Tools used in mineral exploration for measuring the conductivity and the resistivity in drillholes and on drill core: observations on their range of sensitivity

Abstract

A study has been undertaken to acquire conductivity data using the EM39 low-induction-number conductivity tool. Measurements were taken in three holes in the Sudbury, Ontario, area: at Victoria in the south-west part of the Sudbury structure; at Levack, in the north range; and at the Lady Violet deposit near Copper Cliff. These data were compared with pre-existing data acquired using four other tools and measurements taken on core extracted from the holes. The four tools are the DGI galvanic downhole resistivity tool, the IFG downhole conductivity tool, and the handheld KT-10 and GDD meters. The comparison shows that each tool has a finite range of sensitivity. The resistivity tool used by DGI Geoscience is sensitive to conductivities primarily in the range 0.01 to 100 mS/m; the EM39 tool is sensitive to conductivities in the range of ~30 mS/m to 3000 mS/m and the IFG tool to conductivities greater than 30 mS/m. In the sub-ranges where the ranges of two instruments overlap, one might expect a good correlation between the measurements derived from the two tools. However, this is not always the case, as the instruments can have a different volume of sensitivity: the EM39 has a coil separation of 50 cm and will see material greater than 20 cm away from the hole; whereas the IFG conductivity tool seems to have a smaller spatial scale of sensitivity due to its 10 cm coil size. The handheld instruments used to log the conductivity of the core are sensitive to more conductive material (greater than ~1 S/m). The scale of the sensors of these handheld instruments is a few cm, so they are focussing on a very local estimate. The spatial characteristics of the handheld instruments are similar to the IFG tool, so there is a reasonable linear correlation between the conductivities derived from these three different instruments. However, the slopes are not unity; for example, the GDD instrument gives values three times greater than the KT-10. When selecting tools for measuring the resistivity and conductivity, ensure that the values that you expect to measure fall within the range of sensitivity of the instrument and that the features sought are comparable in size to the volume of sensitivity.

We investigated the range of sensitivity of five tools for measuring the conductivity or resistivity in drillholes and on drill core. Each tool has a limited range of sensitivity, so one or more tools should be used to cover the range of values expected on a particular project.

Key words::

• conductivity, exploration, lithology, logging, resistivity, sensitivity, Sudbury.

Acknowledgements

For financial support we are grateful to the groups funding the Industrial Research Chair in Exploration Geophysics (NSERC, Vale, Sudbury Integrated Nickel Operations – a Glencore company, KGHM International, Wallbridge Mining Co. and the Centre for Excellence in Mining Innovation). In addition, Vale, Sudbury Integrated Nickel Operations – a Glencore company, and KGHM International assisted by providing access to their properties, drillholes, drill core and previous measurements.