needed to begin with.

For temperatures below 75 C and pressures below 4,500 pounds per square inch (PSI), the Gravilog can be used in holes deeper than two kilometres, says Nind.

Eric Gilbert of Abitibi Geophysics takes readings with an assembly of ARMIT sensors in Matagami, Quebec. The grey box translates electromagnetic flux sensed in thecentre of each black rod, or ARMIT sensor, into digital data | Courtesy of Abitibi Geophysics

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