Cross-borehole radio tomography is playing an increasingly important role in the discovery and definition of new orebodies in the Sudbury Basin.
The technology has been the focus of R&D efforts in the U.S., South Africa and China, but it’s the Russians who have been most successful at commercializing it and Falconbridge’s Sudbury exploration department has been their primary customer.
Cross-borehole radio tomography is a down-the-hole geophysical survey methodology that measures electrical conductivity between two or more boreholes at depths of up to three kilometres.
Using winches and cables, transmitters and receivers are lowered down the holes and the data collected and analyzed by a computer on surface.
The results are graphically displayed in much the same way as a Computerized Axial Tomography (CAT) scan used for medical diagnostics.
“The ore deposits we’re finding in Sudbury are deeper and deeper, so we’re now drilling holes that are one kilometer to two kilometers deep,” said Kevin Stevens, principal geophysicist with Falconbridge’s Sudbury exploration group.
“It’s not uncommon for a single hole to cost between $100,000 to $150,000 and that just provides a sample of a few centimetres across, so it’s good to have other tools that enable you to look around the hole.”
Cross-borehole radio tomography played a key role in accelerating the development of Falconbridge’s Nickel Rim South property, claimed Walter Peredery, president and chief geologist with Fara Systems Canada Ltd., a Sudbury-based company that handles marketing and logistics, but contracts out the actual surveys to a Russian crew.
“Once you make a discovery, it can take 10 years to develop it, but by using this technology, you can develop a deposit in half the time.”
Defining the size of the Nickel Rim South orebody would have been much more expensive and time consuming without the use of cross-borehole tomography.
Many more boreholes would have had to be drilled at appropriate spacing, said Stevens.
“We had very few holes at the beginning of our discovery, so we maximized the information we got out of them. Radio imaging was one of the technologies we used. It allowed us to understand the basic orientation of the orebody and whether or not it was continuous.”
The Russian company, Geofara Limited, has been kept busy by Falconbridge, but has also done some work for Inco, FNX and Wallbridge in the Sudbury area.
Inco began experimenting with radio tomography 10 years ago, said Al King, Inco’s chief geophysicist.
“We looked around for suppliers and there were four of them at the time: one Russian group, one Chinese group, an American group and a group in South Africa. Over time, we brought them all here and tested their systems.
“We supported the development of the technology, but none of them got to the production stage except for the Russians.”
The Americans got sidetracked by defense applications, the Chinese had difficulty commercializing their technology and the South African group disbanded and resurfaced in Australia, said King.
Falconbridge has been using the technology for nine or 10 years, said Stevens, but it has yet to catch on in the rest of the world.
“It’s not a technology that has been easy to implement. We’ve had our problems along the road and it has been a matter of persevering. There are no Wal-Mart solutions where you can just walk in and pick up something off-the-shelf. You have to be willing to have a vision and put the effort in.
“We’ve been their primary customer and kept them pretty busy,” he said. “There’s no doubt about that. We have a very unique environment in Sudbury and the rocks are conducive to the application of this technology.”
Inco’s King is keen about the technology, which comes out of the Russian Academy of Sciences, but sees barriers to its adoption.
“The biggest barrier is getting good, reasonably priced surveys, getting people over here, getting crews and access to equipment. Everybody agrees on the potential for these systems.”
In the meantime, Inco is also experimenting with similar systems, including cross-borehole seismic tomography and pulse radio systems, or radar.
Seismic tomography, which is commonly used in the oil industry and for assessing potential nuclear waste disposal sites, also has applications in mineral exploration.
Each has its strengths and weaknesses. According to King, seismic imaging works best for detecting massive sulfide deposits, while radio imaging works better with disseminated material.
“They’re complementary,” he said.
Inco has recently purchased a seismic system from a Finnish company and will be using it at its Voisey’s Bay property in Labrador this year.
Peredery is optimistic that the demand for radio tomography will increase.
“Once it catches on, there’s going to be no stopping it,” he said.