zhou and fullagar_2001_applied geophysics

.Journal of Applied Geophysics 47 2001 261269www.elsevier.comrlocaterjappgeo

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

Binzhong Zhou a,), Peter K. Fullagar b,1a CMTErCSIRO Exploration and Mining, P.O. Box 883, Kenmore, QLD 4069, Australia

b Fullagar Geophysics Pty. Ltd., Leel 1, 1 Swann Road, Taringa, QLD 4068, AustraliaAccepted 8 May 2001

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, generalformulae are introduced, relating velocity and attenuation coefficient to conductivity and dielectric constant. The formulaeare valid for lossy media as well as high-resistivity materials. The transformation of velocity and absorption to conductivityand dielectric constant is illustrated via application of the formulae to radar tomograms from the Hellyer zincleadsilvermine, Tasmania, Australia. The resulting conductivity and dielectric constant tomograms constructed at Hellyer demon-strated the potential of radar tomography to delineate sulphide ore zones. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: Electrical conductivity; Cross-hole; Dielectric constant; Mining; Radar; Tomography; Imaging

1. Introduction

Borehole radar can map lithology, structure, andvoids around and between boreholes by measuringthe traveltimes and amplitudes of the electromag-

.netic EM waves propagating from a transmitter toone or more receivers. Broadly speaking, boreholeradar 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

) Corresponding author. Fax: q61-7-3327-4455. .E-mail addresses: [email protected] B. Zhou ,

[email protected] P.K. Fullagar .1 Fax: q61-7-3377-6701.

.Suhler, 1980 , fracture mapping within potential nu-clear 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 . Theprincipal applications of cross-hole radar to date

.have been tunnel detection Lytle et al., 1979 , hy-drological property mapping Hubbard et al., 1997;

.Paprocki and Alumbaugh, 1999 , monitoring mois-ture migration Eppstein and Dougherty, 1998; Peter-

.son 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-evaporitemines is still relatively uncommon, notwithstanding

0926-9851r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. .PII: S0926-9851 01 00070-2

( )B. Zhou, P.K. FullagarrJournal of Applied Geophysics 47 2001 261269262

the strong commercial incentive to accurately defineore boundaries and structures. Conventional GPRreflection has found application in underground coal

.mines Coon et al., 1981; Yelf et al., 1990 and isemployed routinely to define auriferous zones in the

.Witwatersrand Campbell, 1994 and at the Sixteen .to One Mine in California Raadsma, 1994 . En-

couraging experimental applications of boreholereflection 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 to-mography in a base metal mine in Australia.

In base metal sulphide mines, the extreme con-ductivity of the ore zones renders ore contacts asalmost perfect radar reflectors. Therefore, if the hostrock is highly resistive, radar reflection presentspotential means for accurate orebody delineation.GPR is not always effective in these notionallyfavourable environments because small concentra-tions of disseminated sulphide minerals can exertdisproportionate 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 andcommercial issues surrounding borehole radar appli-cations in metalliferous mines. Accordingly, bore-hole radar trials were included as a component in aresearch project which investigated applications ofgeophysical techniques at seven Australian mines inthe mid-1990s. In particular, cross-hole radar datawere acquired at the Hellyer Mine, Tasmania in 1995using a RAMAC system.

Velocity and attenuation tomograms are the nor-mal outputs from inversion of cross-hole radar data.However, in the mining context, rock types are oftenmore interpretable in terms of their conductivity anddielectric constant. Formulae for transformation ofvelocity and attenuation into conductivity and dielec-tric constant are given below, and are applied to theradar velocity and attenuation tomograms generatedat Hellyer data. Encouraging results were obtained inmapping the distribution of electrical properties be-tween boreholes, illustrating the potential of radartomography to delineate ore zones.

