Geophysics for the Mineral Exploration Geoscientist.pdf

Geophysics for the Mineral Exploration Geoscientist

High global demand for mineral commodities has led to increasing application of geophysical technolo-gies to a wide variety of ore deposits. Co-authored by a university professor and an industry geophysicist,this state-of-the-art overview of geophysical methods provides a careful balance between principles andpractice. It takes readers from the basic physical phenomena, through the acquisition and processing ofgeophysical data, to the creation of subsurface models and their geological interpretation.

Presents detailed descriptions of all the main geophysical methods, including gravity, magnetic,radiometric, electrical, electromagnetic and seismic methods.

Provides the next-generation tools, essential to the future of the mineral exploration and miningindustry, to exploit blind mineral deposits by searching deeper.

Describes techniques in a consistent way and without the use of complex mathematics, enabling easycomparison between various methods.

Gives a practical guide to data acquisition and processing including the identication of noise indatasets, as required for accurate interpretation of geophysical data.

Presents unique petrophysical databases, giving geologists and geophysicists key information onphysical rock properties.

Emphasises extraction of maximum geological information from geophysical data, providing explan-ations of data modelling and common interpretation pitfalls.

Provides examples from a range of 74 mineral deposit types around the world, giving studentsexperience of working with real geophysical data.

Richly illustrated with over 300 full-colour gures, with access to electronic versions for instructors.

Designed for advanced undergraduate and graduate courses in minerals geoscience and geology, thisbook is also a valuable reference for geologists and professionals in the mining industry wishing to makegreater use of geophysical methods.

Michael Dentith is Professor of Geophysics at The University of Western Australia and a research themeleader in the Centre for Exploration Targeting. He has been an active researcher and teacher of university-levelapplied geophysics and geology for more than 25 years, and he also consults to the minerals industry.Professor Dentiths research interests include geophysical signatures of mineral deposits (about which hehas edited two books), petrophysics and terrain scale analysis for exploration targeting using geophysical data.He is a member of the American Geophysical Union, Australian Society of Exploration Geophysicists, Societyof Exploration Geophysicists and Geological Society of Australia.

Stephen Mudge has worked as an exploration geophysicist in Australia for more than 35 years, and currentlyworks as a consultant in his own company Vector Research. He has worked in many parts of the world andhas participated in a number of new mineral discoveries. Mr Mudge has a keen interest in data processingtechniques for mineral discovery and has produced several publications reporting new developments. He is amember 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 theMineral ExplorationGeoscientist

Michael DentithThe University of Western Australia, Perth

Stephen T. MudgeVector Research Pty Ltd, Perth

AngloGoldAshanti Limited

CarpentariaExploration Limited

Centre forExploration Targeting

First QuantumMinerals Ltd

MMG Ltd Rio Tinto Exploration St Barbara Limited

University Printing House, Cambridge CB2 8BS, United Kingdom

Published in the United States of America by Cambridge University Press, New York

Cambridge University Press is part of the University of Cambridge.

It furthers the Universitys mission by disseminating knowledge in the pursuit of

education, learning and research at the highest international levels of excellence.

www.cambridge.org

Information on this title: www.cambridge.org/9780521809511

Michael Dentith and Stephen Mudge 2014

This publication is in copyright. Subject to statutory exception

and to the provisions of relevant collective licensing agreements,

no reproduction of any part may take place without the written

permission of Cambridge University Press.

First published 2014

Printed in the United Kingdom by XXXX

A catalogue record for this publication is available from the British Library

Library of Congress Cataloguing in Publication data

ISBN 978-0-521-80951-1 Hardback

Additional resources for this publication at www.cambridge.org/dentith

Cambridge University Press has no responsibility for the persistence or accuracy of

URLs for external or third-party internet websites referred to in this publication,

and does not guarantee that any content on such websites is, or will remain,

accurate or appropriate.

CONTENTS

List of online appendices ix

List of figure credits xi

Preface xv

Acknowledgements xvi

1

Introduction 1

1.1 Physical versus chemical characterisation of the

geological environment 2

1.2 Geophysical methods in exploration and mining 31.2.1 Airborne, ground and in-ground surveys 31.2.2 Geophysical methods and mineral deposits 41.2.3 The cost of geophysics 5

1.3 About this book 7

Further reading 11

2

Geophysical data acquisition, processingand interpretation 13

2.1 Introduction 13

2.2 Types of geophysical measurement 142.2.1 Absolute and relative measurements 142.2.2 Scalars and vectors 152.2.3 Gradients 15

2.3 The nature of geophysical responses 16

2.4 Signal and noise 172.4.1 Environmental noise 182.4.2 Methodological noise 22

2.5 Survey objectives 232.5.1 Geological mapping 232.5.2 Anomaly detection 242.5.3 Anomaly denition 25

2.6 Data acquisition 252.6.1 Sampling and aliasing 252.6.2 System footprint 272.6.3 Survey design 272.6.4 Feature detection 31

2.7 Data processing 322.7.1 Reduction of data 322.7.2 Interpolation of data 34

2.7.3 Merging of datasets 382.7.4 Enhancement of data 38

2.8 Data display 482.8.1 Types of data presentation 482.8.2 Image processing 51

2.9 Data interpretation general 582.9.1 Interpretation fundamentals 592.9.2 Removing the regional response 60

2.10 Data interpretation qualitative analysis 632.10.1 Spatial analysis of 2D data 632.10.2 Geophysical image to geological map 67

2.11 Data interpretation quantitative analysis 702.11.1 Geophysical models of the subsurface 702.11.2 Forward and inverse modelling 742.11.3 Modelling strategy 782.11.4 Non-uniqueness 79

Summary 81

Review questions 82

Further reading 82

3

Gravity and magnetic methods 85

3.1 Introduction 85

3.2 Gravity and magnetic elds 863.2.1 Mass and gravity 873.2.2 Gravity anomalies 883.2.3 Magnetism and magnetic elds 893.2.4 Magnetic anomalies 93

3.3 Measurement of the Earths gravity eld 943.3.1 Measuring relative gravity 963.3.2 Measuring gravity gradients 983.3.3 Gravity survey practice 98

3.4 Reduction of gravity data 993.4.1 Velocity effect 993.4.2 Tidal effect 993.4.3 Instrument drift 1003.4.4 Variations in gravity due to the Earths

rotation and shape 1003.4.5 Variations in gravity due to height and

topography 1023.4.6 Summary of gravity data reduction 1063.4.7 Example of the reduction of ground gravity data 106

3.5 Measurement of the Earths magnetic eld 1063.5.1 The geomagnetic eld 1093.5.2 Measuring magnetic eld strength 1123.5.3 Magnetic survey practice 114

3.6 Reduction of magnetic data 1163.6.1 Temporal variations in eld strength 1163.6.2 Regional variations in eld strength 1173.6.3 Terrain clearance effects 1173.6.4 Levelling 1173.6.5 Example of the reduction of

aeromagnetic data 117

3.7 Enhancement and display of gravity and

magnetic data 1183.7.1 Choice of enhancements 1223.7.2 Reduction-to-pole and pseudogravity

transforms 1233.7.3 Wavelength lters 1243.7.4 Gradients/derivatives 125

3.8 Density in the geological environment 1273.8.1 Densities of low-porosity rocks 1273.8.2 Densities of porous rocks 1293.8.3 Density and lithology 1303.8.4 Changes in density due to metamorphism

and alteration 1313.8.5 Density of the near-surface 1333.8.6 Density of mineralised environments 1333.8.7 Measuring density 1343.8.8 Analysis of density data 134

3.9 Magnetism in the geological environment 1353.9.1 Magnetic properties of minerals 1363.9.2 Magnetic properties of rocks 1383.9.3 Magnetism of igneous rocks 1403.9.4 Magnetism of sedimentary rocks 1443.9.5 Magnetism of metamorphosed and

altered rocks 1453.9.6 Magnetism of the near-surface 1513.9.7 Magnetism of mineralised environments 1513.9.8 Magnetic property measurements and their

analysis 1553.9.9 Correlations between density and magnetism 159

3.10 Interpretation of gravity and magnetic data 1603.10.1 Gravity and magnetic anomalies and their

sources 1603.10.2 Analysis of gravity and magnetic maps 1633.10.3 Interpretation pitfalls 1643.10.4 Estimating depth-to-source 1653.10.5 Modelling source geometry 1673.10.6 Modelling pitfalls 167

3.11 Examples of gravity and magnetic data from

mineralised terrains 1693.11.1 Regional removal and gravity mapping of

palaeochannels hosting placer gold 1693.11.2 Modelling the magnetic response associated

with the Wallaby gold deposit 1723.11.3 Magnetic responses from an Archaean granitoid

greenstone terrain: Kirkland Lake area 175

3.11.4 Magnetic responses in a Phanerozoic Orogenicterrain: Lachlan Foldbelt 179

3.11.5 Magnetic and gravity responses frommineralised environments 186

Summary 188

Review questions 189

Further reading 189

4

Radiometric method 193

4.1 Introduction 193

4.2 Radioactivity 1944.2.1 Radioactive decay 1944.2.2 Half-life and equilibrium 1954.2.3 Interaction of radiation and matter 1964.2.4 Measurement units 1974.2.5 Sources of radioactivity in the natural

environment 198

4.3 Measurement of radioactivity in the eld 1994.3.1 Statistical noise 1994.3.2 Radiation detectors 2014.3.3 Survey practice 204

4.4 Reduction of radiometric data 2054.4.1 Instrument effects 2054.4.2 Random noise 2064.4.3 Background radiation 2074.4.4 Atmospheric radon 2074.4.5 Channel interaction 2084.4.6 Height attenuation 2084.4.7 Analytical calibration 208

4.5 Enhancement and display of radiometric data 2094.5.1 Single-channel displays 2094.5.2 Multichannel ternary displays 2094.5.3 Channel ratios 2104.5.4 Multivariant methods 210

