For years, mining companies watched enviously from the sidelines as their counterparts in the oil industry perfected a powerful tool to search for petroleum: seismic, or sound wave, technology. Now miners are becoming active participants in the development of seismics for resource exploration.
Since the 1930’s, petroleum explorers have used artificially generated vibrations to penetrate the earth’s crust and outline reservoirs of oil and gas. But the rules of nature prevented North American mining companies from taking advantage of the same technique to explore for their main targets, gold and base metals.
The sedimentary basins that contain petroleum deposits are ideal sound reflectors because they are usually large, flat-lying and spatially continuous. Metal deposits, on the other hand - especially in the volcanic terrain of the Canadian Shield – are relatively small, spotty and tend towards a vertical alignment that complicates sound reflection.
So what has changed? Field equipment and processing, particularly the ability to digitize data, has advanced by leaps and bounds. Meanwhile, mining companies have been forced to probe deeper and deeper for new ore.
Now seismic reflection technology, which relies on sound waves to map underground geology at depths of half a kilometre and more, is emerging as an exciting new tool in the search for deep orebodies.
“Literally hundreds of millions of dollars have been spent by the oil industry,” says Larry Matthews, Noranda Inc.’s seismic technology co-ordinator. “Field equipment and processing technology have finally arrived at a level where some of the more difficult technical problems can be tackled.”
Seismic exploration involves recording sound waves as they are reflected from different rock layers. The sound waves, or vibrations, are generated by a number of different sources – dynamite, dropped weights, or huge vibrating trucks - depending on the target and the environmental conditions. Receivers called ‘geophones’ record echoes from the waves as they bounce back to surface. The digital information is then interpreted by geophysicists like Matthews.
Although the South African mining industry has traditionally used seismics to explore for flat-lying sedimentary gold reefs in the Witwatersrand basin, Canadian deposits were thought to be too discontinuous and steeply dipping to generate useful data.
But in the late 1980’s the Lithoprobe program, Canada’s multidisciplinary effort to piece together the evolution of the continent, began picking up strong signals whenever it’s seismic surveys transected a base metal mining area.
At that point, miners like Inco, Falconbridge and Noranda began to see the possibilities of using seismic exploration techniques to find deep orebodies in established camps like Sudbury, Ont.
Noranda has emerged as a frontrunner to develop the deep exploration tool if only because many of its zinc and copper mines are depleted or are on the verge of closing, including operations near Manitouwadge, Ont., Matagami, Que. and Bathurst, New Brunswick.
“Noranda has a vested interest in making this technology work,” says Matthews. “If there is any opportunity to find other potential sources of ore in those camps, at depths we haven’t explored before, then we will pursue that.”
Noranda has yet to find any new deposits, but the company has demonstrated that seismic techniques can provide an accurate reflection of the deep subsurface, including the location of metal-bearing sulphides.
This is a significant breakthrough for mineral exploration. Although many of the shallower orebodies in Canada have been mined out, it may be economic to develop orebodies as deep as three kilometres below surface in camps where infrastructure is already in place. The trick is to find them.
“There’s a lot of potential for seismics,” says Matthews “What it has that other technologies don’t have is the ability to penetrate below 500 metres with some reliability.”
Seismic exploration is expensive. Costs average about $50,000 per sq. km, at least 10-fold the cost of a magnetic survey over a similar area. On the other hand, compared with a $150,000-250,000 price tag per hole for wildcat drilling, the only real alternative for deep exploration, seismics looks like a bargain.
It may be years before seismic techniques, with their complexity and high costs, find practical applications in the search for new mineral-rich regions. But in the meantime, the technology may serve a much more urgent need: keeping Canada’s aging base metal mining camps - and the communities that depend on them – alive a little bit longer.