Most mineral explorationists are by now familiar with the spectacular successes of borehole transient electromagnetics (TEM), as seen in advertisements and case histories; e.g. "Massive sulphide ore lens detected at 1000 m depth from exploratory drill hole." But there are other, less glamorous, applications of borehole TEM that can just as dramatically affect the cost effectiveness and ultimate success of a massive sulphide drilling program.

By closely coordinating borehole TEM surveys with the drilling program, it is possible to provide continuous guidance to the drilling and thereby reduce the length and number of holes necessary to fully explore the targets on a property. The procedure requires that the borehole TEM survey crew work in tandem with the drillers: laying out transmitter loops around the drill rig and probing the hole before the drill is moved to the next hole.

The most significant advantage of this approach is that it helps the supervising geologist decide when drilling has adequately tested a conductive target. Has the conductor already been intersected but is just not obvious in the drill core? Should the hole be lengthened a few more meters to possibly intersect the conductor? Has the conductor been missed entirely and a new hole is required? Maybe the EM target is caused by conductive overburden or some other shallow structure and the hole should be abandoned? The borehole TEM survey results will answer these questions -- and by answering them before the drill rig has moved off, appropriate action can be taken.

An example is shown in Figure 1. A hole was drilled to test an 8-channel, surface TEM anomaly (Fig 1a). The conductor depth was interpreted to be 120 m and the hole was located so as to intersect about 20 m below the top edge. Unfortunately, the hole was misplaced by 100 feet due to a grid chaining error. Once discovered, the drill was immediately placed on standby and a borehole TEM survey was carried out.

The negative (off-hole) response at the bottom of the hole (Fig1b) indicated that the hole had missed the conductor, so the drill rig was move back 100 feet and a second hole was drilled. The large positive anomaly in this hole (Fig 1c) confirmed that this hole had intersected the conductor. It turned out to be due to a thin pyrrhotitic iron formation. The drill hole was shut down within a few meters of this intersection.

A different type of example is shown in Figure 2. Here, a large-amplitude, 5-channel surface TEM anomaly (Fig 2a) was tested by a short drill hole. After drilling a few meters beyond the expected target depth, without any indication of a conductor, the drill crew went on standby and a borehole TEM survey was carried out using three different positions of the transmitter loop.

There was no indication of a conductor at or near the bottom of the hole using any of the transmitter loops. (The primary field induction vectors from the different loops are depicted by small arrows.) However, anomalous responses were observed at the top of the hole using both the east and west loops. The positive response with the west loop (Fig 2b) and the negative response with the east loop (Fig 2d) was interpreted to be caused by a patch of conductive overburden near the collar of the hole, which probably also caused the surface anomaly. No further drilling was carried out at this site.

An example of multiple drill holes and borehole TEM surveys used to locate deep conductors is shown in Figure 3. The surface TEM survey produced a broad, 8-channel anomaly (Fig 3a) indicating one or more conductors at greater than 200 metres depth. The initial drill hole failed to intersect any significant conductive mineralization.

Borehole TEM survey results from this hole (Fig 3b) indicated the presence of a conductor below and behind the hole, as seen by the weak negative response in the middle of the profile. Also, the positive build-up toward the bottom of the hole was interpreted to be due to another conductor farther to the east.

The second drill hole also failed to intersect any conductive mineralization, however the borehole TEM survey (Fig 3c) confirmed the presence of the conductor beneath the hole. The final hole was drilled to test this conductor at 350 metres depth, and hopefully to intersect the second conductor originally interpreted from the first drill hole. A non-economic copper zone was intersected near the bottom of the third hole. Subsequent borehole TEM survey results from this hole (Fig 3d) indicated that the deep conductor was in fact composed of two parallel conductive horizons. The profile also clearly confirmed the presence of the second conductor farther to the east. This conductor has yet to be drilled.

Another advantage of coordinating borehole TEM surveys with the drilling program is that surveys can be carried out in drill holes which might otherwise collapse shortly after the drill rig is moved. If the hole is found to be blocked, it can be immediately cleared by the drillers. If there is a particularly bad section of the hole that typically collapses when the drill rods are removed, then the lower portion of the hole can be surveyed through the drill rods, which are pulled back to a point just below the bad ground. More rods are then pulled and the upper section of the hole is surveyed down to the blockage.

An example is shown in Figure 4. A possible down-plunge extension of a known massive sulphide ore body was tested by a deep drill hole designed to intersect the favourable horizon at about 500 metres depth. No significant mineralization was intersected. A borehole TEM survey of the hole (Fig 4b) identified two off-hole conductors: one at about the expected location of the ore horizon and another unexpectedly to the east.

A subsequent hole was drilled a few years later from the opposite direction to intersect the ore horizon at about 300 metres depth. This drill hole cut a thin section of massive sulphides but did not find ore. The borehole TEM survey results from this hole (Fig 4c) indicated that the drill hole had intersected the edge of the more massive ore body, which was interpreted to lie above the drill hole and along strike. A blockage at the ore horizon prevented surveying of the entire hole.

The most recent drilling was carried out to test the ore horizon between the two previous holes, and to investigate the off-hole conductors discovered by the original borehole TEM survey a few years earlier. Sulphides were intersected at about the expected depth of the ore horizon, but no significant intersections were encountered near the bottom of the hole. The results of a borehole TEM survey from the upper and lower portions of this hole are combined into a single common profile (Fig 4d).

Above the blockage zone, the ore deposit produces a similar edge anomaly as from the shallower hole (Fig 4c). Below the blockage zone, the additional sulphide intersections are observed as in-hole anomalies. Near the bottom of the hole, a weaker off-hole anomaly confirms the existence of the eastern conductor original discovered by the borehole TEM survey in the first hole. This deep, eastern conductor has yet to be tested.

Coordinating borehole TEM surveys with diamond drilling is an effective means of insuring the maximum benefit from a massive sulphide drill program. There is considerable scope for cost savings by cutting short, or even eliminating, unnecessary drilling. And the ultimate success of discovering an ore deposit is enhanced by focusing efforts, and budget, on only the most promising targets.