4.6 Radioelements in the geological environment 2104.6.1 Disequilibrium in the geological environment 2124.6.2 Potassium, uranium and thorium in

igneous rocks 2164.6.3 Potassium, uranium and thorium in altered

and metamorphosed rocks 2164.6.4 Potassium, uranium and thorium in

sedimentary rocks 2174.6.5 Surcial processes and K, U and Th in the

overburden 2174.6.6 Potassium, uranium and thorium in

mineralised environments 219

4.7 Interpretation of radiometric data 2204.7.1 Interpretation procedure 2224.7.2 Interpretation pitfalls 2224.7.3 Responses of mineralised environments 2234.7.4 Example of geological mapping in a fold and

thrust belt: Flinders Ranges 2294.7.5 Interpretation of -logs 230

vi Contents

Summary 231


Further reading 233

5

Electrical and electromagnetic methods 235


5.2 Electricity and magnetism 2375.2.1 Fundamentals of electricity 2375.2.2 Fundamentals of electromagnetism 2435.2.3 Electromagnetic waves 246

5.3 Electrical properties of the natural environment 2475.3.1 Conductivity/resistivity 2475.3.2 Polarisation 2535.3.3 Dielectric properties 2555.3.4 Properties of the near-surface 255

5.4 Measurement of electrical and electromagnetic

phenomena 2575.4.1 Electrodes 2585.4.2 Electrical and electromagnetic noise 258

5.5 Self-potential method 2605.5.1 Sources of natural electrical potentials 2605.5.2 Measurement of self-potential 2625.5.3 Display and interpretation of SP data 2635.5.4 Examples of SP data from mineral deposits 265

5.6 Resistivity and induced polarisation methods 2665.6.1 Electric elds and currents in the subsurface 2685.6.2 Resistivity 2695.6.3 Induced polarisation 2715.6.4 Measurement of resistivity/IP 2735.6.5 Resistivity/IP survey practice 2755.6.6 Display, interpretation and examples of

resistivity/IP data 2785.6.7 Interpretation pitfalls 2895.6.8 Resistivity/IP logging 2935.6.9 Applied potential/mise--la-masse method 294

5.7 Electromagnetic methods 2995.7.1 Principles of electromagnetic surveying 2995.7.2 Subsurface conductivity and EM responses 3065.7.3 Acquisition of EM data 3125.7.4 Processing and display of EM data 3165.7.5 Interpretation of EM data 3185.7.6 Interpretation pitfalls 3265.7.7 Examples of EM data from mineral deposits 328

5.8 Downhole electromagnetic surveying 3305.8.1 Acquisition of DHEM data 3305.8.2 Display and interpretation of DHEM data 3335.8.3 Examples of DHEM responses from mineral

deposits 3375.8.4 Induction logging 339

5.9 Airborne electromagnetic surveying 3395.9.1 Acquisition of AEM data 3405.9.2 AEM systems 3425.9.3 AEM survey practice 344

5.9.4 Display and interpretation of AEM data 3455.9.5 Examples of AEM data from mineralised terrains 345

Summary 347


Further reading 349

6

Seismic method 351


6.2 Seismic waves 3526.2.1 Elasticity and seismic velocity 3536.2.2 Body waves 3536.2.3 Surface waves 354

6.3 Propagation of body waves through the subsurface 3546.3.1 Wavefronts and rays 3546.3.2 Fresnel volume 3556.3.3 Seismic attenuation 3566.3.4 Effects of elastic property discontinuities 357

6.4 Acquisition and display of seismic data 3636.4.1 Seismic sources 3636.4.2 Seismic detectors 3646.4.3 Displaying seismic data 364

6.5 Seismic reection method 3666.5.1 Data acquisition 3676.5.2 Data processing 369

6.6 Variations in seismic properties in the geological

environment 3836.6.1 Seismic properties of common rock types 3846.6.2 Effects of temperature and pressure 3876.6.3 Effects of metamorphism, alteration and

deformation 3886.6.4 Seismic properties of mineralisation 3896.6.5 Seismic properties of near-surface environments 3906.6.6 Anisotropy 3916.6.7 Absorption 3916.6.8 Summary of geological controls on seismic

properties 3926.6.9 Measuring seismic properties 392

6.7 Interpretation of seismic reection data 3936.7.1 Resolution 3936.7.2 Quantitative interpretation 3966.7.3 Interpretation pitfalls 3976.7.4 Examples of seismic reection data from

mineralised terrains 398

6.8 In-seam and downhole seismic surveys 4016.8.1 In-seam surveys 4026.8.2 Tomographic surveys 403

Summary 405


Further reading 406

References 408

Index 426

Contents vii

ONLINE APPENDICESAvailable at www.cambridge.org/dentith

Appendix 1 VectorsA1.1 Introduction

A1.2 Vector addition

Appendix 2 Waves and wave analysisA2.1 Introduction

A2.2 Parameters dening waves and waveforms

A2.3 Wave interference

A2.4 Spectral analysis

References

Appendix 3 Magnetometric methodsA3.1 Introduction

A3.2 Acquisition of magnetometric data

A3.3 Magnetometric resistivityA3.3.1 Downhole magnetometric resistivity

A3.4 Magnetic induced polarisationA3.4.1 Example: Poseidon massive nickel sulphide

deposit

A3.5 Total magnetic eld methods

Summary

Review questions

Further reading

References

Appendix 4 Magnetotelluric electromagneticmethodsA4.1 Introduction

A4.2 Natural source magnetotelluricsA4.2.1 Survey practice

A4.3 Controlled source audio-frequency

magnetotelluricsA4.3.1 Acquisition of CSAMT dataA4.3.2 Near-eld and far-eld measurementsA4.3.3 Survey design

A4.4 Reduction of AMT/MT and CSAMT dataA4.4.1 Resistivity and phase-differenceA4.4.2 Static effect

A4.5 Display and interpretation of MT dataA4.5.1 Recognising far-eld responses in CSAMT data

A4.5.2 Model responsesA4.5.3 Interpretation pitfallsA4.5.4 Modelling

A4.6 MT versus other electrical and EM methods

A4.7 Examples of magnetotelluric dataA4.7.1 AMT response of the Regis Kimberlite pipeA4.7.2 CSAMT response of the Golden Cross

epithermal AuAg deposit

A4.8 Natural source airborne EM systemsA4.8.1 AFMAGA4.8.2 ZTEM

Summary

Review questions

Further reading

References

Appendix 5 Radio and radar frequency methodsA5.1 Introduction

A5.2 High-frequency EM radiation in the geological

environment

A5.3 Ground penetrating radar surveysA5.3.1 Acquisition of GPR dataA5.3.2 Processing of GPR dataA5.3.3 Display and interpretation of GPR dataA5.3.4 Examples of GPR data from mineralised

terrains

A5.4 Continuous wave radio frequency surveysA5.4.1 Example RIM data Mount Isa copper

sulphide

Summary

Review questions

Further reading

References

Appendix 6 Seismic refraction methodA6.1 Introduction

A6.2 Acquisition and processing of seismic

refraction dataA6.2.1 Picking arrival times

A6.3 Interpretation of seismic refraction dataA6.3.1 Travel times of critically refracted arrivals

A6.3.2 Analysis of travel time dataA6.3.3 Determining subsurface structure from travel

timesA6.3.4 Interpretation pitfallsA6.3.5 Example mapping prospective stratigraphy

using the CRM

Summary

Review questions

Further reading

References

Appendix 7 Sources of information onexploration and mining geophysicsA7.1 Journals and magazines

A7.1.1 Exploration Geophysics and PreviewA7.1.2 Geophysics and The Leading EdgeA7.1.3 Geophysical Prospecting and First BreakA7.1.4 Journal of Applied GeophysicsA7.1.5 Other periodicals

A7.2 Case-histories/geophysical signatures publications

A7.3 Internet

x List of online appendices

FIGURE CREDITS

The following publishers and organisations are gratefully acknowledged for their permission to useredrawn gures based on illustrations in journals, books and other publications for which they holdcopyright. We have cited the original sources in our gure captions. We have made every effort to obtainpermissions to make use of copyrighted materials and apologise for any errors or omissions. Thepublishers welcome errors and omissions being brought to their attention.

Copyright owner Figure number

Allen & UnwinImage Interpretation in Geology 2.31b

American Association of Petroleum GeologistsAAPG Bulletin 5.62

Australasian Institute of Mining and MetallurgyGeology of the Mineral Deposits of Australia and Papua NewGuinea

3.76c

Australian Society of Exploration GeophysicistsExploration Geophysics 2.9, 3.17, 5.57b, 5.81a,b,c, 5.89a,b,c, 5.93,

A3.3, A5.6, A5.8, A6.9, A6.10d

Preview 3.54

Cambridge University PressFundamentals of Geophysics 4.2

Canadian Institute of Mining, Metallurgy and PetroleumCIM Bulletin 3.74, 5.49, A5.3

Methods and Case Histories in Mining Geophysics,Proceedings of the Sixth Commonwealth Mining andMetallurgical Congress

3.77a

Canadian Society of Exploration GeophysicistsCSEG Recorder 5.89d

Centre for Exploration TargetingGeophysical Signatures of South Australian Mineral Deposits 5.55, 5.61

Geophysical Signatures of Western Australian MineralDeposits

4.24d, 4.30, 5.59

Predictive Mineral Discovery Under Cover (ExtendedAbstracts), SEG 2004

6.41a,c

Elsevier BVJournal of Geodynamics 2.8, 2.13


Journal of Applied Geophysics/Geoexploration 2.43b, 3.77d, 5.29g, 6.19, A5.5a,b

Elements 3.7

Geochimica, Cosmochimica Acta 3.34

Earth and Planetary Science Letters 3.49

Tectonophysics 3.63a

European Association of Geoscientists and EngineersFirst Break 2.37c

Geophysical Prospecting A5.2

Geological Association of CanadaGeophysics in Mineral Exploration: Fundamentals andCase Histories

