Geophysics for the Mineral Exploration Geoscientist

516 Pages·2014·34.895 MB·English

by  Dentith M., Mudge S.T.

Geophysics for the Mineral Exploration Geoscientist
Highglobaldemandformineralcommoditieshasledtoincreasingapplicationofgeophysicaltechnolo-
giestoawidevarietyoforedeposits.Co-authoredbyauniversityprofessorandanindustrygeophysicist,
this state-of-the-art overview of geophysical methods provides a careful balance between principles and
practice. It takes readers from the basic physical phenomena, through the acquisition and processing of
geophysical data, to the creation of subsurface models and their geological interpretation.
(cid:129) Presents detailed descriptions of all the main geophysical methods, including gravity, magnetic,
radiometric, electrical, electromagnetic and seismic methods.
(cid:129) Provides the next-generation tools, essential to the future of the mineral exploration and mining
industry, to exploit ‘blind’ mineral deposits by searching deeper.
(cid:129) Describes techniques in a consistent way and without the use of complex mathematics, enabling easy
comparison between various methods.
(cid:129) Gives a practical guide to data acquisition and processing including the identification of noise in
datasets, as required for accurate interpretation of geophysical data.
(cid:129) Presents unique petrophysical databases, giving geologists and geophysicists key information on
physical rock properties.
(cid:129) Emphasises extraction of maximum geological information from geophysical data, providing explan-
ations of data modelling and common interpretation pitfalls.
(cid:129) Provides examples from a range of 74 mineral deposit types around the world, giving students
experience of working with real geophysical data.
(cid:129) Richly illustrated with over 300 full-colour figures, with access to electronic versions for instructors.
Designed for advanced undergraduate and graduate courses in minerals geoscience and geology, this
bookisalsoavaluablereferenceforgeologistsandprofessionalsintheminingindustrywishingtomake
greater use of geophysical methods.
Michael Dentith is Professor of Geophysics at The University of Western Australia and a research theme
leaderintheCentreforExplorationTargeting.Hehasbeenanactiveresearcherandteacherofuniversity-level
applied geophysics and geology for more than 25 years, and he also consults to the minerals industry.
Professor Dentith’s research interests include geophysical signatures of mineral deposits (about which he
haseditedtwobooks),petrophysicsandterrainscaleanalysisforexplorationtargetingusinggeophysicaldata.
HeisamemberoftheAmericanGeophysicalUnion,AustralianSocietyofExplorationGeophysicists,Society
of Exploration Geophysicists and Geological Society of Australia.
StephenMudgehasworkedasanexplorationgeophysicistinAustraliaformorethan35years,andcurrently
works as a consultantin his own company Vector Research. He has worked in many parts of theworldand
has participated in a number of new mineral discoveries. Mr Mudge has a keen interest in data processing
techniquesformineraldiscoveryandhasproducedseveralpublicationsreportingnewdevelopments.Heisa
member of the Australasian Institute of Mining and Metallurgy, Australian Institute of Geoscientists,
Australian Society of Exploration Geophysicists, Society of Exploration Geophysicists and European Associ-
ation of Engineers and Geoscientists.
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Geophysics for the
Mineral Exploration
Geoscientist
Michael Dentith
TheUniversityofWesternAustralia,Perth
Stephen T. Mudge
VectorResearchPtyLtd,Perth
AngloGold Carpentaria Centre for First Quantum MMG Ltd Rio Tinto Exploration St Barbara Limited
Ashanti Limited Exploration Limited Exploration Targeting Minerals Ltd
UniversityPrintingHouse,CambridgeCB28BS,UnitedKingdom
PublishedintheUnitedStatesofAmericabyCambridgeUniversityPress,NewYork
CambridgeUniversityPressispartoftheUniversityofCambridge.