2. Multi-parameter reconstruction of radar tomo-graphic data

The propagation and attenuation of EM waves aregoverned by conductivity, s , dielectric constant, ,and magnetic permeability, m. In a homogeneousisotropic medium, the EM attenuation is governed bythe absorption coefficient a , where:

1r21r22 2 v m s~ as 1q y1 . 1 .2 2 / 2 vThe radar phase speed n is given by:v

ns , 2 .b

where vs2p f is the angular frequency and wherewave number b takes the form:

1r21r22 2 v m s~ bs 1q q1 . 3 .2 2 / 2 vThe greatest variation in all the physical proper-

ties of rocks and minerals is exhibited by the resistiv-ity or conductivity. The resistivity of metallic miner-als may be as small as 10y5 V m, for pyrrhotite,while the resistivity of rocks may be as large as 1013

.V m, for dry salt Telford et al., 1990 . In conduc-tive rocks, EM attenuation is high, phase speed isreduced, and wavelengths are shorter. Natural varia-tions in the relative dielectric constant are fairlysmall, varying between 2 and 80 for most mineralsand rocks, depending on the amount of water pre-sent.

Radar velocity, n , can be obtained from inversionof the first arrival times of a cross-hole radar tomo-graphic survey, while the attenuation coefficient, a ,can be constructed from inversion of the amplitudes.Given values of a and n , it follows from Eqs. . . .1 3 that the conductivity s or resistivity r canbe expressed as:

1 2ass s . 4 .

r mn

The corresponding expression for dielectric con-stant, , is:

21 1 as y . 5 .2 /m vn

( )B. Zhou, P.K. FullagarrJournal of Applied Geophysics 47 2001 261269 263

Thus, from measurements of traveltimes and am-plitudes of radar waves, we can construct tomogramsfor four parameters, viz. phase velocity, attenuationcoefficient, conductivity, and dielectric constant. Re-sistivity and dielectric constant are usually muchmore interpretable for geologists and mining engi-neers than velocity and absorption. To the best of ourknowledge, the above general conversion formulaehave not been published elsewhere, although approx-imate expressions for high Q media are frequently

.quoted, e.g. Davis and Annan 1989 .

3. Data reduction procedures

3.1. Traeltime picking

Both traveltimes and amplitudes are strongly de-pendent on the conductivities and dielectric constantsof the media that the waves pass through. The travel-times and amplitudes of the direct arrivals on eachcross-hole radar trace were manually picked, trace bytrace, on a computer. The arrival time of the maxi-mum trough radar amplitude was picked instead ofthe actual onset time of the radar pulse in this studybecause the data were quite noisy and the onset wasnot clearly defined.

Borehole radar systems commonly do not recordabsolute traveltimes. Assuming negligible zero-timedrift, the absolute traveltime can be recovered from atraveltime versus propagation distance plot for direct

.arrivals Mason, 1981; Olsson et al., 1992 . For ahomogeneous medium, the traveltimes should clustertightly around a straight line, and the time interceptafter extrapolation back to zero-distance constitutesthe correction to be subtracted from the picked trav-eltimes. This procedure can also be adopted forinhomogeneous media provided the timedistanceplot defines a linear trend. If the traveltime versusdistance graph does not define a straight line, albeitwith a number of outliers, either the data are of poorquality or the medium is highly heterogeneous.

The assumption implicit in picking the maximumamplitude time as the arrival time instead of theonset time is that the frequency content is invariantfrom trace to trace, to ensure constant time differ-ence between the onset and the amplitude maximum.

3.2. The reduction of the amplitude data

Prior to tomographic reconstruction, the ampli- .tude data must be corrected for: 1 geometrical

. .spreading, 2 radiation pattern of the transmitter, 3 .the angular sensitivity of the receiver, and 4 the

strength of the source. The reduction procedureadopted for the Hellyer data is quite conventional,and assumes far-field propagation and dipole anten-nae. Under this assumption, the electric field ampli-tude, A , can be expressed by:mA sA eyHa l .d lsin u sin u rr 6 . . .m 0 t r

.where sin u defines the radiation pattern for thet .dipole transmitter; sin u is the receiver sensitivityr

function; r is the ray length; A is the source0 .strength; and a l is the absorption coefficient at

distance l along the raypath. The polar angles, utand u , of the AraysB with respect to the transmitterrand receiver antenna axes are defined in Fig. 1. Thefactor 1rr accounts for the geometrical spreading ofthe far-field wave in a 3D homogeneous medium.The above equation can be linearized by taking20log of both sides and viewing the integral as10

