borehole logging techniques applied to … w.e. and nelson, p.ll., borehole logging techniques...

12. BORLHOlE lOGGING TECHNIQUES APPLIED TO BASE MLTAL ORE DEPOSITS

W.E. GlennUURI-I arth Science laboratory, Salt lake City, Utah

P.H. Nelsonlawrence Berkeley laboratory, Berkeley, Cali fornia

Glenn, W.E. and Nelson, P.ll., Borehole logging techniques applied to base metal ore deposits; !!3:Geophysics and Geochemistry in the Search for Metallic Ores; Peter J. Hood, editor, GeologicalSurvey of Canada, Economic Geology Report 31, p. 273-294, 1979.

Abstract

Interest in the application of borehole logging to metallic mineral exploration and depositevaluation has grown substantially in recent years. However, borehole logging tools andtechniques were developed primarily by the petroleum industry and neither the tools nor theirapplication are directly suited to the needs of the metallic mineral industry. Lack of generallyavailable commercial logging services for small diameter holes and for the measurement ofquantities Kuch as magnetic susceptibility, induced polarization, and ore grade has inhibiLed thegrowth of borehole logging in metallic mineral mining.

The need to study new mining technology and to search at greater depths for new oredeposits has created a demand for a slim-hole logging capability. The requirements range fromelemental and mineralogical analyses, through properties used in geophysical exploration to bulkrock properties for either conventional or novel mining techniques. This paper reviews thepresently available capabilities for downhole analyses for geological information, for geophysicalproperties and for fluid flow. Much of the review is based upon direct experience with a facilityoperated in-house by a major metals mining company.

Resume

n y a eu au cours des dernU~res annees un interet croissant pour l'application de ladiagraphie par trou de sonde a l'exploration des mineraux metalliques et a l'evaluation desgiKements. Toutefois, les outils et les techniques de diagraphie par trou de sonde ont ete mis aupoint principalement par l'industrie petroliere; ni les outils ni leur utilisation n'etaientdirectement adaptes aux besoins de l'industrie des mineraux metalliques. Le manque de servicescommerciaux de diagraphie accessibles en ce qui concerne les trous de petit diametre et lamesure de quantizes comme la susceptibilite magnetique, la polarisation provoquee et la teneuren minerai a empeche Ie developpement de la diagraphie par trou de sonde dans Ie secteur desmineraux metalliques.

Le besoin d'etudier de nouvelles techniques d'exploitation et de creuser i:z des profondeursplus grandes afin de trouver de nouveaux gisements de minerai a fait nai1:re une demande pour ladiagraphie en diametre reduit. Les besoins vont des analyses elementaires et mineralogiquesjusqu'aux proprietes utili sees en exploration geophysique et aux proprietes de la roche prise dansson ensemble, en ce qui a trait aux techniques d'exploitation classiques ou nouvelles. I.e presentdocument etudie les methodes actuelles d'analyse, au fond du trou, de l'information geologique,des proprietes geophysiques et de l'ecoulement des fluides. La majeure partie de l'etude estbasee sur des experiences pratiques a l'aide d'une installation exploitee par une importantesociete d'exploitation de mineraux metalliques.

INTRODUCTION

Research on the Clpplication of well logs to porphyrycopper exploration and development began at KennecottCopper Corp. in 1971. logging research wa" justififld formany reasons, among which was the need to developadditional techniques for 10cClting and evaluating deepermineral deposits. Due to a lack of a logging service orientedto metallic minflral applications both in terms of tools anddata analyses, Kennecott decided to develop its own loggingsystem in addition to carrying out research on well log

. applications. This approach allowed a great deal of flexibilityfor sLudying what tool specificntions were needed, wh"t fieldtechniques were best and what kinds of daLa Wflre important.

In general, the use of logging in the base mineralsindustry lags behind th"t of almost all other sectors of theexploration and qeotechnical disciplines. Vie will not nttemptto diagnose reasons for this deficiency, but it means Lhat ifthe mining industry finds the incentives to apply boreholemeasurements on a widespread basis tomorrow, it facesreduced start-up costs in terms of tool development. This isnot to S<lY that the development and deploymenL still required

will be inexpensive, buL with the exception of techniques formineralogiral and elemental analysis, many useful tools havealready been developed and are avail<lble to base metalexplorationists on an "off-the-shelf" basis. Moreover thedevelopment of small diameter tools has been accelerating ata rapid pace in th" last few years. The stimulaLion comes notfrom the base meLal mining indusLry but from oLher sectors,including,

- production logging for oil and gas,- uranium and coal exploration and exploitation,- geoLhermal exploration and exploit<'ltion,- geological flngineering sLudies, such as power plant

si tinfj, tunnelling, etc.

We hope we have made it clear that the true "state-ofthe-art" in mineral logfjing comes not so much from themilleral irldustry as from the whole spectrum of users ofdownhole measurements. Since a true overview of all sourcesand users of slimhole borehole technology is not possiblfl, andwould quickly become out-dated, we will simply list somerecent and on-going dflvelopments in logging technologyapplicable La Lhe mining industry and devote the remainder of

274 W.E. Glenn and P.H. Nelson

temperature logginglogs (for example,type using thermal

the paper to our own experience in applying differenttechniques to the exploration and engineering problemsencountered within the base metal mining industry.

Recent Developments

1. The development of a borehole assaying tool for nickel,with very encouraging indications of its applicability tocopper (Seigel and Nargolwalla, 1975).

2. Recent improvements in the stability of a magneticsusceptibility logging instrument: less.than 10 pegs maximum drift in an operating environmental temperature of70 oC, by the U.S. Geological Survey 0. Scott, pcrs.comm.).

3. The availability of "high resolution" focussed density toolsin slim-hole versions, applicable to the logging of thinseams, veins, and fractures (Fishel, 1976).

4. The continued efforts at quantifying the electricalresistivity and induced polarization response in terms ofthe sulphide and clay content of the rock (Clavier, et aI.,1976; Snyder et aI., 1977; Patchett, 1975) and the currenton-going development of several commercial andindustrial groups of tools for the measurements ofpolarization and dielectric properties in boreholes.

5. Continuing refinement of precisionimd the analysis of continuousConaway, 1977) to correlate rockconductivity.

6. A calibration facility oriented to base mineralenvironments under development at the U.S. Bureau ofMines.

The above listing is almost a random sampling of recentdevelopments which might easily be adapted for specificpurposes in base mineral exploration and engineering.Another approach is to consider physical and chemicalproperties which might be measured in situ and applicationsfor which the data will be most useful. Table 12.1 represents

one such compilation; it intentionally includes engineeringapplications as well as exploration. This extensionemphasizes our belief that downhole logginq will eventuallyhave as much importance for the mining engineer as for theexplorationist.

Application of Techniques

In the remainder of this paper we describe an in-housedownhole logging facility used for a variety of research anddevelopmental purpuses and describe some specificapplications and results. Our discussion is keyed to the tenproperties listed in Table 12.1; the underlined entries indicatethe categories for which examples are given in this paper.

It should be stated that in order to limit the paper wehave excluded exploration techniques which combine surfaceand borehole methods (see Hohmann et cd., 1977, for electromagnetic prospecting examples) and cross-hole techniques forinvestigating large volumes of rock between boreholes (seeLytle et al., 1976, for electromagnetic probing example;R.L. Aamodt, 1976, for examples using acoustic energy; andDaniels, 1977, for IP examples). That is, we confine ourpresentation to probes in a single borehole with a radius ofinvestigation determined by the rock/fluid properties and theprobe geometry.

Keys and MacCary (1971) reviewed the application ofborehole geophysics to water resources investigations andmuch of the material they covered is appropriate to metallicmineral mining applications. From the geotechnical vantagepoint Van Schalkwyk (1976) reviewed a wide range ofequipment which is useful for rock mechanics and engineeringstudies. Other authors have reviewed the use of boreholeinstrumentation for various slim-hole applications. Forexample, Dyck et al. (1975) gave an excellent overview ofborehole geophysics applied to metallic rnineral prospecting.Each of these three review articles is interesting in terms ofits scope and perspective and each contains a useful list ofreferences.

