BURIED TREASURES; Time-domain electromagnetic methods led to some

Explorationists made several so-called “blind” discoveries in the 1980s. At depths of 230 to 1,300 metres, these deposits are hidden well beyond the range of conventional air-borne and ground geophysical techniques. What technological marvel, then, accounts for the increased perceptiveness of explorationists? In at least one case, no new gadgets were needed. The most recent base metal discovery by Brunswick Mining & Smelting in Bathurst, N.B., for example, was based solely on sound geological reasoning. In November, 1989, Brunswick announced the results of an 1,100- metre-deep drill hole put down from surface from a collar location two kilometres north of the company’s No. 3 shaft. The hole, numbered A-248, cut 13.4 metres of massive sulphides with ore grades of 4.6% lead, 9.9% zinc, 0.22% copper, 5.9 oz. silver and 0.05 oz. gold per tonne over 8.5 metres.

Brunswick’s chief mine geologist, Bill Luff, says the discovery was based on geological common sense. C. Van Staal and P.F. Williams published a paper in a 1984 edition of Economic Geology, explaining the basis of this theory. Brunswick had been drilling in the general area since 1983; but using Van Staal’s structural and stratigraphic interpretation of the area and through persistent field work and some three-dimensional block diagrams, Brunswick intersected a new sulphide zone. The full extent of the find is not yet known. Follow-up drilling is in progress.

However, other recent finds can be attributed to a combination of geological reasoning and some innovative geophysical techniques. Among them: Aur Resources in Louvicourt Twp., Que.; Falconbridge on the Lindsley property in Sudbury; Hudson Bay Mining & Smelting at Chisel Lake, Man.; Cameco in the Athabaska Basin of northern Saskatchewan; and several uneconomic deposits in Ungava and the Northwest Territories. Finding major near-surface orebodies outcropping within existing camps is increasingly rare. The latest exploration techniques allow geologists to expand their search for deep-seated elephants by letting them “see” electromagnetic (EM) conductors at greater depths and well beyond the walls of conventional diamond drill holes. Most companies involved in base metals exploration have accepted the new methods, though one geophysicist told us that most explorationists are at least 15 years behind in adopting new techniques.

For those looking for deep-seated orebodies, there are two ways to use these so-called time-domain (as opposed to transient) EM surveys. First, a ground time-domain EM survey will provide a broad picture of EM anomalies, with sufficient detail to suggest a drill target. Miss the target and a down-hole time-domain EM survey will yield a more detailed picture of the anomaly’s location, obviating the need for another expensive deep drill hole.

There are also two methods of conducting both surface and down-hole time-domain EM surveys — the pulse EM method and the step-response method. Both have their advantages and disadvantages. Neither, for example, works well close to railways and power lines, which are sources of EM noise. “We have done considerable work near these sources of noise and the effects can be stripped out with pretty good accuracy,” says Patrick McGowan, president of Lamontagne Geophysics. Layers of conductive waste rock, such as shale, can also create challenges for geophysicists, but results using time-domain EM methods are typically better than those using other methods. Both methods have boosted recent deep-seated discovery rates, and we may have just begun to see their effects on the industry.

Pulse EM

The first prototype of a pulse EM instrument was built by Crone Geophysics of Mississauga, Ont. and tested in the Sudbury Basin in 1963. That unit is now a museum piece in Ottawa. But full credit for promoting the funding of research into the development of down-hole geophysical surveying techniques goes to the Geological Survey of Canada (gsc). Down-hole techniques are now widely used and accepted by serious base metals exploration companies. And today Crone and Geonics, also of Mississauga, supply the industry with pulse EM equipment.

In the late 1960s, at a time when the industry showed little interest in the new time-domain EM technology, Lenard Collett, then the head of the Electrical Methods Section of the gsc, proved instrumental in getting the technology off the ground. He strongly endorsed Crone’s application to the National Research Council for research funds under the federal government’s Industrial Research Assistance Program (irap). The money expanded the depth capabilities of the prototype pulse EM instrument to 1,000 metres. (Collett has been so successful at steering federal funds into the mining exploration sector over the years that he was awarded the J. Tuzo Wilson Award in 1989 by the Canadian Geophysical Union.)

In 1978, Mike Knuckey, who was with Falconbridge Copper, commissioned the geophysical company to test the unit at the Corbet mine, near Rouyn-Noranda, Que.

“It was a good but tough test, with three separate ore lenses and with each of the six test holes missing all three,” Duncan Crone, president of Crone Geophysics, recalls. “Two of the three lenses were pinpointed with pulse EM and a follow-up survey resulted in the first pulse EM find — another extension lens at the 980-metre depth.” The survey results extended the mine life by about two years and attracted the attention of other base metals explorationists.

Crone recently logged a hole 2.5 km deep for Falconbridge in Sudbury, Ont.

Step-response EM

Toronto-based Lamontagne Geophysics manufactures another time- domain system, which is a step-response (not a pulsed) system. This system was born at the University of Toronto and is known as UTEM (after the university). The surface version was instrumental in the 1983 discovery of the 25-million-tonne Hellyer lead-zinc-silver-gold deposit by Aberfoyle Resources (a subsidiary of Cominco) in Tasmania. The down-hole version, first developed in 1984, has a depth capacity of 2.43 km, because the company can lay out large transmit loops which carry relatively little current (about which, more later) and has a fibre-optic cable up to 2.43 km long. For this reason, step-response EM has been chosen more often than pulse EM by uranium exploration companies in the deeper parts of Saskatchewan’s Athabaska Basin, where 900-metre holes are common (at Close Lake and Tucker Lake, for example). Only Crone and Lamontagne actually conduct contract field surveys for exploration companies and assist in interpreting the results. Geonics only manufactures equipment.