2.37a

Geological Society of AmericaGeological Society of America Bulletin 3.51

Handbook of Physical Constants 6.38

Geological Society of LondonJournal of the Geological Society of London 3.47

Geological Survey of CanadaGeophysics and Geochemistry in the Search for Metallic Ores 2.37b, 5.56

Geological Survey of IndiaIndian Minerals 5.31

GeometricsApplications Manual for Portable Magnetometers 3.22

Geonics LtdTechnical Note TN-7 5.72

Geoscience AustraliaAGSO Journal of Australian Geology & Geophysics 3.39, 3.41, 3.42, 3.43, 4.3a, 4.6, 4.16, 4.18, 4.19

Airborne Gravity 2010 Abstracts from the ASEG-PESAAirborne Gravity 2010 Workshop

3.11

Harper & Row (HarperCollins)Solutions, Minerals and Equilibria 3.53

Institute of Materials, Minerals and MiningUranium Prospecting Handbook 4.23

International Research Centre for Telecommunications,Transmission and Radar, DelftProceedings of the Second International Workshop onAdvanced Ground Penetrating Radar

A5.5c

Leibniz-Institut fr Angewandte GeophysikGroundwater Resources in Buried Valleys. A Challenge forthe Geosciences

2.19

xii List of figure credits


McGraw-Hill IncIntroduction to Geochemistry 3.53

Natural Resources CanadaMining and Groundwater Geophysics 1967 3.77c

Northwest Mining AssociationPractical Geophysics for the Exploration Geologist II A4.4, A4.5

PG III Northwest Mining Associations 1998 PracticalGeophysics Short Course: Selected Papers

1.2, 1.3

NRC Research PressCanadian Journal of Earth Sciences 3.44, 3.56, 3.69

Prospectors and Developers Association of CanadaProceedings of Exploration 97: Fourth DecennialInternational Conference on Mineral Exploration

5.90, 5.96, 5.100, A5.1, A5.4

Pergamon (Elsevier)Applied Geophysics for Geologists and Engineers 2.49a

Physical Properties of Rocks: Fundamentals and Principles ofPetrophysics

3.33, 5.18

Geophysical Case Study of the Woodlawn Orebody, NewSouth Wales, Australia

5.66

Plenum Press (Springer Kluwer Academic)Electrical Properties of Rocks 5.13

Society of Economic GeologistsEconomic Geology 3.40, 3.45, 3.76c, 4.20, 4.21, 4.25, A4.9

Society of Exploration GeophysicistsGeophysics 3.77b, 4.9b,d, 5.17, 5.21, 5.26a,b, 5.83, 5.88,

6.40, 6.47, 6.48, 6.49, A3.2, A6.2a

Geotechnical and Environmental Geophysics, Volume 1 5.24

Hardrock Seismic Exploration 2.26, 3.37a, 6.13c, 6.14c, 6.41b

An Overview of Exploration Geophysics in China 3.64

Electromagnetic Methods in Applied Geophysics 5.80, A4.2b.c.d, A4.6, A4.10

Extended Abstracts, SEG Conference, Salt Lake City (2002) 6.51

SpringerPure and Applied Geophysics 2.43c

Studia Geophysica et Geodaetica 6.4d

Landolt-Bornstein: Numerical Data and FunctionalRelationships in Science and Technology

5.14

Taylor & FrancisAustralian Journal of Earth Sciences 3.74

List of figure credits xiii


University of Arizona PressGeology of the Porphyry Copper Deposits: Southwestern NorthAmerica

5.32

W.H. Freeman and CompanyInside the Earth 6.2, 6.3

Wiley/BlackwellA Petroleum Geologists Guide to Seismic Reection 2.21

xiv List of figure credits

PREFACE

This book is about how geophysics is used in the search formineral deposits. It has been written with the needs of themineral exploration geologist in mind and for the geo-physicist requiring further information about data inter-pretation, but also for the mining engineer and otherprofessionals, including managers, who have a need tounderstand geophysical techniques applied to mineralexploration. Equally we have written for students of geol-ogy, geophysics and engineering who plan to enter themineral industry.Present and future demands for mineral explorers

include deeper exploration, more near-mine explorationand greater use of geophysics in geological mapping. Thishas resulted in geophysics now lying at the heart of mostmineral exploration and mineral mapping programmes.We describe here modern practice in mineral geophysics,but with an emphasis on the geological application ofgeophysical techniques. Our aim is to provide an under-standing of the physical phenomena, the acquisition andmanipulation of geophysical data, and their integrationand interpretation with other types of data to produce anacceptable geological model of the subsurface. We havedeliberately avoided presenting older techniques and prac-tices not used widely today, leaving descriptions of these toearlier texts. It has been our determined intention to pro-vide descriptions in plain language without resorting tomathematical descriptions of complex physics. Only theessential formulae are used to clarify the basis of a geo-physical technique or a particular point. Full use has been

made of modern software in the descriptions of geophys-ical data processing, modelling and display techniques. Thereferences cited emphasise those we believe suit therequirements of the exploration geologist.We have endeavoured to present the key aspects of each

geophysical method and its application in the context ofmodern exploration practice. In so doing, we have sum-marised the important and relevant results of manypeoples work and also included some of our own originalwork. Key features of the text are the detailed descriptionsof petrophysical properties and how these inuence thegeophysical response, and the descriptions of techniquesfor obtaining geological information from geophysicaldata. Real data and numerous real-world examples, froma variety of mineral deposit types and geological environ-ments, are used to demonstrate the principles and conceptsdescribed. In some instances we have taken the liberty ofreprocessing or interpreting the published data to demon-strate aspects we wish to emphasise.M.D. has been an active researcher and teacher of

university-level geology and applied geophysics formore than 25 years. SM has been an active mineralsexploration geophysicist and researcher for more than35 years. We hope this book will be a source of under-standing for, in particular, the younger generation ofmineral explorers who are required to embrace andassimilate more technologies more rapidly than previousgenerations, and in times of ever increasing demand formineral discoveries.

ACKNOWLEDGEMENTS

This project would not have been possible without thegreat many individuals who generously offered assistanceor advice or provided materials. Not all of this made itdirectly into the nal manuscript, but their contributionshelped to develop the nal content and for this we are mostgrateful. They are listed below and we sincerely apologisefor any omissions:

Ray Addenbrooke, Craig Annison, Theo Aravanis, GaryArnold, William Atkinson, Leon Bagas, Simon Bate, KirstyBeckett, John Bishop, Tim Bodger, Miro Bosner, BarryBourne, Justin Brown, Amanda Buckingham, AndrewCalvert, Malcolm Cattach, Tim Chalke, Gordon Chunnett,David Clark, John Coggon, Jeremy Cook, Kim Cook,Gordon Cooper, Jun Cowan, Terry Crabb, Pat Cuneen,Giancarlo Dal Moro, Heike Delius, Mike Doyle, MarkDranseld, Joseph Duncan, Braam Du Ploy, David Eaton,Donald Emerson, Nicoleta Enescu, Brian Evans, PaulEvans, Shane Evans, Derek Fairhead, Ian Ferguson, KeithFisk, Andrew Fitzpatrick, Marcus Flis, Catherine Foley,Mary Fowler, Jan Francke, Kim Frankcombe, PeterFullagar, Stefan Gawlinski, Don Gendzwill, Mark Gibson,Howard Golden, Neil Goulty, Bob Grasty, Ronald Green,David Groves, Richard Haines, Greg Hall, Michael Hallett,Craig Hart, John Hart, Mike Hatch, Phil Hawke, NickHayward, Graham Heinson, Bob Henderson, LarissaHewitt, Eun-Jung Holden, Terry Hoschke, David Howard,Neil Hughes, Ross Johnson, Steven Johnson, GregoryJohnston, Aurore Joly, Leonie Jones, John Joseph,Christopher Juhlin, Maija Kurimo, Richard Lane, TerryLee, Michael Lees, Peter Leggatt, James Leven, Ted Lilley,Mark Lindsay, Andrew Lockwood, Andrew Long, JimMacnae, Alireza Malehmir, Simon Mann, Jelena Markov,Christopher Martin, Keith Martin, Charter Mathison, CamMcCuaig, Steve McCutcheon, Ed McGovern, StephenMcIntosh, Katherine McKenna, Glen Measday, JaysonMeyers, John Miller, Brian Minty, Bruce Mowat, ShaneMule, Mallika Mullick, Jonathan Mwenifumbo, HelenNash, Adrian Noetzli, Jacob Paggi, Derecke Palmer, Glen

Pears, Allan Perry, Mark Pilkington, Sergei Pisarevski,Louis Polome, Rod Pullin, Des Rainsford, Bret Rankin,Emmett Reed, James Reid, Robert L. Richardson, MikeRoach, Brian Roberts, Chris Royles, Greg Ruedavey,Michael Rybakov, Lee Sampson, Gilberto Sanchez, IanScrimgeour, Gavin Selfe, Kerim Sener, Nick Sheard, RobShives, Jeff Shragge, Richard Smith, John Stanley, EdgarStettler, Barney Stevens, Ian Stewart, Larry Stolarczyk, NedStolz, Rob Stuart, Nicolas Thebaud, Ludger Timmen, AllanTrench, Jarrad Trunfell, Greg Turner, Ted Tyne, PhilUttley, Frank van Kann, Lisa Vella, Chris Walton, HerbWang, Tony Watts, Daniel Wedge, Bob Whiteley, ChrisWijns, Ken Witherley, Peter Wolfgram, Faye Worrall,Simon van der Wielen and Binzhong Zhou. Particularthanks are due to Duncan Cowan of Cowan GeodataServices for creating almost every image in the book and toAndrew Duncan of EMIT for creating the EM modelcurves.

We also thank Simon Tegg for his work colourising thegures. From Cambridge University Press, we thank LauraClark, Susan Francis, Matthew Lloyd, Lindsay Nightingaleand Sarah Payne.