Table 12.1

Physical and chemical in-situ properties obtained byborehole logging with regard to mining applications

I PorphyryCopper Dump Mine

Exploration Characterization Leaching Development

I

Elemental andmineralogical analysis X X X X

Rock type identificationand correlation X X X X

Rock quali ty andfractures X X X

Density X X

Porosity X X

Electrical andmagnetic properties X X

Temperature andthermal properties X X X

Fuid flow X X X

vVater sampling X X X X--_.._--

Underlined entries indicate the categories for which examples are given in this paper.

Borehole Logging 275

KENNECOTT COPPER CORPORAnON BOREHOLELOGGING SYSTEM

Kennecott Copper Corp. currently (1977) has threelogging trucks, of which only one is designed to measure abroad range of rock properties. The other two are devoted tohydrologic and environmental logging. A brief description ofthe Kennecott Copper Corp. logging truck (Fig. 12.1 and 12.2)is given here. It is a four-wheel drive Ford F600 truck chassiswith a 14 feet (4.3 m) long, 6 feet (1.8 m) wide, 6 feet (1.8 m)high custom-built van on the bed. The van portion is dividedinto a forward operator/recorder cubicle and a rearmechanical/storage area. A boom is located over the rearcentre of the vehicle. The truck is equipped with 1830 m offour-conductor 4.8 mm, steel armour cable with a GearhartOwen four-conductor cable head and an extensive set ofelectronic and mechanical equipment. The electrical powerunit is a 6.5 kW gasoline generator. The truck typicallycarries fourteen different logging tools. The various toolsand their primary function(s) are shown in Table 12.2. Alltools are 2.25 inches (5.7 cm) or less in diameter.

Many of the tools require only one conductor and thearmour for operation, but others such as the IP tool requireall four plus the armour. Except for the resistivity andsingle-point resistance tools, all tools accomplish someelectronic signal processing and enhancement downholebefore the signal is transmitted to the surface. Upholemodules are mounted in removable nuclear instrumentationmodules and do various amounts of signal processing andoutput the analog signals to one or more channels of a fourchannel Texas Instruments chart recorder. A plug-in patchpanel (Fig. 12.2) and programmed patch boards (Fig. 12.3)allow the operator to connect tool functions, modulefunctions and auxiliary electronic equipment such as anoscilloscope in any fashion desired or to operate the system insome predetermined fashion by simply plugging in a board.The flexibility of the patch panel is essential for a researchand development logging system in that the electronicequipment can be quickly interconnected in a modified or newconfiguration and a new tool can quickly become operational.A block diagram of the system just described is given inFigure 12.4.

ELEMENTAL AND MINERALOGICALANALYSES IN BOREHOLES

One of the most exciting and promising frontiers inmineral logging is the development of tools to analyze forspecific elements in the borehole. In the specific case thelogging would be a borehole assay for particular ore minerals.In the general case the type of logging would provide an insi tu geochemical description of the mineral deposi t.

Czubeck (1979) describes the current activity inborehole assaying, most of which is based upon radioactiveand fluorescent techniques. A neutron activation method forthe borehole assay of copper and nickel has been described byNargolwalla et al. (1974) and Seigel and Nargolwalla (1975).Their system, called Metalog, available as a routine service,was tested in Anaconda's porphyry copper mine nearYerington, Nevada (Staff, Scintrex Limited, 1976). The testresults are very envouraging and are presented in Figure 12.5.Because of limited experience with this technology we willnot attempt to describe progress in the field of nuclearanalysis, but will discuss a few examples obtained with moreconventional methods. It is likely that the results from theseother methods will supplement and constrain the nuclearanalytical methods when they become available.

The most readily available analytical technique inboreholes is the measurement of the natural gammaradiation. In the mining environment the total count canoften be directly correlated with the potassium content fromthe laboratory analysis of core material as shown inFigure 12.6. In fact, based upon this kind of directcorrelation, our logs are routinely scaled in K 2 0 content inmost environments. Where the uranium or thorium content issignificant this straightforward procedure does not apply, andspectral information is required to clistinquish the threeelements. Indeed, spectral logging is applied to uraniumexploration and is discussed elsewhere in these proceedings(Killeen, 1979).

~~....~-----

Figure 12.1. Kennecott Copper Company's logging truck. Figure 12.2. Interior view of operator sectionof Kennecott Copper Company's logging truck.


CHARTr-;:;;~::;-I ~RECORDER

r-----..,'--";:--~---I I

ICOMPUTER Il- .J

PROBEFigure 12.3. Program patch board,

Figure 12.4. Block diagram of logging system

86

-c

~ Analytic Data--rComparison of analytic and well log analysis

750

1000

oOt-----'---~cr_-'-------L..----'

250

1000

;;2E

,~.cii .cQ) 500 C.'0 Q)

'0

2000

3000

Examples of the dependence of the neutron log on bothpore water and water bound in hydrous minerals have beenpresented before (Savre and Burke, 1971; Snyder, 1973;Nelson and Glenn, 1975) and the example of Nelson and Glennis reproduced in Figure 12.7. The chemically bound water canbe so high that it can be directly measured by the neutrontool and hence mask the pore water contributions to theneutron log. We will show in the section on porosity thatwhen the two contributions are mixed, it is still possible toremove the bound water contribution. In any case the boundwater can be quantitatively assessed, giving a measure of thehydrated mineral content of the rock, usually translatable asthe altered, clay mineral fraction.

Figure 12.6.of [(20.

o 6o 5o 4

Borehole assaying.

% CU

020

Figure 12.5.

o 1'I

Core1- --II11

10 I Metalog

III

II

.~ L_-T.c

C. IQ) I'0

I

II

--lII__I

I20 ~

IIIT

II

III

40

30

70

80

o 020

50

60

.~


MAGNETIC SUSCEPTIBILITY <,.u c. g. s. )

Figure 12.8. Plot of magnetic susceptibility from logsversus volume per cent magnetite.

A third fairly direct measurement is the magnetitecontent estimated from the magnetic susceptibility tool.Figure 12.8, taken from Snyder (1973), demonstrates thecorrelation. Here again, as in the case of potassium, is one ofthe rare cases where a single element or mineral typepossesses a distinctive physical property which can be readilymeasured and is unique to that mineral type in many cases ofinterest.

Response of drillhole logging tools to rock properties

100000100001000

);/""" .. /," .

"'/to:"-/-"

/~. ". ...

. ..7(

//'

VOL. % MAGNETITE = K/3000

I

I

10

0,1100

.Jo>

*" 1.0

WI;:::WZCJ«::.

Table 12.2

LOGGING TOOLS

Temperature I 4

Caliper I 4 4

Neutron 2 2 3 4

Natural Gamma 2 4

Gamma-Gamma(density) 2 4 3 I 2--

Veloci ty (ll T) 2 3 2

Resistivity 3 4 4 3 2

IP 4 2

Magnetic Susceptibility I

Fluid Flow II Quantitative measure - high success2 Quantitative measure - moderate success3 Always responds qualitatively4 Often responds qualitatively

Figure 12.7. Comparison of the neutron log with boundwater analysis on pulps.

ROCK TYPE IDENTIFICAnON AND CORRELAnON

An obvious need in mining applications is a directmeasure of the sulphide content. Our attempts at thismeasurement are detailed in subsequent sections on densityand electrical properties.

