DTIC ADA513408: LOAPEX: The Long-Range Ocean Acoustic Propagation EXperiment

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IEEEJOURNALOFOCEANICENGINEERING,VOL.34,NO.1,JANUARY2009 1
Peer-Reviewed Technical Communication
LOAPEX: The Long-Range Ocean Acoustic Propagation EXperiment
JamesA. Mercer, JohnA. Colosi, BruceM. Howe, MatthewA.Dzieciuch, Ralph Stephen,and PeterF. Worcester
Abstract—Thispaperprovidesanoverviewoftheexperimental OfficeofNavalResearch(ONR)workshopatLakeArrowhead,
goalsandmethodsoftheLong-rangeOceanAcousticPropagation CA[1].Theseissuesincluded:1)theunexpectedphasestability
EXperiment(LOAPEX),whichtookplaceinthenortheastPacific of long-range acoustic signals, 2) the evolution of space-time
Ocean between September 10, 2004 and October 10, 2004. This
signalcoherencewithrange(distance),3)theacousticscattering
experiment was designed to address a number of unresolved
physicsresponsiblefortheverticalextensionofacousticenergy
issues in long-range, deep-water acoustic propagation including
theeffectofoceanfluctuationssuchasinternalwavesonacoustic farintothegeometricshadowzonesbeneathcaustics(shadow
signal coherence, and the scattering of low-frequency sound, in zonearrivals)[2],and4)theeffectsofbottominteractionnear
particular,scatteringintothedeepacousticshadowzone.Broad- bottom-mountedsourcesandreceivers[3],[4].Inaddition,the
bandacoustictransmissionscenterednear75Hzweremadefrom distributionofacousticsourceand receiverlocationsavailable
various depths to a pair of vertical hydrophone arrays covering
duringLOAPEXconstituteanexampleofmovingshiptomog-
3500 m of the water column, and to several bottom-mounted
raphy [5], making it possible to infer a thermographic “snap-
horizontallinearraysdistributedthroughoutthenortheastPacific
OceanBasin.Pathlengthsvariedfrom50kmtoseveralmegame- shot” of the northeast Pacific Ocean Basin at the time of the
ters. Beamformed receptions on the horizontal arrays contained experiment.
10–20-ms tidal signals, in agreement with a tidal model. Fifteen LOAPEX was one of three closely coordinated, jointly de-
consecutive receptions on one of the vertical line arrays with a signed ONR experiments collectively called the North Pacific
source range of 3200 km showed the potential for incoherent
Acoustic Laboratory 2004 (NPAL04): LOAPEX, led by J.
averaging. Finally, shadow zone receptions were observed on an
MerceroftheAppliedPhysicsLaboratory,UniversityofWash-
oceanbottomseismometeratadepthof5000mfromasourceat
3200–250-kmrange. ington (APL-UW, Seattle, WA); BASSEX (Basin Acoustic
Seamount Scattering EXperiment), led by A. Baggeroer of
IndexTerms—Acousticscattering,acoustictomography,coher-
the Massachusetts Institute of Technology (MIT, Cambridge,
ence,lowfrequency,propagation,underwateracoustics.
MA); and SPICEX (SPICE EXperiment), led by P. Worcester
oftheScrippsInstitutionofOceanography(SIO,Universityof
California, La Jolla, CA). BASSEX used a towed horizontal
I. INTRODUCTION
receivingarraytostudytheeffectsofseamountsonlong-range
acousticpropagation.SPICEXused250-Hztransmissionsand
T HELong-rangeOceanAcousticPropagationEXperiment
fixed ranges of 500 and 1000 km to: 1) elucidate the relative
(LOAPEX)tookplaceinthenortheastPacificOceanbe-
roles of internal waves, ocean spice (buoyancy compensated
tweenSeptember10,2004andOctober10,2004.Thisexperi-
watermasseswithsoundspeedsdifferentthanthesurrounding
mentwasdesignedtoaddressunresolvedissuesinlong-range,
water masses), and internal tides in causing acoustic fluctua-
deep-wateracousticpropagationthatwereidentifiedata1998
tions; 2) understand the acoustic scattering into the geometric
shadow zone beneath caustics (shadow-zone arrivals); and 3)
explore in a limited way the range dependence of the fluctu-
Manuscript received May 09, 2007; revised October 09, 2008; accepted
ation statistics [6]. SPICEX and LOAPEX complement one
November25,2008.CurrentversionpublishedMarch20,2009.
AssociateEditor:J.Buck. anotherbyprovidinginformationonthefrequencydependence
J.A.MerceriswiththeAppliedPhysicsLaboratory,DepartmentofEarthand of the scattering. BASSEX utilized transmissions from both
SpaceSciences,UniversityofWashington,Seattle,WA98105USA(e-mail: LOAPEX and SPICEX, while LOAPEX utilized two vertical
[email protected]).
linehydrophonearrays(VLAs)installedbySPICEX.
