application of elctric method

8/3/2019 Application of Elctric Method

1/15

GEOPHYSICS, VOL. 65, NO. 6 (NOVEMBER-DECEMBER 2000); P. 19461960, 12 FIGS., 1 TABLE.

The application of geophysics during evaluationof the Century zinc deposit

Andrew J. Mutton

ABSTRACT

During the period 1990 to 1995, experimental pro-

grams using high-resolution geophysics at severalAustralian operating mines and advanced evaluationprojects were undertaken. The primary aimof those pro-grams was to investigate the application of geophysicaltechnology to improving the precision and economics ofthe ore evaluationand extractionprocesses. Geophysicalmethods used for this purpose include:

1) borehole geophysical logging to characterize oreand rock properties more accurately for improvedcorrelations between drill holes, quantification ofresource quality, and geotechnical information

2) imaging techniques between drill holes to mapstructure directly or to locate geotechnical prob-

lems ahead of mining3) high-resolution surface methods to map ore con-tacts and variations in ore quality, or for geotech-nical requirements

In particular, the use of geophysics during evalua-tion of the Century zinc deposit in northern Australiademonstrated the potential value of these methods tothe problems of defining the lateral and vertical ex-tent of ore, quantitative density determination, predic-tion of structure between drill holes, and geotechnicalcharacterization of the deposit. An analysis of the po-

tential benefit of using a combination of borehole geo-physical logging and imaging suggested that a moreprecise structural evaluation of the deposit could beachieved at a cost of several million dollars less thanthe conventional evaluation approach based on anal-ysis from diamond drill-hole logging and interpolationalone.

The use of geophysics for the Century evaluation alsoprovided substance to the possibility of using systematicgeophysical logging of blast holes as an integral part ofthe ore extraction process. Preliminary testsindicatethatore boundaries can be determined to a resolution of sev-eral centimeters, and ore grade can be estimated directlyto a usable accuracy. Applying this approach routinelyto production blast holes would yield potential benefitsof millions of dollars annually through improved time-liness and accuracy of ore boundary and quality data,decreased dilution, and improved mill performance.

Although the indications of substantial benefits result-ing from the appropriate and timely use of geophysics atRio Tintos mining operations are positive, some chal-lenges remain. These relate largely to the appropriateintegration of the technology with the mining process,and acceptance by the mine operators of the economicvalue of such work. Until the benefits are demonstratedclearly over time, the use of geophysics as a routine com-ponent of evaluation and mining is likely to remain at alow level.

INTRODUCTION

Geophysics has been used by the metalliferous mining in-dustry primarily as a tool for exploration. By contrast, thepetroleum and coal industries have exploited geophysics toa far greater extent in terms of using the data to quantify thevalue, size, or production capabilities of their resource. The ap-plication of geophysics to mineral-deposit evaluation and min-ing is not developed well in general. Some possible reasons forthis are:

Manuscript received by the Editor February 5, 1999; revised manuscript received May 12, 2000.Rio Tinto Technical Services, P.O. Box 2207, Milton, Queensland 4064, Australia. E-mail: [email protected] 2000 Society of Exploration Geophysicists. All rights reserved.

1) insufficient knowledge by mining managers, engineers,and operators of the existence of geophysical method-ologies, coupled with a high techhigh cost perceptionof geophysics

2) perceived infrastructure and logistical difficulties in usinggeophysics in a working mine environment

3) lack of knowledge of the physical properties of the oreand host rocks which may be exploitable in a miningenvironment

1946

Downloaded 16 Nov 2011 to 76.120.23.222. Redistribution subject to SEG license or copyright; see Terms of Use at http://segdl.org/


2/15

Geophysical Applications at Century Zinc Deposit 1947

4) limited research effort applied to standard geophysi-cal methods to develop higher-resolution acquisition orinterpretation tools applicable to mining applications,rather than tools applicable to exploration

5) scarcity of geophysicists with access to mine problems orwith sufficient knowledge of mining culture or opera-tional requirements to champion the use of geophysics inthis area

During the period 1990 to 1995, CRA Limited (nowRio Tinto) undertook experimental programs using high-resolution geophysics at several Australian operating minesand advanced evaluation projects, to investigate opportunitiesto develop and test geophysical applications related directly toresource definition and mining problems. Using some exam-ples of results obtained during the evaluation by CRA of theCentury zincdeposit in northern Australia,this paperdescribesthe opportunities available and the rationale for applying geo-physics beyond the exploration stage of mineral development.

WORK UNDERTAKEN

A rangeof geophysical surveys, some conventional and someadapted for specific problems, has been tested in recent yearsat several CRA evaluation projects and mining operations toenhance ore-boundary definition and recoveries. The surveysfall into several categories, including:

1) High-resolution surface surveys to map remotely thelateral and vertical limits of ore or other geotechnicalparameters required for mine design. Successful applica-tion of such methods potentially can reduce the amountof closely spaced expensive drilling generally needed forthese requirements, and can enable improved reservescalculation and mine design. Examples of such workincorporate the use of standard methods such as mag-netics, electromagnetics (EM), and IP/resistivity usingimproved modeling and data enhancements, or new-generation methods such as borehole EM imaging andground-penetrating radar (GPR).

