instrumentation for study of natural … · 2019. 6. 24. · j.h. filloux, oceanicelectromagnetism...

Physicsof theEarth andPlanetaryInteriors, 7(1973)323—338© North-HollandPublishingCompany,Amsterdam—Printedin TheNetherlands

TECHNIQUES AND INSTRUMENTATION FOR STUDY OF NATURALELECTROMAGNETIC INDUCTION AT SEA

JEANH. FILLOUXScrippsInstitution ofOceanography,LaJolla, Calif (USA.)

Acceptedfor publicationFebruary2, 1973

Electromagneticfluctuationsin the oceanhaveexternalsourcesabove(ionospheric)andbelow(secularvaria-tion of theearth’smagneticfield), andinternal,purelyoceanicsourcesassociatedwith interactionbetweenwatervelocity fieldsandtheearth’sfield. Energydiagramsindicativeof theelectromagneticactivity in theseaarepres-ented.From thelatter,estimatesof theresolutionrequiredin electromagneticresearchat seacanbemade.Absoluteminima of 17 and0.05MV/rn arenecessaryfor magneticandelectricfields, respectively.Becausetheoceanshieldsoverheadsourcesatfrequenciesaboveafewhundredc/h andbecausemotional fieldshaveweaksignatures,ares-olution atleast10 thneshigherwould considerablyenhancethe scopeof suchresearch.

Theresponseof electricfield instrumentsto motionallyinducedfieldsdependsuponwhethertheyarefixedordrifting, butboth typesrespondsimilarly to fieldsof externalbrigin.

Themoststringentlimitation to electricfield samplingin theseais the difficulty in achievinglow-noiseelec-tricalcontinuitybetweenmeasuringcircuitsandseawater.Even thebestmatchedsilver—silverchlorideelectrodesintroducevariableelectrochemicalsignalshardto maintainbelowamillivolt. Thesemaskvery low frequencysignalsunlesssophisticatedtechniquessuchaselectrodeswitchingareused.

I. Introduction waterstreamsandhisattemptsto detectIt acrossthe Thames,failing by lackof adequateinstrumen-

Exceptingthe ancientdiscoveryof the earth’s tatlon. This field wasobservedin 1851 by Wollastonmagneticfield, the earliestidentificationof terrestial while studyingtelluric currentsacrosstheEnglishelectromagnetismwas thediscoveryby GellibrandIn Channelusinga telegraphiccable.Thepresenceof1635of thesecularvariationof the earth’sfield, lunarperiodicitlespuzzledWollastonwho postponedAlmost a centurylaterGrahamdiscoveredtheexis- his report.Hence,In themid 1800’sthemixed naturetenceof quasi-continualrapidmagneticfluctuations of telluric currentsat sea,thoseInducedby magneticthatAndreasCelsiusand1-Liorta relatedto aurorae. pulsations,andthoseinducedby watermotionAnothercenturylaterwhentelegraphlinesbeganto causedhesitationsIn Interpretation.flourishBarlowdemonstratedtheexistenceofspon- Early this century,Young et al. (1920)demon-taneousearthelectriccurrentsandtheelectricvs.mag- stratedthat the oceanicelectricfield couldbe record-neticstormcorrelationwasrecognised.Fundamentalto ed aboarda steamingship by electrodestowedasternthesefindingswerethe compass,of uncertainorigin, andthatsuchsignalsare relatedto watermotion.perhapsusedIn Chinathousandsof yearsago and This introducedan alternateelectricfield instrumentknownin Europaduring theMiddle Ages, thegalvano- concept,the “drifting” type,In oppositionto themeterandtelegraphlines.It is noteworthythat all “fixed electrodes”typeusedonland.The propertiesthesediscoveriesoriginatedon land andalsothat the of the two, however,differ considerably.earliestinductionprocessobservedin theoceanwas Thelatebeginningof electromagneticinductionreportedlessthana centuryago(Adams, 1881).One discoveriesat seawas followed by slow initiation ofshould,however,rememberFaraday’sspeculations oceanicresearchin this field andour presentefforts(1.832),abouta motionally inducedelectricfield in to exploit electromagneticinductionat seacontinue

324 JH. Filloux, Oceanicelectromagnetism

I I I I I I2~,’’—. I II II I I I~~~(i/ •—.

8 Yearlyand semiyearly modulation — - — - —-— Internal SourcesRotation of the sun

6 - So/ar daily variôtibn Oceanic SourcesllUfl~ Ionospheric Sourcesuuuiirl

~ 4 - llllllllllllt~ (land and ocean surface) -

o IJllllhIIIIIIIT)’~ ___________I ~Ut#JjIIIIT’ I Ionospheric Sources2 - Quasigeostroph/ci JJJ.4jjn.rt ~‘ (main ocean floor) -

— currents Ui’-’ ~

Borotropic tidal71 4~h. Intermittent

Baroc//nic tidal—’ Uncertain° -2 - Turbulence (u~qoerboundarid \

Continental She/f only) Pearls

(!)Wind generated ~,

>.. 6 waves arid swells \

C~ -8 Earth cavity

resonances-10 Whistlers -

Recovery from 4c magnetic stormsLii —12 I0 Magnetic Storms and bays

—14 Micropulsations

.) \..~-16-

~ ,~ “8

E—Is- = _Iq _Iq — — —

I II II I I I—20 I I I I

—7 -6 —5 —4 —3 —2 -1 0 1 2 3 4 5 6 7 8 9 10Log10 Frequency) Cycle / Hour

Fig.1. Energylevelof magneticpulsationsin the ocean.field. Fortunately,in returnfor this complexItytwo

to lag their counterpartonland.ThissituationIs subjectscanbeenlightenedby electromagneticundoubtedlyduetothehostileanddiscouragingnature observationsat sea;(1) Inductionby magneticof the oceanicenvironmentwith its Intenseand pulsationswhich bearson the earth’sstructure;anddestructivemotIons,the strongchemIcalactivity of (2) oceanicwatermotions.Both areinterrelatedandits water,itsabyssalpressureand theremotenessof mustbeconsideredtogether.This contributçstothe only stableplace,the seafloor. Such factors extendthecoverageandvarIetyof observationsmakethe simplestobservationsvery difficult and necessaryto achieveadequateInterpretation.Therenderuselessalmostall land-typeinstrumentation. Instrumentationto fulfill this goal is in Its infancy.