J. A. Colosi was with the Woods Hole Oceanographic Institution, Woods
Hole,MA02543USA.HeisnowwiththeDepartmentofOceanography,Naval There have been a number of well-controlled, long-range
PostgraduateSchool,Monterey,CA93943USA(e-mail:[email protected]). propagation experiments, e.g., SLICE89 [7]–[9], the Acoustic
B. M. Howewas withthe AppliedPhysics Laboratory and the Schoolof
ThermometryofOceanClimate(ATOC)AcousticEngineering
Oceanography, University of Washington, Seattle, WA 98105 USA. He is
nowwiththeDepartmentofOceanandResourcesEngineering,Universityof Test [10], the Alternate Source Test (AST) [11], [12], and
Hawai’iatManoa,Honolulu,HI96822USA(e-mail:[email protected]). the 1998–1999 North Pacific Acoustic Laboratory experiment
M. A. Dzieciuch and P. F. Worcester are with the Scripps Institution of
(NPAL98)[13].Nonehadbeendesignedtoexamine,however,
Oceanography, University of California, La Jolla, CA 92093-0225 USA
(e-mail:[email protected];[email protected]). the detailed range dependence of coherence and scattering as
R.StepheniswiththeWoodsHoleOceanographicInstitution,WoodsHole, LOAPEX. Section II provides a description of the LOAPEX
MA02543USA(e-mail:[email protected]). experimentdesign,theacousticassetsthatweredeployed,and
Colorversionsofoneormoreofthefiguresinthispaperareavailableonline
the engineering methodologies. Section III presents examples
athttp://ieeexplore.ieee.org.
DigitalObjectIdentifier10.1109/JOE.2008.2010656 ofthedataandconcludingremarksaregiveninSectionIV.
0364-9059/$25.00©2009IEEE
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4. TITLE AND SUBTITLE 5a. CONTRACT NUMBER
LOAPEX: The Long-Range Ocean Acoustic Propagation Experiment
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Applied Physics Laboratory,Department of Earth and Space REPORT NUMBER
Sciences,University of Washington,Seattle,WA,98105
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2 IEEEJOURNALOFOCEANICENGINEERING,VOL.34,NO.1,JANUARY2009
Fig.1. GeographicallocationsofthevariousassetsdeployedduringLOAPEX.
II. EXPERIMENTDESIGN,ACOUSTICASSETS,ANDMETHODS nominally50,250,500,1000,1600,2300,and3200kmfrom
apairofVLAacousticreceiversshownasasingleyellowdot
A. Overview (see Table I for precise locations). The VLAs were separated
The primary science objective for this experiment was to by only 5 km. The eighth transmission station was near the
betterunderstandhowoceansound-speedfluctuations(e.g.,due bottom-mountedacousticsourceonthenorthernslopeofKauai.
tointernalwavesandspice)affectspace-timesignalcoherence This source is cabled to a shore facility and is remotely con-
asafunctionofrangeanddepth.Animportantemphasisforthis trolledbyAPL-UWfromSeattle,WA.Theopencircleslabeled
experimentwastoobtainabetterunderstandingofthephysics withsinglealphabeticcharactersindicatetheapproximateloca-
responsible for the previously observed “deep shadow zone” tionsofbottom-mountedhorizontallinearrayreceivers.These
arrivals—long-rangeacousticsignalsthatappearwiththesame fixedreceiversarealsocontrolledfromAPL-UW.Thereddia-
travel times as the deterministic lower turning point caustics, mondshowsthelocationoftheKermitSeamountaboutwhich
but at significantly greater depths [2], [14]. An important and BASSEXdatawerecollected,andfinally,thetwoblackdots500
relatedproblemistheextensionofthefinalecausticattheend and1000kmfromtheVLAreceiversonthemainLOAPEXpath
of the reception when the source is significantly off the sound indicate the locations of two moorings with 250-Hz acoustic
channel axis. This requires an experimental configuration that transceiversthatwereinstalledforSPICEX.Notshownonthis
includes awater-column-spanningVLA andsource thatcould figure are the locations of four ocean bottom seismometer/hy-
be positioned at multiple ranges and depths. This drove the drophone(OBS/H)packagesdeployedaroundthedeeperofthe
design to a ship-suspended source that occupied stations at twoVLAs(seeFig.7).
distancesfromaVLArangingfromkilometerstomegameters.