ItfurtherstheUniversity’smissionbydisseminatingknowledgeinthepursuitof
education,learningandresearchatthehighestinternationallevelsofexcellence.
www.cambridge.org
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©MichaelDentithandStephenMudge2014
Thispublicationisincopyright.Subjecttostatutoryexception
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Firstpublished2014
PrintedintheUnitedKingdombyXXXX
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ISBN978-0-521-80951-1Hardback
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CONTENTS
List of online appendices ix 2.7.3 Mergingofdatasets 38
List of figure credits xi 2.7.4 Enhancementofdata 38
2.8 Data display 48
Preface xv
2.8.1 Typesofdatapresentation 48
Acknowledgements xvi
2.8.2 Imageprocessing 51
2.9 Data interpretation –general 58
1
2.9.1 Interpretationfundamentals 59
Introduction 1 2.9.2 Removingtheregionalresponse 60
2.10 Data interpretation – qualitativeanalysis 63
1.1 Physical versus chemical characterisation of the
2.10.1 Spatialanalysisof2Ddata 63
geological environment 2 2.10.2 Geophysicalimagetogeologicalmap 67
1.2 Geophysical methods in exploration and mining 3 2.11 Data interpretation – quantitative analysis 70
1.2.1 Airborne,groundandin-groundsurveys 3 2.11.1 Geophysicalmodelsofthesubsurface 70
1.2.2 Geophysicalmethodsandmineraldeposits 4 2.11.2 Forwardandinversemodelling 74
1.2.3 Thecostofgeophysics 5 2.11.3 Modellingstrategy 78
1.3 About thisbook 7 2.11.4 Non-uniqueness 79
Further reading 11 Summary 81
Review questions 82
2 Further reading 82
Geophysical data acquisition, processing
3
and interpretation 13
Gravity and magnetic methods 85
2.1 Introduction 13
2.2 Types of geophysical measurement 14 3.1 Introduction 85
2.2.1 Absoluteandrelativemeasurements 14 3.2 Gravity andmagnetic fields 86
2.2.2 Scalarsandvectors 15 3.2.1 Massandgravity 87
2.2.3 Gradients 15 3.2.2 Gravityanomalies 88
2.3 The natureof geophysical responses 16 3.2.3 Magnetismandmagneticfields 89
2.4 Signalandnoise 17 3.2.4 Magneticanomalies 93
2.4.1 Environmentalnoise 18 3.3 Measurementof the Earth’s gravityfield 94
2.4.2 Methodologicalnoise 22 3.3.1 Measuringrelativegravity 96
2.5 Surveyobjectives 23 3.3.2 Measuringgravitygradients 98
2.5.1 Geologicalmapping 23 3.3.3 Gravitysurveypractice 98
2.5.2 Anomalydetection 24 3.4 Reduction of gravitydata 99
2.5.3 Anomalydefinition 25 3.4.1 Velocityeffect 99
2.6 Data acquisition 25 3.4.2 Tidaleffect 99
2.6.1 Samplingandaliasing 25 3.4.3 Instrumentdrift 100
2.6.2 Systemfootprint 27 3.4.4 VariationsingravityduetotheEarth’s
2.6.3 Surveydesign 27 rotationandshape 100
2.6.4 Featuredetection 31 3.4.5 Variationsingravityduetoheightand
2.7 Data processing 32 topography 102
3.4.6 Summaryofgravitydatareduction 106
2.7.1 Reductionofdata 32
3.4.7 Exampleofthereductionofgroundgravitydata 106
2.7.2 Interpolationofdata 34
vi Contents
3.5 Measurement of the Earth’s magnetic field 106 3.11.4 MagneticresponsesinaPhanerozoicOrogenic
3.5.1 Thegeomagneticfield 109 terrain:LachlanFoldbelt 179