Anotherprobleminherentin theoceanis theexistencethereof two inductionprocesseswith 2. Energy levelsof the naturalfieldsbroadfrequencyoverlap,onehavingfor sourcesexternalpulsations(ionospheric),theother involving In thefollowingweusetherationalizedM.K.S.-sys-local interactionofvelocIty fields with the earth’s tem of units.We referto magneticfield thevector

J.H. Filloux, Oceanicelectromagnetism 325

TABLE I electromagneticwave diffusinginto a conductingSeawaterskindepthasafunctionof frequency(assumed mediumfalls to e~its initial value.D = (irfpa)~conductivity= 4 fi m ) wheref is the frequencyin hertzandathe electrical

conductivity.)Theskin depthof seawateris tabulat-Frequency D

(meters) ed for vanousfrequenciesin Table I. A skin depthCyclesperhour hertz of 4.5km, theaverageoceanicdepth,corresponds.

to 10 c/h. Beyondthis frequencythe heavyline that,3.6 2 8’lO’ ~52 in Fig. 1, representsthe level of magneticactivity

3 6 10210-i 7~ from overheadsources,branchesdownand falls102 2.8 102 1480 rapidly.Exceptfor shallowseas,theoceanis shielded

3.6 ~10’ 10_2 2520 from ionosphericactivity at frequenciesgreaterthan10 2.8 - io-~ 4780 afewhundredcyclesperhour.

3.6 i0~ 7800 The energydensityoriginatedby motionalproc-

esseswithin the oceanis outlinedin Fig. 1 by dottedB classicallydefinedby B = pH, where,for our pres- lines.Few dataareavailableto compiletheseesti-ent purposesthepermeabilityp doesnot differ mateswhichmustbe regardedas guidelines. Exceptanywherefrom that in avacuum(4 - lO~henry/ at the semidiurnallunar frequencywhereoceanicmeter).B is thenin weber/m

2.Since thisunit is tidescreatea sharpandnarrowspectralpeak,mo-inconvenientlylarge,a sub-unithasbeenselected, tionally inducedfluctuationsremainbelowiono-the gamma(7) equalto onenanoweber/ni2.For sphericsignals.the electricfield themicrovolt/mhasbeensubstitu- Althoughmagneticsignalsfrom oceanicsourcested for theM.K.S.-unitsor volt/rn. havebeenusedto investigateoceantides(Larsenand

Cox, 1966;Larsen,1968) theprospectof extracting2.1. Magneticsignals extensiveinformation from suchobservationsappears

In Fig. 1 we summarizethemagneticactivity level uncertain.observedat the earth’ssurface..Theheavyline indi-catesroughly theaveragemagneticenergyfromatmo~ 2.2. Electric signalssphericandionosphericoriginatmid-latitudes.Several The estimatedlevel of oceanicelectric fluctuationsimportantfeaturesareidentified.Theseare cherished isillustratedin Fig. 2.by investigatorsbecauseof their highenergydensity The mostprominent ionosphericsignalsare thoseand their simplerfield geometry.Fromthe pearls dueto magneticstormsandbayswhichcoverthemicropulsationsaround300 c/h and towardlower range0.02—50c/h — typical spectra,Fifioux (1967b)frequenciesthe magneticstormscovera wide fre- — andthe daily solarvariationCox et al. (1971).quencyrange,followed by the solardaily variation Again no appreciableenergyremainsat frequenciesandits harmonics,the recurrenceof solarstormsat aboveafew hundredcyclesperhour.sunrotation periodicitiesand finally the 11-yearcycle Dueto thegreatvariety of velocity fields in theof solar activity. At this point theheavyline merges ocean,motionally inducedfields therearewidespread,into the dash-dotline that characterizesthe secular activeandcomplex(seeFig. 2).Quantitativedescrip-variationsof the earth’smain field. tionsof thesefieldshaveto beconsistentwith the

At frequencieslower than a few cyclesperhour frame of referencein use,namelystationarywiththe level of magneticactivity is essentiallythesame respectto theseafloor or movingwith thewater.in theoceanasit is at theearth’ssurface.However, Interpretativeguidancein that respectis providedbyat higher frequenciesthe currentsinducedin thesea Fig. 2.reflect awaytheincidentelectromagneticwaves, Of all motionalelectric fields in the oceanthepreventingtheirdownwardpenetration.Theeffect mostconspicuousare thoseassociatedwith: (1)windbecomesappreciablewhenthe waterdepthbecomes drivensurfacedrift; and(2)turbulenteddiesoflargerthantheseawaterskindepthD. (The skin majoroceancurrents.Large signalsare alsogenerateddepthis thedistanceoverwhich theamplitudeof an on thecontinentalshelf,(Cox et al., 1971),by turbu-

326 JH. Filloux, Oceanicelectromagnetism

o I I~ 1111 I j I III I I II

~~nosp~e~jce4

so(M2)

0~7 ,._ fSCfllfrCØ~ ~-Quas*stmp4iicBa’sChWiC currentsE4

2 ?id &ilt, *-Seen by fixed instrumentk,thuiMteddi.s~~ ~-TurbuIence

-~ ~ **-Seenbydriftinginsfrument~ t- Far offshoreThrbulence, q~

Ca~ -2 ~ °“~‘ tt-Coostal (enhanced)

~ (“\ç’ sw//s**a,C ~ ~. >-~‘ “ ~= .~

2 i Ti II I T IC,O I I I I I I I I I I

-6 -5 —4 —3 —2 —i 0 1 2 3 4 5 6 7 8 9 ~O

Log10 Frequency, Cycle / HourFig. 2. Energylevelof electric~u1sationsin the ocean.

