Predicting the path of drill holes can be a real challenge

As many geologists can attest, drilling is not a predictable task. And millions of dollars are spent annually on drilling programs to test exploration targets and to extract rock samples for assaying.

Despite the strategic importance of this information, there are few reliable and cost effective methods of determining the exact subsurface location of exploration drill holes.

It’s almost axiomatic that there is no such thing as a straight drill hole. Drill holes tend to bend right and upwards as they increase in depth, or they may obstinately veer to the left, detour downwards, or corkscrew. A host of mechanical and geologic factors, such as drilling technique, changes in rock hardness or the occurrence of fractures and shear zones, cause drill holes to deviate.

Predicting the path of drill holes more accurately is a challenge that is becoming increasingly important, especially since many near surface exploration targets in North America have been exhausted. Mining companies are routinely drilling targets that lie 500 metres, or even up to 2,000 metres, below surface. And, in these deep holes, the degree of deviation can be surprising — one exploration company drilled a 1,500-metre hole that was collared 4 from the vertical and shallowed to an angle of less than 20 from horizontal at the bottom.

Many solutions have been developed to address the problem of determining the true position of drill holes in mineral exploration. Instruments that evaluate drill hole deviation are based on measurement of one or more of the following parameters, including the earth’s gravitational field, the earth’s magnetic field, or the actual physical curvature of the drill hole or drill rods.

Definite tradeoffs characterize each type of system. For example, instruments for measuring the gravitational field, such as gyroscope devices, are accurate but expensive. Instruments which measure both the gravitational and magnetic field, using a dip meter and magnetic compass, for example, are less accurate but are relatively inexpensive. Alternately, instruments which measure drill hole curvature accumulate measurement errors (because each reading is relative to the previous one) but are not affected by magnetic interference.

To stretch scarce exploration dollars, geologists have typically used the inexpensive magnetic-based devices. The more expensive gravity-based devices are the last-resort solution in situations where high accuracy is essential, whereas the curvature measuring devices can be effective in highly magnetic rocks.

In the last two years, a Brampton, Ont.-based manufacturer of borehole instruments has developed an orientation probe that the company claims combines the best of all worlds. IFG Corp. has integrated both magnetic and gravitational principles into the design of a fully digital orientation probe for mineral exploration applications.

The company’s orientation probe functions as both a digital compass and a digital dipmeter. The compass is actually three fluxgate sensors which measure the drill hole azimuth (or bearing). Two electrolytic tilt sensors are used to establish the dip of the drill hole. All sensors are housed in a 40 mm diameter non-magnetic housing that is rated to a depth of 2,000 metres. To date, the IFG probe has been used to acquire data in Africa, Canada, Germany, Japan and the U.S. In Canada, surveys have been conducted for a number of mining companies.

In one survey, the probe was used to determine azimuth and dip along the full length of a hole known to have a distinctive kink, a result of directional drilling that was applied to steepen the hole. With samples taken every 10 cm, the irregular path of the hole was clearly mapped out. Moreover, the bottom hole position was in agreement with the result from an independent seismic triangulation survey within the 2% survey accuracy claimed by both methods.

In another survey, the probe was used to evaluate dip and azimuth in a hole known to have a significant amount of magnetite and pyrrhotite — a common scenario since many exploration holes are drilled to intersect magnetic anomalies outlined from surface geophysical surveys.

While magnetic interference can play havoc with conventional magnetic-based instruments, this is less of a problem when the orientation probe is used. Because the probe records the three components of the earth’s magnetic field, any magnetic interference can be identified and most often removed. In the summer of 1992, the new IFG orientation probe will be further evaluated in a head-to-head battle with a variety of mechanical instruments. The Geological Survey of Canada is planning an objective testing program that will determine what level of accuracy can be achieved with commercially available deviation probes. Existing holes with known coordinates, including breakthrough holes in one or more operating mines, will be used for the tests. Geologists and mining companies will have a vested interest in the results of the testing program. As near-surface deposits dwindle, the industry, now more than ever, is searching for answers to some of the most enigmatic, yet basic, questions in exploration, such as “What is the exact path of the drill hole?” and “Where is the mineralization located with respect to the drill hole?”. For the mining industry’s geologists who continue to probe for deeper deposits, the stakes in answering these questions are high.

— Greg Hollyer is a geophysical engineer and corporate communications consultant based in Toronto.