3.5.2 Measuringmagneticfieldstrength 112 3.11.5 Magneticandgravityresponsesfrom
3.5.3 Magneticsurveypractice 114 mineralisedenvironments 186
3.6 Reduction of magneticdata 116 Summary 188
3.6.1 Temporalvariationsinfieldstrength 116 Review questions 189
3.6.2 Regionalvariationsinfieldstrength 117 Further reading 189
3.6.3 Terrainclearanceeffects 117
3.6.4 Levelling 117
3.6.5 Exampleofthereductionof 4
aeromagneticdata 117
Radiometric method 193
3.7 Enhancement and display of gravity and
magnetic data 118 4.1 Introduction 193
3.7.1 Choiceofenhancements 122 4.2 Radioactivity 194
3.7.2 Reduction-to-poleandpseudogravity 4.2.1 Radioactivedecay 194
transforms 123 4.2.2 Half-lifeandequilibrium 195
3.7.3 Wavelengthfilters 124 4.2.3 Interactionofradiationandmatter 196
3.7.4 Gradients/derivatives 125 4.2.4 Measurementunits 197
3.8 Densityin the geological environment 127 4.2.5 Sourcesofradioactivityinthenatural
3.8.1 Densitiesoflow-porosityrocks 127 environment 198
3.8.2 Densitiesofporousrocks 129 4.3 Measurementof radioactivity in the field 199
3.8.3 Densityandlithology 130 4.3.1 Statisticalnoise 199
3.8.4 Changesindensityduetometamorphism 4.3.2 Radiationdetectors 201
andalteration 131 4.3.3 Surveypractice 204
3.8.5 Densityofthenear-surface 133 4.4 Reduction of radiometric data 205
3.8.6 Densityofmineralisedenvironments 133 4.4.1 Instrumenteffects 205
3.8.7 Measuringdensity 134 4.4.2 Randomnoise 206
3.8.8 Analysisofdensitydata 134 4.4.3 Backgroundradiation 207
3.9 Magnetism inthe geological environment 135 4.4.4 Atmosphericradon 207
3.9.1 Magneticpropertiesofminerals 136 4.4.5 Channelinteraction 208
3.9.2 Magneticpropertiesofrocks 138 4.4.6 Heightattenuation 208
3.9.3 Magnetismofigneousrocks 140 4.4.7 Analyticalcalibration 208
3.9.4 Magnetismofsedimentaryrocks 144 4.5 Enhancement and displayof radiometric data 209
3.9.5 Magnetismofmetamorphosedand 4.5.1 Single-channeldisplays 209
alteredrocks 145 4.5.2 Multichannelternarydisplays 209
3.9.6 Magnetismofthenear-surface 151 4.5.3 Channelratios 210
3.9.7 Magnetismofmineralisedenvironments 151 4.5.4 Multivariantmethods 210
3.9.8 Magneticpropertymeasurementsandtheir
4.6 Radioelementsin the geologicalenvironment 210
analysis 155
4.6.1 Disequilibriuminthegeologicalenvironment 212
3.9.9 Correlationsbetweendensityandmagnetism 159
4.6.2 Potassium,uraniumandthoriumin
3.10 Interpretation of gravityandmagnetic data 160 igneousrocks 216
3.10.1 Gravityandmagneticanomaliesandtheir 4.6.3 Potassium,uraniumandthoriuminaltered
sources 160 andmetamorphosedrocks 216
3.10.2 Analysisofgravityandmagneticmaps 163 4.6.4 Potassium,uraniumandthoriumin
3.10.3 Interpretationpitfalls 164 sedimentaryrocks 217
3.10.4 Estimatingdepth-to-source 165 4.6.5 SurficialprocessesandK,UandThinthe
3.10.5 Modellingsourcegeometry 167 overburden 217
3.10.6 Modellingpitfalls 167 4.6.6 Potassium,uraniumandthoriumin
3.11 Examples of gravity and magnetic data from mineralisedenvironments 219
