A cross-hole RFEM (Radio Frequency Electromagnetic) tomographic survey was conducted at the Mt. Isa Copper Mine in 1995 as part of a CMTE/AMIRA project investigating the application of geophysics in metalliferous mines. The primary objective of the survey was to evaluate the capability of RFEM for orebody delineation, in a section of the mine where a correlation had previously been established between conductivity and copper grade.An absorption tomogram constructed from the limited 52.5 kHz data set demonstrated that RFEM has potential in this environment for resolving orebody boundaries and establishing ore continuity between drill holes. The calculated absorption coefficients on the tomogram lie between 0.94 and 5.165 dB/m, consistent with laboratory absorption measurements on rock samples from the survey site.The continuity of the footwall orebody, paralleling the Paroo Fault, was not well represented in the tomogram, due to low ray coverage in the comer of the image. However, a simple amplitude mask, depicting only the less attenuated ray paths, provided evidence for continuous ore between the holes. This provides encouragement for efforts to combine amplitude masking with tomography.

The role of geophysics in mineral exploration has expanded rapidly in recent decades, but in mining its importance is only just being recognised. The barriers to greater acceptance of geophysics in mining are more 'cultural' than technical, insofar as mining companies have historically (albeit unconsciously) effectively quarantined geophysicists from mine development and production personnel. The principal motivation of this paper is therefore to raise the geophysical awareness of mining geologists, engineers, and managers.In resource definition and mine development, major capital expenditures are committed on the basis of very sparse information. Local inaccuracies in mine models based on incomplete datasets are not infrequently the root cause of unexpected and sometimes costly production shortfalls, through lost ore or bad ground. When suitable physical contrasts exist, geophysics has the potential to reduce the risks in mine development decision-making via timely and cost-effective mapping of the orebody and its environment. Geophysics, appropriately applied, can underpin mine performance improvements in a number of spheres, including cost per tonne, safety, and environmental impact.Geophysical methods can be classified into two broad categories: borehole logging, for determination of in situ physical properties of the borehole wall rocks; and geophysical imaging, for mapping features at tens or even hundreds of metres from the sensors. Imaging methods can be applied from the pit floor, or underground from individual holes, between holes, or from hole to roadway. Petrophysical borehole logs can expedite ore boundary delineation, rock mass characterisation, and (sometimes) grade estimation. Benefits during production include substitution of core drilling with cheaper percussion drilling, and blasting pattern optimisation. Less well recognised is the value of petrophysical measurements during feasibility studies and at the onset of mine development. Sonic velocity logging of delineation holes can yield a continuous in situ record of rock strength, for example. The benefit of petrophysical data at an early stage in a project is enhanced three-dimensional understanding of the geological and geomechanical environment in the appropriate time frame to influence major mine design decisions. Geophysical imaging techniques can be applied in mines for a variety of applications, including orebody delineation, hazard detection, and exploration. Radio-frequency tomography, for example, is used to map conductive orebodies between drill-holes. Ground-penetrating radar can generate detailed images of potential hazards such as cavities or aquifers, while passive seismic monitoring reduces safety risks by delineating zones of high rock stress. Downhole electromagnetics and borehole magnetics are employed both for detecting off-hole mineralisation and for ground sterilisation.Geophysics is not a panacea at mining operations, but should be viewed as an additional source of tools to deploy in the continuous struggle to maximise overall performance. The benefits flowing from application of geophysics may be direct, as immediate cost reductions, or indirect in the form of an enhanced ore recovery, optimised blasting pattern, or early warning of a safety hazard.

In resource definition and mine development, major capital expenditures are committed on the basis of sparse information and even in production surprises such as bad ground are not uncommon. Drill holes constitute the principal source of information about the orebody and its environment and optimisation of drilling implies maximising the yield of useful information per dollar expended. Geophysical borehole logging provides new flexibility for optimising the drilling budget.Economic benefits can flow from geophysical logging at all stages of the mining cycle. The most commonly cited benefit is substitution of diamond delineation drilling with cheaper percussion or reverse circulation drilling in cases where geophysical logs can substitute for core. This approach can deliver an attractive direct saving in drilling costs (and time) if drill meterage is unchanged, or a potentially greater indirect benefit from better ore control if more holes are drilled within the original drilling budget. More ore contact intercepts per dollar can translate into lower dilution and enhanced ore recovery, thereby increasing revenue as well as reducing mining costs.Substitution of diamond drilling with percussion drilling is not always feasible. However, geophysical logging of diamond holes is often highly advantageous in its own right, both for geotechnical characterisation as well as ore delineation. Operational advantages of logging include data objectivity, speed of interpretation and reduced core handling and analysis costs. Geophysical logging assists mining engineers by providing a continuous measure of in situ rock strength, even over intervals with poor core recovery.The additional expense of geophysical logging in blast holes can be justified in terms of more accurate ore boundary delineation and rock strength evaluation. Precise ore boundary delineation permits refinement of charge placement, to minimise dilution and maximise recovery. Detailed knowledge of rock strength can enable optimisation of blast design. Rock strength can be evaluated in real-time via analysis of drill performance data recorded with measurement-while-drilling (MWD) technology.To successfully apply geophysical logging in mines, its technical, operational and economic viability must all be established. This entails verification of the existence of appropriate physical property contrasts, investigation of the best way to integrate the geophysical results with the mining method and an objective assessment of the value of the resulting mine performance benefit.Geophysical logging is already well established in many mining operations. The greatest impediments to its wider utilisation at mines are: lack of appreciation of geophysics by most mine geologists, engineers and managers; limited interpretational aids; and the necessity to re-enter completed drillholes. In this paper we endeavour to address these issues by informing mine geologists about potential applications of geophysics, by illustrating automated interpretation of geophysical logs and by highlighting the importance of emerging MWD and logging-while-drilling (LWD) technology.