Identification of rock types is facili tated where there isa direct correlation between an easily identified mineralspecies, such as magnetite, and the rock unit. A goodexample of this is evident at one of Kennecott's porphyrydeposits. The copper mineralization is in an andesite pilecrosscut by numerous latite dykes. The dykes and andesitesshow contrasting expressions on both the natural gamma(potassium) and magnetic susceptibility (magnetite) logs•Figure 12.9 illustrates this contrast. The latite between163 and 186 m shows a high natural gamma response and anear-zero magnetic susceptibili ty, whereas the andesiteexhibits a lower natural gamma response and magneticsusceptibili ty averaging around 3000 ].l cgs. Note that thedensity of the latite is also lower than the density of theandesite. A short interval between 151 and 159 m has a verysimilar response on all logs as does the latite except that themagnetic susceptibili ty is not zero. We interpret this intervalto be a near miss of the same dyke intersected at 163 m. Theresult suggests that the drillhole is parallel to a dyke overthis interval.

We have noted a striking correlation between resistivityvariation and sulphide zoning at one of Kennecott's porphyrycopper deposits. The highly variable, low to high, resistivitypattern evident in the upper part of the long and short normallog* for each hole shown on a cross-section in Figure 12.10 iscoincident with the pyrite halo in this deposit. The lowercut-off of this variable resistivity is closely correlated with aparticular copper grade. Some detailed features of theresistivity logs can be correlated among the drillholes. Thesecorrelated features have been associated with both avariation in sulphide veining and the degree of sulphide

.S

.cC.

500 '""C

400

600

41000

.......- weight %

chemically bound water

21500

1

neutron

wt ,counts/secJ.-__.L-__-L__---l i.-_----,

o2500

1800

1400

1200

+'Q)Q)...~ 1600.....c+'0.Q)

'0

* Industry standard: 40 and 16 inches (1.22 and 0.488 m) respectively.


H-5H·4H- 3H-2

'"'"<:><:>

~ hr---+--+-f--+-"-----l-----f-.l.,-+---I:2- -j---h~---I

T

GEOLOGYH -,

.O' Andesite'.::~:~~'..,

....;1 Andesi te breccia~r"" Andesite'..,.,'.~-:.':~.~ .. Latite porphyry

~~ Andesite

;'~-,:'~~~:..,./

Fault zone~: Andesite

;It Andesite breccia.'~,~."If"

MAGNETICSUSCEPTIBILITY NATURAL GAMMA

\.lc.g.s. % K20o 5000 5 10

DENSITYgm/cc

3.10 2.60 2.30

200

100

400

~

~;;

600Ec 200

-'= "·c. ·'" '"" .~ BOO

""'"·c300 1000

1200

Figure 12.9. Latite dyke correlation on natural gamma andmagnetic susceptibility logs.

Figure 12.11. Natural gamma logs.

o

At jlsec/teeto ,.--,,-_8-l.0_..------L._......-"4~O'--_

50 100RQD

1500

500

1....<Il<Il Ul- ~

;;.S E

~0

~ ~--------- '".J:::;; .... L _____

1E C. ----<Il-0

1000 ~ .6t

~=-

Long and short normal resistivity logs.

--

Figure 12.10.

RDH-1.I\-m

2

o

1

1

Figure 12.12. Rock quality designation(RQ D) from core versus compressionalwave transit time measured in a drillhole.


DENSITY

grn/cc

3.12.5 23

c

100

300

290

At (TRANSIT TIME) LOG

22

NEUTRON

total water (vol. %)

20 10 2

20

CALIPER LOGHole Oiomeler lin.) psee/f!.

8 920 50

em

VELOCITY

jJsec/ft50 60 70

CALIPERhole dia.

inches4

10 14

1000

'c

TEMPERATURE

OF

75 95 1153

24 35

NS961' 66° E

,N75W

Q; 966 66

~

J:

~Ol

"~1"0

N45982' 37 0 N

I

" ~992' 420 :

BOREHOLE SEISVIEWER LOG

A BIMagnetic North, Strike direction relative to MN.

Figure 12.14. Fracture location and orientation obtainedfrom an acoustic seisviewer log.

Figure 12.13. Suite of logs showing rock fracture signatureon several well logs.

200

1200

400

~. ·:i ·c'00 ·~ E

:i 200 c

'"~ r.

'"~800

1000 300

oxidation. Also, at the same deposit, the natural gamma logshows a substantial increase in potassium as the causativeintrusive is approached. This feature is depicted inFigure 12.11 where logs from five different drillholes areshown. An interval of high potassium can be correlated on alllogs as indicated by the dashed lines in Figure 12.11. Theincreased potassium is seen in the core as an increase insericite and orthoclase mineralization.

We have numerous examples of rock-type identificationand correlation that we have accomplished at several ofKennecott's porphyry copper deposits and prospects.However, we feel the ones given here are sufficient toillustrate the ability to use well logs both to differentiaterock types and to correlate rock units across a deposit. Thisinformation is useful for structural geology studies and forthe understanding of the porphyry system geometry.

ROCK QUALITY AND FRACTURE LaCAnON

With logging techniques it is possible to determine themechanical properties of rocks and to study the structuralgeology. In this section logging relevant to both topics willbe described.

Rock Quali ty Designation (RQD) as used by minegeologists is the measure obtained by summing the length ofcore pieces greater than 10 cm and dividing the total lengthof sample, typically 1.5 m to 3 m of core. If all the pieces ofcore in a 1.5 m section, for example, were greater than10 cm, the RQD would be 100%, if all pieces were smallerthan 10 cm the RQD would be 0%. RQD is an empiricalmeasure of rock competence. We have found that thevelocity (reciprocal of the compressional wave transit time)log gives a fairly good measure of RQD where the rock ismoderately to well fractured and one example is shown inFigure 12.12. In this case, the velocity log responds primarilyto the cracks or fractures in the rock.

Fractures or fractured intervals of rock commonly givea characteristic response on several logs: caliper, velocity,neutron and density. An example is shown in Figure 12.13.For illustration note the response of each log at 308 m wherethe geologist has noted a fault zone in the core. The calipershows an increased hole diameter which indicates thatmaterial has fallen from the fracture or was plucked out bythe bit during drilling. The velocity log shows a low velocity,the neutron shows a high porosi ty and the densi ty log shows alow density at this depth. Several fractures have been pickedfrom these logs and are indicated by an f beside the caliperlog at the appropriate places.

The preceding analysis gives a location only forfractures which appear open in the hole and does not providestrike or dip information.

We have used the seisviewer log offered as a service byBirdwell to determine fracture orientation in drillholes. Thistool was first described by Zamanek et al. (1970). Anexample of this log along with the caliper and velocity logs isshown in Figure 12.14. The interpreted fractures and theirorientation are noted beside the seisviewer log. Theseisviewer tool sends acoustic pulses toward the boreholewall. The tool rotates and sends a signal to the surface whenit passes through magnetic north. The acoustic energyreflected from the borehole wall is monitored and sent to thesurface where it is presented on an oscilloscope andphotographed. The oscilloscope triggers on the north signal.A smooth borehole wall will generate a strong reflection, thebright areas on the log, and a rough, caved borehole wall willgenerate a weak reflection, the dark areas on the log. Hencefractures are indicated by dark traces on the log. However,if the tool is not centred in the hole or the hole deviates verymuch from <l rvlindrical cross-section, the log will exhibit a


Or, equivalently, the following perturbations are required toproduce an error in 5' of 1.0 weight per cent:

(0-4)

L',pb=O.O] L',Pg=O.Ol L', ps=O.Ol L', <[l= 1%

+0.75 -0.81 -0.02 +1.30

Perturbations:

[ITOI' in 5' (wt. %)

Solving for 5' we get

Pb

-p + 0.01<1> (p - 1)5' = 100 9 g

Pb(l - Pg/ps)

Equation (0-4) is used to compute the sulphide logs and ispresented Cjraphically in Figure 12.16. Figure 12.16 allowsquantitative inspection of the dependence of 5' on <1>, Pb andp. In addition, a sensitivity analysis for 5' on the fourv~riables P , P , P and <I> was carri~d out by evaluatingthe derivatPves %Jitt\' respect to each of the four variables.Selecting base values of Pb = 2.70, Pg = 2.65, PoS = 5.0 a~d

<[l =4% (equivalent to 5' = 9.1 wt. %), the followmg errors mthe sulphide estimate result:

vertical zebra pattern as seen in Figure 12.14. Often thispattern will obscure the fractures. The tool is alsocontinuously pulled vertically and the resultant log is a planarrepresentation of a cylindrical borehole with north at the leftand right of the record and depth along the side. A planeintersecting the drillhole will apear as a sinusoid on the flatsurface. The dip magnitude and direction are obtained fromthe excursion and location of the maximum and the minimumof the sinusoid. The data from the seisviewer log can bestudied in a conventional way as illustrated in Figure 12.15.This figure shows a contoured equal-area plot of poles to thefractures and I~ose diagrams of strike and dip directions. Aplot (not shown) was also made for fractures measured on thesurface around the drill site and the drlta agree extremelywell with one exception; understandably, the surface studyshowed very few near-horizontal fractures.