2) Geophysical logging of evaluation drill holes to quan-tify mineralogical, geotechnical, and structural parame-ters of ore and host rocks in situ. Many of the standardpetroleum logging tools, such as density, sonic, and dip-meter, have been adapted successfully to the hard-rockenvironment, along with developments in new tool tech-nology such as magnetic susceptibility and conductivity.Improved confidence and demonstrated success with thisapproach is aimed mainly at providing significant costsavings by the future routine use of noncore rather thancore drilling.

3) Interborehole imaging methods usingelectromagneticorseismic sources. Examples of suchmethods include radio-imaging (RIM) surveys between holes to map ore con-tinuity and structure. The use of these methods is de-signed to improve the knowledge and reliability of theore boundaries at an earlier stage of evaluation, whichshould reduce the time and cost of the evaluation pro-cess.

4) Geophysical logging of blast holes to define ore-wastecontacts to a resolution of a few centimeters and, in somecases, to estimate ore grade directly. Applying this ap-

proach routinely to production blast holes is designed toyield substantial cost benefits through improved timeli-ness and accuracy of resource information, less dilution,and improved mill performance.

Much of CRAs investigation of the use of geophysics formining and mine evaluation was carried out at the site ofthe Century zinc deposit in northern Australia, discovered by

CRA Exploration in 1990. Since 1997, it has been owned andoperated by Pasminco Pty Limited. The discussion and exam-ples presented below will focus on the work at Century and therole geophysics played in the various phases of the evaluationof the deposit prior to 1995.

RESOURCE DEFINITION GEOPHYSICS AT THE CENTURY

ZINC DEPOSIT

Geology

The Century Zn-Pb-Ag deposit is located about 250 kmnorth-northwest of Mount Isa, Queensland. Substantialdrilling (about 500 diamond core holes) from 1990 to 1995 out-

lined a geologic resource containing about 120 million metrictons of 10.5% zinc, 1.5% lead, and 35 g/ton silver.The Century deposit occurs within sediments of the

Proterozoic MountIsa inlier and is locally unconformablyover-lain by Cambrian rocks comprising dolomitic limestone, chert,and chert breccia (Figure 1). Dolomiticsiltstones and carbona-ceous shales host the deposit. The mineralized sequence is

FIG. 1. Century deposit location and geology.



3/15

1948 Mutton

about 40 m thick and consists of four laterally continuous sub-divisions (units 1 to 4). The bulk of the mineralization occursas strata-bound, banded sphalerite, galena, and pyrite withinblack carbonaceous shales of units 2 and 4. A barren dolomiticsiltstone bed within unit 3 represents a semicontinuous markerbed throughoutmuch of the deposit. Themineralized sequencehas excellent grade continuity but is disrupted by late faulting.A detailed description of the geology and mineralogy of thedeposit is included in Waltho et al., 1993.

Figure 2 shows a schematic north-south cross-sectionthrough the deposit. The deposit consists of a smaller, shallowsouthern block which subcrops in the southwestern margin ofthe orebody, and a larger, deeper northern block completelyconcealed beneath Cambrian limestone and recent alluvium.Bounding surfaces to the deposit comprise either postmineral-ization faults or Cambrian and younger erosional surfaces.

Physical characteristics of the ore and host

Comprehensive laboratory petrophysical measurements onCentury core were made on several samples as part of anAustralian Mineral Industry Research Association (AMIRA)project (project P436) to investigate the application of geo-physics in mine planning and mining (Fullagar et al., 1996a;Fullagar and Fallon, 1997). These data have been evaluated inconjunction with extensive geophysical log data collected froma large proportion of theholesdrilled. A typical compositegeo-physical log through the Century ore sequence and host rocksis presented in Figure 3.

The petrophysical results show that the Century orebodyis typical of many shale-hosted sulfide base-metal deposits

FIG. 2. Century deposit cross-section 46800E.

throughout the world. In particular, the ore is characterizedby distinctive physical properties, including higher density, lownatural radioactivity, and low magnetic susceptibility.

The main difference from many other shale-hosted depositsis that the Century ore, consisting mainly of sphalerite withrelatively low iron sulfide, is not very conductive and in parts ismore resistive than barren host shales. However, petrophysi-cal measurements on core and downhole induced-polarization(IP) surveys show the ore has high electrical chargeability.Figure 4a shows a typical downhole IP log through the deposit,and Figure 4b is a statistical summary of the IP and resistivityresponse recorded in several drill holes. These results clearlyindicate how chargeable the ore units are compared with otherlithologies.

The analysis of physical properties also indicates diagnos-tic contrasts among some of the host lithologies. Althoughthis knowledge is not specifically useful for ore evaluation,it has helped to address other problems arising during theproject development. For example, the high resistivity of theCambrian limestone has been exploited to assist with a hydro-logic problem associated with development of a large pit (seebelow).

Role of geophysics

The discovery of Century was based largely on testing ofa zinc soil geochemical anomaly which was delineated onregional gravity, ground magnetic, and soil sample traverses(Thomas et al., 1992; Broadbent, 1996). No anomalous re-sponse was detected on the gravity or magnetic data or onother geophysical surveys attempted prior to the discovery.



