1. Controlled-source electromagnetic (CSEM) and magnetotelluric profiling (MT) are two key geophysical methods that use electromagnetic waves for offshore oil exploration. CSEM works by transmitting low frequency EM waves from a source and analyzing the resistivity contrasts in the subsurface based on readings from receiver arrays. MT uses a broader range of frequencies over time to create a smooth profile of the subsurface that accounts for anisotropic properties. 2. While CSEM is better for exploring a large area due to its deep penetration, MT provides more accurate detail on the orientation and location of reservoirs due to its consideration of anisotropy. Together, the methods can complement each other in offshore oil exploration.

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ChrisEstevez Geophysics
Using Electromagnetic Waves to Discover Offshore Petroleum Deposits
Introduction:
Hydrocarbonsplaya crucial role intwentyfirstcenturyprocesses.Everythingfromthe
manufacturingof automobiles,airplanes,batteries,andpoweringthe electrical gridwhichmillionsrely
on forday today functioningisa directresponse tohydrocarbons.One of the hydrocarbonsthispaper
will focusonisoil or petroleum.Petroleumisanelusive fossilfuel whichchallengingtomine for,and
evenmore challengingtodiscover.Manydifferentmethodshave beendevolvedforonshoremethods
such as oil rigsor wells,butwhataboutoffshore oil exploration,andwhatmethodsare the most
effective fordiscoveringdepositsof petroleum?The aimof thispaperisto provide aconcise lookatthe
fieldof geophysics,particularlythe geophysical conceptof ElectromagneticWaves,andhow theyplaya
role inthe offshore petroleumindustry,the geophysicalproperties,andhow tointerpretvarioussetsof
data withrespecttooil explorationoffshore.
There are two keymethodsthatare basedonElectromagneticWavesorEM Wavesthat are used
inthe oil industrytoday foroffshore exploration.These two methodsare Controlled-source
Electromagnetic(CSEM),andMagnetolluricprofiling(MT).EachmethoddealswithEMwavesdifferently,
and usesa differentprotocol fordiscoveringoffshore oil deposits.The firstof these isControlled-source
Electromagnetic.
Controlled-source Electromagnetic (CSEM)
CSEM hasbeenused inthe oil industry since the early 1930’s to analyze the oceanfloorforany
tracesof petroleumwhichmaybe drilledintoforharvesting(Chave,2009).CSEMworksby relyingona
horizontal electrical dipole source orHED.Thisdipole source isplacedroughlynearthe bottomof the

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seabeddingbutnottouchingit.Nearthe dipole source lies numerous arraysof electrical andmagnetic
dipole field receivers spreadaroundthe seafloorand touchingthe seabedding.Once the apparatusis
setup overthe distance thatistoo be sampled,the measurementscanoccur,and these measurements
use a lowfrequencyEMWave,usuallyaround1Hz (Kurang,Nabighian,Li 2005). Once the datais
collected,itcanbe plottedina2D or 1D Model.
CSEM isunique ina geophysical sense thatitfunctionsina extremelysimilarmannertoDC
resistivity.Inessence itisthe deep waterversionof alandbasedDC resistivitybasedsurveys,anduses
the propertiesof resistivity(ρ), conductivity(ohm-m),andtakesthese valueseitherdecreaseorincrease
inresolutionasthe transmittertoreceiverdistance (m) increasesordecreases.The wholeobjective of
CSEM dataonce gatheredistoanalyze the thickness, layers,andvariouszones of resistivitydifferences
whichvarybasedon depth,andlateral distance fromthe electrical dipole source (Chave,2009). Depth
beingthe distance belowthe seafloor. Hydrocarbonzoneslye betweenareasof highandlow contrast
betweenthe actual petroleumdeposit,andthe surroundinglayers.The surroundinglayersof anoil
depositsuchas the seabeddingwill havealowerconductivity versusthe petroleumdepositwithahigh
conductivity.The conductivityof the petroleumdepositishigherbecause itisinfluencedbythe
chemistryof the carbon-carbonbondswhere the surroundinglayersare simply saturatedwithwater,
and thischemical alterationcausesthe contrastof conductivitytooccur.It is thissuddenshiftin
conductivityoveracoveredsite thatisthe focusof CSEMdata analysis(Chave,2009).
Once a fieldtestisconducted,the dataisplottedonaCartesianplane basedonthe following
parameters:X-axisisthe range fromthe Electrical dipolesource tothe transmitterover “x”distance,and
the y-axisisthe logof the electrical fieldratio(figure 1).The logis takentonormalize the electrical field
to adjustforanynoise.SimilartoDCResistivity,CSEMdealswith interpretingthe subterrainovera
certainhorizontal distance andvertical depth,butinsteadof makingthe y-axisdepth,since CSEMdata
dealswithEMWaves, an electrical field isaccounted for.Thiselectrical fieldisnormalizedtoaccountfor