We believe logging for structure crln benefit both theexploration and the mine geologist. Both could usc theinformation to develop a better understanding of the threedimensional structure of a mineral deposit.

Expressing the sulphides as weight per cent (5') and theporosity as a volume per cent gives

where P , Pf' Pg are the sulphide, fluid and nonsulphidegrain derYsities, and 5, <I> are the sulphide content and poresp ace as v olume fractions.

Rewriting and setting the fluid density Pf

= 1 gives

Sulphide Estimates from Den.sity Logs

Because the sulphide minerals are almost twice as denseas the silicates, it should be possible to estimate the sulphidecontent using the densi ty log, provided that the silicate graindensity is constant and the porosity can be determined fromanother log.

The bulk density of a rock containing three components- sulphides, water-filled pore space, and nonsulphide matrix is given by

DENSITY LOGGING

Routine Den.sity Estimates

We routinely employ a single-detector, gamma-gammadensity probe in NX-size boreholes. The maximum tooldiameter is 5.5 cm. In operation the entire tool is heldagainst the borehole wall by ;] decentralizing arm whichextends from the tool. Because only a single detector is used,the measurement is uncompensated, that is, no correction ismade for borehole rugosity. In cored holes in hard rock thisdoes not present as great a problem as in soft rock becausezones of severe rugosity are relatively infrequent and notmuch data is lost, although recently it has become possible toapply compensation algorithms to slim-hole density logs(Scott, 1977). We have found that it is a relativelystraightforward proc~ss to acquire reliable density data in NXboreholes, using core to establish the calibration andreferencing the tools with calibration blocks during routinelogging operations. Merkel and Snyder (1977) discuss some ofthe problems of calibrating a slim-hole density tool.

(0-5)

(0-6)

L', <I> = 0.77%L',P b = 0.013 L',Pg = -0.012 L',Ps = -0.50

1Ps = (l + R) (4.2 + 5.02R)

Ps = 4.2c + 5.02p

and

where c and p denote the chalcopyrite and pyrite fractionsand c + p = l.

Calling pic = R gives

c=1!(1+R)

p = ] - c = RI (l + R)

Field Examples of Sulphide Estimationfrom Den.sity Logging

Figure 12.18 presents an empirical test of the methodof sulphide ~stimation. The data were obtained in asedimentary sequence penetrated by a quartz latite porphyry.The hole is unusually high in pyrite as shown by the heavyliquids analyses as well as by a visual estimate which checkedreasonably well with the laboratory analysis. Magnetiteoccurs only between 381 to 401 m. The density data werereduced using reasonable values in equation 0-4, butunfortunately the new density tool utilized had not beenproperly calibrated at the time the log was obtained so no

Figure 12.17 is a graph of this function.

Finally, the porosity variations rlre most easily assessedby using the neutron log to compute a neutron porosity foruse in the expression which defines the sulphide estimate.Since the neutron porosi ty incorporates bound water, theporosity eslimate will usually be somewhat high andconsequently the sulphide estimate will be high unless boundwater is taken into account.

Of the four unknowns, changes in the nonsulphide grrlindensi ty will probrlbly produce the largest errors because thereis at present no way to assess grain density in the borehole.The bulk density measurement itself also can be a problem ifthe count rate is insufficient.

The third variable, the sulphide grain density, is lesscritical because the sulphide estimate is less sensitive to itsfluctuations. The influence CRn be further reduced if anestimate of the pyrite-to-chalcopyrite ratio is available andno other sulphides <Ire present. Since the pyrite grain densityis 5.02 glcc and chalcopyrite about 4.2 glee, we have

(0-2)

(0-1)

(0-3)

Pb = Sp + <l>Pf + (1 - <1>- S)ps g

PbP

b= P + 0.01-5' (p - P ) - O,Ol<l>(p - 1)

g Ps s g g

Pb = P + S(p - P ) + <1>(1 - P )g s g g

a)Equal area net projection of polesto fractures observed on televiewerborehole log

IO,-----f.....


Contoured equal area net projectionof poles to fractures observed onteleviewer borehole log

10

Figure 12.15. 1':1:ample of structure analyses from seisviewer log.

scale could be attached to the resulting sulphide estimate.The density-derived sulphide estimate tracks the laboratoryvalues well, and it is reasonable to expect that the agreementcan be improved with a proper' calibration and judicious use ofequation fJ-4. At least for high sulphide contents (3 to 8weight per cent), useful estimates of sulphide content shouldresult and the combination of electrically-derived (see nextsection) and density-derived estimates should be useful.

Figure 12.19 is a second field example whichquali tatively illustrates both the sulphides and porosi tyeffects on the density log. The cali per log shows fractures inseveral intervals but the reader should examine just two, onebetween 348 to 360 m and the other between 245 to 280 m.rhe neutron log shows a high porosity for both intervals. Thedensity log shows a lower density for the upper interval whichreflects the increased porosity whereas only a part of thelower interval has a lower density. In fact, between 259 mand 265 m the density is relatively high despite the higherporosity indicated by the neutron log. The higher densityreflects the galena and sphalerite veining over this interval.A second interval of sulphide veining where no high porosityexists is evident in the density 10C] between 101 m and 131 m.We found that this mode of qualitative log inspection andinterpretation is enhanced greatly when several log types are

available. Given the present strlte-of-the-art, we recommendthat the suite of logging tools employed in a given hole be ascomplete as possible.

Other applications of the densi ty tool include 1) controldata for the interpretation of gravity surveys, 2) to establishthe bulk modulus in combination with acoustic velocity data,and 3) to obtain bulk tonnage estimates.

POROSITY LOGGING

Porosity is commonly estimated from one or more ofthe following four logs: neutron, grlmma-gamma, resistivityand velocity logs. The methods used are well documented inthe literature (e.g. Pirson, 1963; Pickett, 1960; Savre andBurke, 1971). However, each log responds to several rockcharacteristics including porosity. There is an extensiveliterature describing the response of these logs in varioussedimentary rock sequences but little information exists onthe response of these tools in igneous or metamorphic rocks(e.f. Ritch, 1975; Nelson and Glenn, 1975; Merkel and Snyder,1977; Snyder, 1973). One problem is the calibration of tools.No stand8rd facility such as that available for sedimentaryrock environments with the API pits at the University ofHouston is available for the igneous and metamorphic rock


124 8pyrile/cho!copyrile rOfio R

Figure 12.17. Sulphide grain densitydependence on pyrite/chalcopyrite ratio R.