4/15


Despite the lack of success of geophysical techniques inthe initial detection of ore, the recognition after the discov-ery of contrasts in the physical properties of the ore andhost sediments provided some confidence that geophysicalmethods could have a role in further exploration and evalu-ation of the deposit. Geophysical surveys subsequently carriedout over and within the Century deposit since its discoveryhave included detailed ground and airborne surveys (gravity,magnetics, electromagnetics, IP/resistivity, reflection seismic),borehole geophysical logging, and interborehole geophysicalimaging

The work resulted in some successes, in particular relatingto defining the overall lateral and vertical extent of mineraliza-tion, mapping lithologic boundaries and structure within thedeposit, providing data for ore-reserve estimates and mineplanning, supporting geotechnical studies for pit design, andaiding ongoing exploration in the area

Detailed surface surveys

Lateral and vertical extent of mineralization.After the ini-tial discovery of zinc-rich, low-sulfide mineralization and therealisation that EM methods did not detect the mineralization,the IP method was suggested as a possible way of mapping theextent of the mineralization beneath the Cambrian cover. A

FIG. 3. Century deposit composite geophysical log.

limited number of IP/resistivity traverses subsequently werecompleted over the Century deposit and immediate environs,indicatingthepresenceofagoodIPanomalyapproximatelyco-incident with themineralizationas known at that time (Thomaset al., 1992). IP/resistivity logging of selected drill holes sub-sequently confirmed the source of the surface IP anomalies.Figure 4a presents an example of downhole IP results in onehole, which clearly indicates the high chargeability response ofthe mineralized zone.

The downhole IP/resistivity surveys thus provided the reas-surance that IP anomalies represented good exploration tar-gets. In fact, initial drilling into the deeper northern ore zone(Figure 2), which is masked completely by hundreds of metersof barren limestone and sediments, essentially was guided bythe presence of the IP anomaly in that area. Although only alimited number of IP lines were completed over the deposit,the IP results, in combination with drilling, helped to confirmthe lateral limits of mineralization far sooner in the evaluationprocess than if IP surveying had not been attempted.

As well as mapping the lateral extent of the orebody, laterwork by CRA Exploration demonstrated that it is possible totransform the IP/resistivity data into an approximate depthimage of the mineralization (representing the vertical distri-bution of chargeable material). This work used inversion algo-rithms developed by the Geophysical Inversion Facility group



5/15

1950 Mutton

at the University of British Colombia (Oldenburg et al., 1997).Figure 5 shows the image derived from unconstrained inver-sion of surface IP/resistivity data adjacent to the cross-sectionpresented in Figure 2. The outline of the orebody derived fromsubsequent drilling is superimposed.

This work clearly demonstrates the potential mappingcapabilitiesboth laterally and verticallyof a surface geo-

a)

FIG. 4. Century downhole IP/resistivity results: (a) log for DDH LH117; (b) statistical summary for all IP/resistivity logs fromCentury deposit.

physical methodfor which the physical property contrasts havea close association with the mineralization, defining the ap-proximate extent of ore prior to extensive drilling. The imageof the orebody provides the explorer with improved capabilityfor successful drill targeting. However, it also has the poten-tial, by the application of constraints from initial drilling, todefine the ore boundaries and estimate reserves substantially



6/15


at a far earlier stage of the evaluation process than hithertopossible.

Geotechnical applications.Although the predominantlysphaleritic Century ore has little electrical conductivity con-trast with the host sediments, surface electromagnetic methodshave been applied successfully, based on the conductivity con-

trast between the Proterozoic sediments and overlying lime-stone (Figure 4), to assist geotechnical work associated withpit design. Two phases of Controlled Source Audio-FrequencyMagnetotelluric (CSAMT) surveys were completed over thedeposit to (1)locate large blocksof detached Proterozoic shalewithin the limestone (the shale presents a geotechnical hazardfor pit-slope stability during initial excavation of the limestoneportion of the proposed pit, and (2) determine the thicknessof the surrounding water-saturated limestone to estimate thelikely water flow into the open pit as it is excavated.

The CSAMT method was chosen in preference to conven-tional TEM methods because of logistical considerations as-sociated with the limestone topography, and the belief thatCSAMT is more sensitive than TEM methods to delineat-

ing contrasts in the more resistive lithologies present in theCentury area.The first work was only partially successful, in that only very

large blocks of shaletypically greater than 50 50 50 mcould be delineated. The survey results did not offer sufficientresolution to locate smaller blocks confidently, but improvedsurvey design may have provided better resolution.

FIG. 5. Century deposit Line 46800E IP pseudosection and inversion model compared with orebody location from drilling. IP dataare from 100-m dipole-dipole frequency, domain survey.

The secondphase of work proveduseful in mapping thebaseof the limestone and hence assisting with the hydrologic studyfor the pit design. Figure 6 (after Mayers and Bourne, 1994)shows a plan of the resistivity model obtained from inversionof the CSAMT data and collated at a vertical depth of 100 m.The intense blue represents the presence of resistive limestoneat this depth, and the warmer colors indicate the presence ofless resistive shale and siltstone.