We are also very grateful to the following organisations forproviding, or allowing the use of, their data or access togeophysical software:

Barrick (Australia Pacic) LimitedCGGDepartment of Manufacturing, Innovation, Trade,Resources and Energy, South AustraliaEMIT Electromagnetic Imaging TechnologyEvolution MiningGeological Survey of BotswanaGeological Survey of NSW, NSW Trade & InvestmentGeological Survey of Western Australia, Department ofMines and PetroleumGeometrics

GeonicsGeoscience AustraliaGeotech Geophysical SurveysGPX SurveysGround Probe (SkyTEM)Haines SurveysMines Geophysical ServicesMontezuma Mining CompanyNatural Resources Canada, Geological Survey ofCanadaNorthern Territory Geological SurveyOntario Geological Survey

University of British Colombia, Geophysical InversionFacility (UBC-GIF)

Finally we are most grateful to the six industry sponsors:Carpentaria Exploration, First Quantum Minerals, MMG,Rio Tinto Exploration, AngloGold Ashanti and St Barbara,plus the Centre for Exploration Targeting at the Universityof Western Australia, whose nancial support has allowedus to produce a textbook with colour throughout, greatlyimproving the presentation of the data.

Mike Dentith and Stephen Mudge

Acknowledgements xvii

CHAPTER

1 IntroductionGeophysical methods respond to differences in the

physical properties of rocks. Figure 1.1 is a schematic

illustration of a geophysical survey. Over the area of

interest, instruments are deployed in the field to meas-

ure variations in a physical parameter associated with

variations in a physical property of the subsurface. The

measurements are used to infer the geology of the

survey area. Of particular significance is the ability of

geophysical methods to make these inferences from a

distance, and, for some methods, without contact with

the ground, meaning that geophysics is a form of

remote sensing (sensu lato). Surveys may be conducted

on the ground, in the air or in-ground (downhole).

Information about the geology can be obtained at

scales ranging from the size of a geological province

down to that of an individual drillhole.

Geophysics is an integral part of most mineral

exploration programmes, both greenfields and

brownfields, and is increasingly used during the

mining of orebodies. It is widely used because it can

map large areas quickly and cost effectively, delineate

subtle physical variations in the geology that might

otherwise not be observed by field geological investi-

gations and detect occurrences of a wide variety of

mineral deposits.

It is generally accepted that there are few large ore-

bodies remaining to be found at the surface, so mineral

exploration is increasingly being directed toward

searching for covered and deep targets. Unlike geo-

chemistry and other remote sensing techniques,

geophysics can see into the subsurface to provide

information about the concealed geology. Despite this

advantage, the interpretation of geophysical data is

critically dependent on their calibration against geo-

logical and geochemical data.

Folded massive nickel sulphide mineralisation in the Maggie Hays mine, Western Australia. The eld of view is 1.2 m wide.Photograph: John Miller.

1.1 Physical versus chemicalcharacterisation of the geologicalenvironment

The geophysical view of the geological environmentfocuses on variations in the physical properties withinsome volume of rock. This is in direct contrast with thegeological view, which is primarily of variations in the bulkchemistry of the geology. The bulk chemistry is inferredfrom visual and chemical assessment of the proportions ofdifferent silicate and carbonate minerals at locations wherethe geology happens to be exposed, or has been drilled.These two fundamentally different approaches to assessingthe geological environment mean that a particular area ofgeology may appear homogeneous to a geologist but maybe geophysically heterogeneous, and vice versa. The twoperspectives are complementary, but they may also appear

to be contradictory. Any contradiction is resolved by thechemical versus physical basis of investigating thegeology. For example, porosity and pore contents arecommonly important inuences on physical properties,but are not a factor in the various schemes used bygeologists to assign a lithological name, these schemesbeing based on mineralogical content and to a lesser extentthe distribution of the minerals.Some geophysical methods can measure the actual

physical property of the subsurface, but all methods aresensitive to physical property contrasts or relativechanges in properties, i.e. the juxtaposition of rocks withdifferent physical properties. It is the changes in physicalproperties that are detected and mapped. This relativistgeophysics approach is another fundamental aspect thatdiffers from the absolutist geological approach. Forexample, one way of geologically classifying igneous rocks

A AB

Location

Responsefrom mineralisation

Geo

phys

ical

resp

onse

Responsefrom contact

A

Transmitter

a)

b)

c)

Receiver(airborne survey)

A

Mineralisation

Responsefrom

mineralisation

Geophysical response

Dep

th

Responsefrom

contact

B

B

BEnergy originatingfrom mineralisation

Artificial energy

Receiver(downhole

survey)Mineralisation

BA

B

Receiver(ground survey)

Natural energy

A

Figure 1.1 Geophysical surveying schematicallyillustrated detecting mineralisation and mapping acontact between different rock types. Instruments(receivers) make measurements of a physical parameterat a series of locations on or above the surface (AA0) ordownhole (BB0). The data are plotted as a function oflocation or depth down the drillhole (a). (b) Passivegeophysical surveying where a natural source of energy isused and only a receiver is required. (c) Activegeophysical surveying where an articial source of energy(transmitter) and a receiver are both required.

2 Introduction

is according to their silica content, with absolute valuesused to dene categories such as felsic, intermediate,mac etc. The geophysical approach is equivalent to beingable to tell that one rock contains, say, 20% more silicathan another, without knowing whether one or both aremac, felsic etc.The link between the geological and geophysical

perspectives of the Earth is petrophysics the study ofthe physical properties of rocks and minerals, which isthe foundation of the interpretation of geophysical data.Petrophysics is a subject that we emphasise stronglythroughout this book, although it is a subject in whichsome important aspects are not fully understood and moreresearch is urgently required.

1.2 Geophysical methods in explorationand mining

Geophysical methods are used in mineral exploration forgeological mapping and to identify geological environ-ments favourable for mineralisation, i.e. to directly detect,or target, the mineralised environment. During exploit-ation of mineral resources, geophysics is used both indelineating and evaluating the ore itself, and in the engin-eering-led process of accessing and extracting the ore.There are ve main classes of geophysical methods,

distinguished according to the physical properties of thegeology to which they respond. The gravity and magneticmethods detect differences in density and magnetism,respectively, by measuring variations in the Earths gravityand magnetic elds. The radiometric method detectsvariations in natural radioactivity, from which the radio-element content of the rocks can be estimated. The seismicmethod detects variations in the elastic properties of therocks, manifest as variations in the behaviour of seismicwaves passing through them. Seismic surveys are highlyeffective for investigating layered stratigraphy, so they arethe mainstay of the petroleum industry but are compara-tively rarely used by the minerals industry.The electrical methods, based on the electrical properties

of rocks and minerals, are the most diverse of the veclasses. Electrical conductivity, or its reciprocal resistivity,can be obtained by measuring differences in electricalpotentials in the rocks. When the potentials arise fromnatural processes the technique is known as the spontan-eous potential or self-potential (SP) method. When theyare associated with articially generated electric currentspassing through the rocks, the technique is known as the

resistivity method. An extension to this is the inducedpolarisation (IP)method which measures the ability of rocksto store electric charge. Electrical properties can also beinvestigated by using electric currents created and measuredthrough the phenomenon of electromagnetic induction.These are the electromagnetic (EM) methods, and whilstelectrical conductivity remains an important factor, differentimplementations of the technique can cause other electricalproperties of the rocks to inuence the measurements.The physical-property-based categorisation described

above is complemented by a two-fold classication of thegeophysical methods into either passive or active methods(Fig. 1.1b and c).Passive methods use natural sources of energy, of which

the Earths gravity and magnetic elds are two examples, toinvestigate the ground. The geophysical measurement ismade with some form of instrument, known as a detector,sensor or receiver. The receiver measures the response of thelocal geology to the natural energy. The passive geophysicalmethods are the gravity, magnetic, radiometric and SPmethods, plus a form of electromagnetic surveying knownas magnetotellurics (described in online Appendix 4).Active geophysical methods involve the deliberate

introduction of some form of energy into the ground, forexample seismic waves, electric currents, electromagneticwaves etc. Again, the grounds response to the introducedenergy is measured with some form of detector. The needto supplement the detector with a source of this energy,often called the transmitter, means that the active methodsare more complicated and expensive to work with. How-ever, they do have the advantage that the transmission ofthe energy into the ground can be controlled to produceresponses that provide particular information about thesubsurface, and to focus on the response from some region(usually depth) of particular interest. Note that, confus-ingly, the cause of a geophysical response in the subsurfaceis also commonly called a source a term and context weuse extensively throughout the text.

1.2.1 Airborne, ground and in-ground surveys

Geophysical surveying involves making a series of meas-urements over an area of interest with survey parametersappropriate to the scale of the geological features beinginvestigated. Usually, a single survey instrument is used totraverse the area, either on the ground, in the air or withina drillhole (Fig. 1.1). Surveys from space or on water arealso possible but are uncommon in the mining industry. In

1.2 Geophysical methods in exploration and mining 3

general, airborne measurements made from a low-yingaircraft are more cost-effective than ground measurementsfor surveys covering a large area or comprising a largenumber of readings. The chief advantages of airbornesurveying relative to ground surveying are the greaterspeed of data acquisition and the completeness of thesurvey coverage.As exploration progresses and focuses on smaller areas,

there is a general reduction in both the extent of geophys-ical surveys and the distances between the individualreadings in a survey. Airborne surveys are usually part ofthe reconnaissance phase, which is often the initial phaseof exploration, although some modern airborne systemsoffer higher resolution by surveying very close to theground and may nd application in the later stages ofexploration. Ground and drillhole surveys, on the otherhand, offer the highest resolution of the subsurface. Theyare mostly used for further investigation of areas targetedfrom the reconnaissance work for their higher prospectiv-ity, i.e. they are used at the smaller prospect scale.Methods that can be implemented from the air include

magnetics, known as aeromagnetics; gravity, sometimesreferred to as aerogravity or as currently implementedfor mineral exploration as airborne gravity gradiometry;radiometrics; and electromagnetics, usually referred to asairborne electromagnetics (AEM). All the geophysicalmethods can be implemented downhole, i.e. in a drillhole.Downhole surveys are a compact implementation ofconventional surface surveying techniques. There are twoquite distinct modes of making downhole measurements:downhole logging and downhole surveying.Downhole logging is where the in situ physical proper-

ties of the rocks penetrated by a drillhole are measured toproduce a continuous record of the measured parameter.Downhole logs are commonly used for making strati-graphic correlations between drillholes in the sedimentarysequences that host coal seams and iron formations.Measurements of several physical parameters, producing asuite of logs, allow the physical characterisation of the localgeology, which is useful for the analysis of other geophysicaldata and also to help plan future surveys, e.g. Mwenifumboet al. (2004). Despite the valuable information obtainable,multiparameter logging is not ubiquitous in mineral explor-ation. However, its use is increasing along with integratedinterpretation of multiple geophysical datasets.