A common area of application for DHEM is in defining extensions or additional lenses on the edges of current economic mineralisation. This may demand surveys in close proximity to infrastructure being conducted during mining operations. Numerous spurious responses may occur which do not necessarily fall into the three categories of noise as defined by McCracken et al (1986): namely, electromagnetic, geologic and observational. The response from an unnatural feature can have two general forms. The first is passive, as a spurious conductor, processed metal is generally far more conductive than our target commodities, albeit with a significantly reduced volume. The second is active, as a primary source of electromagnetic radiation.

An efficient and effective exploration strategy for further resources is important for large scale mining operations such as Mt Isa where the depth and scope of the operations means that a proportionately larger volume of highly prospective rock is available for exploration.The application of drillhole electromagnetics (DHEM) allows the effective radius of exploration from a single drillhole to be significantly increased, thus decreasing the cost of discovery. Typically this type of near- and in-mine exploration will utilise directional drilling technology from the surface or underground. Such holes commonly have sections with inclinations too shallow for conventional DHEM, which relies on gravity to move the probe. This paper describes trials undertaken by MIM Isamine which were designed to overcome the problem of surveying shallow dipping to upwardly inclined drillholes. Various logistical, operational and safety aspects are also addressed which cumulatively produce a working system and help minimise noise.

Drillhole electromagnetics (DHEM) is extensively used as part of the ongoing exploration program at Mount Isa Mine Limited’s copper/lead/zinc mine, the Isa Mine. Exploration targets at Isa Mine are deep, extending to more than 2000m below surface and are tested by both surface and underground drilling.Physical property measurements indicate that although the Urquhart Shale (the host rock) is conductive, the lead/zinc and copper orebodies are approximately two orders of magnitude more conductive. Sirotem axial component, Crone multi-component and the Utem axial component DHEM systems have been used during recent exploration programs. Due to the depth of exploration, large surface loops were needed to generate adequate primary signal levels. However received signal levels were low in the conductive environment at depth and in some cases a total loss of signal resulted from all transmitter loops.Field examples and modelling have shown that significant off-hole economic mineralisation can be detected in the highly conductive environment at Isa Mine. Some responses in the DHEM profiles, that might reasonably be interpreted as being due to off-hole target conductors, have been shown by the modelling to be ‘apparent’ or ‘false’ responses produced by deviating drillholes beneath a conductive overburden.

In past years, regional to detailed gravity and aeromagnetic surveys have been employed to develop a greater understanding of the geology in and around the Mount Isa copper and silver-lead-zinc ore bodies. In recent years, several downhole electromagnetic (DHEM) surveys have been conducted, primarily to target further massive sulphide mineralisation. This paper will present some of the results from these geophysical surveys.Gravity data show a complex but clearly anomalous effect over the mineralisation. A large negative anomaly is correlated with a zone of deep oxidation and leaching in the footwall, with a complex but strong positive anomaly associated with the silica dolomite alteration and silver-lead-zinc mineralisation.Aeromagnetic data over the ore zone are partly obscured by the cultural effects of the city and mine. The removal of the cultural effects has been attempted by measuring the field at a number of terrain clearances and then calculating an actual vertical gradient. These vertical gradients were then used to compute a ‘culture’ filter. The data, after filtering the cultural effects, suggested that the silver-lead-zinc ore bodies produce an anomaly with a magnitude of approximately 35 nT.Two holes have been probed using DHEM. The results show a complex superposition of responses from many sources and demonstrate the applicablilty of DHEM in exploring for base-metal mineralisation, within the conductive environment at Mount Isa. They also show the heterogeneous nature of the system given the differing responses from both holes.