The depth to the base of the limestone is shown in selecteddrill holes. In general, there is a good correlation between theintersected thickness of limestone and depths predicted fromthe CSAMT resistivity model. The results show that postde-positional faulting has led to large variations in the limestonethickness in the vicinity of Century and, importantly, the lime-stone is necked to the north and east of the deposit. TheCSAMT result thus gives some confidence that the total vol-ume of water flow into a proposed pit will be significantly lessfrom the thinning of the water-saturated limestone than if thelimestone had been uniformly thick to the north and east ofCentury.

Borehole surveysgeophysical logging

Although geophysics did not feature strongly in the initialdiscovery of ore, downhole geophysical characterization log-ging wascarriedout at Century in several campaignscommenc-ing soon after the initial drill holes were completed. The workwas commissioned for several reasons:



7/15

1952 Mutton

1) to help geologists and geophysicists gain a better under-standing of the in situ physical properties of the depositand its host rocks as an aid to interpretation of explo-ration and geotechnical data

2) to provide data to assistwith the correlation of lithologiesand definition of the structure of the ore zones betweendrill holes

3) to provide quantitative in situ density information to as-sist with estimation of the ore reserves

4) to provide rock-strength information to assist in thegeotechnical assessment of the deposit by miningengineers

The logging programs used standard suites of electrical andnuclear tools (resistivity, resistance, self potential [SP], naturalgamma,density, andneutron), as well as tools such as dipmeter,magnetic susceptibility, and sonic velocity, which had not beenused commonly in base-metalgeophysical logging prior to thattime.

Lithologic correlations.Interpretation of the downholelogs indicates that the major lithologic units and the mineral-ized zones canbe distinguished readily by their density, natural

FIG. 6. Century deposit region showing smooth-model inver-sion of CSAMT resistivity at 100-m depth,compared with lime-stone depth from drilling. The more resistive areas (blue) rep-resent limestone greater than 100 m thick.

Table 1. Summary of Qualitative Log Responses.Prot. Prot. shale and Prot. shale

Cambrian sandstone siltstone Ore zone and siltstonelimestone (h/wall) (h/wall) (Units 14) (footwall)

Natural gamma (API cps) very low (25) high (160) moderate (125) low-mod (70150) high (200)Magnetic Susceptibility very low (1000) low (60) low (75) variable (50200) low (80)Density (g/cm3) moderate (2.7) low (2.6) moderate (2.7) mod-high (2.83.0) moderate (2.62.7)Neutron (API cps) high (1500) high (1600) moderate (1200) low (700-variable) moderate (1200)Sonic velocity (m/s) very high moderate moderate (4000) mod-high (40005000) moderate (4000)

(50006000) (4500)

gamma, neutron, resistivity, sonic, and magnetic susceptibilityproperties. Figure 3 shows a composite log of a typical drillhole through the Century deposit. Table 1 summarizes the re-sponses that characterize the main lithologic units for each ofthe main log types.

The mineralized sequence (units 1 to 4) is differentiated wellby the density, natural gamma,and magnetic susceptibilitylogs.The density contrast is attributed to the presence of sulfide andan increase in iron carbonate (siderite) content. The internalwaste unit within the ore zone (unit 3.2) is characterized bylower density and generally higher gamma, magnetic suscepti-bility, and resistivity, compared with the mineralized intervals.

Quantitative analysis.The quantitative analysis of the logdata, which attempts to transform such data into usable physi-cal or chemical quantities such as density, grade, rock strength,etc, is dependent on the equivalence of data from the differentlogging tools used at various times and by various contractors.Differences arise from differences in tool specifications andalso from environmental differences, including borehole di-ameter, fluid content and properties, temperature, weathering,

smoothness of the hole wall, etc.Comparisons to a calibration standard, such as the APIstan-dard, are used by most logging contractors but do not accountfor most of the environmental factors. For example, an inspec-tion of different API-standard calibrated natural gamma logscan show some marked offsets in absolute values recorded.

The most successful form of calibration achieved at Centuryhas been the use of a calibration drill hole, which was resur-veyed at regular intervals during each logging program. Suchactionensuredthat therepeatability of logs over a known inter-val could be compared and quantitatively adjusted by meansof a correction factor, if necessary.

Quantitative analyses undertaken on the Century data haveincluded automated lithologic and grade-prediction studies but

primarily have been aimed at determining in situ density androck-strength variations.

Automated lithologic prediction.Experimental work de-signed to test the possibility of predicting lithology and gradedirectly from a combination of geophysical logs wascarried outon selected geophysical logs from Century. Three approacheswere attempted: (1) artificial neural networks (ANN), (2) lin-ear regression and discriminant analysis, and (3) cluster analy-sis (lithology prediction only).

Some encouragement was obtained from the initial predic-tion tests, but it was clear that factors such as measurement



8/15


uncertainty (calibration differences, depth discrepancies in thetraining sets, etc.) or the level of data conditioning (such asfiltering) can have a large influence on the predictions. Theresults produced from the ANN tests were not sufficiently en-couraging in either lithology or grade prediction for this to beconsidered a viable method at present.