4.8

4.0 -/---,-,---,-,----.-,----,--r----,--r--,--+o-260 ]'

2~9 -265 !--270 ~

31

0-215

-2.BO -0

-2.85 ~

8ulphlde grain denali)'1";,- 4,8 ,.vee

3~O,

1.8 ;

,'8,

19,

,18

18, ,19

Bulk densily fa Igm/ee)2 ~8

,17

i1.5

.'":8 JOf--~'---7L--+/-~'-- ,,L-+--7"---r'-- ..__,L.j--;':;;

Figure 12.16. Sulphide content versus bulk density, with porosity and graindensity varying parameters. Graph is accurate for p = 2.70, most accuratefor other p at <!J =8%. Worst S' error is 1.3'!1:> for 6: =2.85, <!J = 16%.

g 9

environment with its typically very low porosity. The APIcalibration pits are still used to calibrate tools for the miningindustry but it becomes necessary to further calibrClte thetools with core analyses. We will discuss tool calibration inmore detail in a later section. A second problem is that theresponse of certuin logging tools is significantly influenced byrock properties usually ignored in nonmineral applications.

To be specific, we have found for the low porosity ofigneous rocks, typically less than 8 per cent, the water inhydrated minerClls accounts for a substantial part andsometimes the total response of the neutron tool (refer toFig. 12.7). The gamma-gamma densi ty and resistivity logdetermination of porosity in mineralized rock is significantlyaffected by the variation in base mineral concentration(Fig. 12.17, 12.18 and 12.19). A porosi ty determined fromeither log would be in substantial error. The velocity log isstrongly affected by fractures and in low porosity igneousrocks the fractures are as important as "pore" porosi ty onvelocity response (Fig. 12.12). Each of these effects on thefour logs has been discussed in previous sections andrefcrence has been made to the appropriate figures in thosesections.

by another effect that it will be useless. If a porosityestimate is desired, the recommended approach is to utilizeall four of the bnsic tools.

ELECTRICAL PROPERTV LOGGING

Induced polarization (IP) measurements ilre the mostwidely applied geophysical exploration technique used in thesearch for disseminated sulphide mineralization. For thisrenson alone it is useful to continue the measurement to thesubsurfacc as drilling progresses in order to check the testingof a surface anomaly. Other purposes include the use ofelectrical properties as a correlation tool (see previousexample in Fig. 12.10) and as a direct indicator of the totalsulphide content in the borehole. In spite of the obviousbenefits, IP logging has not yct been widely applied inexploration programs, partly because of the relativedifficulty in carrying out the measurement. Snyder et al.(1977) presented an overview of the IP method in boreholesand also address the difficulties of implementing a reliablesystem.

Figure 12.20 shows a porosity determination from thevelocity and neutron logs. The porosity estimate from thevelocity tool is based upon the time-avernge equntion (Wylieet aI., 1956). On the basis of substantial laboratory analysesof core for bound water at this mining property we haveconfidence in uniformly subtracting a 1.35 per cent boundwater contribution to the neutron log. The only significantdeviation of the two logs occurs over the top 30 m of holewhich suggests that the bound water contribution here is lessthan the 1.35 per cent used to correct the log. The orenspike in the porosity determination from the velocity log at72 m is due to a fractured interval of rock between 70 m and73 m.

Generally it is difficult to anticipate rock conditionswell enough to know which of the four tools will be bestsuited to estimating porosi ty and which will be so dominated

The Continuous Resistivity and Induced PolarizationLogging Tool of Kennecott Copper Corporation

Kennecott has developed a resistivity-IP logging toolwhich operates over 1500 m of armoured four-conductorcable and is compatible in every respect with the overallsystem outlined in figure 12.4. To overcome the problem oftransmitting phase information over long lengths of cable, thesignal and reference measurements are made in the downholesonde, frequency modulated in the sonde, transmitted throughthe cable, and then demodulated and detected by the surfaceelectronics equipment.

The system provides continuous amplitude nnd phaseinformation at a nominal logging speed of 9 m per minute.Although the system design does not constrilin the operatingfrequency, continuous sinusoidal trnnsmission and detectionat a 10 Hz frequency are used. The phase measurement is

Borehole Logging

WEIGHT " SULPHIDES BY HEAVY LIQUIDS

283

2 4 6 8 10 12 14

1000

...wWIL.

~ 1200x:...lLWo

1400

1600

r\ reduced den.lty log (no .c.le>

~r

I

r -==: I \sulphide. by heavy liquids

(/)wa:...w

C:I..GIl

,r-- -==

~L...,

~I

,I I I I I I

Figure 12.18. Comparison between sulphide estimates from reduced density log (unsealed due to lackof calibration) and total sulphide.

accurate to ±0.1 degree, or about ±2 milliradians, at temperatures up to 77°C. Resistivity is recorded on alogarithmic scale. A unique feature is the ability of theoperator to change the downhole amplifier gain from thesurface. This is necessary because the phase accuracy islimited to an amplitude range of 35 decibels. Operationallythis becomes a nuisance only if the resistivity varies by morethan two orders of magnitude within a short vertical distance.With the gain change capability, the dynamic range of thesystem is 100 decibels at a fixed transmitted current. Theabili ty to control the current level gives another 30 decibelsof flexibility. We have made continuous logs in various rocktypes with resistivities ranging from one ohm-metre togreater than 10 000 ohm-metre (40db). The system canaccommodate any electrode array which uses one current

electrode on the surface, one current electrode downhole, andtwo potential electrodes downhole. Most of Kennecott'smeasurements are made with a one-metre pole-pole (normal)array.

Correction Curves for Borehole IP Measurements

Electrical data from boreholes must be corrected toeliminate the contribution of the borehole fluid. Suchborehole correction charts (often called departure curves) fora variety of electrode arrays were computed years ago forresistivity logging (e.g. Schlumberger, 1955). The corresponding chart for induced polarization corrections, whichamounts to a derivative of the resistivity chart, was publishedby Brant et a1. (1966). We have recomputed the resistivity


1200

.J-_L- ....L_--"'----'==ce...:='--_.Ll.. --'

and IP departure curves for the pole-pole array in a boreholeof resistivity Rm surrounded by an infinite medium ofresistivity R

t(F ig. 12.21 and 12.22). The method of

computing the resistivity was taken from Gianzero and Rau(1977). The factor B2 on Figure 12.21 is

In practice, a fifth-order polynomial was fitted to thecurves and the logs were corrected to R t Flild <l>t on a digitalcomputer. Note that the correction to the measured phasecan be anywhere in the range of 0 to 30 per cent dependingupon the resistivity contrast between the rock anrl theborehole fluid.

This correction procerlure requires that the boreholefluid resistivity, R ,be known. We obtained water samplesfrom the boreholemwith a water sampler Gnd measured Rwith a fluid conductivity apparatus. m

Induced Polarization Logs

Figures 12.24 and 12.25 display IP and resistivity datafrom two holes containing sulphide mineralization in thesouthwestern Uni ted States. Both logs were obtained in8.3 em diameter, water-filled boreholes using the one-metrenormal electrode array. DOH-SF 1 penetrates andesite andandesite breccia. Sulphide mineralization is pyrite andchalcopyrite, with the pyrite to chalcopyrite ratio rangingfrom less than one to as high as ten. The borehole fluidresistivity was 14 ohm-metres. Sulphides occur predominantly along veins and also as disseminations.

In Figure 12.24, the erratic nature of the resistivity andphase logs is attributed to the veined character of thesulphirlcs. Estimates of the sulphides content is based uponlaboratory analysis (heavy liquid separation) on 3 mcomposites of the cored samples. These analyses are plotterlin the left-hand column on Figure 12.24 and also used toestablish the absciss8 in Figure 12.26. The data points inFigure 12.26 are 3 m averages of amplitude and phase overthe corresponding intervals. The [lhase values do not displayany particular trend as the sulphide content increases. As aresult the ratio ¢/R (also in Fig. 12.26) increases with thesulphide content, roughly following the trendline indicated bythe square root of 10 <lJ /R. This correspondence is emphasized by overlaying the computed log of~ onto thesulphide content in the left-hand column of Figure 12.24,establishing a rough but adequate estimator of the sulphidecontent based upon the electrical measurements.

