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The Down-hole Magnetometric Resistivity (DHMMR) technique uses a grounded dipole to inject current into the ground and a downhole sensor to measure the resultant magnetic fields. Until recently, the sensor was usually a single component induction coil probe. Recently a 12-hole DHMMR program conducted on the North Mine, Broken Hill, used a 3-component B-field `Atlantis' probe. The results from this survey were spectacular, particularly when considering the target and location: The target was the narrow discontinuous ribbons of low-conductivity Zinc Lodes, located 20-50m above the highly-conductive main North Mine orebody, and underneath/next to the North Mine infrastructure and development. One risk facing the survey was that the main orebody would act as a short-circuit, causing the impressed current to avoid the Zinc Lodes entirely. To mitigate this, one transmitting dipole electrode was placed down a deep drill hole in a Zinc Lodes intersection and the other was dug into a surface expression of the Zinc Lodes, ~1.5km south of the drill hole electrode. This layout very effectively isolated and energized the Zinc Lodes mineralisation. The 3-component B-field probe has a noise level, at the frequencies used in the Broken Hill survey, which is significantly below that of sensors used previously for DHMMR. This resulted in better data and faster acquisition times. Additionally, there is relatively little processing required after the survey to present data in a meaningful manner for interpretation. One final product of the survey was vector geomagnetic data (not utilized to date). The success and accuracy of this survey using new equipment in difficult conditions is expected to lead to a wider and better appreciation of DHMMR's capabilities. |