On the other hand, reasonable predictions of lithology wereobtained from the cluster analysis method, using the LogTrans

FIG. 7. Lithology-basedLogTrans geologic interpretation of Century drill hole LH643 derived by usingstatistics fromnearby controldrill holes. The interpretation has predicted successfully a normal fault across which part of the orebody is missing.

software developed as part of the AMIRA P436 project(Fullagar et al., 1996a). Figure 7 shows an example of predic-tion of lithology at Century using this method. The predictionfor the hole presented(LH643) is based on a statistical analysisof data from holes nearby, so in effect, the prediction is uncon-strained by thelog results andgeology from LH643.The resultscompare favorably with the mapped lithologies, even thoughpartoftheoresequenceinLH643ismissingbecauseoffaulting.



9/15

1954 Mutton

Improvements to and routine use of such software in thefuture could save considerably on the cost of manual loggingof core, but the greatest saving would result from the use ofpercussion rather than core drilling for a large proportion ofthe drill holes. This approach would be applicable to a depositsuch as Century in which the need for closely spaced drilling isrelated more to structural interpretation than to grade control.

Density.The most extensive quantitative analysis of theCentury log data has been done for the purpose of densitydetermination for ore-reserveestimation. Several independentmethods were considered and tested to determine a suitableapproach. The methods included:

1) Archimedes-type measurements on whole core at siteor by a laboratory on core fragments submitted for geo-chemical analyses

2) laboratory measurement of density of pulverized and ho-mogenized drill-core samples by acetone titration

3) physical measurement (dimensions and weight) of wholedrill core at site

4) stoichiometry, based on base metal and sulfur assay data5) geophysical (gamma-gamma) logging of drill holes

Of these methods, geophysical logging is considered to pro-vide the most consistent measure of bulk density, althoughphysical measurement and stoichiometric calculations basedon zinc, lead, iron, manganese, and sulfur assay data also

FIG. 8. Comparison of gamma-gamma log-derived density (showing differences in calibration standards) and density measurementson core for Century drill hole LH483.

proved useful. The titration method proved to be the mostunreliable, yielding many spurious values probably caused bysmall variations in the measurement method.

All methods have limitations. For example, the chemicalassaybased methods yield data restricted to the assay inter-vals only, and the physical-measurement methods are moresubject to measurement error arising from slight variations inthe procedure. Geophysical logging appears to yield very con-sistent results. However, the conversion of the measured countrate to a density value is subject to error because of differencesbetween thematerial containedwithin thevolume of rock sam-pled by the logging tool and the smaller volume of rock in thecore used as the reference value.

Calibration factors supplied by the logging contractor shouldbe used with caution in hard-rock mining environments, be-cause most of the calibration standards are devised for loggingin lower-density sedimentary environments hosting petroleumor coal deposits. Figure 8 shows the comparison between thecontractors derived density and the recalculated density logusing the calibration derived from the physical measurementof core. The result of using the former is a large overestimationof the ore density and therefore the contained tonnage of ore.

The major advantages of using geophysical logging for den-sity determination are that (1) the data are available and con-sistent through the length of the drill hole rather than just overan assayed interval, and (2) data can be obtained for noncoredholes, reducing the requirement for cored holes. However, thenoncored holes must be drilled well so that the hole walls are



10/15


reasonably smooth. Cavingand irregularities in thewall arethemain sources of error with this method. Calliper data collectedin conjunction with the density logs can be used to correct orexclude data affected in this manner.

The main limitation of density logging is the use of a strongradioactive source and the possibility that such a source couldbecome stuck in a drill hole, thus requiring some effort andexpense to recover the source. For the work at Century, a pro-cedure was implemented to use the density tool only after oneor more successful runs with other nonradioactive tools. In thisway, thesafety risk wasconsidered manageable,and a high pro-portion of density logs from the available holes was achieved.

Rock strength.Measurement of the strength and fractur-ing characteristics of both the ore and the host rock (includ-ing the overlying shale, sandstone, and carbonate) is essentialfor mine planning at Century and has implications for pit de-sign, blasting requirements, and milling of the ore. Initially, atraditional approach to prediction and modeling of the rock-strength variations within the deposit was taken, using stan-dard procedures such as measurement of rock quality descrip-

tor (RQD) on all cores. These data then were compared withtest measurements of unconfined compressive strength (UCS)and other strength parameters on selected pieces of core.

To assistwith this problem, sonic velocity data acquired froma standard slimline sonic tool were investigated to determineif such data could provide more uniform and extensive infor-mation than reliance on RQD and limited core measurements.

FIG. 9. Comparison of geotechnical data derived from laboratory measurements and sonic logs for Century drill hole LH643.

This approach has been used widely in rock-strength analysesin Australian coal-deposit evaluation for many years (Asten,1983; McNally, 1990; Davies and McManus, 1990).

A study (Duplancic, 1995) was initiated as part of theAMIRA P436 research project to address this requirement.The focus of this specific study was to determine the relation-ship between rock strength (measured as UCS) and sonic ve-locity, measured both on core samples in the laboratory and insitu in the borehole.