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the average seabedconductivity,andshows the amplitude of the seafloororthe geological featuressuch
as foldsorhills,anditis thisamplitude candistinguishbetweenstrongorweakhydrocarbon reservoirs
overa certaindistance orrange (Chave,2009).
In termsof confirminganddetermininghydrocarbon reservoirs,the peaksof the electrical field
oversome distance indicatethe sharpcontrastof resistivityvalues(figure2),andthisis the keyto
discoveringdeposits(Kurang,Nabighian,Li 2005). Keepinmindthatthese depositsmaynot necessarily
be petroleum,butitisa stepforwardif acompanyforexampleswantstoinvestinadrillingoperation.
Thisoil to watercontrastiswhatcompaniessuchas BritishPetroleumorExxonMobileuse,andisa key
Geophysical methodappliedrelatingtoEMWaves (Hoversten,Newman,Geier,Flanagan,2006)
Thisdata can furtherbe forwardmodeledintoapseudo-sectionwhichshowsthe resistivityof
the amplitudesorpeakswhichindicateeitherstrong,weak,ormoderate hydrocarbonsreservoirs.The
higherthe conductivity,the greaterprobabilitythereisthatsome depositexistsatacertaindistance on
the sea floor(Kurang,Nabighian,Li 2005). Figure 3 showsa psuedosectionof the electrical fieldovera
certaindistance.
The amplitude of the wave isthe one of the keyfactorsthatprovesreliable inoffshore
exploration.Mathematically,the amplitude of the wavesinthe electricalfieldgeneratedisbasedon
formula1, andskindepthisbasedon formula2:
E air = e ^(−2h/δ) / 2πσr3 (1)
δ = (2/σωµ) (2)
Where:
E air is the amplitude of the EMwave overanelectrical fieldthatisnormalized,histhe water
depth, δis the skin depth, σis conductivity, ωisfrequency, risthe distance betweenthe source dipole

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and a receiverata certaindistance, andµis the permeabilitywhichisbasedonthe frequencyused
(Constable,Weiss 2006).
Formula1 showsthat the air wave isnot dependent onthe resistivity of the site beingtested.
Alsoskipdepth orequation2 playsa role inwhat type of frequencymustbe used.WithEMwaves,the
resolutionwhichisdesiredisbasedonthe frequencyused.Lookingatformula1, itis exponential,so the
higherthe skindepth,anda largerh or waterdepth,the smallerthe amplitudeof the airwave.Smaller
amplitude of the airwave meansthathydrocarbondepositsmightbe missedif theyare deeper,andthe
airwave getsreducedenoughtogive apoor resolution. Inorderto findthe slope,simplytakingthe
inverse of the distance betweenthe source dipoleandreceiverwouldwork.Thiscanalsoprovide
boundariesof conductivitycontrastwhenanalyzingagraph of fielddata;Figure 1 and 2 for example
(Constable,Weiss2006).
Whenadjustingthe frequency, skindepth isanimportantfactorto consider.WithCMES,the
skindepthisroughly100m at frequency’s around25Hz or greater.Lowerfrequencies wouldyielda
lowerskindepth.Thisreductioninthe strengthof the frequencyisdue tothe air wave beingabsorbed
by the surroundingwatermolecules,andreducinginstrengthonce itcontactsthe ocean floor
(Hoversten,Newman,Geier,Flanagan,2006).Substitutingformula2intoone wouldgive an
approximationforthe amplitude thatmightbe expectedonce the electricfieldisnormalized.
One of the mainadvantagesof usingthismethodsimilarlytoDCResistivityisthe depthatwhich
a studycan occur. Usingthe lowfrequencywavesallowsforamuch deeperpenetrationintothe ground
aroundthe dipole source andnearestreceivers.TypicallyforCSEMit isideal touse approximately
2000m as a testdistance because asanelectrical fielddispersesoveracertain distance (Constable,
Weiss2006), itlosesenergy,andthisenergyisresponsibleforthe resolutionwhichisneededfor

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detectionof resistivitycontrasts.Thatiswhyitisan ideal methodfordepth basedstudies.Yetitcanstill
provide accurate informationaboutthe geological formationof the hydrocarbonlayerby loweringthe
frequency.Thisallowsformore accurate confirmationof depositorientationoveradistance that
exceeds2000mhorizontally(Chave,2009). If depthisthe main focus,skindepthmustbe considered,
and that iswhysuch a lowfrequencyisusedinthese casesforCSEM.
Magnetolluric profiling (MT)
One of the keypointstonote withMagnetolluricprofilingisthatitprovidesavery
accurate profilingof the actual physicorientation,size,andothersurface featuresinananisotropic
manner.Measurementsare takeninthe +x, -x,+y,and –y directions (Pellerin,Hohmann1990).
Magnetolluricprofiling orMT reliesonelectromagneticwavesinasimilarfashionasCSEMbut
overa broad range of frequenciesrangingfromafractionof .01 Hz toup to 20,000 + Hz inorderto
measure the electricandmagneticfieldspresentinthe ground.Thismethodcanbe appliedoffshorefor
petroleumexploration (Torres-Verdin,1992). The setupusesa receiverwhichislocatedonthe bodythat
isto be tested.Inthiscase a suspectedspotwhere oil maybe inthe Gulf of Mexicoforexample.The EM
emitterispositioneddeadcenterof a testsite,andvariousreceiversare placedinasquare orientation
(Figure 4).If it isin a offshore setting,floatingdevicesorwaterproof equipmentcouldbe used. Itisin
thissquare perimeterthataprofile will be createdusingasetfrequencythatisgovernedbycertain
propertiesand condition (Matsushima,Honkura2002).
Propertiesthatgovernthe use of MT are conductivity,resistivity,andthe earth’snatural
magneticfield.Withmagnetolluricprofiling,the complexityordetail of observationthatcan be seen
aroundthe testsite increase due tothe considerationthatMT recognizesanisotropy,isotropic,
heterogeneity,andhomogeneity conditions.These conditionsmustbe thoughtof before astudycanbe
done,andthe properset upshouldinplace before astudycan be conducted. If these considerationsare

