Application of geophysics in metalliferous mines

Abstract

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 geo‐physicists 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.