mineralised terrains 169 4.7 Interpretation of radiometric data 220
3.11.1 Regionalremovalandgravitymappingof 4.7.1 Interpretationprocedure 222
palaeochannelshostingplacergold 169 4.7.2 Interpretationpitfalls 222
3.11.2 Modellingthemagneticresponseassociated 4.7.3 Responsesofmineralisedenvironments 223
withtheWallabygolddeposit 172 4.7.4 Exampleofgeologicalmappinginafoldand
3.11.3 MagneticresponsesfromanArchaeangranitoid– thrustbelt:FlindersRanges 229
greenstoneterrain:KirklandLakearea 175 4.7.5 Interpretationofγ-logs 230
Contents vii
Summary 231 5.9.4 DisplayandinterpretationofAEMdata 345
Review questions 232 5.9.5 ExamplesofAEMdatafrommineralisedterrains 345
Further reading 233 Summary 347
Review questions 348
5 Further reading 349
Electrical and electromagnetic methods 235
6
5.1 Introduction 235
Seismic method 351
5.2 Electricityand magnetism 237
5.2.1 Fundamentalsofelectricity 237 6.1 Introduction 351
5.2.2 Fundamentalsofelectromagnetism 243 6.2 Seismic waves 352
5.2.3 Electromagneticwaves 246 6.2.1 Elasticityandseismicvelocity 353
5.3 Electricalproperties of the natural environment 247 6.2.2 Bodywaves 353
5.3.1 Conductivity/resistivity 247 6.2.3 Surfacewaves 354
5.3.2 Polarisation 253 6.3 Propagation of body waves through thesubsurface 354
5.3.3 Dielectricproperties 255 6.3.1 Wavefrontsandrays 354
5.3.4 Propertiesofthenear-surface 255 6.3.2 Fresnelvolume 355
5.4 Measurementof electricaland electromagnetic 6.3.3 Seismicattenuation 356
phenomena 257 6.3.4 Effectsofelasticpropertydiscontinuities 357
5.4.1 Electrodes 258 6.4 Acquisition anddisplayof seismic data 363
5.4.2 Electricalandelectromagneticnoise 258 6.4.1 Seismicsources 363
5.5 Self-potentialmethod 260 6.4.2 Seismicdetectors 364
5.5.1 Sourcesofnaturalelectricalpotentials 260 6.4.3 Displayingseismicdata 364
5.5.2 Measurementofself-potential 262 6.5 Seismic reflection method 366
5.5.3 DisplayandinterpretationofSPdata 263 6.5.1 Dataacquisition 367
5.5.4 ExamplesofSPdatafrommineraldeposits 265 6.5.2 Dataprocessing 369
5.6 Resistivity and induced polarisation methods 266 6.6 Variationsin seismic properties in the geological
5.6.1 Electricfieldsandcurrentsinthesubsurface 268 environment 383
5.6.2 Resistivity 269 6.6.1 Seismicpropertiesofcommonrocktypes 384
5.6.3 Inducedpolarisation 271 6.6.2 Effectsoftemperatureandpressure 387
5.6.4 Measurementofresistivity/IP 273 6.6.3 Effectsofmetamorphism,alterationand
5.6.5 Resistivity/IPsurveypractice 275 deformation 388
5.6.6 Display,interpretationandexamplesof 6.6.4 Seismicpropertiesofmineralisation 389
resistivity/IPdata 278 6.6.5 Seismicpropertiesofnear-surfaceenvironments 390
5.6.7 Interpretationpitfalls 289 6.6.6 Anisotropy 391
5.6.8 Resistivity/IPlogging 293 6.6.7 Absorption 391
5.6.9 Appliedpotential/mise-à-la-massemethod 294 6.6.8 Summaryofgeologicalcontrolsonseismic
5.7 Electromagnetic methods 299 properties 392
5.7.1 Principlesofelectromagneticsurveying 299 6.6.9 Measuringseismicproperties 392
5.7.2 SubsurfaceconductivityandEMresponses 306 6.7 Interpretation of seismic reflection data 393