B. TheAcousticSources
ApathwaschosenintheNorthPacificthatwasageodesicre-
movedfromstrongfronts(e.g.,theCaliforniaCurrentsystem) TheacousticsourcethatwassuspendedfromtheR/VMelville
andsignificantbathymetry.Atthesametime,giventhein-place (Fig.2)is identicalto thebottom-mountedsourcenear Kauai.
cabledassetsofNPAL(abottom-mountedacousticsourcenear Bothwere purchased for the Acoustic Thermometryof Ocean
theHawaiianislandofKauaiandhorizontallinearrayreceivers Climate (ATOC) project [15] and were made-to-order by Al-
around the basin), we could addresstwo additional topics: the liant Techsystems, Inc. (Mukilteo, WA, now out of business)
effects of bottom interaction near bottom-mounted sources [16]. The source design is based upon the proven barrel-stave
andreceivers[3],[4],andademonstrationofathermographic bender-bar transduction design. When deployed to a specific
“snapshot”ofthenortheastPacificOceanBasin. depth,theinternalcavityofthe“barrel”isfilledwithgastothe
The geographical locations of the various assets employed ambient pressure to provide the necessary compliance for ef-
duringLOAPEXareillustratedinFig.1.Thereddotsindicate ficientperformance.AteachLOAPEXstation,thesourcewas
the eight stations at which the acoustic source was suspended initiallydeployedtothedeepestdepthplannedforthatstation.
from the R/V Melville. Approximately 24–48 h were spent at Forthe firstfew stationsthiswas 800m. Oncethe sourcehad
eachstation.SevenofthesestationsareonthemainLOAPEX reachedthedesireddepth,anacousticsignalfromasmalltrans-
path shown as the solid black line. These seven stations were ducer suspended near the surface activated an acoustic valve
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MERCERetal.:LOAPEX:THELONG-RANGEOCEANACOUSTICPROPAGATIONEXPERIMENT 3
TABLEI
STATIONCOORDINATES,WITHNOMINALRANGETOTHEDEEPVLA
Fig.3. Sourcedeploymentwinchwith1.7-cmcable(standard0.68-inoceano-
Fig.2. AcousticsourcebeingdeployedfromthefantailoftheR/VMelville. graphiccable),andoneoftwoaircompressors,allmountedinonehalfofa
20-ftvan.Theotherhalfofthevanhousedthepoweramplifier,signalgenera-
tionelectronics,andacomputer.
mountedonthesource.Thevalvereleasedthegasstoredinfour
high-pressure (41.37 MPa, 6000 lbf/in ) bottles mounted in a receiverthatalsogatedthetriggeringclocktotheD/Aconverter.
frame underneath the source. As the gas was released into the
cavity,theimpedanceofthesourcewasmonitoredviathesignal Three different types of acoustic signals were transmitted
cableuntilitceasedtochangemeasurably.Thiswastheindica- fromthe suspendedsourceduringLOAPEXand allwere pro-
tionthattheinternalpressurehadreachedtheambientlevel.The cessed by digital replica correlation (details in Section II-C4).
gas valve was then closed by another acoustic signal from the The most frequently transmitted signal was an M-sequence
ship. When the source was raised to the shallow transmission (TableII).TheM-sequencemostoftenusedintheexperiment
depth (350 m), the excess gas voided through an open port in isaphase-modulatedcarrierwithtwocyclesofthecarrierfre-
thebottomofthecavity.Midwayinthecruise,problemswith quencymaking1bofa1023-bcode.Thecarrierfrequencyof
the pressurization system limited the deeper depth to 500 m. thesuspendedsourcewhendeployedto800mwas75Hz,and
The problem was eventually traced to a gas filter and 800 m 68.2Hzwhensuspendedto500or350m.Becausethecharac-
wasagainattainedatthefinalstation. teristicimpedanceofthesourcechangedsomewhatwithdepth,
The source was lowered from the ship on a 1.7-cm cable the carrier frequency was selected to optimize the transmit
(standard 0.68-in oceanographic cable) that also served as the waveformwhileminimizingelectricalandmechanicalstresses.
