Field Geophysics

249 Pages·2004·3.84 MB·English

by  Milsom, John.

Field Geophysics
THIRDEDITION
John Milsom
UniversityCollegeLondon

Field Geophysics
TheGeologicalFieldGuideSeries
BasicGeologicalMapping,Thirdedition JohnBarnes
TheFieldDescriptionofMetamorphicRocks NormanFry
TheMappingofGeologicalStructures KenMcClay
FieldGeophysics,Thirdedition JohnMilsom
TheFieldDescriptionofIgneousRocks RichardThorpe&GeoffBrown
SedimentaryRocksintheField,Secondedition MauriceTucker
Field Geophysics
THIRDEDITION
John Milsom
UniversityCollegeLondon
Copyright2003by JohnMilsom
Published2003by JohnWiley&SonsLtd,
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LibraryofCongressCataloging-in-PublicationData
Milsom,John,1939–
Fieldgeophysics/JohnMilsom.–3rded.
p.cm.–(Thegeologicalfieldguideseries)
Includesbibliographicalreferencesandindex.
ISBN0-470-84347-0(alk.paper)
1.Prospecting–Geophysicalmethods.I.Title.II.Series.
TN269.F4452002
622(cid:1).15–dc21 2002191039
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ISBN0-470-84347-0
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Contents
PrefacetotheFirstEdition vii
PrefacetotheSecondEdition ix
PrefacetotheThirdEdition xi
1 Introduction 1
1.1 Fields 1
1.2 GeophysicalFieldwork 5
1.3 GeophysicalData 10
1.4 BasesandBaseNetworks 22
1.5 GlobalPositioningSatellites 25
2 GravityMethod 29
2.1 PhysicalBasisoftheGravityMethod 29
2.2 GravityMeters 31
2.3 GravityReductions 38
2.4 GravitySurveys 41
2.5 FieldInterpretation 46
3 MagneticMethod 51
3.1 MagneticProperties 51
3.2 TheMagneticFieldoftheEarth 53
3.3 MagneticInstruments 58
3.4 MagneticSurveys 62
3.5 SimpleMagneticInterpretation 67
4 RadiometricSurveys 71
4.1 NaturalRadiation 71
4.2 RadiationDetectors 75
4.3 RadiometricSurveys 78
5 ElectricCurrentMethods–GeneralConsiderations 83
5.1 ResistivityandConductivity 83
5.2 DCMethods 88
5.3 VaryingCurrentMethods 91
6 ResistivityMethods 97
6.1 DCSurveyFundamentals 97
6.2 ResistivityProfiling 107
6.3 ResistivityDepth-sounding 108
6.4 CapacitativeCoupling 113
v
CONTENTS
7 SPandIP 117
7.1 SPSurveys 117
7.2 PolarizationFundamentals 120
7.3 Time-domainIPSurveys 122
7.4 Frequency-domainSurveys 124
7.5 IPData 126
8 ElectromagneticMethods 129
8.1 Two-coilCWSystems 129
8.2 OtherCWEMTechniques 140
8.3 TransientElectromagnetics 144
9 VLFandCSAMT/MT 149
9.1 VLFRadiation 149
9.2 VLFInstruments 155
9.3 PresentationofVLFResults 158
9.4 NaturalandControlled-sourceAudio-magnetotellurics 162
10 GroundPenetratingRadar 167
10.1 RadarFundamentals 167
10.2 GPRSurveys 171
10.3 DataProcessing 175
11 SeismicMethods–GeneralConsiderations 179
11.1 SeismicWaves 179
11.2 SeismicSources 183
11.3 DetectionofSeismicWaves 188
11.4 RecordingSeismicSignals 192
12 SeismicReflection 197
12.1 ReflectionTheory 197
12.2 ReflectionSurveys 201
13 SeismicRefraction 207
13.1 RefractionSurveys 207
13.2 FieldInterpretation 211
13.3 LimitationsoftheRefractionMethod 216
Appendix TerrainCorrectionsforHammerZonesBtoM 223
Bibliography 225
Index 229
vi
PREFACE TO THE FIRST EDITION
Thepurposeofthisbookistohelpanyoneinvolvedinsmall-scalegeophys-
icalsurveys.Itisnotatextbookinthetraditionalsense,inthatitisdesigned
for use in the field and concerns itself with practical matters–with the-
orytakingsecondplace.Wheretheorydeterminesfieldpractice,itisstated,
not developedor justified. For example,no attempt is madeto explain why
four-electroderesistivityworkswheretwo-electrodesurveysdonot.
The book does not deal with marine, airborne or downhole geophysics,
nor with deep seismic reflection work. In part this is dictated by the space
available, but also by the fact that such surveys are usually carried out by
quite large field crews, at least some of whom, it is to be hoped, are both
experiencedandwillingtospreadthebenefitofthatexperiencemorewidely.
Where appropriate, some attention is given to jargon. A field observer
needsnotonlytoknowwhattodobutalsotherightwordstouse,andright
in this context means the words which will be understood by others in the
samelineofbusiness,ifnotbythecompilersofstandarddictionaries.
Awordsofapologyisnecessary.Thefieldobserverissometimesreferred
to as ‘he’. This is unfortunately realistic, as ‘she’ is still all too rare, but is
not intended to indicate that ‘she’ is either unknown or unwelcome in the
geophysical world. It is hoped that all geophysical field workers, whether
male or female and whether geophysicists,geologists or unspecialized field
hands,willfindsomethingusefulinthisbook.
Finally,awordofthanks.PaulHaystonofBPMineralsandTimLangdale-
SmithofTerronicsreadearlydraftsofthetextandmadenumerousinvaluable
suggestions.Tothem,toJanetBaker,whodrewmanyofthesketches,andto
thecompanieswhichprovideddataandillustrations,Iamextremelygrateful.
vii