Figure 9 shows a portion of a sonic velocity log fromCentury, with the laboratory velocity (P-wave measured at1 MHz) and UCS measurements superimposed. The velocitydata show a reasonable correlation, but the correlation withUCS is poor. The analysis found that the relationship betweenvelocity and rock strength is a function of the lithology andthe rock porosity, more than the intrinsic variation in strength.Figure 10 demonstrates this relationship schematically, sug-gesting that it would not be feasible to predict UCS fromthe borehole velocity data without first accounting for theporosity and lithologic variation.

The study also noted that sample quality, laboratory mea-surement problems, and insufficient samples to gaugestatisticalreliability could affectthese results adversely. However, it mustbe concludedfrom this work that at best, theprediction of UCSfrom velocity can be regarded as indicative only.

Structural prediction from geophysical logs.One of theearliest problems recognized at Century was the presence of



11/15

1956 Mutton

small-scale faulting within the deposit (Waltho et al., 1993).Thesefaults aremostly steeply dipping andtrendin several pre-ferred directions. Because surface exposures of these featuresare masked by the irregular structure of the Cambrian lime-stone cover, drill-hole intersections provided the only indica-tion of the presence of these faults initially. However, becausemost of the drill holes are vertical, only limited informationabout the structures is available from drilling.

Some thought was put into methods that may assist withthe prediction of the presence and geometry of such faults be-tween drill holes. Seismic and electromagnetic imaging werepostulated and are discussed below. Because of lack of confi-dence and uncertainty of the availability of these techniques atthe time, an alternate method was sought that used individualboreholes and made predictions from the information in theborehole.

Measurement of the core orientation and the dip of thestrata in the core is the traditional approach to interpretingstructure between drill holes in stratiform deposits. However,the problem at Century was that because most of the holeswere vertical, the reliability of the available core-orientationdata was questionable. To address this issue, tests using a com-bined dipmeter/hole deviation tool developed by BPB Instru-ments were carried out at Century. The results demonstratedthat good-quality oriented dip information could be obtainedon the more laminated units (shales and siltstones), includ-ing the ore sequence. The amount and quality of the data ob-tained enabled the estimationof structuraloffsets between drillholes.

Blast-hole logging.Therecognition that lithologiccontactscould be delineated readily from borehole geophysical logs ledto the possibility that the blast holes planned for the benchmining of the deposit could be logged geophysically to pro-

vide more accurate information about ore-waste contacts thanconventional geologic logging from blast cuttings. Such infor-mation could provide a substantial benefit to the mining eco-nomics by optimizing the blast design and the mining of oreand waste. Most of the benefit would come in reduced dilutionof ore for grade-control purposes.

Figure 11 shows the result of logging a test blast hole withina small pit excavated to obtain a bulk sample of the ore (Fig-

FIG. 10. Approximate relationships between sonic velocity,rock strength (UCS), and porosity, Century deposit (afterDuplancic, 1995).

ure 1). Only natural gamma and magnetic susceptibility logswere obtained for this test. The prediction of the ore unit con-tacts was based on a comparison with curve shapes, using atemplate overlay, from nearby reference core holes. The mainore units, as mapped from the pit face and extrapolated to theblast hole,are shown also.Thecomparison with theinterpretedlithologies suggests that the ore contacts can be predicted toa vertical precision of about 10 cm. This result was confirmedfromthesubsequentgeophysicalloggingofabout50blastholesdrilled in the trial pit.

The possibility of measuring ore grade in blast holes hasbeen considered also and some testing has been conducted.The key to grade estimation is the measurement of density, butthe use of a highly radioactive source in an active mining areawas discouraged strongly. A test program was conducted bythe Commonwealth Scientific and Industrial Research Organ-isation Australia (CSIRO) to try a low-activity spectrometricprobe developed by the CSIRO (Charbucinski et al., 1997)to measure zinc grade directly in blast holes. Such a probewould be an alternative to laboratory chemical analysis forgrade control. Information from such a probe would be avail-able in a much shorter time frame than chemical analyses, andpotentially should provide more accurate information on ver-tical grade variations than would be possible from analysis ofsamples taken from drill cuttings.

Initial results of such tests were promising. The tests suggestthat such a tool could achieve better than 2.5% Zn deter-mination, with substantial opportunity for improvement (to acalculated limit of about 1.5% Zn). Ore boundaries can belocated to a vertical resolution of 10 cm. Although the preci-sion of this grade prediction is not as good as with a chemicalassay, it is unlikely that the result obtained from a bulk assayof drill cuttings will reflect the actual grade of the material toany better accuracy than indicated by the probe. Further workon the assessment and development of this technology is rec-ommended.

Borehole surveysinterborehole imaging

Because the Century deposit is faulted locally, the delin-eation of faults within the deposit and the impact these wouldhave on mine design and extraction of the ore have been subject to much investigation. To determine possible options toaddress this problem, some analysis of the potential applica-tion of seismic tomography, radio frequency electromagnetic(RFEM) imaging, and borehole radar was undertaken to de-termine if any of these methods can define the structure of theore zone between drill holes at Century.

