Electromagnetic techniques help probe at depth

Until recently, a systematic minerals exploration program would include surface mapping and soil sampling, provided the overburden was responsive in the latter case and if there was sufficient rock exposure for the former.

One or more geophysical techniques would typically follow. If a sufficient degree of coincidence could be established between soil and geophysical anomalies, or between two different types of geophysical anomalies, then, and only then, would the drills be brought in to investigate. Drilling programs are expensive. They also have the same characteristic as government initiatives — they tend to overrun their budgets. There is always compelling evidence for the geologist to drill one more last hole. Times are changing in Canada. The mining industry has reached maturity, if not middle age, and deposits are deeper and more difficult to find. Surface geophysical systems cannot scan beyond a few hundred feet into bedrock. But technology has changed too, and a new generation of geophysical systems is now able to perform within the limited confines of the drill hole itself. This is a marvel in miniaturization in itself yet it is only the means to an end: the application of electromagnetic (EM) techniques at previously unheard of depths.

The most recent discovery in this connection is that of Inco (TSE) at Victor on the east rim of the Sudbury Basin in northeastern Ontario. Here, massive nickel-copper sulphides were identified by down-the-hole geophysics at hole depths of 8,000 ft.

This particular survey was carried out by Lamontagne Geophysics of Kingston, Ont., a highly specialized entrepreneurial outfit, as most geophysical companies are. It was founded by Yves Lamontagne to commercialize the findings of his Ph.D. research undertaken at the University of Toronto. (There are many companies active in borehole geophysics, each having its own area of expertise.)

How these systems work is comprehensible only to a geophysicist. What happens in the field is nevertheless understandable. A loop of wire 3,000-5,000 ft. in length is laid on the ground forming the shape of a polygon. (The borehole is in the centre.) This is the transmitting coil. A generator pulses an intermittent flow of current through the coil and the response is picked up by a receiver in the borehole itself. The receiver measures the decay of the EM response in the interval between the pulses. The data are transmitted to surface and then processed by computer.

If there is conductive sulphide in the rock mass combed by the pulsating power, it will distort the electrical impulse and consequently reveal its presence.

The big question is, how far away from the borehole can the system detect an orebody that is scarcely quantifiable? A small body of sulphide will give the same response as a large one much farther away. Likewise, the shape of the body and how it lies with respect to the borehole will have a big impact, for example, if the orebody is broadside-on it will

create a more discernible

response than one which is end-on.

Regardless of the qualifications, a borehole EM survey has accomplished its purpose when it has traversed the length of the borehole. If there is an EM response, another hole is justified nearby.

If there is no response, much judgment will have to be brought into play. Thus, an orebody at a depth of 8,000 ft. would have to be at least “x” million tons in size to be a payable proposition. That size of an orebody would have been detected about 500 ft. away from the first drill hole. Consequently, the next hole should be about 1,000 ft. distant from the first to cover all bets.

Need it be said that mining companies roll dice, too?