.pseudo-traveltime t in dB in analogue to the seis-mic traveltime Jackson and Tweeton, 1994; Zhou et

.al., 1998

ts20log e a l d lsa r .H10 ar

s20log A yay20log10 0 10 /sinu sinut rsa yy 7 .0

where a is the apparent absorption coefficient ina.dBrm ; a s20log A is the source strength;0 10 0

as20log A is the measured amplitude in dB; y10 mis the reduced amplitude after the correction of the

.geometry spreading and radiation pattern effects .Using the above formula, the effects of geometricalspreading and antenna radiation patterns can be re-moved, given source and receiver locations and bore-hole trajectories. However, it should be noted thatantenna radiation patterns vary in conductive media,where secondary lobes may develop or where inextreme cases signal can be completely nulled. Thelikelihood of imperfect radiation pattern correction is


Fig. 1. Schematic illustration of ray polar angles at the transmitterand receiver.

greatest for AraysB, which make an acute angle with .the transmitter or receiver axis Pears, 1997 .

Accounting for the effect of source strength, A0is rarely straightforward. The term, a s20log A ,0 10 0can be estimated in homogeneous media by linearfitting the pseudo-traveltime against the sourcere-ceiver distance after correction for geometricalspreading and radiation pattern. The offset inpseudo-traveltime at zero-distance is the desired esti-mate of a . Subtracting the a estimate from the0 0data will compensate for the effect of source strength,and for other systematic effects such as gain setting.In practice, accurate correction of source strength isvery difficult, both because surveys are not normallyundertaken in homogeneous media and because theperformance of a transmitter in a borehole is influ-enced by the electrical properties of the borehole

.fluid and surrounding rocks Fullagar et al., 1996 .Although, in fact, the effective source strength,

a , will vary with transmitter site, likewise, receiver0sensitivity is degraded within conductive stratigra-phy. Ideally, the transmitter performance should bemonitored, e.g. by recording input current andimpedance to guide A correction in the data. How-0ever, the RAMAC radar system does not record suchtransmitter parameters. It is conventional practice toassume a constant source strength during reductionof amplitude data. Subtracting a constant a Astatic0shiftB from the t-values will leave some sourcestrength effects in the reduced data. However, thismay yet prove to be the most practical approach; ifthe A value derived in resistive host rocks is ap-0plied everywhere, the effect in conductive zones,

where the actual source strength drops, will be toincrease the apparent absorption. This is in factappropriate, since the degradation of the transmitterperformance is due to the increased conductivity, andhence, higher absorption, close to one or both anten-nae. To AfullyB correct for source strength variationswould entail explicit modelling of the transmitter.

It is possible to pursue source corrections furtherif reciprocal data is available, i.e. if data were col-lected with the positions of transmitter and receiver

.reversed. Following McGaughey 1990 , Fullagar et .al. 1996 introduced individual source strength cor-

rections at each transmitter site, their magnitudeschosen to enforce reciprocity. Reciprocity is implic-itly assumed in tomographic algorithms. To forcereciprocity is to acknowledge that the issue is notsimply variation in source strength, but also in re-ceiver sensitivity. Once a reciprocity correction hasbeen applied, a fixed source strength is assumed fordata reduction and image construction. Unfortu-nately, this approach was not open to us at Hellyerbecause, as is usually the case, no reciprocal datawere collected.

4. Imaging results at Hellyer

The Hellyer zincleadsilver massive sulphidedeposit is located in the Cambrian Mount Reid Vol-

canic belt in western Tasmania McArthur and Dron-.seika, 1990 . The stratigraphy consists of a flat-lying

.sequence of footwall andesites FPS overlain by .hangingwall volcaniclastics HVS and basaltic pil-

.low lavas PLS . The main mineralised unit is de- .noted BMS, capped in places by barite-rich Ba and

. .glassy silicapyrite GSP zones Fig. 2 .Crosshole tomographic radar data were collected

at Hellyer Mine in 1995 using a RAMAC 20 MHzborehole radar system. Single-hole reflection andcross-hole transmission radar data were recorded in

.three holes HL798, 800 and 801 in drill section .10340N Fig. 2 . The transmitter spacing was 4 m

while the receiver interval was 2 m. The wavelengthis about 5 m. The principal objective of the trial atHellyer was to evaluate whether borehole radar couldassist in detecting and mapping fault offsets of the

.hangingwall HVSrBMS ore contact.