Downhole surveying is designed to investigate the largerregion surrounding the drillhole, with physical propertyvariations obtained indirectly, and to indicate the direction

and even the shape of targets. That is, downhole electricalconductivity logging measures the conductivity of the rocksthat form the drillhole walls, whereas a downhole electro-magnetic survey detects conductivity variations, perhapsowing to mineralisation, in the volume surrounding thedrillhole. Downhole geophysical surveys increase the radiusof investigation of the drillhole, increase the depth of inves-tigation and provide greater resolution of buried targets.Geophysical surveys are sometimes conducted in

open-pit and underground mines; measurements are madein vertical shafts and/or along (inclined) drives, usually todetect and delineate ore horizons. There exists a rathersmall literature describing underground applications ofgeophysics, e.g. Fallon et al. (1997), Fullagar and Fallon(1997), and McDowell et al. (2007), despite many success-ful surveys having been completed. Application and imple-mentation of geophysics underground tend to be unique toa particular situation, and survey design requires a fairdegree of ingenuity to adapt the arrangement of transmit-ter and receiver to the connes of the undergroundenvironment. They are usually highly focused towardsdetermining a specic characteristic of a small volume ofground in the immediate surrounds. Electrical and mech-anical interference from mine infrastructure limits thesensitivity of surveys, which require a high level of plan-ning and coordination with mining activities. Also, datafrom in-mine surveys require particular skills to interpretthe more complex three-dimensional (3D) nature of theresponses obtained: for example, the response may eman-ate from overhead, or the survey could pass through thetarget. The generally unique nature of underground geo-physical surveys and our desire to emphasise the principlesand common practices of geophysics in mineral explor-ation restrict us from describing this most interestingapplication of geophysics, other than to mention, whereappropriate, the possibilities of using a particular geophys-ical method underground.

1.2.2 Geophysical methods and mineral deposits

The physical properties of the geological environmentmost commonly measured in mining geophysics aredensity, magnetism, radioactivity and electrical properties.Elastic (seismic) properties are not commonly exploited. Ingeneral, density, magnetism and radioactivity are used tomap the geology, the latter when the nature of the surfacematerials is important. The limited use of electrical prop-erties is due to their non-availability from an airborne

4 Introduction

platform, although AEM-derived conductivity measure-ments are becoming more common. Direct detection of amineralised environment may depend upon any one ormore of density, magnetism, radioactivity, electrical prop-erties and possibly elasticity. Table 1.1 summarises howcontrasts in physical properties are exploited in explorationand mining of various types of mineral deposits, and ingroundwater and petroleum studies.

1.2.3 The cost of geophysics

The effectiveness and cost of applying any tool to theexploration and mining process, be it geological,

geochemical, geophysical, or drilling, are key consider-ations when formulating exploration strategies. Afterall, the ultimate aim of the exploration process is todiscover ore within the constraints of time and cost;which are usually determined outside the realms of theexploration programme. In both exploration and pro-duction the cost of drilling accounts for a large portionof expenditure. An important purpose of geophysicalsurveying is to help minimise the amount of drillingrequired.The cost of a geophysical survey includes a xed

mobilisation cost and a variable cost dependent upon thevolume of data collected, with large surveys attracting

Table 1.1 Geophysical methods commonly used in the exploration and exploitation of some important types of mineral deposits.

Brackets denote lesser use. Also shown, for comparison, are methods used for petroleum exploration and groundwater studies.

L downhole logging, M geological mapping of prospective terrains, D detection/delineation of the mineralised environment.

The entries in the density column reflect both the use of ground gravity surveys and anticipated future use of aerogravity. Developed

from a table in Harman (2004).

Deposit type Density Magnetism

Electrical

properties Radioactivity Elastic properties

Iron formation (associated Fe) M D L M D D M (L)

Coal (M) L M D L L M D L

Evaporite-hosted K L M D L

Fe-oxide CuAu (IOCG) M D M D D D

Broken Hill type AgPbZn M (D) M D

Volcanogenic massive sulphide (VMS) CuPbZn M (D) M D D

Magmatic Cu, Ni, Cr and Pt-group M D M D D

Primary diamonds M M (M)

Uranium M M M D L

Porphyry Cu, Mo M M D D D

SEDEX PbZn M M (D) D

Greenstone belt Au M M

Epithermal Au M M M

Placer deposits M (M) M M

Sediment-hosted CuPbZn M M D

Skarns M M D (D)

Heavy mineral sands M D M D

Mineralisation in regolith and cover materials, e.g. Al, U, Ni D M D

Groundwater studies M D L L M

Petroleum exploration and production (M) L (M) (M) L L M (D) L

1.2 Geophysical methods in exploration and mining 5

favourable economies-of-scale. Additional costs can beincurred through lost time related to factors such asadverse weather and access restrictions to the survey area,all preventing progress of the survey. Local conditions arewidely variable, so it is impossible to state here the costs ofdifferent kinds of geophysical surveys. Nevertheless, it isuseful to have an appreciation for the approximate relativecosts of various geophysical methods compared with thecost of drilling. Drilling is not only a major, and often thelargest, cost in most exploration and mining programmes,it is often the only alternative to geophysics for investi-gating the subsurface.

Following the approach of Fritz (2000), Fig. 1.2 showsthe approximate relative cost of different geophysicalmethods. Of course the gures on which these diagramsare based can be highly variable owing to such factors asthe prevailing economic conditions and whether thesurveys are in remote and rugged areas. They shouldbe treated as indicative only. The seismic method is byfar the most expensive, which is one reason why it islittle used by the mining industry, the least expensivemethods being airborne magnetics and radiometrics. Theareas over which information is gathered for eachmethod are compared in Fig. 1.3, noting that cost

Drillhole(too small to show to scale)

IP/resistivity

Groundmagnetics

Fixed-wingaeromagnetics/radiometrics

Fixed-wingtime-domain AEM

a)

Airbornegravity gradiometry

Gravity

3.6 km2

10 km2

4 km2

50 km2

160 km2 of fixed-wing aeromagnetics with radiometrics (100 m line spacing).50 km2 of airborne gravity gradiometry (100 m line spacing).20 km2 of fixed-wing TDEM with magnetics and radiometrics (100 m line spacing).20 km2 of helicopter TDEM with magnetics and radiometrics (100 m line spacing).10 km2 of differential GPS-controlled ground magnetics (50 m line spacing, 1 m stn spacing).4 km2 of gradient array IP/resistivity (100 m line spacing, 50 m dipoles).3.6 km2 ground gravity stations (differential GPS-controlled, 100 m grid).

25 line km of fixed-loop TDEM profiles.10 line km of 50 m dipole 12-frequency CSAMT sections.810 line km of dipole-dipole IP/resistivity (50 to 100 m dipoles).6 km coincident-loop TDEM soundings (100 m stn spacing).2 line km of detailed shallow seismic data.

160 km2

20 km2 20 km2

Helicoptertime-domain AEM

Fixed-loop TDEMCSAMTIP/resistivityTDEM soundingsShallow seismic

b) Drillhole (too small to show to scale)

25 km10 km

8-10 km6 km

2 km

Figure 1.2 Approximate relative costs of differentkinds of geophysical surveys in terms of (a) area and(b) line-length surveyed for the cost of a singledrillhole. AEM airborne electromagnetics, CSAMT controlled source audio-frequency magnetotellurics,EM electromagnetics, IP induced polarisation.Redrawn with additions, with permission, fromFritz (2000).

6 Introduction

estimates are equated to the estimated total cost of asingle 300 m drillhole, including logging, assaying, reme-diation etc. The drillhole provides reliable geologicalinformation to a certain depth, but only from a verysmall area. Drilling on a grid pattern at 25 m intervalsover an area of 1 km2 would cost a few tens of millionsof dollars, but would only sample 3 ppm of the volume.Geophysical methods provide information from vastlygreater areas and volumes, albeit in a form that is notnecessarily geologically explicit and will not necessarilydirectly identify mineralisation. Despite this, appropri-ately designed geophysical surveys and appropriatelychosen data analysis are highly effective for optimallytargeting expensive drillholes.

1.3 About this book

Our focus is an explanation of the principles and modernpractice of geophysics in the search for mineral deposits.The explanations are presented from a perspective relevantto a mining industry geologist.