0000 DEC. 3, 1961 0300 0600 0900

‘VI ~

I ~.-

I \ .V.\ / \~ •i~~.~i’\~~/ f\i- .1 —._-~ / .._~...bOy \fF

V

I’I’ I \ -.

.i’~ 1 ‘-~I \ -‘

\~ / \_ ( \ .1 11\./~Jj11V

...— 1 ‘1’.,

- / ~ I E.\ - .

I ‘. I 4mv/km~1!I’’

‘~IvSI-~I

Fig. 3. Totalmagneticvariations,F,aftercorrection,a few milesoff California’scontinentalslope,andelectricfield paralleltothecoaston theseafloor below,E.

J.H~Filoux, Oceanicelectromagnetism 327

lent tidal energydissipationandathigher frequencies, ing on the oceanwere madelastcenturyat islandora prominentbumpcorrespondsto swell andwind coastalobservatories:Theseusedsuspendedmagnetwaves(Weaver,1965) Emergingfrom the solarcluster, variographswith theodolitereading.Representativea narrowpeakat lunar semidiurnalfrequencyrepre- modelsarethoseof SchmidtandLaCour(Chapmansentsthe effectsof tidal currents,Coxet al. (1971). andBartel,1940).Remarkson thecoastaleffect

Large-scalequasi-geostrophicvelocity fields are as originatedmainlyfrominterpretationoftheir recordsyetverypoorly understood.Tentativeestimatesof (Parkinson,1959).Improvedvariographsofthesametheirassociatedfieldshavebeenmadeby Cox(un- principle,reducedinsizeandweightandrecordingonpublished).Thebackgroundenergyof gravitational film havereachedportablecapabilities.Arraysof suchandionospherictidesduringmagneticallyquietdays stations,usingAskaniamodels,wereusedfor studiesalsoprovidesanupperlimit to thesignatureof these of the coastaleffectinPeruandCaliforniabylittle knownvelocity fields. Schmucker(1964).More recentlyGoughandReitzel

(1967)havedevelopedamodel morereadilycon-2.3. Resolutionand stability structedby experunentalists.

For observationsto be useful: (1) tha instrumental The first magnetometerwithouta movingmagnetresolutionmustprovidea faithful descriptionof the usedat seawas thefluxgateor saturablereactorsignals;and(2) thenoisemustbe smallenoughnot magnetometer.Improvementsof this instrumentbyto distortthesesignalsappreciably.Minimum least Vacquier(Dobrin, 1952)3broughtits resolutiontocountandmaximumallowabledrift for various theneighborhoodof 17 openingthepossibilityofstudiesareshownin TableII. The assumptionhave airbornemagneticsurveying(in this configurationbeenthat: (1) samplingaroundfrequiresa least the fluxgatewasusedduring the SecondWorld Warcountof 10—1of thepeaktopeakamplitudein theband asa submarinedetectingservice).0.5fto 1 .5f; and(2) thedrift during the timeinterval The fluxgateprinciple allows miniaturization,lowf~shouldnotexceedthe leastcount. power,anddirectdigital translation.Thesearevalu-

TableII stressesthestringentlong-termstability ableadvantagesin oceanographyandseveralseafloorrequired,andshowsthatmeaningfulresearchbegins fluxgatemagnetometershavebeenbuilt (Pepper,witharesolutionof theorderof 1 ~yfor themagnetic 1968;OwenandSik, 1972).Measurementsoff Call-field and0.05pV/rn for theelectric field. Tenfold forniaarereportedby Greenhouse(1972).improvementcouldbetakenadvantageof readily. Of varioustypesof magneticresonancemagneto-.

Magneticobservationssufferanotherlimitation. meters,only the protonmagnetometerappearstoThemeasurementsmustbecarriedout in the presence havebeenusedat sea.Inventedin 1953 (Packardandof themain field (averageintensityof4. 104’y). If Varian,1954),the instrumentisbasedon the natural•notrigidly supported,rotationof the sensorsmay precessionof protonsseekingequilibriumin thecauseerrorsashighas4~104’y/rad.Attemptswere magneticfield to be measured,afterhavingbeenmade,AlldredgeandFitz (1964), touse oceanic temporarilypolarizedin adifferentdirection. Thestableplatforms.Tilts aslargeas 102 radhavebeen precessionfrequencyis givenbyf = (I’/2ir)B whererecorded,discouraginghopeof sufficientimprove- I’ is thegyromagneticratio of theproton.With Bmentof this techniquewith realisticbudgets. in gammas(7) andfin hertzthenB = 23,487f

More successfulwere experimentscarriedoutnear Protonmagnetometerstherefore,measurethetotalthenorthpoleby Russianresearchers,Zhigalov field andwifi detectchangesof themain field only.(1960),Trofimov et al. (in preparation),who sup- Sensitivityto variationsin directionsat right anglesportedtheir magnetometerson icebergs. canbe obtainedby addingfields which resultin the

wantedtotal field orientation.The stability of suchbiasingfields, however,mustmatchthe resolution

3. Instrumentation and techniques wanted. The challengeis greatandevenif met thepowerrequirementwould remainoverwhelming.