Fig. 2. Hellyer drill section 10340N, showing the holes used for .radar surveys blue . Tomographic panel locations are indicated in

orange. Geological interpretation is superimposed in green.

Two tomographic panels were recorded, between .holes HL800HL801 and HL798HL801 Fig. 2 .

Only the HL800HL801 panel will be discussed

Fig. 3. Raypath geometry for the tomographic radar survey be-tween holes HL800 and HL801 at Hellyer. Numbers at thetransmitter locations in HL800 identify the shot gathers in Fig. 4.

Fig. 4. The nine shot gathers for HL800HL801 cross-hole radarpanel. The picked traveltimes are superimposed in red. The varia-tion in radar amplitude is evident, especially for shot 7.

here as similar result was achieved for the otherpanel. Fig. 3 shows the raypath configuration for thispanel; path lengths vary between 9 and 25 m. Theshot gathers for the nine transmitter sites in HL800are shown in Fig. 4, with picked traveltimes for themaximum trough event superimposed in red. For the

Fig. 5. Traveltime distribution for HL800HL801.


7th shot gather, the transmitter was located withinrelatively conductive HVS; consequently, the ampli-tude is attenuated strongly, and the traveltime in-creased substantially.

The picked traveltimes were plotted against trans-mitterreceiver distance as shown in Fig. 5. The lineof best fit has equation ts11.4 dq23.8 ns. Theaverage radar velocity in this panel is s88.0avmrms. The intercept time should be subtracted frompicked data to compensate for the source timingerror.

The pseudo-traveltimes derived via reduction ofthe amplitude data can also be plotted against trans-

.mitterreceiver separation see Fig. 6 after the ef-fects of geometrical spreading, and source and re-ceiver radiation patterns have been removed, as per

.Eq. 6 . The line of best fit is given by tsa dyaav 0neper, where the average apparent attenuation isa s0.338 neperrms2.933 dBrm. The interceptavis the source strength estimate: a sy15.07 neper0sy131 dB. The a estimate was subtracted from0the pseudo-travel -times prior to tomographic recon-struction.

Velocity and absorption tomograms were gener-ated using the picked traveltimes and amplitudeswith a SIRT straight ray tomographic reconstruction

.procedure Jackson and Tweeton, 1994 . A cell sizeof 2=2 m2 was used during the inversion. The

.reconstructed velocities Fig. 7 span a relatively . large range, from 78.5 in HVS to 94.7 mrms in

.Fig. 6. Reduced amplitude pseudo-time, t distribution forHL800HL801.

.PLS . The corresponding attenuation coefficients . .Fig. 8 lie between 4.57 black and 1.95 dBrm .white . The overall appearance of these two tomo-grams is very similar, engendering confidence in thedata acquisition and reduction procedures.

.Borehole resistivity logs red and geological con- .tacts green are superimposed on the tomograms. It

is clear that the overall velocity trends correlatedirectly with the resistivity logs, while attenuationand logged resistivity are inversely correlated. There

are significant resistivity and hence velocity and.absorption variations within the PLS.

The velocity and attenuation tomograms can betransformed into dielectric constant and radar resis-tivity images by using the expressions given by Eqs. . .4 and 5 . The results are presented in Figs. 9 and10, respectively. A dominant recorded frequency of15 MHz and free-space permeability were assumedfor the conversion.

The magnetic susceptibility of most minerals isnegligible. The exceptions are magnetic mineralssuch as magnetite, pyrrhotite, and titano-magnetite.In metalliferous mines, the effect of magnetic perme-ability on radar propagation is potentially significant.At Hellyer, there is minor magnetite in the ore, butthe host rocks are virtually non-magnetic. Therefore,free-space permeability has been assumed here.