Throughout the text we emphasise the key aspects ofmineral exploration geophysics, in particular those aspectsthat affect the interpretation of geophysical data. Theseinclude petrophysics, the foundational science of geophys-ics; numerical processing of the data; the creation andinterpretation of raster imagery; problems presented bydeeply weathered environments; geophysical characteris-tics of geologically complex basement terrains; and theinability to remove noise completely from the measure-ments. We introduce the term geophysical paradox, whereto fully understand the geophysical signal (the informationof interest) and the noise (the interference producinguncertainty in the signal) requires information about thesubsurface, but the purpose of the geophysical survey is toacquire this very information. We emphasise the need tounderstand this fundamental aspect of geophysics whenworking with geophysical data.There have been many developments in geophysics in

recent years. We have deliberately avoided presenting oldertechniques and practices not used widely today, leavingdescriptions of these to earlier texts.The text is structured around the main geophysical

methods with each described in its own separate chapter.General aspects of the nature of geophysical data, theiracquisition, processing, display and interpretation,common to all methods, are described rst in a generalchapter, Chapter 2. Essential, and generally applicable,details of vectors and waves are described in the onlineAppendices 1 and 2, respectively. The other chapters aredesigned to be largely self-contained, but with extensivecross-referencing to other chapters, in particular toChapter 2. We have responded to the widespread comple-mentary use of gravity and magnetics by describing themin a single combined chapter, Chapter 3. Geophysicalmethods less commonly used by the mining industry aredescribed in online Appendices 3 to 6. Appendix 7 listssources of information about mineral exploration geophys-ics, especially case histories. The principles described aredemonstrated by examples of geophysical data and casestudies from a wide variety of mineral deposit types fromaround the world. All deposits referred to are listed inTable 1.2 and their locations shown on Fig. 1.4.At the conclusion of each chapter we provide a short list

of appropriate resource material for further reading on thetopic. The references cited throughout the text emphasisethose we believe suit the requirements of the explorationgeoscientist.

App

roxi

mat

e re

lativ

e co

st

x 10

x 100

x 1000

1

x 10000

Regional-scaleProspect-scaleTarget-scale

Seismic

IP/resistivity

CSAMTLarge-loopEM

Helicopter AEM

Fixed-wingAEM

Fixed-wingaeromagnetics/radiometrics

Reconnaissancegravity/aeromagnetics

Gravity/aeromagnetics(public domain)

Magnetics

Gravity

IP/resistivity(gradient array)

Aerogravity

Aerogravity gradiometry

100 101

Area (km2)101

Figure 1.3 Approximate relative (a) areas and (b) line lengthssampled by geophysical surveys costing the equivalent of a single300 m deep diamond drillhole. The area of the drillhole is shownfor comparison. AEM airborne electromagnetics, CSAMT controlled source audio-frequency magnetotellurics, GPS globalpositioning system, IP induced polarisation, TDEM time domainelectromagnetics. Redrawn with additions, with permission,from Fritz (2000).


Table 1.2 Locations of deposits and mineralised areas from which geophysical data are presented. IOCG Iron oxide copper gold,

MVT Mississippi Valley-type, SEDEX sedimentary exhalative, VMS volcanogenic massive sulphide.

Number Deposit name Commodities Deposit style/type Country Section

1 Adams Fe Iron formation Canada 3.11.3

2 Almora Graphite India 5.5.4.1

3 Balcooma CuAgAu VMS Australia 5.8.3.1

4 Bell Allard ZnCuAgAu VMS Canada 6.7.4.2

5 Blinman Cu Sediment hosted Australia 4.7.4

6 Bonnet Plume Basin Coal Canada 3.10.6.2

7 Broken Hill area PbZnAg Broken Hill type Australia 3.7

8 Buchans ZnPbCu VMS Canada 4.7.5

9 Butcherbird Mn Supergene Australia 5.9.5.1

10 Cluff Lake area U Unconformity style Canada 4.7.5

11 Cripple Creek district AgAuTe Epithermal USA 3.4.7

12 Cuyuna Iron Range Fe Iron formation USA 5.5.3.2

13 Dugald river ZnPbAg SEDEX Australia 4.7.5

14 Eloise CuAu SEDEX Australia 5.7.7.1

15 Elura ZnPbAg VMS Australia 2.6.1.2

16 Enonkoski (Laukunkangas) Ni Magmatic Finland 5.8.4

17 Ernest Henry CuAu IOCG Australia 5.7.7.1

18 Estrades CuZnAu VMS Canada 5.6.6.3

19 Franklin U Sandstone type USA 5.6.8.2

20 Glalana Cr Magmatic Turkey 3.11.5

21 Golden Cross/Waihi-Waitekauriepithermal area

AuAg Epithermal New Zealand 3.9.74.6.64.7.3.2A4.7.2

22 Goongewa/Twelve Mile Bore PbZn MVT Australia 5.6.7

23 Goonumbla/North Parkes area CuAu Porphyry Australia 3.11.44.6.6

24 Iron King PbZnCuAuAg VMS USA 4.6.6

25 Jharia Coaleld Coal India 3.11.55.5.3.2

26 Jimblebar Fe Iron formation Australia 4.7.5

27 Joma FeS Massive pyrite Norway 2.9.25.5.3.1

28 Kabanga Ni Magmatic Tanzania 3.9.8.2

29 Kerr Addison Au Orogenic Canada 3.11.3

30 Kimheden Cu VMS Sweden 5.5.3.2

8 Introduction

Table 1.2 (cont.)


31 Kirkland Lake Au Orogenic Canada 2.8.1.13.11.3

32 Las Cruces CuAu VMS Spain 3.7

33 Lisheen ZnPbAg Carbonate-hosted Eire 5.7.4.25.7.4.3

34 London Victoria Au Lode Australia 3.11.4.1A6.3.5

35 Maple Creek Au Placer Guyana A5.3.4.1

36 Marmora Fe Skarn Canada 2.6.43.11.5

37 Mirdita Zone Cu VMS Albania 5.5.3.1

38 Mount Isa PbZnCu SEDEX Australia 5.8.2A5.4.1

39 Mount Keith area Ni Magmatic Australia A3.3.1.1

40 Mount Polley Cu-Au Porphyry Canada 2.8.2

41 Murray Brook CuPbZn VMS Canada 2.9.2

42 New Insco Cu VMS Canada 5.5.3.1

43 Olympic Dam CuUAuAgREE IOCG Australia 2.7.2.35.6.6.3

44 Pajingo epithermal system (ScottLode, Cindy, Nancy and Vera)

Au Epithermal Australia 5.6.6.4

45 Palmietfontein Diamond Kimberlite-hosted South Africa 5.6.6.15.6.6.2

46 Pine Point PbZn MVT Canada 2.9.25.6.6.4

47 Port Wine area Au Placer USA 3.11.1

48 Poseidon Ni Magmatic Australia A3.4.1

49 Prairie Evaporite K Evaporite Canada 4.7.56.5.2.5

50 Pyhsalmi Ni Magmatic Finland 2.10.2.3

51 Qianan District Fe Iron Formation China 3.10.1.1

52 Red Dog ZnPb SEDEX USA 5.6.6.3

53 Regis Kimberlite Diamond Kimberlite-hosted Brazil A4.7.1

54 Rockys Reward Ni Magmatic Australia A5.3.4.2

55 Safford Cu Porphyry USA 5.5.4.2

56 Sargipalli Graphite India 5.5.3.1

57 Silvermines ZnPbAg Carbonate-hosted Eire 5.6.6.2


40

652 46

35

53

41

47

869

4

19

68

1842

3629

65

3111

73

6763

55

12

24

49

59

10

32

57331

28

27

20

50 1630

71

61

45

37 51

25

21

2 58

60

56

15

43

62

7

26

13

70 5749

64

38317

14 44

23

22

39

66

5448

34

72

180W 120W

120W

60W

60W

60E 120E

120E

180E0

60S

30S

30S

30N

60N

90N

0

90S

30N

60N

Figure 1.4 Locations of deposits and mineralised areas from which geophysical data are presented.

Table 1.2 (cont.)


58 Singhblum Cu Disputed India 5.5.3.2

59 South Illinois Coaleld Coal USA 6.7.4.1

60 Sulawesi Island Ni Lateritic Indonesia A5.3.4.1

61 Telkkl Taipalsaari Ni Magmatic Finland 2.10.2.3

62 Thalanga ZnPbCuAg SEDEX Australia 2.8.1

63 Thompson Ni Magmatic Canada 3.11.5

64 Trilogy CuAuAgPbZn VMS Australia 5.7.7.1

65 Tripod Ni Magmatic Canada 5.7.7.1

66 Uley Graphite Australia 5.6.8.1

67 Uranium City area U Unconformity style Canada 4.7.3.1

68 Victoria Graphite Canada 5.6.9.5

69 Voisey Bay Ni Magmatic Canada 6.8.2

70 Wallaby Au Orogenic Australia 3.11.2

71 Witwatersrand Goldeld Au Palaeoplacer South Africa 6.7

72 Woodlawn CuPbZn VMS Australia 5.6.9.4

73 Yankee Fork Mining District AgAu Epithermal USA 3.8.63.9.7

74 Yeelirrie U Calcrete-hosted Australia 4.7.3.1

10 Introduction

FURTHER READING

Blain, C, 2000. Fifty year trends in minerals discovery commodity and ore types. Exploration and Mining Geology,9, 111.

Nabighian, M.N. and Asten, M.W., 2002. Metalliferous mininggeophysics state of the art in the last decade of the 20thcentury and the beginning of the new millennium. Geophys-ics, 67, 964978.

A summary of the state of the art of mining geophysics, stillrelevant even though more than 10 years old.

Paterson, N.R., 2003. Geophysical developments and mine dis-coveries in the 20th century. The Leading Edge, 22, 558561.

This and the previous paper provide data on mineraldeposit discovery rates and costs, and their relationshipswith the use and development of geophysical methods.


CHAPTER

2 Geophysical data acquisition,processing and interpretation2.1 Introduction

The use of geophysical methods in an exploration pro-

gramme or during mining is a multi-stage and iterative

process (Fig. 2.1). The main stages in their order of

application are: definition of the survey objectives,

data acquisition, data processing, data display and

then interpretation of different forms of the data.