5.7.3 AcquisitionofEMdata 312 6.7.1 Resolution 393
5.7.4 ProcessinganddisplayofEMdata 316 6.7.2 Quantitativeinterpretation 396
5.7.5 InterpretationofEMdata 318 6.7.3 Interpretationpitfalls 397
5.7.6 Interpretationpitfalls 326 6.7.4 Examplesofseismicreflectiondatafrom
5.7.7 ExamplesofEMdatafrommineraldeposits 328 mineralisedterrains 398
5.8 Downhole electromagnetic surveying 330 6.8 In-seam and downhole seismic surveys 401
5.8.1 AcquisitionofDHEMdata 330 6.8.1 In-seamsurveys 402
5.8.2 DisplayandinterpretationofDHEMdata 333 6.8.2 Tomographicsurveys 403
5.8.3 ExamplesofDHEMresponsesfrommineral Summary 405
deposits 337
Review questions 406
5.8.4 Inductionlogging 339
Further reading 406
5.9 Airborneelectromagnetic surveying 339
5.9.1 AcquisitionofAEMdata 340
References 408
5.9.2 AEMsystems 342
5.9.3 AEMsurveypractice 344 Index 426

ONLINE APPENDICES
Available at www.cambridge.org/dentith
Appendix 1 Vectors A4.5.2 Modelresponses
A1.1 Introduction A4.5.3 Interpretationpitfalls
A4.5.4 Modelling
A1.2 Vector addition
A4.6 MT versus other electrical and EM methods
A4.7 Examplesof magnetotelluric data
Appendix 2 Waves and wave analysis
A4.7.1 AMTresponseoftheRegisKimberlitepipe
A2.1 Introduction
A4.7.2 CSAMTresponseoftheGoldenCross
A2.2 Parameters definingwaves andwaveforms epithermalAu–Agdeposit
A2.3 Waveinterference A4.8 Natural source airborne EM systems
A2.4 Spectral analysis A4.8.1 AFMAG
References A4.8.2 ZTEM
Summary
Appendix 3 Magnetometric methods Review questions
A3.1 Introduction Further reading
A3.2 Acquisition of magnetometric data References
A3.3 Magnetometricresistivity
A3.3.1 Downholemagnetometricresistivity Appendix 5 Radio and radar frequency methods
A3.4 Magnetic induced polarisation A5.1 Introduction
A3.4.1 Example:Poseidonmassivenickelsulphide A5.2 High-frequency EM radiation in thegeological
deposit
environment
A3.5 Total magnetic field methods
A5.3 Ground penetrating radar surveys
Summary A5.3.1 AcquisitionofGPRdata
Review questions A5.3.2 ProcessingofGPRdata
Further reading A5.3.3 DisplayandinterpretationofGPRdata
References A5.3.4 ExamplesofGPRdatafrommineralised
terrains
A5.4 Continuouswaveradio frequency surveys
Appendix 4 Magnetotelluric electromagnetic
A5.4.1 ExampleRIMdata–MountIsacopper
methods
sulphide
A4.1 Introduction
Summary
A4.2 Natural source magnetotellurics
Review questions
A4.2.1 Surveypractice
Further reading
A4.3 Controlled source audio-frequency
References
magnetotellurics
A4.3.1 AcquisitionofCSAMTdata
Appendix 6 Seismic refraction method
A4.3.2 Near-fieldandfar-fieldmeasurements
A4.3.3 Surveydesign A6.1 Introduction
A4.4 Reduction of AMT/MT and CSAMT data A6.2 Acquisition and processingof seismic
A4.4.1 Resistivityandphase-difference refraction data
A4.4.2 Staticeffect A6.2.1 Pickingarrivaltimes
A4.5 Display and interpretation of MTdata A6.3 Interpretation of seismic refraction data
A4.5.1 Recognisingfar-fieldresponsesinCSAMTdata A6.3.1 Traveltimesofcriticallyrefractedarrivals