The appearance of the dielectric constant image .Fig. 9 is reversed from the other images, i.e. highervalues in the HVS than the PLS. The derived relativedielectric constants range from 9.3 to 12.7, which isconsistent with laboratory measurements made onHellyer drill core of 10.2 for PLS and 12.913.4 forHVS samples.

Although the resistivity logs were noisy, theirbroad trends are well correlated with the variations

.in derived resistivity Fig. 10 . The derived resistivi-ties are in the range of 100280 V m, consistentwith the resistivity range 88438 V m measured onHVS core samples. However, in the high-resistivitylog interval at the elevations 610620 m along theborehole HL801, near the lower left corner of theimage, the tomogram shows relatively low resistiv-ity. This is probably a consequence of the lowdensity of tomographic rays in that region. Similarphenomenon was observed in our radio-frequencytomographic imaging with similar configuration .Zhou et al., 1998 .


.Fig. 7. Velocity tomogram. The geologic interpretation green .and borehole resistivity logs red are overlaid on the image. The

values of resistivity log increase to the right for HL801 and to theleft for HL800.

Examination of the velocity, absorption, dielectric .constant and radar resistivity tomograms Figs. 710

suggests that the HVS body intersected by borehole

Fig. 8. Radar attenuation tomogram. The geologic interpretation . .green and borehole resistivity logs red are overlaid on theimage. The values of resistivity log increase to the right forHL801 and to the left for HL800.

Fig. 9. Permittivity tomogram constructed from the velocity Fig.. . . .7 and attenuation Fig. 8 tomograms via Eqs. 4 and 5 . The

. .geologic interpretation green and borehole resistivity logs redare overlaid on the image.

HL801 pinches out between the holes, while theHVS intersected by borehole 800 is gently west-di-pping, in keeping with the geologic interpretation

Fig. 10. Resistivity tomogram constructed from the velocity Fig.. . . .7 and attenuation Fig. 8 tomograms via Eqs. 4 and 5 . The

. .geologic interpretation green and borehole resistivity logs redare overlaid on the image.


.green . It is possible that there is a previouslyunrecognized steep fault between these two bore-holes.

5. Conclusions

In this paper, we have presented general formulaerelating resistivity and dielectric constant to radarvelocity and attenuation. These expressions consti-tute a convenient means for transformation of veloc-ity and attenuation tomograms into dielectric con-stant and resistivity tomograms in high-loss as wellas low-loss media. Such transformation is advanta-geous in the context of mining since resistivity anddielectric constant are more readily interpretable bygeophysicists and geologists.

Resistivity tomograms constructed from cross-holeradar data recorded at the Hellyer Mine, Tasmaniaexhibit variations along their edges which are quali-tatively consistent with borehole resistivity logs.

The trial at Hellyer indicated that, in principle,borehole radar could delineate offsets in the sedi-

.mentrore HVSrBMS hangingwall contact pro- .vided: a that the borehole radar were operated from

.within lava PLS at a range of less than 35 m and . .b that the lavarsediment PLSrHVS contact par-alleled the hangingwall ore contact. Further test workis required at a well-controlled site to confirm thepractical merit of the radar tomography for faultdetection and, hence, ore boundary delineation atHellyer.

Acknowledgements

This work was part of AMIRArCMTE ProjectP436rMM1, A Application of geophysics to mineplanning and operationsB, sponsored by AberfoyleResources, Acacia Resources, CRA Exploration,Mount Isa Mines, Normandy Mining, OutokumpuMining, and Pasminco Mining. We are grateful toGreg Marshall, Chris Davies, David Shipp, BevanMcWilliams and other Aberfoyle staff for their helpduring our visit to Hellyer. Likewise, we wish toacknowledge the professionalism of ChristerGustafsson and Inge Naslund of Mala Geoscience

during acquisition of the borehole radar data. Thanksalso goes to two anonymous referees for their thor-ough reviews and insightful comments.

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