The geologist should help to define the objectives of

the survey and should have a significant contribution

during interpretation of the survey data, but to ensure

an optimum outcome, an understanding of all the

other stages highlighted by Fig. 2.1 is required. Survey

objectives dictate the geophysical method(s) to be

used and the types of surveys that are appropriate,

e.g. ground, airborne etc. Data acquisition involves

the two distinct tasks of designing the survey and

making the required measurements in the field. Data

processing involves reduction (i.e. correcting the

survey data for a variety of distorting effects),

enhancement and display of the data, all designed to

highlight what is perceived to be the most geologically

relevant information in the data. The processed data

can be displayed in a variety of ways to suit the nature

of the dataset and the interpreters requirements in

using the data. Data interpretation is the analysis of

the geophysical data and the creation of a plausible

geological model of the study area. This is an indeter-

minate process; an interpretation evolves through the

iterative process as different geological concepts are

tested with the data. It is often necessary to revise

aspects of the data enhancement as different charac-

teristics of the data assume greater significance, or as

increased geological understanding allows more accur-

ate reduction.

The interpreter needs to have a good understanding

of the exploration strategy which was the basis for

defining the survey objectives. Ideally the interpreter

should also have a working knowledge of the geophys-

ical acquisitionprocessing sequence since this impinges

on the evolving interpretation of the data. The type of

survey and the nature of the data acquisition affect the

type and resolution of the geological information

obtainable, whilst the interpretation of geophysical

data is dependent on the numerical methods applied

to enhance and display the data. Analysis of the data

involves its processing and interpretation. We empha-

sise that interpretation is not a task to be undertaken in

isolation; it is an inextricable part of the iterative and

multi-stage analysis shown in Fig. 2.1.

Figure 2.1 illustrates the framework for this chapter.

We discuss in turn the various stages in using a geophys-

ical method, but before doing so we discuss some gen-

eral aspects of geophysical measurements, geophysical

responses, and the important concepts of signal and

noise.

Airborne magnetic survey aircraft. Image courtesy of New Resolution Geophysics.

2.2 Types of geophysical measurement

The parameters measured in the various types of geophys-ical surveys described in Section 1.2 are continuous, i.e.they vary in time or space and without gaps or end. Thevariations are an analogue representation of the physicalproperty variations that occur in the subsurface. Measuringor sampling an analogue signal at discrete times or atdiscrete locations is known as digitisation. The continuousvariation is then represented by a series of data samplesforming a digital series, a form most convenient for storageand processing by a computer.A geophysical survey consists of a series of measure-

ments made at different locations; usually different

geographic locations, or different depths in a drillhole.The location assigned to the measurement is usually thesensor location but may be some point between the trans-mitter and the sensor. The resultant measurements, i.e. thedataset, comprise a spatial series in the spatial domain.Each of the measurements may comprise a single reading,or may be a series of readings made over an interval of timeto form a time series in the time domain, or over a range offrequencies to form a frequency series in the frequencydomain. In some geophysical methods (e.g. electrical meas-urements), time- and frequency-series data provide theinformation about the nature of the rocks at the measure-ment location; and in other methods (e.g. seismic andsome kinds of electromagnetic measurements) they areused to infer variations in the geology with distance fromthe measurement location. This might be lateral distancefrom a drillhole, but is most commonly depth below asurface reading. The latter are then known as soundings.Series of all types of geophysical data can be conveni-

ently treated as waves, and we use wave terminologythroughout the text. It is strongly recommended that thosereaders unfamiliar with waves and their properties consultonline Appendix 2 for details.

2.2.1 Absolute and relative measurements

Most kinds of geophysical surveys make absolutemeasurements of the parameter of interest. This is notalways necessary; for some kinds of survey, notably gravityand magnetic surveys, relative measurements provide suf-cient information. In general, relative measurements havethe advantage of being cheaper and easier to make thanabsolute measurements.A survey comprising relative measurements requires one

or more reference locations, called base stations, and themeasurements are said to be tied to the base stations. Theabsolute value of the parameter at the base stations may beknown, in which case making comparative measurementsat other locations allows the absolute values to be deter-mined elsewhere. For example, when we say that thestrength of Earths magnetic eld at a base station is50,000 nanoteslas (nT), we are referring to the absolutevalue of the eld. If the eld strength at a second station is51,000 nT, then its relative value with respect to the basestation is +1000 nT (and the base station has a relativevalue of 1000 nT with respect to the second station). If themagnetic eld at a third station has a relative strength of+2000 nT with respect to the base station, then it has an

Logistical &cost constraints

Expectedresponses

Topography

Instrumentresponses

Noisecharacteristics

Explorationmodel

Geophysicalmodel

Survey design

Define survey objectives(mapping vs targeting)

Additionalgeophysical

surveys

Field surveys

DATA ACQUISITION

Othergeoscientific

data

DATA PROCESSING

DATA INTERPRETATION

Multiple products

DATA DISPLAY

Multiple products

Modifybased onevolving

interpretation

Outcomes(pseudo-geological map)

(drilling targets)

Reduction

Enhancement

Qualitative

Quantitative(modelling)

1D/2D/3D Products

Figure 2.1 The principal stages of a geophysical programme in mineralexploration: from identifying the objectives of the geophysical survey(s)through to providing an interpretation of the subsurface geology.

14 Geophysical data acquisition, processing and interpretation

absolute value of 52,000 nT. In terms of relative values, thebase station is assigned a value of zero. In large surveysthere may be a master base station from which a series ofsubsidiary base stations are established. This facilitatessurveying by reducing the distance that needs to be trav-elled to the nearest base station. Note that the accuracy ofthe absolute value of a parameter obtained by relativemeasurement from a base station is dependent on theaccuracy of the absolute value at the base station and theaccuracy of the relative measurement itself.

2.2.2 Scalars and vectors

Physical quantities are classied into two classes. Thosethat have magnitude only are known as scalar quantities orsimply scalars. Some examples include mass, time, densityand speed. Scalar quantities are described by multiples oftheir unit of measure. For example, the mass of a body isdescribed by the unit of kilogram and a particular mass isdescribed by the number of kilograms. Scalar quantities aremanipulated by applying the rules of ordinary algebra, i.e.addition, subtraction, multiplication and division. Forexample, the sum of two masses is simply the addition ofthe individual masses.Some physical quantities have both magnitude and dir-

ection and are known as vector quantities or simply vectors.Some examples are velocity, acceleration and magnetism.They are described by multiples of their unit of measureand by a statement of their direction. For example, todescribe the magnetism of a bar magnet requires a state-ment of how strong the magnet is (magnitude) and itsorientation (direction). The graphical presentation andalgebraic manipulation of vectors are described in onlineAppendix 1.Measuring vector parameters in geophysics implies that

the sensor must be aligned in a particular direction. Oftencomponents of the vector are measured. Measurements inperpendicular horizontal directions are designated as the Xand Y directions, which may correspond with east andnorth; or with directions dened in some other referenceframe, for example, relative to the survey traverse alongwhich measurements are taken. Usually the X direction isparallel to the traverse. Measurements in the vertical aredesignated as Z, although either up or down may be takenas the positive direction depending upon accepted stand-ards for that particular measurement. We denote the com-ponents of a vector parameter (P) in these directions as PX,PY and PZ, respectively.

2.2.3 Gradients

Sometimes it is useful to measure the variation in theamplitude of a physical parameter (P) over a small distanceat each location. The difference in the measurements fromtwo sensors separated by a xed distance and oriented in aparticular direction is known as the spatial gradient of theparameter. It is specied as units/distance in the measure-ment direction, and so it is a vector quantity. As themeasurement distance decreases, the gradient convergesto the exact value of the derivative of the parameter, aswould be obtained from calculus applied to a functiondescribing the parameter eld. For the three perpendiculardirections X, Y and Z, we refer to the gradient in the Xdirection as the X-derivative and, using the notation ofcalculus, denote it as P/x. Similarly, we denote theY-derivative as P/y and the Z-derivative as P/z.Gradients may be measured directly using a gradi-

ometer, which comprises two sensors positioned a shortdistance apart (Fig. 2.2a). Alternatively, it is usuallypossible to compute gradients, commonly referred to asderivatives, directly from the non-gradient survey meas-urements of the eld (see Gradients and curvature inSection 2.7.4.4).

Gradientdistance

Gradientdistance

Verticalgradient

Horizontalgradient

Sensors

a)

b)

X

Y

Z

PZZPZY

PZX

PYZ

PZPY

PXPXZ

PXY

PXX

PYYPYX

Figure 2.2 Gradient measurements. (a) Vertical and horizontalgradiometers. (b) The three perpendicular gradients of each of thethree perpendicular components of a vector parameter P forming thegradient tensor of P, shown using tensor notation; see text for details.

2.2 Types of geophysical measurement 15

Gradient measurements have the advantage of not beingaffected by temporal changes in the parameter being meas-ured; the changes affect both sensors in the same way so anydifference in the parameter at each sensor is maintained.Gradient data are very sensitive to the edges of sources.They comprise variations that are more spatially localisedthan non-gradient data and so have an inherently greaterspatial resolution (Fig. 2.3). The main disadvantage of gra-dient measurements is that they are very sensitive to vari-ations in the orientation of the sensor. Also, long-wavelength variations in the parameter, which produce verysmall gradients, are often not large enough to be detected.The derivatives in the three perpendicular directions of

each of the three components of a vector parameter (P)(Fig. 2.2b) completely describe the parameter at the

measurement point. We denote the derivative of the Xcomponent of P (PX) in the X direction as PX/x, andthe derivatives of the same component in the Y and Zdirections are PX/y and PX/z, respectively; andsimilarly for the Y and Z components. They form a tensorand are displayed and manipulated in matrix form:

PXx

PXy

PXz

PYx

PYy

PYz

PZx

PZy

PZz

0BBBBBBBB@

1CCCCCCCCA

orPXX PXY PXZPYX PYY PYZPZX PZY PZZ

0@

1A 2:1

Several components of the tensor are related as follows: PXY PYX, PXZ PZX and PYZ PZY, so it is not necessary tomeasure all of them. This means that less complex sensorsare needed and measurements can be made more quickly.The full-gradient tensor of nine components, i.e. the gra-

dients in the three components in all three directions, pro-vides diagnostic information about the nature of the sourceof a geophysical anomaly. Tensormeasurements aremade inairborne gravity surveying (see Section 3.3.2) but are other-wise comparatively rare in other geophysical surveys atpresent. It seems likely that they will become more commonin the future because of the extra information they provide.

