GravityandMagneticExploration Principles,Practices,andApplications Thiscombinedstudyandreferencetextprovidesa fields.Hehasauthoredorco-authoredmorethan130journal comprehensiveaccountoftheprinciples,practices,and publications,andfourbooks,andhasservedasAssociateEditor applicationofgravityandmagneticmethodsforexploringthe ofGeophysicsandSeniorEditorofTheJournalofGeophysical subsurfaceusingsurface,subsurface,marine,airborne,and Research–SolidEarth.ProfessorHinzehasbeenamemberof satellitemeasurements.Keycurrenttopicsandtechniquesare numerousgovernmentandscientificpanelsdealingwithkey described,includinghigh-resolutionmagneticinvestigations, issuesfromthegeophysicsandgeologyofcontinentstonuclear time-variationgravityanalysisfromsurfaceandsatellitegravity wastedisposalandgeophysicaldata.Heisamemberofthe measurements,absoluteandgradientgravimetry,andtheroleof SocietyofExplorationGeophysicists,theAmerican GPSinmappinggravityandmagneticfields.Thebookalso GeophysicalUnion,andtheGeologicalSocietyofAmerica. describesthephysicalpropertiesofrocksandotherEarth RALPH R. B. VON FRESEisProfessorofEarthSciencesat materialsthatarecriticaltotheeffectivedesign,implementation, TheOhioStateUniversity,wherehehastaughtundergraduate andinterpretationofsurveys,andpresentsanoverviewofdigital andgraduatecoursesacrossgeophysicsandEarthsystemssince dataanalysismethodsusedtoprocessandinterpretanomalies 1982.Hisresearchhasfocusedmostlyonarchaeologicaland forsubsurfaceinformation. planetaryapplicationsofgravityandmagneticfields,andhehas Eachchapterstartswithageneraloverviewandconcludes authoredorco-authoredmorethan100journalpublicationsand withalistofkeyconceptsthathelpreadersreviewwhatthey servedonseveralgovernmentandscientificpanels.Professor havelearned.Anappendixprovidesagroundingonbasicdata vonFreseisthefoundingco-chairoftheAntarcticDigital analysisusingsimpleandaccessiblemathematicalnotation. MagneticAnomalyProject(ADMAP),aninternational Studyquestionsandproblemsetsonanaccompanyingwebsite, collaborationoftheScientificCommitteeforAntarcticResearch togetherwithcomputer-basedexercisesavailableonline,give (SCAR)andtheInternationalAssociationofGeomagnetismand readershands-onexperienceofprocessing,modeling,and Aeronomy(IAGA).HeisamemberoftheSocietyof interpretinggravityandmagneticanomalydata.A ExplorationGeophysicists,theAmericanGeophysicalUnion, comprehensivesuiteoffull-colorcasehistoriesonthebook’s andtheGeologicalSocietyofAmerica. websiteillustratesthepracticalutilityofmoderngravityand magneticsurveysinenergy,mineral,environmental, AFIF H. SAADisaGeophysicalConsultanttotheoilandgas archaeological,andengineeringexplorationandlithospheric explorationindustry,specializinginintegratedinterpretation, studies,aswellastheirpotentiallimitations. modeling,magneticdepthestimation,softwaredevelopment, Thisbookisanidealtextforadvancedundergraduateand andtraining.Hehasover40yearsofbroadexperienceinthe graduatecoursesbutalsoservesasareferenceforresearch theoryandpracticeofgravityandmagneticmethodsin academics,professionalgeophysicists,andmanagersof exploration,including25yearsintheoilindustry,andhas explorationprogramsthatusegravityandmagneticmethods.It providedtrainingatseveralschoolsandworkshopsinbasic, isavaluableresourceforallthoseinterestedinpetroleum, intermediate,andadvancedgravityandmagneticsintheUnited engineering,mineral,environmental,geological,and Statesandoverseas.DrSaadhasauthoredorco-authoredseveral archaeologicalexplorationofthelithosphere. publications,haspeer-reviewednumerouspapersandtextbooks ingravityandmagneticsforGeophysicsandtheGeophysical