The f,stimator ,I 10 <I> !R was first suggested byG.D. Van Voorhis following an exhaustive in-house study atKennecott Exploration, Inc. of data obtained frommeasurements on samples from several porphyry copperdeposits. The estimator normalizes the polarizability, cI>, withrespect to the resistivity which in turn can vary tremendouslydepending upon the pore structure of the rock and the natureof the mineralization. The idea is not new, having beenpreviously introduced 8S the so-called metal fnctor infrequency-domain IP terminology (see Madden and Cantwell,1967). What is suggested here, however is that the measureor some variation of it, may be quantitatively useful,providing that the statistical variatiorls can be establishedand that the exceptions to the rule can be understood. Thenext example is one such exception.

Figures 12.25 and 12.27 display the results of phase andampli tude measurements in 8 Ii thic tuff in drillhole SFZ. Thesulphides are mostly pyrite occurring as fine disseminationsand as blebs, with occasional veins. Magnetite is present onlyin trace amounts. The borehole fluid is water with aresistivity of 7.1 ohm-metres.

The electrical properties in DDH-SF2 are quitedifferent from those of DOH-SF 1. The resistivity is high,ranging between 550 and 11150 ohm-metres and as a resultthe~ estimator is much too low, greatly underestimating the actual sulphide content (see both Fig. 12.25and 12.27). However the ph8se in Figure 12.25 is plotted todemonstrate the qood correIa lion between phase and sulphide

100

NEUTRONvol. 'I; tote.:" water

1:) 20

DENSITYgm!cc

2.59 2.25 2.16

Correlation of density log with sulphide

CALIPERhole dia. (in.)

4 5 3.11

400

600

200

m0.

f100 Ec

~

Q."

1000300

Figure 12.19.veining.

Rt

,l[{

B2'a

~ aRt-

a

similarly,

R 3RBl

m a- R 3R

a m

Borehole Corrections for Resistivity Measurements

The IP logs can be corrected to give the true orformation value of resistivity if the curves of Figures 12.21and 12.22 are converted to functions Rt/R.mfor the pertinentratio of electrode spacing to hole diameter, AM/d. One suchconstruction is shown in Figure 12.23 for the case of a onemetre array in an NX borehole. NX boreholes range from7.6 to 8.3 cm in diameter; we used AM/d = 12.6 for this case.

Conceptually, Figure 12.23 would be used by entering onthc right-hand ordinate with the value of log (Ra/Rm), movinrj horizontally to the Ra/Rmcurve, then downwards to getthe value of log (Rt/R m ). From this point move vertically tointersect the B2 curve then horizontally to the left to obtainB2. The phase shi ft of the rock is then

where Ra

is the apparent resistivity.

A check for numerical RccurClCY found that Bl + 82summed te· unity to within three decimal places. Theresulting resistivity departure curves checked relatively wellwith the Sehlumberger curves. The IP departure curveschecked fairly well with the results of Brant et cil. (1966)except for small values of the ratio of electrode spacing tohole diameter, where we believe the curves of Brant ct a!. tobe in error.

~30.5 metres---toi

<o

-"'>1d cS'3>=

"lN-et>;s

"' ....::l~",.>=~N-Q..,.o::l

Q.."::l 0~..,

<~'" -.0':;'"~(\)'<",_N

0-''03'" Q'N-

~

~100 feet -.I

.j:l.

<Xl

"'0oJJo

~(J)-i-<

NenV1

15'..,en~

g,enro

I.CI.CS'

I.C

1000

.4 .-......I 1 1 .-/ A '\I-R ~Mud resistivity

m 1/ .... 1:>< AI \ 1\I-Rt~Formation resistivity l.- i'..1.--..,. V 1/ D< X /\ 1\ \I- AM =Po1e- 001 e seoara ti on /'

d ~Hole diameter L V .........hi' ~i'.. / "- 1/ V f\ \ f\2 .- ./ ......v~ / ~/ "/

1/1 IJi\. / '\. "- \v ~- f\.

I..--"'" V 1/ j<..r-..V )(.... /'J"'" "-1/ X. "- ~ '\ '\

~ 7' r-

".,-v/' 1/ V r--. k: Y-. t/<-- v .........k" ........ / "r-...l/ .............. ~ ~ """7r-- "" .......... I........

// /' V V -7 "- 7---2t:--L r---. --J. r-1-7"------ t:::--:t:--~ r-- -.:.-~

0l,.1f /' / / 1/ V V / / / / V,

~o' /' ./ / V // / // /

/

./ /' / V V V / / / /~\o

0/ ./ /' V .....,v / V / /---

IV '" .....

/ /' ......V V V /' / ,//' /

.-&0/ .//

./,/ '/ ,/ ./ ./' Vi/ i/

_,00"'" ___ ,.,./~ ~v

i'" .....i'" ~ ,....../I-'

-~!Soo-----i---- >- _f- -------S 2-eooo

10000

I'(Values 01 of

'm

AM •ilo....d j II""""

I

I.

o.

Q~ W 100

RATIO OF POLE SEPARATION (AM) TO HOLE DIAMETER (d)Figure 12.21. IP departure curves. (Beds of infinite thickness.) Centred pole-pole array.

I.

o.

82


000

30000

10000

100

10

10 100 eDrRATIO OF SPACING (AM) TO HOLE DIAMETER (d)

Figure 12.22. Resistivity departure curves. (Beds of infinite thickness.)

1 I .1 1..11111 I T

~ R =Apparent resistivityV~I- -.......a .

I-Rm~Mud resistivityV

i,,;'

"'"~ Rt

=Resistivity of formation

~V I-

~ '"V i'-d ~Drillhole diameter

'" r---~AM =Spacing (normal device) ./ V r--.... 1---1-

~ ,...... ........ "./ ./ ./ .....// 1./ ........... 1-1-"7 "7 .-

V V'/ 7 .............. ~

1/ ~ V / ...............I.--" -V / );' 1//V~

...............--/ /

~~ v""v~v ......1--

"'-."-........f- V ---/ to-.

~V V v"":t;; r---. r-... ~

/ / i'-.I-- r--f- V V

~~/. V l/ r-.......,tl'/ / V Vi; r...... r-...

IWOO/

/ ./ /o / / -.- '- ..........

~7/ 1/ /' r---. --~o / // / v ......

lL.-... o' / / V 1/ -- -- --0°..../ / V V vI.-- ...........~"J 0 '/ r-....."PffP / V/V V ......t---~

1/' , V I.-- I- r-..... '-

~,~~V V V v ..............r-.......

./ V v~ i'-...... r......r-~/ ./

~oo/ V V .....t-V r...... r-[/f!J _/ V I- --r----.ftoo 0/V VV I'-

/' fl,0./ l-t-..-./ ./ ./ I--'"' ......

~o" ./ ~

,fj / /' I---

V 0/ V L--- -..........17( /' ./ ..........I--,,0' /0' '/ --r---./0

f- ~o/~

10- t-

v' ./ --- >-.../~ -I-

k'.~/V,6

0-- r-

./

-"~V

~o-

~ • R, AMYolues 01

»t-III d .


Im no rma I In 3.12 ho Ie.

AMid 12.6

of IP parameters upon sulphide content divides into twoclasses governed by the geometrical distribution of sulphidesin rock:

0.6-+------r--------.------.-----+-0.00.0 1.0 2.0 3.0 4.0

log R, IR m

Figure 12.23. Borehole correction factors for the pole-polearray in a borehole penetrating a uniform medium.

A. Mineralization Predominantly Disseminated

Electrical conduction is controlled by the pore fluids sothe resistivity is independent of the sulphide content. Hencethe resistivity obeys Archie's Law after the fluid resistivity ismodified to account for surface conduction. The phase shiftis proportional to the sulphide surface area exposed to thepore fluid, usually manifested by a linear dependence of phaseupon sulphide content. Such a dependence of phase uponsulphide content is substantiated by our experience withlaboratory measurements on artificial samples and also byprediction from simple mathematical models.