3.1. Magneticfield measurements Evenwhenusedas total field instruments,protonTheearliestobservationofmagneticvariationsbear- magnetometersrequireconsiderableelectricpower

328 J.H. Filloux, Oceanicelectromagnetism

TABLE IIResolutionanddrift compatiblewith varioustype investigations

Typeof measurement Magnetic Electricresolution maximum resolution maximumrequired drift required drift(ay) (7/h) (pV/m) (~V/m/h)

a Ionosphericeffectsduringmoderatemagneticstorm,0.1_i c/h 1.5 5.0 0.05 0.05

b Sameas(a)but for averageionosphericactivity 0.3 1.0 0.01 0.01

c Sameas(a)but 1—10 c/h 1.0 i.5 0.005 0.05

d Sameas(c) but for averageionosphericactivity 0.2 0.3 0.001 0.01

e Wind drift, eddiesmeasuredwithGEK, (frequenciesaround1 c/h) — — 0.3 0.3

f Quasi-geostrophicvelocity fieldsaroundiO3 c/h 0.2 0.0002with fixedelectrodessystem — — 0.1 0.0001

to activatethepolarizationcoils. Sincetheyare Becausehigh-frequencymagneticsignalsin theinsensitiveto positionalstability they areideally seaare cut off, air andiron-coreinductionmagneto-suitedto surveythestatic anomaliesof the sea meterscannotbeeffectiveat sea,except,perhapsfloor (WarrenandVacquier,1961).With adequate on theshelf.precautionstheycanalsobe usedto recordtemporal OthersensitivedevicessuchasHall effectsensorstotal field variations(Filloux, 1 967b).An exampleof andmagneticresistorshavebeeninvestigatedbuta total field recordmadeoff Californiaby meansof notusedprobablybecauseof long-termstabilitya towedprotonmagnetometeris given in Fig.3. Suc- limitations.cessfulremovalof thenoisedueto bottom anomalies,necessitated:(1) alocal surveyof themagneticanom- 3.2. Electricfieldmeasurementsalies(seeFig.T4); and(2) continuoustrackingof the Thesignificanceof motionalelectric fields in theship’sposition(seeFig. 5). oceanand their relationto measuringmethodshas

The fragility of suspendedmagnetvariographs beenexaminedformally by Longuet-Higginset al.canbe reducedby usingsensitiveangulardetectors (1954).Thecasewheremagneticallyinducedfieldsthat relaxthecompliancerequirementson the sus- are alsopresentis treatedin Coxet al. (1971).pensionfibers. A singlecomponentmagnetometer Two typesof electric field devices(Fig.8) areof this kind hasbeensuccessfullyoperatedon the usedin theocean.In the first one,Fig.8a, twoseafloor (Fifioux, l967a,b). Although thestiff electrodesin contactwith seawaterat A andB aretungstensuspensionreducestheangularsensitivity connectedto themeasuringmeterby an insulatedof the device,a grid-typeoptical leverinspiredby cablec. In thesecondcase,Fig.8b, the electrodesJones(1961)allows a resolutionof one‘y. A section arelaid nearthe meter,within nonconductingtubesof sucha recordis shownin Fig. 6. A peculiarityof t, openedatA andB (suchelectrolyticconductorstheinstrumentis a simpleorientationcontrol that are calledsaltbridges).insuresproperorientationon theseafloor (seeFig. It canbe shownthat thesignalu registeredby7). the infinite resistancemeterMis in eithercase:

Althoughthe fragility of suspensionsshouldnotbeoverlookedthe simplicity andlow drainof sus-pendedmagnetvariographsare attractive.

J.H. Filoux, Oceanicelectromagnetism 329

122’55 12250

iE~1~~S\\S*.’SSS,S,..S, I‘‘S /

6 -- ABLE. “, I.,’ ~ /5-’ 555 S ~‘1 ‘HIIIII~

8 --. l’~ ~_----- /- ‘~ ~ I ‘ IUUII ‘ / —t

- ~“JS 5~\\\I. /~ /~‘t-.5~ti ?~~\\\\~.\~-----i /\‘.~%%t i ~X~X~\\ / ---- /

‘S ~\‘~t ‘jSI~t ‘ ~~j\\ )c.----- / /5._St / //~‘S ‘0 --

-. ~. ~ -~. ~ ~\~\ .1--v-- r‘-~ ‘..O ‘‘\.~\~~- \ 1~-l-~( 4,~

5S, ~ .‘- ~ \.l1--’rtl Is-s‘N ‘N

‘_5 ..~ ~4~V\ \ ~ ~-If _\t‘~55, / % BAKER4-\-~-

1-~-~I/ ~ \.-V\\l~-~

----5-

// S/ - - -//A\ 0~-~0

.~,

—I I —~

Fig.4. Contoursof total anomaly(sy)dashedlines,depthcontours(fathoms)darkcontinuouslines.

it E=!—VXB (2)f (_a_(V_Vc)XB)‘dz (1)c(ortr while anInstrumentdrifting with thewater(V~= F)

whereV is thewatervelocity, P~the cable(or tube) recordsthe field:velocity, I thecurrentdensityandds anelementofthecircuit. Thevalidity of eq. 1 is generalandcovers Ed = ~- (3)thecasewheremagneticallyinducedpulsationsarepresent.An instrumentfixed on theseafloor (~= 0) (thesubscriptd refersto drifting). Hencethesignalsrecordsthefield: measuredby the fixed or drifting Instrumentsdiffer

by themotionalfield VXB. If oneof thetwo senses


12255’ I22°50

N

05:OI

— 06:01 —05:05 44

07:01—...~

(.OTO;”NN5~ 06:05

02:12 3~75”,.,., 4—04:0I~ 4

02:I403:IO—’