WILLIAM J. HINZEisEmeritusProfessorofGeophysicsat JournalInternational,andhasservedasAssociateEditorfor PurdueUniversity,andhastaughtexplorationgeophysicsfor MagneticExplorationMethodsforGeophysics.Heisamember 40years.Hisextensiveteachingandresearchexperienceis oftheSocietyofExplorationGeophysicists(SEG),theSEG complementedbyindustrialconsultingexperienceinthe GravityandMagneticsCommittee,theSEGGlobalAffairs geologicalandengineeringapplicationsofgravityandmagnetic Committee,andtheGeophysicalSocietyofHouston. “Writtenbythreeleadingresearchers,thisisacomprehensivetextbookthattakesitsread- ersfromthefundamentalsofpotentialfieldsthroughmoderndataacquisition,processing, modelingandinversiontopracticalinterpretation.Thetheoryandmathematicalderivations are suitable for both beginners and experienced geophysicists. Well-organized and nicely illustrated,itisbothinformativeandclearlywritten.” –ProfessorDrAlanGreen InstituteofGeophysics,ETH-SwissFederalInstituteofTechnology “This extensive work is much more than just a textbook: it includes detailed discussions, suchasthephilosophyofmodelingandthenatureoferrors,whicharecriticaltoproperly interpretinggravityandmagneticdata,butareoftenglossedover.Averyusefuladditionto practitioners’referenceshelvesandanexcellenttextbookforadvancedstudents.” –RogerC.Searle,ProfessorEmeritus DepartmentofEarthSciences,DurhamUniversity “Thegeophysicalapplicationsofgravityandmagnetictechniqueshaveadvancedagreatdeal in the twenty-first century. Thus, this rigorous book covering the physical basis, analysis, interpretation,andapplicationsofthesetechniquesisatimelyandimportantcontribution. It is designed to serve both the student and practitioner and is enhanced by an innovative website.” –ProfessorG.RandyKeller GeologyandGeophysics,UniversityofOklahoma; DirectorofOklahomaGeologicalSurvey Gravity and Magnetic Exploration Principles, Practices, and Applications William J. Hinze Purdue University Ralph R. B. von Frese The Ohio State University Afif H. Saad Saad GeoConsulting cambridgeuniversitypress Cambridge,NewYork,Melbourne,Madrid,CapeTown, Singapore,Sa˜oPaulo,Delhi,MexicoCity CambridgeUniversityPress TheEdinburghBuilding,CambridgeCB28RU,UK PublishedintheUnitedStatesofAmericabyCambridgeUniversityPress,NewYork www.cambridge.org Informationonthistitle:www.cambridge.org/9780521871013 (cid:2)C WilliamJ.Hinze,RalphR.B.vonFreseandAfifH.Saad2013 Thispublicationisincopyright.Subjecttostatutoryexception andtotheprovisionsofrelevantcollectivelicensingagreements, noreproductionofanypartmaytakeplacewithoutthewritten permissionofCambridgeUniversityPress. Firstpublished2013 PrintedandboundintheUnitedKingdombytheMPGBooksGroup AcatalogrecordforthispublicationisavailablefromtheBritishLibrary ISBN 978-0-521-87101-3Hardback Additionalresourcesforthispublicationatwww.cambridge.org/gravmag CambridgeUniversityPresshasnoresponsibilityforthepersistenceor accuracyofURLsforexternalorthird-partyinternetwebsitesreferred tointhispublication,anddoesnotguaranteethatanycontentonsuch websitesis,orwillremain,accurateorappropriate. Contents Preface pageix Objectivesofthisbook ix Relatedbooks x Organizationofthisbook x Studyquestionsandexercises xi Units xi Acknowledgements xii 1 Introduction 1 1.1 Overview 1 1.2 TheEarthanditsplanetaryforcefields 2 1.3 Basisofthegravityandmagneticmethods 3 1.4 Foundationsofgeophysicalmethods 5 1.5 Geophysicalpractices 7 1.6 Natureofgeophysicaldata 11 1.7 Keyconcepts 14 PartI Gravityexploration 17 2 Thegravitymethod 19 2.1 Overview 19 2.2 Roleofthegravitymethod 20 2.3 