B. Mineralization Predominantly Along Veinlets

In this case, the sulphide mineralization is distributedalong intersecting planar features and is sufficientlycontinuous that electrical conduction is controlled by thesulphide content. Empirically, the resistivity decreasesinversely with the square of the sulphide content. The phaseis relatively independent of sulphide content, because theabundance of sulphide-electrolyte interfaces does not changemuch as the sulphide content increases. Empirically, thefactor /10<1> /R or some modification of it provides anestimate of the sulphide content.

These two categories are rather general and areprobably not distinct; that is, there will be many instanceswhere the mineralization is equally disseminated andcontrolled by veining. In fact, using a prototype version ofthe equipment described herein, Snyder (1973) observed acube root dependence of the <1> /R factor upon sulphidecontent, but suggested that the dependence will be a functionof rock type and sulphide distribution. The eventual form ofa quantitative relation and the determination of its usefulnessmust await further collection and correlation of empiricaldata. Because of sampling problems this must be done in thefield rather than in the laboratory. Although it is possible toperform surface studies in mines, we believe that boreholemethods provide the most expedient means of pursuing thisgoal.

4.0

3.0

E0::........

0

2.0 0::

""~

1.0

./'1

4/\/ .

/. \;I. .

./

1.1

Lz-.------.-7\.

/ \../ ../.

1.0

0.8

0.7

~0.9

content, a correlation which did not occur in DOH-SF 1. Thesame data, replotted on a linear graph show a fairly linearcorrelation between the phase and sulphide content(Fig. 12.28) with about 15 milliradians of phase shift producedby 1.0 weight per cent sulphides. To emphasize theindependence of resistivity from sulphide content,Figure 12.29 displays the porosity computations based uponthe neutron log, <1>N' and upon the resistivity log, <1>. The<1>R trace is based 'upon Archie's Law using a "cerrfentationexponent" of 2.0 and a pore fluid resistivity of 1.75 ohmmetres. The 2.0 value of the cementation exponent is inaccord with the values found by Brace and Orange (1968) forlaboratory samples of igneous rocks. The 1.75 value is onefourth the 7.1 ohm-metres value of the borehole fluid, a factwhich can be attributed at least in part to the effects ofsurface conduction in the rock, as also observed by Brace andOrange. The main point, however, is that the resistivitydetermined porosity values are reasonably close to theneutron-porosity, which is not the case when conduction iscontrolled by the sulphide content.

Dependence of IP Parameters upon SUlphide Content

Although the data base is Ii mited (only three other rocktypes were examined in the same detail as the two casesdiscussed above), we propose that the functional dependence

MAGNETIC SUSCEPTIBILITY LOGGING

The magnetic susceptibili ty tool, like the inducedpolarization tool, is more useful in mining applications than inother geotechnical applications. A quali tative use, for rocktype identification and correlation, has been demonstratedabove (Fig. 12.9). The system used by Kennecott for loggingis similar to that described by Broding et al. (1952) andZablocki (1966).

The magnetic susceptibili ty tool is also used forquantitative studies in exploration. Figure 12.30 is oneexample where the magnetic susceptibili ty measurement inseveral drillholes yielded the thickness and magneticsusceptibility of the Gila conglomerate (southwestern UnitedStates). The volume susceptibili ty is relatively uniformaround 1000 \l cgs. These data enabled the explorationgeophysicist to improve his aeromagnetic interpretation forthis prospect.

A second quantitative use, the direct measurement ofthe magnetite content has been demonstrated by Snyder(1973) and also in Figure 12.8, but we have not pursued thisapplication any further.


GEOLOGY TOTAL SULPHIDES /1 O~/R PHASE RESISTIVITY Rtweight ..• • 15

mllilradiansso 75 100

ohm-melres31 l()() 316 100;)

andesite

fraclure zone'":,'

r '

r',;', :: IBOO

,JJ """=-~

~-..~.