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not met,distortionsinthe datacan occur, and cause an error (Pellerin,Hohmann1990). Anexample of
a distortionornoise thatcan occur basedonthe landscape isif there isananomalyinthe ground.If a
measurementof twoEMcurveson a graph basedondifferentequipmentplacementslooksthe same,
mostlikelythere issome sortof subterraneanbody. MT Profilingisverysensitivetothistype of noise
more so then controlled-source methods(Pellerin,Hohmann1990). Luckily,thisdilemmacanbe fixed,
by takingdata inwhichthe apparentresistivity(ohm-m) isonthe y-axis,time (s)isonthe x-axis,and
invertinga1D model.Thisinversioncorrectsthe typical noiseassociated withchangesinthe testingsite
whetheritisan unequal distributionof soil oranomalies.
Since the MT methoddatatakesdata overa large range of frequenciesoverasettime,the x,
and y axis’snowaccountsfortwo conditions.Time,andfrequency forthe x-axis,andthe frequency
phase angle,andapparentresistivityonthe y-axis(Figure 5) (Pellerin,Hohmann1990).The frequency
correspondstothe electrical,andmagneticfieldbeingapplied.
The electrical/magneticfield inbothcasesmathematicallyare governed by:
Ey(w) =Zyx(w)Hx(w) +Zyy(w)Hy(w) (3)
Ex(w) = Zxx(w)Hx(w) + Zxy(w)Hy(w) (4)
Where: The equationconsidersanisotropy.The rightside equations3,4 correspondtothe electrodes
placedina square orientation,andE is the electrical field.
Thiselectrical fieldiswhatallowsthe receiver tomeasure the resistivityvalues.Ittakesinto
considerationformula1,and 2 that CSEM considersonlywithafrequencyrange of anywhere from0.01
Hz to 30,000 Hz. The electrical fieldoverasquare orientationthusallowsamultitude of skindepthsto
occur withoutaffectingthe depthpenetrationbecause MTusesthe frequencyrange overtime which
will yieldanice profile thatshowsasmoothcurve comparedto the angular andridgedCSEMprofile.
The smoothridge attributedtothe anisotropicpropertiesof MT.

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Conclusion:
Offshore oil explorationbasedonelectromagneticwavesreliesontwokeymethods: Controlled-
source Electromagnetic(CSEM),andMagnetolluricprofiling(MT).Eachmethodhasitsunique use,and
one methodcan complementthe other.If detail,orientation, andexactlocationare the goals,
Magnetolluricprofilingisthe methodpreferreddue toitsbroadrange of frequency’swhichovertime
create a smoothprojectionof a petroleumreservoir.If range of the area coveredisthe goal,CSEM
wouldbe a more appropriate method.
Figures:
Figure 1(CSEM):(Chave,2009)

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Figure 2(CSEM):(Kurang,Nabighian,Li 2005)
Figure 3 (CSEM):(Kurang,Nabighian,Li 2005).
Figure 4 (MT): (Matsushima,Honkura2002)

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Figure 5 (MT): (Pellerin, Hohmann1990)

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References:
1) Chave,AlanD."Onthe ElectromagneticFieldsProducedbyMarine FrequencyDomainControlled
Sources."GEOPHYSICALJOURNALINTERNATIONAL179.3 (2009): 1429-457. AcademicSearch Alumni
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2) Constable,S. andWeiss,C. (2006). ”Mappingthinresistorsandhydrocarbonswithmarine EM
methods:Insightsfrom1Dmodeling.” GEOPHYSICS,71(2),G43–G51.
3) Gamble,T., Goubau,W., and Clarke,J. (1979). ”Magnetotelluricswitharemote magnetic
reference.”GEOPHYSICS,44(1),53–68.
4) Mehta,Kurang,Misac Nabighian,andYaoguoLi.(2005) "ControlledSource Electromagnetic(CSEM)
Technique forDetectionandDelineationof HydrocarbonReservoirs:AnEvaluation." UBC-GIF.Center
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deepwaterEMexplorationsurvey.”GEOPHYSICS,71(5),G239–G248. Web.9 April 2013
6) Pellerin,L. andHohmann,G. (1990). ”Transientelectromagneticinversion:A remedyfor
magnetotelluricstaticshifts.”GEOPHYSICS,55(9),1242–1250.
7) Torres-Verdín,C. andBostick,F.,Jr. (1992). ”Principlesof spatial surface electricfieldfilteringin
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