TheEarth’sgravityfield 21 2.4 Historyofthegravitymethod 27 2.5 Implementingthegravitymethod 32 2.6 Keyconcepts 36 3 Gravitypotentialtheory 38 3.1 Overview 38 3.2 Introduction 38 3.3 Gravityeffectsofapointmass 39 3.4 Gravityeffectsofanextendedbody 41 3.5 Idealizedsourcegravitymodeling 48 3.6 Generalsourcegravitymodeling 53 3.7 Gauss’law 59 3.8 Gravityanomalyambiguity 60 3.9 Poisson’stheorem 61 3.10 Pseudoanomalies 61 3.11 Keyconcepts 62 v vi Contents 4 DensityofEarthmaterials 64 4.1 Overview 64 4.2 Introduction 64 4.3 Typesofdensities 65 4.4 DensityoftheEarth’sinterior 66 4.5 Rockdensities 68 4.6 Densitymeasurements 75 4.7 Densitytabulations 85 4.8 Keyconcepts 86 5 Gravitydataacquisition 88 5.1 Overview 88 5.2 Introduction 89 5.3 Measuringgravity 89 5.4 Gravitysurveying 107 5.5 Gravitymeasurementsfromspace 113 5.6 Keyconcepts 120 6 Gravitydataprocessing 122 6.1 Overview 122 6.2 Introduction 122 6.3 Extraneousgravityvariations 123 6.4 Gravityanomalies 143 6.5 Anomalyisolationandenhancement 155 6.6 Keyconcepts 173 7 Gravityanomalyinterpretation 175 7.1 Overview 175 7.2 Introduction 175 7.3 Interpretationparameters 181 7.4 Simplifiedinterpretationtechniques 189 7.5 Modelinganomalysources 200 7.6 Keyconcepts 211 PartII Magneticexploration 213 8 Themagneticmethod 215 8.1 Overview 215 8.2 Roleofthemagneticmethod 215 8.3 TheEarth’smagneticfield 216 8.4 Historyofthemagneticmethodinexploration 229 8.5 Implementingthemagneticmethod 231 8.6 Keyconcepts 233 9 Magneticpotentialtheory 235 9.1 Overview 235 9.2 Introduction 235 9.3 Magneticpotentialofapointdipole 236 9.4 Magneticeffectsofapointdipole 236 9.5 Magneticeffectsofanextendedbody 238 9.6 Idealizedsourcemagneticmodeling 243 9.7 Generalsourcemagneticmodeling 244 9.8 Totalmagneticmoment 250 Contents vii 9.9 Magneticsourceambiguity 250 9.10 Combinedmagneticandgravitypotentials 250 9.11 Keyconcepts 251 10 MagnetizationofEarthmaterials 252 10.1 Overview 252 10.2 Introduction 252 10.3 MagnetismofEarthmaterials 253 10.4 Mineralmagnetism 257 10.5 Magneticsusceptibility 259 10.6 Magnetizationofrocksandsoils 261 10.7 Magneticpropertymeasurements 272 10.8 Magneticpropertytabulations 273 10.9 Keyconcepts 274 11 Magneticdataacquisition 276 11.1 Overview 276 11.2 Introduction 276 11.3 Instrumentation 277 11.4 Surveydesignandprocedures 284 11.5 Magneticmeasurementsfromspace 291 11.6 Keyconcepts 297 12 Magneticdataprocessing 300 12.1 Overview 300 12.2 Introduction 301 12.3 Extraneousmagneticvariations 302 12.4 Anomalyisolationandenhancement 314 12.5 Keyconcepts 336 13 Magneticanomalyinterpretation 338 13.1 Overview 338 13.2 Introduction 339 13.3 Interpretationconstraints 342 13.4 Interpretationtechniques 355 13.5 Modelinganomalysources 394 13.6 Keyconcepts 411 PartIII Applications 413 14 Applicationsofthegravityandmagneticmethods 415 14.1 Introduction 415 14.2 Generalviewofapplications 415 14.3 Near-surfacestudies 416 14.4 Energyresourceapplications 417 14.5 Mineralresourceexploration 418 14.6 Lithosphericinvestigations 420 AppendixA Datasystemsprocessing 422 A.1 Overview 422 A.2 Introduction 422 A.3 Databasesandstandards 423 A.4 Mathematicalmethods 424 viii Contents A.5 Anomalyanalysis 444 A.6 Datagraphics 463 A.7 Keyconcepts 475 References 477 Index 502 Preface Objectivesofthisbook ciples. Case histories in supplemental chapters, which are available on an accompanying website, illustrate the Investigations of terrestrial gravity and magnetic fields advantages and limitations of the methods in a variety areamongtheoldestmethodsfordeterminingthenature of applications including