Seismic tomography.Although surface seismic has littleapplication at Century because of the rugged limestone to-pography and weathering (Thomas et al., 1992), analysis ofsome test data suggested that reflectors are associated withlithologies withinthe ore zone.It was postulated therefore thatborehole seismic tomography could map these units and pro-vide a basis for structural interpretation between drill holes.However, seismic tomography was not undertaken at Century,mainly because of the limited support to service such work inAustralia.



12/15


Radio imaging.As an alternative to seismic, RFEM imag-ing was assessed by CRA as having potential application atseveral of itsoperations. These include delineation of thestruc-ture, including offsets caused by faulting, of coal seams or oreunits between drill holes or mine-development access ways,detection and location of geologic or operational hazard zonesbetween drill holes, detection of unknown ore not intersectedby drill holes, assessment of quality of coal or ore between drillholes, and detection and location of large voids (cavities, oldworkings, etc.) between drill holes.

Unlike seismic tomography, several potentially suitable EMtomography systems are available, such as the RIM system(Stolarczyk, 1992), the Russian FARA system, the SouthAfrican RT system, and the Chinese JW-4 system (Fullagaret al., 1996b). The RIM system has advantages in Australianconditions because of its lower frequency capability (down to

FIG. 11. Lithologic prediction from geophysical log in blast hole, based on curve matching from nearby referencehole, Century bulk sample pit.

12.5 kHz). It was also the only commercially available systemin Australia when the work at Century was being considered.

A trial of the RIM system therefore was undertaken atCentury in 1992 to determine its suitability in delineating thestructureof the ore zone between drill holes. A test wascarriedout in the vicinity of an exploration shaft which had been sunkinto the orebody to obtain a bulk sample of ore and to provideaccess for geotechnical investigations and mapping of structurefrom drives within the orebody (Figure 12a, b). This mappingprovided direct evidence of a previously unknown fault withthrow about 1015 m adjacent to holes in which an RIM surveywas carried out.

Initial processed results from this survey did not providemuch encouragement that the method could offer any infor-mation on the structure between holes to the resolution re-quired for mine development. As a result, no further surveys



13/15

1958 Mutton

FIG. 12. Comparison of mappedgeology in underground driveand radio-imaging method(RIM) survey results, Century explorationshaft area: (a)locationplan; (b)geologicsection through drive; (c) reprocessed RIMtomogram (frequency 520 kHz) from drill-holesection oblique to drive.



14/15


have been undertaken. However, alternative data-processingoptions were investigated subsequently, resulting in the im-age shown in Figure 12c. This result was obtained by usingexactly the same data as the initial processing, but in this casethe tomographic inversion used software developed by VIRG-Rudgeofizika for its FARA RFEM system. The main differ-ence with this software is that it uses both the amplitude andphaseinformation. Thereprocessed image clearly indicatesthepresence of the fault very close to its predicted position. It isinteresting to note that no specific a priorigeologicinformationavailable from the drill holes or the subsequent undergrounddevelopment was used to constrain the resulting image.

The use of RIM or equivalent radio-imaging systems there-fore could play an important part in evaluation of the structureof an orebody such as Century in advance of infill drilling andmine development. At Century, this has not happened so for,probably because of unfavorable impressions gained by themine-evaluation staff from the initial processing of the data,and the passage of time before better results were obtained.However, it is estimated that if the radio-imaging technologyand processing had been sufficiently developed, proved, andaccepted at the time of the Century discovery, the structuralevaluation of the deposit might have been achieved at a sig-nificantly lower cost ($5 million to $10 million less) and in ashorter time frame. The basis for this substantial cost benefit isin the reduction of the number and type of boreholes required(i.e., noncore), but the main benefit comes from increased con-fidence in the reserves and the subsequent mine design andmining plan.

Borehole radar.A trial borehole radar survey was carriedout at Century as part of the AMIRA P436 research project.The objectives of this work were to map faults within the Pro-terozoic sediments and determine their geometry; and to de-tect cavities, representing a future mining hazard, within the

limestone.The use of borehole radar to map faults was a possible al-

ternate to radio imaging, on the basis that the higher frequen-cies would give higher resolution of the structures and thatthe radar could be carried out in a single-hole reflection moderequiringless logistical effort. The results of this work were dis-appointing because the penetration distance of the radar signalusing a 60-MHz source frequency through the weakly conduc-tive sediments was very small (


15/15

1960 Mutton

the definition,evaluation, and mining of a mineral deposit. Thisknowledge can be gained only from physical measurementson core or geophysical logging of drill holes. Therefore, as aprerequisite to any mine-evaluation program, it is necessary toacquire a suite of information on a range of physical properties,andthen useeither previous experience or indicative modelingstudies to determine what geophysical methods may achievethe required goals of the resource definition.

CONCLUSIONS

Experience gained by CRA in the early 1990s during theevaluation of the Century deposit and at other Australianmining operations suggests that successful application of geo-physics in mine evaluation or during mining is achievable incertain circumstances. These circumstances are controlled bythe contrasts in the physical properties of the materials beingmined. To use geophysics successfully in mining, it is there-fore necessary to acquire information on a range of physicalproperties, and then use this information to determine whatgeophysical methods may assist the evaluation process or min-ing operation.