Time-domain EM

In time-domain EM, a single-turn transmit loop is laid out on surface in the shape approximating a square; they are usually rectangular. The size of the square can vary anywhere from 100×100 metres up to 2×2 km. In the case of the Lamontagne step-response system (which has a very efficient 1,100-W transmitter), the loop can be up to four kilometres square.

Pulse EM and step-response EM are dissimilar in that the waveform of the transmitted electric current sent by each system is different. In the case of pulse EM, a square-wave electric current charges the loop with a peak current in the order of 10 to 20 amps. And in the case of the step-response system, the transmitted electric waveform is triangular rather than square. Instead of being on and off for equal time periods (as is the case with the square wave), a graph of the current versus the time of a triangular waveform would look like the triangular teeth on a saw.

Regardless of the characteristics of the electric current transmitted, the current sets up a primary magnetic field. This field then creates electric currents in any buried conductive body nearby. These currents produce their own magnetic field (called a secondary field) which, in the case of pulse EM, is detected when the primary field is shut off. In the case of step-response EM, the secondary field is measured continuously by subtracting the step current.

In both cases, the sensor coil senses the rate of change
of the secondary field with respect to time (or, in the jargon of the geophysicist, the time derivative of the transmitted wave form). In the case of pulse EM (a square wave), this time derivative is called a pulse. The time between pulses is about 15 to 20 milliseconds. Because a conductor is present in the ground, this pulse modifies into what geophysicists call a transient. The transient is sampled at various times called windows or channels (usually eight to 20). The rate of decay of the transient is determined by what geophysicists call the k constant of the anomaly. The larger this number is (i.e. the longer the decay time), the more conductive is the target.

In the case of step-response, the time derivative of the transmitted triangular wave picked up by the sensor coil is a square wave. “This makes the interpretation of the step-response system more reliable,” says Lamontagne’s McGowan. The key to Lamontagne’s success has been its ability to develop the complicated electronics to accomplish this.

Surface Surveys

If a ground survey is being conducted, all instrumentation is kept on surface. The transmitter is left unattended and the reciever and tripod-mounted coil are moved around the property along survey lines, usually in a backpack. The tripod sensor, connected to the receiver by a 2-metre-long shielded cable, is set up at stations spaced an equal distance apart along the survey lines and readings are taken.

To design a surface time-domain EM survey, the size of the target and the depth of interest must be taken into account. The spacing between survey lines and between sample stations on those lines is tailored to the assumed shape of the anomaly to be found. For deep-seated orebodies (below 75 metres), lines are typically spaced 200 metres apart and samples are taken every 50 metres. Surface surveys can be conducted either inside the transmit loop or at a distance up to two times the loop dimension outside of the loop.

Down-hole Transient EM

Down-hole surveys are a different matter. The receiving coil is housed in a down-hole probe attached to the receiver (which remains on surface) by a long cable. Crone and Geonics use a shielded metal cable while Lamontagne incorporates a lightweight fibre optic cable. The sensor coil and cable are lowered down a diamond drill hole. A common method of laying out transmit coils is to lay out three to five loops in such a way that any given hole can be surveyed from a number of different loops. This way, more detailed information on the shape and location of the conductor can be gathered. In this type of survey, the size of the loop is usually about one-third to one-half as large as the depth of the hole to be surveyed.

From surface, as a rule-of-thumb, a conductive body (usually assumed to be a disc on its side) of a given diameter can be detected to a depth twice its diameter. However, an in-hole sensor coil responds better to a buried conductor because of the elementary fact of its proximity to the conductor. Such a down-hole sensor is typically housed in a 1.6-metre-long, down-hole probe, measuring 3.02 cm in diameter and weighing about four kilograms and lowered down the drill hole with a winch. (For depths ranging from 1,300 m to 2,000 m, a larger receiver probe, 3.15 cm in diameter and weighing 3.8 kg is available.) A special steel tube with locking jaws retrieves probes that become stuck in drill holes. It is rare to lose a probe in the hole, but some holes are blocked or are too shallow for down-hole surveys.

Interpretation

No matter what transient system is used (Crone’s or Geonics’ pulsed system or Lamontagne’s step response system), intrepretation is based on the shape and amplitude of the response. The location and size of the conductor are interpreted using response curves, type curves (which are system specific) and primary field diagrams. Computer models and an atlas of primary fields help exploration geologists interpret anomalous responses. Prof. James Macnae of the U of T’s Dept. of Physics (who is also a principal in Lamontagne) compiled the atlas of primary fields in 1980 and geophysicist William Ravenhurst (now of Crone) created a personal computer model program called PLATE in 1986. Lamontagne has also created a computer program. Called MULTILOOP (marketed by Geopak), it allows a geophysicist to model as many as 15 conductors simultaneously.

Research

Research money continues to improve the two time-domain systems. Crone is investigating the technical feasibility of using larger surface loops and increasing transmit powers. It is also experimenting with receiver coils oriented orthogonally to the axis of the hole, according to David Watson of Crone. The company’s digital receiver continues to be improved with software developed in house. In addition to programmable channel times, the measurement decay curve and profiles can be graphically displayed on the receiver, providing the operator with valuable on-site information.

Lamontagne is applying its time-domain system to measure the resistivity of the ground. Instead of using electrical probes (as is the case with induced polarization systems), the company can put a current into the ground with its surface loop. The same equipment is used, but the electric field is measured, not the magnetic field. The technique can probe areas covered with thick sheets of glacial overburden.