How `down-the-hole’ geophysics enhances drill-hole data

Drill core presents hard, irrefutable evidence of what lies beneath the earth’s surface, but the cross-sectional area of the core is tiny compared with the prospective area as a whole. Nevertheless, value judgments and program decisions have to be made routinely by geologists on the basis of this information. Questions asked include:

— How extensive is the mineralization intersected by the drill hole?

— In what direction does the mineralized horizon trend?

— Has the drill hole tested the better section of mineralization?

— Are there any economic concentrations of mineralization close to the drill hole that remain untested?

The earth’s crust is a complex environment that rarely presents the same scenario twice. When exploring near surface, mapping, geochemical and geophysical survey results provide useful data. A geologist must rely on his experience and that of others to interpret all the available information and determine where to explore next.

Geophysical borehole techniques can be an invaluable aid in this process providing answers or clues to the answers of many of the often-asked questions. The techniques chosen depend on the exploration target and success is often tied to the number of drill holes available for investigation.

Massive to semi-massive sulphides can often be traced by electromagnetic methods or the simpler mise-a-la-masse technique. Less well-established and accepted by the mining community are induced polarization/resistivity techniques (IP) for outlining disseminated sulphide mineralization.

The components of the electrode string, its configuration and general construction have to be carefully designed to minimize extraneous noise sources while maximizing investigation capabilities and allowing versatility in array types.

The Geological Survey of Canada has been developing and testing a sophisticated borehole probe for several years with results from a number of deposits available in the public domain. MPH Consulting, using custom-made and more versatile equipment, has also tested both surface and underground applications. The results have demonstrated the ability and reliability of borehole logging in:

— determining the continuity of mineralization up to tens of metres away from a given drill hole;

— detecting any substantial off-hole mineralization within tens of metres of the hole;

— tracing the continuity of mineralized horizons for up to 100 metres and more;

— allowing investigations to be completed to depths in excess of 700 metres;

— eliminating the necessity of some fences in underground development drilling;

— in some cases, indicating variations in types of mineralization or IP responses.

A borehole investigation program of a given mineralized horizon may involve one or more of the following techniques:

— directional arrays with the current electrodes on surface to determine overall continuity and orientation;

— short multiple-dipole in-hole array to locate near-hole mineralization and possible variations in polarizable responses within a system;

— larger multiple-dipole in-hole arrays to determine off-hole continuity and dip as well as the presence of any off-hole mineralization;

— hole-to-hole techniques to determine continuity up and downdip as well as along strike;

— spectral measurements to discriminate between different sources of IP responses.

The decision to take measurements in the time or frequency domain mode can be critical to the outcome of the program results. Frequency domain measurements will be subject to coupling noise that increases in severity with increasing string length. The frequency domain, however, is also the preferred mode for obtaining true spectral measurements.

A healthy compromise for larger string lengths is the use of a time domain receiver with the ability of measuring the frequency domain equivalent over a series of different transmit times. While the recording time is longer, the confidence in data quality is also much higher.

The success of any borehole IP investigation depends on how the program is designed and whether information can be obtained from a large number of holes. Ultimately, the acceptance of these techniques as reliable exploration tools will depend upon companies prepared to incorporate routine IP borehole logging in their programs and the publication of case histories. Simon Bate is the senior geophysicist for MPH Consulting, Toronto.