04:03,5—” 45

40

0I:30 DEC 3, I96I—Q5:34.5

- 06:2605:3I:30

03:36 I:3802:39.5

—— —02:36.5

3:31‘—04:23

34.—I I 40’

Fig.5. Trackof ship,positionandtime,for removalof spurioussignalsdueto magneticbottomanomaly.

fully the motlonalfield, theothermustregisterno GEK while barotropicmotionis sensedbestbysignalat all. In practicethesituationis not clearcut stationaryinstallatIons.and bothfixed or drifting Instrumentsregistercorn- Quick statementof exactlywhatsignalis regis-plementaryfractionsof the motlonalfield. Therela- teredby drifting andby fixedInstrumentsis possibletive valueof thesecontributionsdependson thege- only in simplecases.However,thefollowlng.gulde-ometry of the velocity field andonthe shapeand line isuseful: velocity fields of smallsizewith respectconductivity of theboundaries.Thisdual behavioris to adjacentstationarywaterproduceonly smallifiustratedIn Fig. 9 wheretheresponseof-theGEK signalsin fixed instrumentswhile Instrumentsdrifting(geomagneticelectrokinectograph)is comparedwith within themotion cell registerthemajorpartof thethatof fixed electrodesin the caseof a two-layer motlonalsignal. Thereverseis truefor large-scalesystem.Shallowsurfacedrift is bestrecordedby the motionswithin low conductivityboundaries.For

J.H. Filoux, Oceanicelectromagnetism 331

~ 0000 MAY 10, 065 0600 1200 800Fig. 6. Magneticfluctuationseastward,DSF, on theseafloor and600km off California. MagneticvariatIonDCAB alongthecoast(Cambria)andDTU atTucson.

instance,in the two-layer caseof Fig.9: (1) when bridgestype,Fig. 8b. However,dueto the lowha/h -÷0thedrifting GEKregistersVa X B and the conductivityof seawaterwithrespectto copper,fixed electrodesystemregistersnothing;and(2) salt bridgeinstrumentsdo notpermit largeelectrodewhenha/h -+ 1, the GEK registersno signalandthe separations.Their advantageis that theelectrodesfixed installationregisters—~ X B. canbelocatedneartherecordingsystem,a require-

All the statementsmadeearlier for thecabletype ment for drift removalby electrodeswitching(seeof electricalfield recorders,Fig.8a,holdfor the salt later).

With longcablesan averagefield:

,~~CC2 E~p/l (4)

~ ~j-1 ~ is obtainedwhere1 is the electrodeseparation.When~ I the separationis derivedfrom thecablelengthr V properstretchingis highly critical. If theelectrode’s

I - V positionis knownby othermeans,a non-straightSri / V

I M

I ~

D~ B~E~

-. J L V V V

/1/I V V V // ,‘ (~/L Lf~I2, V / /

OT 11r~1FE2~ / (4) /j~ ~rç L~ / //

- -~ ~-i LJC A

Fig. 7. Automaticorientationof magneticsuspensionin sea- A

floor magnetometer.At launchingmagnetsAM in floatFrest ‘ bin tankT. Oil drippingthroughcapillary tubeCTfrom stor- a

agetankSTfills tank T. FloatF risesandconnectswith discs Fig. 8. a. Schematicrepresentationof acable-typeelectricD andmirrorM. Subsequentmirror rotationsarepickedup field sensorandb. saltbridgeversionof thesameinstrument.by grid-typeopticallever. CC1 andCC2 = calibrationcoils; EA,EB= electrodes,C = cables,t = saltbridgetubes,OL objectivelens;0T oilôverflow tank. M meter.

332 .1.H. Filoux, Oceanicelectromagnetism

pathmayresult in errorsdueto signalsinducedin theloop madeby thecableandthe straightline between

— — - theelectrodes.Thiserror remainssmallfor electrode:b) AA1 811 ~ —~ — - separationsundera few tensof km, Cox et al. (1971).

Signalsfrom transoceanictelegraphcablesmay there-VbSO fore requirecorrection.

C) / A, s~x Amonginstrumentsusedto recordelectricfields

/ \ in theoceanwe havealreadyreferredto oceanicJ, - V - telegraphcables.Wehavealso mentionedthe GEK

Ocean F/oo usedin tow behinda steamingshipto estimatethesurfacedrift. A descriptionof this instrument,its

a) ~- b) ~ ~- 4 usesandproblems,15 givenby Von Arx (1950).

Typically, electrodeseparationis lOOm.HenceaFig. 9. Responseof GEKandfixedelectrodesin asimple two- surfacevelocity of 1 cm/sec,at mid-latitudeswherelayerflow. Thesurfacelayerof thicknessha moveswith theverticalcomponentof B is around4 . I 0~~velocity Va’ cuttingverticalmagneticcomponentB to produce generatesa signalof 40pV oftenbelow electrodemotionalfield VaXB.Electriccurrentswith densityta setin sur-face layerreturnin stationarylowerlayer(densityib). Layer voltage.Thetime-independentpart of the lattercanthicknessha (above)andhb (below), a. Responseof GEK; be removedby reversalof thesteamingdirection.b andc. Responseof fixed electrodesin movingandstation- Sincecomponentsat right anglesareneeded,steamingary layers,respectively.

—~ f-,

<>

Et

~� E~—~SI

ML

S2

_5___, G‘0

/ “

TS PS /( ~cj~1~ PL~

Fig.10.FiXed electricfield recorderon theseafloor. PC= recordingvoltmeterin pressurecase;C cable;EA andEB = electrodes;G = gasolinefloat;ML = mooringline;S~,S3 swivels;SB = surfacebuoy;FL = pulling wire;SW = saShweights;P platfonn;PS=

permanentshackle;TS = temporaryshackle.