2.3 The nature of geophysical responses

As described in Section 1.1 and shown schematically inFig. 1.1, geophysical surveys respond to physical propertycontrasts, so changes in the local geology can producechanges in the geophysical response of the subsurface.When the measured property of a target zone is greaterthan that of the host rocks, the contrast is positive; whenlower, it is negative. Typically the changes are localised,arising perhaps from a body of mineralisation or a contactof some kind. These deviations from background values arecalled anomalies. The simplest form of anomaly is anincrease or decrease of themeasured parameter as the surveytraverses the source of the anomaly. Often, though, peaks inthe anomaly are offset from their source and/ormay bemorecomplex in form; for example, the response from magneticsources may comprise both an increase and an adjacentdecrease in response, forming a dipole anomaly.Although the underlying physics of each geophysical

method is different, some important aspects of the measuredresponses are the same. Figure 2.4 shows some general

Source

Response

Horizontal gradient of response

a)

0

Response

Horizontal gradient of response

b)

Source

0

0

0

Figure 2.3 Horizontal gradient data across (a) a localised source,and (b) a contact. Note how the gradient response is localised nearthe source edges.


characteristics of (non-seismic) geophysical responses usinga simple shaped anomaly. A negative contrast produces ananomaly that is an inverted image of that for an identicalpositive contrast of the same source geometry (Fig. 2.4a). Theamplitude (A) of an anomaly depends on the magnitude ofthe physical property contrast and the physical size of theanomalous distribution. In general, increasing the propertycontrast increases the amplitude of the anomaly proportion-ally. Figure 2.4b shows that for two bodies of the same size theanomaly has larger amplitude when there is a larger contrast.

The amplitude of the anomaly decreases the further thesource is from the transmitter (if there is one). Also,increasing sourcedetector separation causes the ampli-tude of the response to decrease and to extend over a widerarea, i.e. there is an accompanying increase in wavelength() of the anomaly. The increased separation could bebecause the source is at a greater depth below the surfaceor because the sensor is at a greater height above thesurface, as in the case of airborne measurements. Whenthe source varies in shape, this variation also affects theanomaly, with increasing source width producing longerwavelength responses (Fig. 2.4d).Figures 2.4 and 2.49a (see Section 2.11.4) illustrate some

important characteristics of many kinds of geophysicalresponses. In general, the deeper the anomalous bodyand/or the smaller its property contrast, the larger its sizemust be in order for it to be detectable against the inevit-able background noise (see Section 2.4). Also, anomalieswith the same amplitude and wavelength can be caused byvarious combinations of source depth, geometry and con-trast with the host rocks. Without additional informationabout these variables, the actual nature of the source of theanomaly is indeterminable. This problem of ambiguity isdiscussed further in Section 2.11.4.In summary, whether or not an anomalous physical

property distribution produces a recognisable geophysicalresponse depends on its size and the magnitude of thecontrast between it and the surrounding rocks. In addition,the physical property contrast of a geological feature canchange markedly as the properties of the surroundingrocks and/or those of the target feature change, both lat-erally and with depth. This can signicantly change thenature of the geophysical response; it can form multiplegeophysical targets related to different parts of the samegeological feature and even change the type of geophysicalmeasurements needed to detect it.

2.4 Signal and noise

A measurement of any kind, but especially one made in asetting as complex and unpredictable as the natural envir-onment, will be contaminated with unwanted information.This unwanted information is known as noise and is asource of error in a measurement, whilst the informationbeing sought in the measurement is known as signal. Therelative amounts of signal and noise in a measurement arequantied by the signal-to-noise ratio (SNR). Ideally, onehopes that the amplitude of the signal, the signal level, is as

Dep

thD

epth

Dep

thD

epth

b)

0

a)

P=Positive

+

+

+

+

P=Negative

0

P=Smaller P=Larger

c)

0

d)

0

A

l

Figure 2.4 The general characteristics of a geophysical response andhow these change with variations in (a) sign of the physical propertycontrast (P), (b) magnitude of the contrast, (c) depth of the sourceand (d) shape of the source.

2.4 Signal and noise 17

high as possible and that the amplitude of the noise, thenoise level, is as low as possible in order to obtain anaccurate measurement of the parameter of interest. As ageneral rule, if SNR is less than one it will be very difcultto extract useful information from the measurement,although data processing techniques are available toimprove the situation (see Section 2.7.4).Suppression of noise is of utmost importance and must

be considered at every stage of the geophysical programme,from data acquisition through to presentation of the datafor interpretation. Active geophysical methods usuallyallow the SNR to be improved by changing the nature ofthe output from the transmitter, e.g. increasing itsamplitude or changing its frequency. This advantage is lostwith passive methods, where the geophysicist has no con-trol over the natural transmitter.The signal depends solely on the objective of the survey,

and geological responses not associated with the objectiveof the survey constitute noise. Of course, any response ofnon-geological origin will always be considered noise. Asdata are revisited the information required from them mayvary, in which case so too do the representations of signaland noise in the data. A useful denition of signal is thenwhat is of interest at the time, whilst noise would then beeverything else; just as the saying goes, One mans trash isanother mans treasure, so also one geoscientists signal isanother geoscientists noise.Two basic types of noise affect geophysical measure-

ments. Firstly, there are effects originating from the localenvironment, i.e. environmental noise. Secondly, there ismethodological noise, which includes unwanted conse-quences of the geophysical survey itself and of the process-ing of the geophysical data. A feature in the data that iscaused by noise is referred to as an artefact. It goes withoutsaying that identication and ignoring of artefacts is crit-ical if the data are to be correctly interpreted.

2.4.1 Environmental noise

The main types of environmental noise affecting thedifferent types of geophysical survey are summarised inTable 2.1. Environmental noise can be categorised by itsorigin; as either geological or non-geological. Geologicalenvironmental noise is produced by the geological envir-onment, including topography. Non-geological environ-mental noise includes sources in the atmosphere andouter space, plus cultural responses associated with humanactivities.

Wind is a common source of noise from the atmos-phere. It causes objects attached to the ground to move,e.g. trees and buildings, which produces noise in seismic,electromagnetic and gravity surveys. The movement ofwires linking sensors to recording equipment may alsocreate noise because of voltages induced by their move-ment through the Earths magnetic eld (see Section5.2.2.2). Wind turbulence also causes variations in theposition and orientation of geophysical sensors duringairborne surveys which affect the measurements.As well as creating noise, natural phenomena may

reduce the amplitude of the signal: for example, radioactiv-ity emitted from soil is attenuated when the soil is satur-ated by rainfall. The variability and unpredictability ofnatural phenomena cause noise levels to vary during thecourse of a geophysical survey.Cultural noise includes the effects of metal fences, rail-

ways, pipelines, powerlines, buildings and otherinfrastructure (see Section 2.9.1). In addition, cultural fea-tures may radiate energy that causes interference, such aselectromagnetic transmissions (radio broadcasts etc.),radioactive fallout and the sound of machinery such asmotor trafc. Mine sites are particularly noisy environ-ments, and noise levels may be so high as to precludegeophysical surveying altogether.The two most troublesome forms of geological environ-

mental noise are those associated with the shallow subsur-face and with topography; the latter are known astopographic or terrain effects. In both cases it is possible,in principle, to calculate their effects on the data andcorrect for them. To do so requires very detailed infor-mation about the terrain and/or physical properties of thesubsurface, which is often lacking. This is an example ofthe geophysical paradox (see Section 1.3). To fully under-stand the geophysical signal, and the noise, requires infor-mation about the subsurface. However, it was to acquiresuch information that the geophysical survey wasundertaken.

2.4.1.1 Topography-related effectsSome examples of topography-related noise are shownschematically in Fig. 2.5a. In rugged terrains, topographycreates noise by causing variations in the distance betweengeophysical transmitters and/or sensors and features in thesubsurface. This changes the amplitude and wavelength ofthe responses (see Section 2.3). These effects can some-times be accounted for by modifying the measurements,during data reduction. The accuracy with which this can be


Table 2.1 Common sources of environmental noise and the forms in which they manifest themselves for the various geophysical

methods. Specific details are included in the relevant chapters on each geophysical method.

Source of noise Gravity Magnetics Radiometrics

Electrical and

electromagnetics Seismic

Regolith Changes inthickness andinternal variationsin density causingspuriousanomalies

Oxidation of magneticmineral speciesFormation ofmaghaemite causingspurious anomalies

Concealment of bedrockresponsesMobilisation of radioactivematerials causingresponses that are notindicative of bedrock

High conductivityleading to poor signalpenetration andelectromagneticcoupling withmeasurement arrayInternal changes inconductivity(groundwater, clays)causing spuriousanomaliesSuperparamagneticbehaviour (maghaemite)

Changes inthickness andinternal changesin velocityaffectingresponses(statics)Reduction inthe energytransmitted fromsource

Glacialsediments

Changes inthickness andinternal variationsin density causingspuriousanomalies

Magnetic detrituscausing spuriousanomalies

Concealment of bedrockresponsesMobilisation of radioactivematerials causingresponses that are notindicative of bedrock

Internal changes inconductivity causingspurious anomalies

Changes inthickness andinternal changesin velocityaffectingresponses frombelow (statics)

Permafrost andsnow cover

Changes in icecontent causingspuriousanomalies

Concealment of bedrockresponses

Internal changes inconductivity causingspurious anomalies

Changes inthickness andinternal changesin velocityaffectingresponses frombelow (statics)

Hydrological Formation o

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