The overall economic benefit of successful implementationof geophysical technology into all phases of resource defini-tion and mining at Rio Tintos operations is estimated at tensof millions of dollars annually. This projected benefit aloneshould stimulate the need for investigating further technolog-ical improvements and evaluation of available technologies atexisting operations.

Despite this positive assessment, some barriers inhibitingthe routine testing and implementing of such technology ex-ist. These barriers relate largely to the appropriate integrationof the technology with the mining process, and acceptance bymine operators of the economic value of such work, comparedwith traditional approaches to resource definition. Some effortby mine management will be required to address these issues

and support further evaluation of the technology before theuse of geophysics will be accepted as a routine component ofresource definition and mining.

Further anticipated technological developments, cost pres-sureson production, and gradual acceptanceby mineoperatorseventually will ensure the use of geophysics in mining. The re-ality may be that those mining operations that evaluate andsuccessfully exploit such developments will be the most effi-cient and economically viable mines of the future.

ACKNOWLEDGMENTS

The assistance of the staff of CRA Exploration and CenturyZinc Limited with the work undertaken at Century from 1990

to 1996 is acknowledged gratefully. In particular, I acknowl-edge the efforts of Barry Bourne and Theo Aravanis in ini-tiating some of the innovative work presented here, and theencouragement of John Main, Andrew Waltho, and the CZLstaff to undertake such work. I also thank the AMIRA P436team for its input. This paper is published with the permissionof Rio Tinto Limited and Century Zinc Limited.

This paper waspublished 1997 in Proceedings of Exploration97: Fourth Decennial International Conference on Mineral Ex-

ploration, edited by A. G. Gubins. Some updates have beenmade to this version at the request of the Special Editor forpublication in GEOPHYSICS.

REFERENCES

Asten, M. W., 1983, Borehole log analysis using an interactive com-puter: Bull. Aust. Soc. Expl. Geophys., 14, 310.

Broadbent, G. C., 1996, The Century discoveryIs exploration evercomplete?: in Mauk, J. L., and St. George, J. D., Eds., ProceedingsPacrim 95 Congress: Aust. Inst. Min. Metall. Publ. 9/95, 8186.

Charbucinski, J., Borsaru, M., and Gladwin, M., 1997, Ultra-low ra-diation intensity spectrometric probe for ore body delineation andgrade control of Pb-Zn ore: Gubins, A. G., Ed., Proceedings of Ex-ploration 97:Fourth Decennial InternationalConferenceon MineralExploration, 631638.

Davies, A. L., and McManus, D. A., 1990, Geotechnical applications ofdownhole sonic and neutron logging for surface coal mining: Expl.Geophys., 21, 7382.

Duplancic, P., 1995, A comparison of mechanical properties of rockwith in situ velocity measurements at the Century deposit: B.E. the-sis, Univ. Queensland.

Fullagar, P. K., and Fallon, G. N., 1997, Geophysics in metallifer-ous mines for orebody delineation and rock mass characterization,Gubins,A. G., Ed.,Proceedingsof Exploration97: FourthDecennialInternational Conference on Mineral Exploration, 573584.

Fullagar, P. K., Fallon, G. N., Hatherly, P. J., and Emerson, D. W., 1996a,Implementation of geophysics at metalliferous minesFinal reportAMIRA Project P436: Cooperative Research Centre for MiningTech. and Equipment Report MM1-96/11.

Fullagar, P. K., Zhang, P., Wu, Y., and Bertrand, M-J., 1996b, Applica-tion of radio frequency tomography to delineation of nickel sulfidedeposits in the Sudbury Basin: 66th Ann. Internat. Mtg., Soc. Expl.Geophys., Expanded Abstracts, 20652068.

Mayers, P. J., and Bourne,B. T., 1994, Geological and geophysical study

of the Lawn Hill limestone annulus: Century Zinc Ltd. Internal Re-port 94/002.McNally, G. H., 1990, The prediction of geotechnical rock properties

from sonic and neutron logs: Expl. Geophys., 21, 6572.Oldenburg, D. W., Li,Y.,and Farquharson,C. G., 1997, Geophysical in-

version:Fundamentals andapplications inmineralexploration prob-lems: in Gubins, A. G., Ed., Proceedings of Exploration 97: FourthDecennial International Conference on Mineral Exploration, 545548.

Stolarczyk, L. G., 1992, Definition imaging of an orebody with theRadio Imaging Method (RIM): IEEE Trans. on Industry Applica-tions 28, 11411147.

Thomas, G., Stolz, E. M., and Mutton, A. J., 1992, Geophysics ofthe Century zinc-lead-silver deposit, northwest Queensland: Expl.Geophys., 23, 361366.

Waltho, A. E., Allnutt, S. L., and Radojkovic, A. M., 1993, Geologyof the Century zinc deposit, northwest Queensland: Proc. Internat.SymposiumWorld Zinc 93, 111130.

application of elctric method

Documents