Delineation of sulphide ore-zones by borehole radar tomography at Hellyer Mine, Australia

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Abstract

Velocity and absorption tomograms are the two most common forms of presentation of radar tomographic data. However, mining personnel, geophysicists included, are often unfamiliar with radar velocity and absorption. In this paper, general formulae are introduced, relating velocity and attenuation coefficient to conductivity and dielectric constant. The formulae are valid for lossy media as well as high-resistivity materials. The transformation of velocity and absorption to conductivity and dielectric constant is illustrated via application of the formulae to radar tomograms from the Hellyer zinc–lead–silver mine, Tasmania, Australia. The resulting conductivity and dielectric constant tomograms constructed at Hellyer demonstrated the potential of radar tomography to delineate sulphide ore zones.

Introduction

Borehole radar can map lithology, structure, and voids around and between boreholes by measuring the traveltimes and amplitudes of the electromagnetic (EM) waves propagating from a transmitter to one or more receivers. Broadly speaking, borehole radar can be operated in two configurations: single-hole reflection or cross-hole transmission. Well-established applications of single-hole reflection borehole radar include cavity detection (Owen and Suhler, 1980), fracture mapping within potential nuclear waste repositories Olsson et al., 1992, Stevens et al., 1994, hydrological investigations (Lane et al., 1994), and stratigraphic mapping within salt mines Mundry et al., 1983, Eisenburger et al., 1993. The principal applications of cross-hole radar to date have been tunnel detection (Lytle et al., 1979), hydrological property mapping Hubbard et al., 1997, Paprocki and Alumbaugh, 1999, monitoring moisture migration Eppstein and Dougherty, 1998, Peterson et al., 1998, Alumbaugh and Paprocki, 2000, mapping hydraulically permeable fractures (Wright and Lane, 1998), and delineating porous zones (Peterson et al., 1999).

Application of borehole radar in non-evaporite mines is still relatively uncommon, notwithstanding the strong commercial incentive to accurately define ore boundaries and structures. Conventional GPR reflection has found application in underground coal mines Coon et al., 1981, Yelf et al., 1990 and is employed routinely to define auriferous zones in the Witwatersrand (Campbell, 1994) and at the Sixteen to One Mine in California (Raadsma, 1994). Encouraging experimental applications of borehole reflection radar have been reported from coal mines (Murray et al., 1998), Witwatersrand gold mines (Wedepohl et al., 1998), and base metal sulphide mines Livelybrooks et al., 1996, Liu et al., 1998. This paper concerns a trial of cross-hole radar tomography in a base metal mine in Australia.

In base metal sulphide mines, the extreme conductivity of the ore zones renders ore contacts as almost perfect radar reflectors. Therefore, if the host rock is highly resistive, radar reflection presents potential means for accurate orebody delineation. GPR is not always effective in these notionally favourable environments because small concentrations of disseminated sulphide minerals can exert disproportionate influence, transforming a resistive host into a strong attenuator of radar waves (Fullagar and Livelybrooks, 1994). Likewise, heterogeneity in the host can scatter radar signals (Fullagar et al., 2000). Nonetheless, the potential economic benefits justify further investigation of both the technical and commercial issues surrounding borehole radar applications in metalliferous mines. Accordingly, borehole radar trials were included as a component in a research project which investigated applications of geophysical techniques at seven Australian mines in the mid-1990s. In particular, cross-hole radar data were acquired at the Hellyer Mine, Tasmania in 1995 using a RAMAC system.

Velocity and attenuation tomograms are the normal outputs from inversion of cross-hole radar data. However, in the mining context, rock types are often more interpretable in terms of their conductivity and dielectric constant. Formulae for transformation of velocity and attenuation into conductivity and dielectric constant are given below, and are applied to the radar velocity and attenuation tomograms generated at Hellyer data. Encouraging results were obtained in mapping the distribution of electrical properties between boreholes, illustrating the potential of radar tomography to delineate ore zones.

Section snippets

Multi-parameter reconstruction of radar tomographic data

The propagation and attenuation of EM waves are governed by conductivity, σ, dielectric constant, ε, and magnetic permeability, μ. In a homogeneous isotropic medium, the EM attenuation is governed by the absorption coefficient α, where:α=ω2με21+σ2ε2ω21/2−11/2.

The radar phase speed ν is given by:ν=ωβ,where ω=2πf is the angular frequency and where wave number β takes the form:β=ω2με21+σ2ε2ω21/2+11/2.

The greatest variation in all the physical properties of rocks and minerals is exhibited by the

Traveltime picking

Both traveltimes and amplitudes are strongly dependent on the conductivities and dielectric constants of the media that the waves pass through. The traveltimes and amplitudes of the direct arrivals on each cross-hole radar trace were manually picked, trace by trace, on a computer. The arrival time of the maximum trough radar amplitude was picked instead of the actual onset time of the radar pulse in this study because the data were quite noisy and the onset was not clearly defined.

Borehole

Imaging results at Hellyer

The Hellyer zinc–lead–silver massive sulphide deposit is located in the Cambrian Mount Reid Volcanic belt in western Tasmania (McArthur and Dronseika, 1990). The stratigraphy consists of a flat-lying sequence of footwall andesites (FPS) overlain by hangingwall volcaniclastics (HVS) and basaltic pillow lavas (PLS). The main mineralised unit is denoted BMS, capped in places by barite-rich (Ba) and glassy silica–pyrite (GSP) zones (Fig. 2).

Crosshole tomographic radar data were collected at Hellyer

Conclusions

In this paper, we have presented general formulae relating resistivity and dielectric constant to radar velocity and attenuation. These expressions constitute a convenient means for transformation of velocity and attenuation tomograms into dielectric constant and resistivity tomograms in high-loss as well as low-loss media. Such transformation is advantageous in the context of mining since resistivity and dielectric constant are more readily interpretable by geophysicists and geologists.

Acknowledgements

This work was part of AMIRA/CMTE Project P436/MM1, “Application of geophysics to mine planning and operations”, sponsored by Aberfoyle Resources, Acacia Resources, CRA Exploration, Mount Isa Mines, Normandy Mining, Outokumpu Mining, and Pasminco Mining. We are grateful to Greg Marshall, Chris Davies, David Shipp, Bevan McWilliams and other Aberfoyle staff for their help during our visit to Hellyer. Likewise, we wish to acknowledge the professionalism of Christer Gustafsson and Inge Naslund of

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