~

‘fff/7f/7/;1///7/////f////f///////////f//////f/[fff///f//////f//7 7) ////11777/7/77117/7117/7771f/////f/////f//ff////////f/I/I///I////f///f/IIf//f/I////f/IIfI/,

Fig. 11. Installationof long cableelectricfield recorderon theseafloor, a. Thecableis stretchedat thesurfaceandballastedtocreatea0.5 m/secnaturalsinking speed.b. Lowering from shipat 0.5 m/secandstretchingfrom smallboatby meansof longpullingwire. c. Releaseof pullingwire andlaunchingof float.

ina squarepatternis commonpracticewhenGEK aroundtheverticalreplaces.theGEK squaresteam-measurementsare in progress. ingpattern.Differentiationof thesignalsat opposed

Thereplacementof cablesby salt bridgeshas orientationsis built in theelectronicswhich alsobeenpioneeredby Mangelsdorf(1968)with aview suppressesotherunwantedsignalsassociatedwithto electrodevoltagesuppressionby electrodeswitch- free-fall velocity androtations.Referenceto magnet-ing. If the recordedvoltagesbefore‘and afterswitch- ic northusesa smallpickupcoil. Thepresentresolu-ing are Ub and U~thenthe electrodenoiseandthe tion is±2 cm/sec,a significant result fora systemelectricfield signalsarerespectively: usinga saltbridge0.3 m long, andtheauthorsexpect

1 1 to improve it. Theinstrumentis sevenfeetlongandUE = ~ (Ui, + L~~) L~= ~ — Uc) (5) weighsabout70 kg. Thefree-fall speedis around

1 m/secwithrotation at 0.1 revolution/sec.TheWith saltbridgeGEK’s the electrodesarekept small silver—silverchlorideelectrodesaremounted

aboard,protectedagainstleakageandtemperature backto backon a beryliumoxideplate to minimizevariations.Switchingis madeby meansof insulated temperaturedifferences.Thehorizontalvelocity invalves, this caseis with respectto the meanor barotropic

A recentdevelopment(Sanford,1967;Dreyer velocity.Thelattercanbeestimatedonly with fixedandSanford,1970),extendsthe scopeof drifting systems.instrumentsto samplingof thehorizontalelectric Attemptsto recordpotentialdifferencesin thefield from surfaceto bottom.The self-contained deepoceanby laying cablesfrom coastalstationsinstrumentis ashort span,saltbridge freefallGEK havedemonstratedthatlandingthe cablesthroughthat rotatesonits waydown(andup). Rotation the surfzoneis amajorobstacle.Mooredsea-floor


000i 0000 INOV 25 26 I 27 28 29 NOV 30 I

~~-“ “~v~~-”~ “-‘~‘ “PL

___V ~V~V V_5

ECN8L I

J\t~ ~ ~iooro~

OF~ ~ I-.

DEC I, 1961 DEC 2 DEC 3 0 V10000 2b0 oo~o oooo :~-. I ________

Fig. 12. Electricfield fromtheseafloor comparedwith V’ .V.VVV: - ~ FGvariousmagneticsignalsEABL = electricfield 100 km offCalifornia;ZFRE,HF~,DF~= magneticvariations(vertical “~

to magneticnorthandto magneticeast)100km inland; V

ZCAB = verticalmagneticvariationalongcoast.Upperbox:duringmagneticallyquiet week.Lowerbox: duringmagneticstorm, Fig. 13. Silver—silverchlorideelectrodecross-section.PCrecordershavebeenmorepractical,Filloux (1967b), porouscontainer;SC= silvergaugecylinder;SS silver

seeinstalledconfigurationon Fig. 10.To lessenthe stem;EF electrodefiller (silver chloride); TC= top cap;EC= electrodehousing;FG = shockabsorber(pyrexwool);

electrodenoise,the cableis aslongaspractical.With FL = plug;IL = intermediatelead.1 km length,magneticstormpulsationshavebeen V

recordedwithamplitudesaslow as0.2pV/m. Full In spiteof theoperabilityof longcablesystems,stretchingof the electrodecableandconfrol of the theavailability of small, two-componentinstrumentsazimuthareprimordial.Fig. 11 illustratesseveral would improveoceanographicresearchconsiderably.stepsof this difficult operation.Successfulstretching Onemajorobstaclein developingtheseisthe grossaswell asachievedazimuthcanbeverified by means mismatchbetweenthe signalsto besensedandtheof orientationcompassesattachedalongthe cable, electrodenoise.Fifioux (1967b).

An exampleof electricfield recordingin deep 3.3. Electrodeswater(4km) isgiven in Fig. 12. At the pointswheremetallic circuitsmakecontact

Anotherelectricfield approachto estimateoceanic with seawaterthemetal—electrolytejunctiongener-velocitiesis describedby Harvey(1972).Theelec- atesanelectrochemicale.m.f. variablefrom metaltrodes,spreadvertically,sensehorizontalvelocities to metalbut alwaysnearonevolt. Evensothe e.m.f.in the magneticeast—westdirections.Becauseno of perfectlymatchedjunctionpairs would cancelappreciableelectriccurrentcanflow throughthe eachother.The degreeto whichthis canbe ap-air—seainterfaceno interpretationdifficulty arises proachedis veryunsatisfactory.from sea-floorconduction.


t I‘900 -400 -300 -200 -100 0 100 200 300 400 500 \Electrode ‘~bltoge(Uvolts)

Fig. 14. Electrodevoltagedistributionof 66 Ag-Ag a. dcc- -l “trodes.Gaussiandistributionwith sameareaandsamestandarddeviationalsoshown.