~ -=- ~

~~ ~ r=--:>1..., ;-

?, eulphldes ::;;= "'----~_.:J- sulphIdes magn lite

~~~• .1

t.,~ /iOi7R~

~p; -5 ::::>----- , t>

'1'« ~ -.~ 1<::::"

~t:>

c:::::z"" ~.,C

~ ~p ~

400

II

~;e.:5DOl

"

600

,":-,<

121086(weight %)

42

0 25 50 75 100 125 1300 1000

f ~..r0-

t... :;;s::

, tesisljyjtJ

L.., <::.......

~ r~>

\SulphiL--P rJl

de ~Q)...-Q)

{E

t:2' ...r---I /hose

co-mnrr- ~

1h

l ~ ~-

~o

.5

800

1000

600'

Figure 12.24. Sulphide data and IP log results corrected for borehole et't'ects in DDH-SFl.

GEOLOGY PHASE RESISTIVITY(millirodions) (ohm - melers)

SULPHIDE ESTIMATES

Figure 12.25. IP logs and sulphide estimates in DDII-SF2.

..

1000+ I I + 1+. I..;. I + 1000+ I . ..1: + I+ I / I +. . .

. . L"O,/R : ". '.~ /500- 05- /. '. • 500- " • O~

'. " .... /. /" ••• " ,,/. , E

:'.. '.... ,~, .: /'200- ' ~ .02-'-' '/' 200- ~ 02- / ' '. . ~ " ~

'00; . . I'.:.' . I + Li:'·:-· + '00+ I I + I 0,1 .'. ......:. +I 2 5 10 - I 2 5 10-

• s' (wg~%) ~ 5' (wgIO!o) ~ ••- -0.05- 0.05-

. ..

100+. • I I + 002- 100+ I I '.' ;",+ 002-

. . . -. - ..

. . .e:, '.' .". . .. ',:.50-;- : .:.' • - 001+ I I + 50- • - 0.01+ I I +

"" ·r 1 2 5 10 1 2 5 10~'. S'(w91 %) ~ S'(w91%)~ • D

I . i, . ,20- - 20--

10+ I I + I 10+ I I +I 2 5 10 1 2 5 10

S'lwgl%1 s' (w91%1L- ---.J

~CD::ro;'ro.c.c5'.c

Figure 12.26. IP parameters and sulphide content, DDH-SFI. Figure 12.27. IP parameters and sulphide content, DDH-SF2.

ND:l

'"


TEMPERATURE LOGGING

CONCLUSIONS

1. 13orehole logging methods will gradually bp,come rnorecommon in the base metals mining industry forexploration, property assessment and engineeringstudies. The demand will be a direct function of theability to obtain in-situ data which is not readily<1Vailable from core, and the abili ty to reduce the needfor core recovery. In other words, the ability toquantify the elemr,nts and minerals which have primeirnportancp, to the mining geologist is the key factor inthe widespread use of borehole techniques.

and a number of the contacts show the greatest coolingeffects on the temperature logs, The direction of anygroundwater flow in these rocks should be strongly influencedby intrusive and volcanic rock contacts.

The American Petroleum Institute has establishedcertain recommended standards for calibration and format ofborehole logs; lor example, nuclear logs (API, 1974) andelectrical logs (API, 1967). API has also estnblished acalibration facility on the Univ8rsity of Houston campus,available to anyone for a set fee. The c8libration facili ty isdesigned for oil field envirownents and for nuclear logs. TheUni ted States Bureau of Mmes hrls established a calibrationfacility for the mining arld engineering logging applications.This facility provides standards for density, magneticsusceptibility, sonic velo::::ity, resistivity and caliper logs(Sn:Jdgrass, 1976) and the facility is available to industry.

Kenrlecott has calibrated nuclear tools at the API pitsand the rp,st of thp,ir tools with core analyses. An example ofcore calibration of the maqnetic susceptibility tool was givenin Figure 12.31. Since it is neither practical to calibrate thevarious tools frequently at the calibration facili ties in ei therHouston or Denver nor to continuously check the calibrationwith c', e analyses, Kennecott spot check with core analysesand also verify the tool calibration before and after each logwilh some fixed reference. For example, the density toolcalibration is checked with two blocks of material of knowndensi ties before and after euch log is made. A geometricarray of ferrite rods is used to check the calibration of themagnetic susceptibility tool. The calibration of slim holetools is not an easy matter and Merkel and Snyder (1977) havedisclJssed this problem in some detail. Waller et al. (1975)examined some basic errors inherent in 1001 calibration.

CALIBRAnON OF LOGGING EQUIPMENT

In the previous section on temperature logging we notedhow the temperature log could indicate points of fluid flow orfluid exit in a drillhole. Some attempt can be mude toquantify this flow from the temperature anomalies but themethod is very imprecise. The rocks will often showtemperature variation beyond the zone of flow andmathematical modelling is not simple (Smith und Steffenson,197U),

A more direct quantitative measure can be made with aspinner log (c. f. Schlumberger Production Log Interpretation,1974). However, a spinner tool at best measures flowaccurately only above 0.75 mimin. Figure 12.33 shows twoflow profiles made in holes used for dump leaching at one ofKennecott's operating properties. The logs show that fluidexit is nonuniform and is exiting primarily out the top portionof the completed hole interval.

FLUID FLOW DETERMINAnON

12104 6 8SULPHIDE (weight %)

..r ph••• : 15.0xS"'phl••

.;/

j/

/o+---,-----------,----,---,------,---_+_

o

20

40

100

120

140+-----,-----,------,-----,-----,---,-------,-

"jI

.j/

./".. j

A temperature log is a continuous record of boreholefluid temperature with depth which will bc the geothermalgradient only if the fluid is in equilibrium with the adjacentrocks and if there is no vertical cireulatiorl of fluid in thedrillhole,

The tool has been calibrated using magneticsusceptibility measurements on core samples and thecalibration is routinely checked with spot core measurements.I"igure 12.31 shows an example of this ckeck in an explorationhole in southern Arizona.

Figure 12.28. Phase values averaged over 10 foot intervalversus sulphide content, DDH-SF2.

Temperature logs are necessary for the correctinterpretation of resistivity logs because of temperatureeffect on electrical resistance (Alger, 1966; Schlumberger,1972). The fluid resistivity needs to be corrected fortemperature before it is used for porosity analyses or for anestimate of total dissolved solids.

Temperature logs can also be used to examine fluid flowin drillholes (Keys and MacCary, 1971; Wilterholt and Tixier,1972). One method is to inject cooler surface water into adrillhole for some period of time and obtain temperature> logsduring injection and at periodic intervals postinjcction. Theselogs are then compared to the preinjection log. The injectedfluid will cool the borehole fluid and the zones of fluid exitfrom the hole. The temperature in the drillhole will recovertoward the preinjection temperature profile with zones offluid exit recovering most slowly. Figure 12.32 illustratesthis logging method.

A preinjection and two postinjection temperatureprofiles are shown in Figure 12.32a and the differencebetween the post- and pre-injection logs are shown inFigure 12.32b. Also shown are fracture locations picked fromthe caliper log. The rocks are volcanics intruded by dacite

UJ(/)«IQ. 60

u;c:

'"'0~ 80

!


o·5

600

2.5

#5

200

OJ.e.5.r:

;;; 0...'0

en<Il...-<IlE

250

260

50003000

Xm II cgs

1000o

Magnetic susceptibility logs in several exploration holes.

820

800 + __..L.....,_--l--..1.----l'---~

840 <..c:- -~

C. <Il<Il <Il

"C -860 ~0

Figure 12.30.

/ C •/ oo.n,

~//

~/v

..c:-C.<Il

"C

en<Il...-<IlE

200

300

10

POROSITY(volume %)

5o600-+-----+-----1

Figure 12.29. Porosity logs in DDH-SF2.Calculated from resistivity using Archie'sLaw with a 2.0 cementation exponent and a1.75 ohm-metre mud resistivity. The <plog is computed from the neutron log. N

800-t-----,::9;".,--f"f-----l

1200'-J------L-------'

..c:-Co

-a

-<Il<Il

'+-

880

270

900 -i. ...L..

o Bison measurements on core

Susceptibility tool measurements

Figure 12.31. Comparison of magnetic susceptibility measurements made in drillhole BB-1 and on BB-1 core samples fromsouthern Arizona.


% Cumulative Flow Loss

20 40 60 80 100

of Perforations200

220

(il) (b) 70TEMPERATURE (OF) TEMPERATURE DIFFERENCE (oF)

70 90 110 20 240 UJ

"'''''''' .... .,~<J ~

I 35 45 10 15 QI -°c v '" "1 QI <ll

\\ ,....,,,~ .... .... E"'''''.;' "c l::,.."'('. '" ..... c

s\J JJ 260; :... ..c: 80 .r:::.,.4,toJ <J

C.0-~ " QI., v 't 0 -el) t'7t J

,J"J

J\ " ',. t., 2803J ."

CALIPER,. ~" FRACTURES.... '...... , ,,- 1'1

~ \ " /' ~ 90II , .... ,300_ \ I,

l\7:lOa.m. 2/21/76 ., (

" ,-. .. 1(

) \ 9//4/74

" t( 'J.... .... ...... 1~ ,1 ....... 1/~ .. 't" (.~ \\

1C ,. 't~ a) Drill hole DU-l,,' k

I::: " of. % Cumulative Flow LossQ, ~~~ ,"~ k" \\

20 40 60 80 100, •, of. ~ 120

I.. .... (

'"

~\·, . ~I... ~,,~

~ ~OStart of Perforations" ... E... , 11

\\ '> -/\ 0-;

" A <0 14

~ lJC ( ~N

2/20/76

1a ~ t •0 \\ • •• 2:55 p.m.In , t

1'1.. 'lII (I•« (

\\ ••• •'1-" ( J(.·., <J

~It. ~ ••

QIQI <ll

~ ( .... E.~ 180 c..c:... .r:::.

~LEGEND 0- -QI Q.

~0 60

<ll... ').. Lati te "CJ

200UTI:,-, Dacite

-Shut in lf t;m~ m/:00 p m. 2/20/76 ., &. Andesite

Figure 12.32. Temperature injection profiles.22

70

b) Drill hole DU-2

Figure 12.33. Cumulative per cent fluid loss withdepth in two mine dump holes.


3.

2.

waterWaterStates

In general, the added information which can be gleanedfrom a combination of logs instead of just one or two iswell worth the added cost of acquisition. This is truewhether the application is a quali tative one such as rocktype and correlation, or a quantitative goal such asestimation of sulphide content.

With present technology, it is now possible in the basemetal environment to make quantitative estimates ofporosity, fracture intensity, bulk density, magnetitecontent, potassium content, and mineralogically boundwater. We feel that with some additional effort andcare it will soon be possible to add sulphide content tothis list.

4. As pointed out by Snyder (1973) these measurementshave particular application in the porphyry copperenvironment where drillholes are often deep and costly.We emphasize that the logs have enhanced value whenthey are applied on a deposit-wide basis and areintegrated into geological and geochemical studies.

5. We agree with an observation made by Dyck et al.(1975) that there is a need for more case historydocumentation and published empirical relationships forthe mining environment such as exists for a wide rangeof logging applications in the oil and gas industry.

ACKNOWLEDGMENTS

D.D. Snyder initiated the Kennecott logging program in1972 and formulated many of the practices and designs,including the initial design of the continuous IP logger.D. Purvance performed the numerical computations for the IPdeparture curves. E.P. Baumgartner acquired and interpretedseveral of the logs presented in this paper. Dale Green,Roy Lenk, Jim Simmons and Dave Ewing contributedsignificantly to the development and maintenance of thelogging systems. Jonathan Jackson assisted in the recordingand analysis of the spinner logs. We thank Kennecott CopperCorporation for permission to publish the material in thispaper.

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