near-surface engineering and and processes of the Earth. Despite the development of archaeologicalstudies,mineralandenergyresourceinves- anincreasingnumberofadditionalcomplementaryinves- tigations,andplanetarycrustalandsubcrustalstudies.A tigative methods, some of which have better subsurface finalchapterofthisbooksummarizestheseapplications. resolution,thegravityandmagneticmethodscontinueto This presentation of gravity and magnetic methods have an important, often decisive role, in a wide vari- differs substantially from existing books. For example, etyofterrestrialinvestigations.Incontrasttotheseveral it takes into account the rapidly accelerating availability available texts on specific topics in gravity and magnet- of subsurface, terrestrial, marine, airborne, and satellite ics,thisbookprovidesanoverall,modernresourceonthe measurements that the current information age is map- principles,practices,andapplicationsofboththegravity pinginprodigiousvolumes.Further,itdevelopsstrategies and magnetic methods to exploring the Earth. Although onthecombineduseoftheseanomalyfieldsforsolving theseaspectsofthegravityandmagneticmethodsarewell subsurface problems. It also describes the newest stan- grounded in widely described and accepted principles, dardsandmethodsforreducinggravityandmagneticdata they are continually undergoing practical improvements to a usable form, as well as modern instrumentation for and expanded understanding. The continued improve- observing both absolute and relative total field and vec- mentsresultfromenhancedtechnologyforacquiring,pro- torialcomponents.Thebookemphasizespracticalproce- cessing, and interpreting data made possible largely by dures and applications, and the physical properties and increasingcomputationalpowerandnewtechniques.Spe- geologicalprinciplesthatconstrainsubsurfaceanalysesof cialemphasisingravityandmagneticexplorationisbeing gravityandmagneticfields.Itdescribesallmajormodern placed on high sensitivity mapping of anomalies at the topicsofgravityandmagneticmethodsthatarelikelyto extremitiesoftheirspectra,bothshorterandlongerwave- be of general interest. More general descriptive material lengths, increasing the vertical and horizontal resolution andproceduresarehighlighted,andreferenceismadeto ofindividualanomalysources,andinvestigatingthetem- alternateproceduresorapproaches.However,necessarily poralvariationsinthegravityandmagneticfieldsofthe numerous computational and interpretational procedures EarthwhichpermitnewinsightsintoEarthprocesses.It describedinthegeophysicalliteraturewhichmayhavea is hoped that this book which sets a benchmark in our morelimitedapplicationarenotincludedbecauseofspace current knowledge of gravity and magnetic exploration limitations. Wherever possible, examples of these meth- willencouragefurtherdevelopmentsinthesemethodsand ods are briefly described and pertinent literature is cited applications. to guide the interested reader to additional information. Boththetheoryandpracticeofgravityandmagnetic Derivationsofequationsarerestrictedtothoseofgeneral methodsappliedtosubsurfaceinvestigationsaredescribed interestanddetailsofmanyproceduresarenotincludedin in this book. The book considers the methods from the thedescriptions.Thereforethereaderofthistextwillneed planning and organization of surveys through the analy- to make abundant use of the many references cited for sis and interpretation of observations by digital compu- moredetailedandcomprehensivetreatmentofspecialized tations incorporating both physical and geological prin- topics. ix