~2V A

Of all theelectrodestried for junctionwithsea \ -

water,thesilver—silverchloridetypehasenjoyedanunchallengedleadin spite of performancesfar

below oceanographers’needs.Ives andJanz(1961) “~ ~

reviewtheproblemof referenceelectrodesand Log10 Frequency(cph)describefabricationmethodsfor threeclassictypes Fig. 15. Energyspectraof electrodesneartheoceanfloor.of Ag-Ag Cl electrodes,namelyelectrolytic,thermal A for low noiseelectrodes,B for marginalelectrodes.andhybrid (thermal-electrolytic).Thesearemainlyusedin laboratoryresearch,chemistryandbiosciences, Seebeckeffect alongthe saltwaterpath.Von Arxwherecareandoptimumutilization arethe rule. (1950)reviewsthis subject.DreyerandSanfordThesegoodpracticescannotbe respectedin oceano- (1970)give for sensitivity figures:350 jN/°Candgraphicwork. Sturdierelectrodesaredescribedby 500pV/% of salinity change.ResearchcapableofVon Arx (1950)andFifioux (1967b).A crosssection resultingin a clear separationof electrodeandwaterof thelatteris shownin Fig. 13.Unfortunately,there pathsensitivityto temperatureandsalinity gradientsdoesnotseemto bea fabricationtechniqueleading would bemostvaluable.If undertaken,this researchto suresuccess.Performancesimprove with: (1) shouldalso evaluatethe effectof diffusion slowinggeneralcare,cleanlinessinparticular;(2) training;(3) by gelatinasadvocatedby someresearchers.largenumbers;and (4) massiveuseof silver and Theunknownsignalbias dueto electrodevoltagesilver chloride.In Fig. 14 the distributionof voltage canbeestimatedby meansof electrodereversal.deviationaroundtheir meanfor 66electrodesis seen Extensionof this techniqueinto regularandrapidto approacha Gaussiandistribution.Properlymatch- electrodeinversionis identicalto the “chopping”ed,newelectrodepairshavee.m.f. well belowone techniqueusedin drift removalof d.c.-amplifiers.millivolt. Noisespectraforelectrodepairsof highand Wethereforegavetothedoublepoledoublethrowvalveof marginalperformance,in their oceanicenvironment, thatperformsthis operationthenameof “waterare given in Fig. 14. chopper”.It is clear that thedrift correctioncapa-

Independentlyfrom their inherentnoise,Ag-AgCl biitiesapply notonly to theelectrodes,but alsotoelectrodescanproducespurioussignalsif the tempera- all circuitsbetweentheelectrodesandthe recorder,tureor the salinity changes.The intrinsic electrode greatlyenhancingthebenefitsof “waterchopping”.sensitivityto changesin temperatureandsalinity is Becauseof cut-offat frequenciesbeyonda fewhardtoseparatefrom furthermanifestationsof the hundredc/h,thehighestchoppingfrequencycom-


V V V salt bridye,n~e’e-

Electrode # 1 V l4jivV _________ ~/ectrodes

mem~4rena

V V V VV VV~V - I~ I ____ ________ V

1 hour I ~ -~~-V I ~fl ~dr,,rn9rod

sea/rn5 ph.’qs

Electrode #2 _________________

salt 6ridqe,~~t’o

iron armaiure

I ________Li’nhi~mM5bar

kit.rna/ ~-inqmaqoeteXtc/77R/ri~i~/fl~,7Qt

Fig. 17. Top: doublepoledoublethrow“water chopper”.V

D 25y Bottom: low powerdriver forabove

noiselevel at thesea-floorinterfacefor a few metersseparationis well belowthe electrodenoise.Morerecenttestsat 1 km depthwith 2.5 m longsalt

Fig. 16. Lowerthreetraces:magneticpulsations(z,H, I) = bridgesorientedin oppositedirectionsfurther con-vertical, northward,eastward).Uppertwo traces: electric firm the feasibility of shortspanbottomelectricpulsationsin oppositedirectionin 2.5 m salt bridge.Depth recorders,seeFig. 16.Thelowerthreetracesre-is 1000 m. presentthemagneticforerunnersof a magnetic

storm.Perfectcorrelationandidentity — exceptpatiblewith resolvingthe existingsignalsis alsoof for sign— of two independentandsimultaneousthis order.Theelectrodenoiseenergyat this frequen- electric signatures,seeuppertwo traces,indicatecy, seeFig. 15,is compatiblewith this choppingrate irrefutably the inducedelectricfield natureof theandelectrodereversaldoesnotneedto be fasterthan signals.Quantitativeconsiderationsleadto resolutionevery 10 seconds. estimatesof about0.1 pV/m for 2.5 m electrode

separations.3.4. Feasibilityofshortspan,freefall electricfield In practice,thetwo main problemsareto insure~

recorder. (1) highopento closedresistanceof thevalves;andTheteststhatproducedtheelectrodenoisespectra (2) minimal powerdrainof themechanicaldrwer.

in situ shownon Fig. 14 alsoindicatethat the natural An approachto switchingandto low powerdriving

J,IL Filloux, Oceanicelectromagnetism 337

is ifiustratedhi Fig. 17. If presentexpectationsare Filloux, J.H., 1967a.An oceanbottom,D V componentfulfilled, it will bepossiblein the nearfurture to magnetometer.Geophysics,32: 978—987.Filloux, J.H., 1967b.OceanicElectric Qirrents,Geomag-constructtwo-componentself-containedbottom netic Variations andtheDeepElectricalConductivityelectricfield recorderswith free fall andbuoyant StructureoftheOcean-ContinentalTransitionofCentral

recoverycapabilities. California, Thesis,Univ. of California,SanDiego,Calif.,166 pp.

Gough,D.I. andReitzel,J.S., 1967.A portablethree-corn-

4. Conclusion ponentmagneticvariometer.I. Geomagn.Geoelectr.,19 (3): 203—215.Greenhouse,J., 1972. GeomagneticTime Variation on the

Thegreatvarietyof theelectromagneticactivity SeaFloor offSouthernCalifornia. Thesis(SlOReferencein the oceanjustifiesextensiveexperimentalresearch. 7 2—67).Univ. of California,SanDiego,Calif.However,this researchrequiresextremelysensitive Harvey,R.R.,1972. OceanicWaterMotionsDerivedfrom

theMeasurementofthe VerticalElectric Field. Hawaiiinstrumentation,deployedin substantialarraysand Instituteof Geophysics,Univ. of Hawaii, Honolulu.capableof working unattendedfor longperiodof Ives, D.J.G.andJanz,G.J. (Editors),1961.Referencetime.Thereis little to be gainedin trying to operate Electrodes,TheoryandPractice.AcadethicPress,Newby meansof simpleadaptationsof land instrumenta- York, N.Y., 651 pp.

tinn to the oceanicrigors.Higher successandlower~ Jones,R.V., 1961.Somedevelopmentsandapplicationsof

costsare in generalderivedfromconceptsthat take optical levers.J. ScL Instrum.,38.Larsen,J.C., 1968. Magneticandelectricfieldsinducedby

the oceaninto accountright at thebeginning.For deepseatides.Geophys..1., 16: 47—70.

thosewho valueexperimentalsciences,research Larsen,J.C. andCdx, C.S.,1966.Lunarandsolardailyaboutor baseduponelectromagneticinductionin variationin themagnetotelluricfield beneaththeocean.theoceanoffersmanychallenges. I. Geophys.Res., 71: 4441—4445.

Longuet-Higgins,M.S., Stern,M.E. andStommel,H., 1954.Theelectricfield inducedby oceancurrentsand waves,with applicationsto the methodof towedelectrodes.

Acknowledgement Pap.Phys.Oceanogr.Meteor., 13 (1): 1—37.Mangelsdorf,Jr.,P.C.,1968. Gulf streamtransportmeas-

Preparationof this review andrecentwork by the urementsusiugthesaltbridgeGEK andLoranC naviga-tion. Trans.Am. Geophys.Union,49: 198 (abstract).

authorfirst reporte-dherewere supportedin partby Owen,T.R.E.andSik,J.M., 1972.A three-componenttheNationalScienceFoundationgrantnumber fluxgatemagnetometerfor seabottomuse.Oceanogr.GB-3l342. mt., 38.

Packard,M.E. ~ndVarlan,R., 1954.Freenuclearinductionin theearth’smagneticfield. Phys.Rev.,93: 941.

Parkinson,WD., 1959.Directionof rapid geomagneticReferencesandbibliography fluctuations.Geophys.1., 2:1—14.

Pepper,C.S., 1968. VehicleSystemDevelopedfor a SeaAdams,A.J.S., 1881.J. Soc. Tel. Eng., 10: 35. Floor RecordingMagnetometer.Univ. of California,Alldredge,L.R. andFitz, J., 1964.Submergedstabilizedplat- SanDiego,Calif., MPL-U-21/168, 13 pp.

form. Deép”SeaRes., 11: 935—942. Sanford,T.B., 1967.MeasurementandInterpretationofChapman,S. andBartels,J., 1940. Geomagnetism,2 Vols. MotionalElectricFields in the Sea.Thesis,M.I.T.,

CiarendonPress,Oxford, 1049pp. Cambridge,Mass: 148 pp.Cox, C.S.,Filloux, JH. and Larsen,J.C., 1971.Electromag- Schmucker,U., 1964.Anomaliesof geomagneticvariations

neticstudiesof oceancurrents andelectricalconductivity - in thesouthwesternUnitedStates..1. Geomagn.below theoceanfloor. In: A. Maxwell (Editor), TheSea. Geoelectr.,15:193—221.Vol. 4, PartI. Trofimov, I.V., Fonarev,G.A.andShneyer,V.S., in prepara-

Dobrin,M.B., 1952.Introductionto GeophysicalProspecting. tion. Someresults of magnetotelluricresearchin theMcGraw-Hill, NewYork,N.Y., 435pp. CentralArtic.

Dreyer,jG. andSanford~i.B.,1970.A free-failelectro- Von Arx, W.S., 1950. An electromagneticmethodfor meas-magnetic current meterinatrumentation. In:Proceedi,*s uringthevelocitiesof oceancurrents from a shipof theLE.R.E. Conferenceon ElectronicEngineeringin underway. Pap. Phys.Oceanogr. Meteor., 11(3): 1—62.OceanTechnology.LE.R.E., London,p. 353370 Warren,R.E. andVacquier,V., 1961.A Ship-towedProton

Paraday,M., 1832.BakerianLecture IV, Experimental Magnetometer.Univ. of California,SanDiego, Calif.researchin electricity.Phios Trans.R Soc.,175. MPL-U-17/61,16 pp.


Weaver,J.T., 1965.Magneticvariationsassociatedwith Zhigalov,L.N., 1960. Somefeaturesof thevariation of theoceanwavesand swell.!. Geophys.Res.,70 (8): geomagneticverticalcomponentin the Artic Ocean.1921—1929. Translatedfrom GeomagneticDisturbances.In: Collection

Young, F.B.,Gerrard,H. andJevons,W., 1920.On elec- of Articles No. 4 relatingto Section3 of I.G.Y. Program.trical disturbancesdue to tidesandwaves.Phios.Mag., Academyof SciencesPress,Moscow.Ser.6, 40: 149—159.

instrumentation for study of natural … · 2019. 6. 24. · j.h. filloux, oceanicelectromagnetism...

Documents