signalcable.Thepurpose-builtcablewinch,theaircompressors The choice of the 350- and 500-m depth carrier frequency
for refilling the gas bottles and to power the deployment “air involvedseveralcompromises.The requirement for a periodic
tuggers,” and lab space housing the transmit electronics were waveformdictatedthatthewaveformcontainanintegernumber
all integrated into a standard 20-ft van for portability (Fig. 3). ofcarrierperiods.Inaddition,theVLAreceivers’(AVATOCs)
LOAPEXacousticsignalsweregeneratedfromdigitalfilesrun scheduleswerepreprogrammedtocollect40M-sequences(for
on an 80486 PC, then converted to analog by a National In- the 20-min transmissions) with 75-Hz carriers. M-sequences
struments(Austin,TX)digital-to-analog(D/A)converterboard, at a slightly different carrier frequency would not fit an in-
andfinallyamplifiedbya48-kVALingpoweramplifier.Trans- teger number of sequences into the preprogrammed collection
mission timing accurate to 1 s was provided by a Spectrum window.Thechoiceofa68.2-Hzcarrierforthe350-and500-m
Instruments(SanDimas,CA)globalpositioningsystem(GPS) depths was considered an adequate compromise since at this
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4 IEEEJOURNALOFOCEANICENGINEERING,VOL.34,NO.1,JANUARY2009
TABLEII
M-SEQUENCESIGNALPARAMETERS
TABLEIII ThescheduleforLOAPEXtransmissionswasbasedupona
PRESCRIPTIONFREQUENCYMODULATED(PFM)SIGNALPARAMETERS predeterminedschedulethatwasprogrammedintotheAVATOC
data acquisition systems on the two SIO VLAs. Because the
linearraysoperatedautonomously,thesuspendedsourcetrans-
mission times had to be adjusted based upon the station loca-
tion; nevertheless, while on station transmissions were sched-
uled once per hour. The only exception to this occurred fol-
lowing the 80-min transmissions, which in fact included the
carrierfrequencytransmitting40M-sequencesof1023bwith time frame for the following hourly scheduled 20-min trans-
two carrier cycles per bit filled the 20-min receiver collection mission. The 80-min transmissions were always preceded in
window. The Kauai acoustic source is on the bottom at 811 the previous hour by a 20-min prescription FM transmission.
m so its carrier frequency was also 75 Hz. However, the bit ThePentalinetransmissionswereonlyusedatthestationnear
code for the Kauai source is “orthogonal” [15] to that used Kauai,wheretwoofthemwereinsertedinthehoursbeforethe
for the suspended source. This allows receptions that overlap prescriptionFMtransmissions.Fig.4providesaschematichis-
in time at distant receivers to be separated from one another tory of the suspended source transmissions during LOAPEX.
after replica correlation. All of the transmissions from the In this figure, the vertical axis is time in year-days from Jan-
KauaisourceduringLOAPEXwere20mininduration;i.e.,44 uary1,2004(notethat2004wasaleapyear).ThelabelsT50,
repetitionsofthe1023-bcode.M-sequencetransmissionsfrom T250,etc.,refertothevarioustransmissionstationsillustrated
thesuspendedsourcewereeither20or80mininduration(44 inFig.1andtotheirapproximatedistancesinkilometersfrom
or176repetitions,respectively). thedeeperofthetwoVLAs.“TKauai”isthesuspendedsource
The second type of transmission used on the suspended stationneartheislandofKauai.Theincreasingperiodoftime
source,andonlyfromthefinalstationneartheislandofKauai, inyear-daysbetweenstationsisduetotheincreasingdistance
is called a “Pentaline” transmission [17]. The Pentaline trans- betweenthestations(requiringmoretransittime)andbecause
mission is not really a different type of transmission, nor a moretimewasspenttransmittingfromthemoredistantstations.
signalwithfivepuretones,butrathera3-bM-sequencewhose Thehorizontal scale is the UTCtime in hoursand the various
spectrumhasfivedistinctpeaks(PL350andPL500inTableII) charactersindicatethetypeandlengthofthetransmissionand
allowing a more direct analysis of the frequency-dependent thedepthofthesuspendedsource.Itisclearthat20-minM-se-
phase coherence. These signals have five cycles of the carrier quences (smaller “M” in Fig. 4) were by far the predominant
frequencyperbitandaphasemodulationof69.3 betweenthe transmissions. All together, there were 228 transmissions to-
“0” and “1” bits. All Pentaline transmissions were 20 min in talingnearly100hduringLOAPEX.
length. The carrier frequency for 350- and 500-m depths was All LOAPEX transmissions were preceded by a ramp-up
68Hzandat800-mdepth,itwas75Hz. to full power. This precursor is not included in the previously
A third transmission used with the suspended source is re- stated transmission durations. The ramp-up started 5 min plus
ferredtoasthe“prescriptionFM”signal(TableIII)[18].These one period (e.g., 300 s 27.28000 s 327.2800 s for the
experimental signals were designed with a variable frequency 75-Hz signal M-sequence signal) before the prescribed start
sweeprate.Thesweepratewaslowatfrequencieswherethere- timeofthetransmissionatalevelof0.26W(165dBre1 Pa
sponseofthesourceisrelativelylowandfastwheretheresponse @ 1 m) and increased in level 6 dB every minute until the
ishigher,providingamoreequalenergydensityacrosstheband desired output level was reached. The ramp-up was intended
andeffectivelybroadeningthebandwidth.Anoptimizationpro- to alert marine life close to the source, and allow sufficient
cedure was used to maximize the source level while keeping time for an animal to increase its distance from the source.
electricalandmechanicalstressesbelowacceptablelevels.For All acoustic transmissions were made at a nominal level of
sourcedepthsof350and500m,thesweepbandwasfrom32to 260W or 195dB re1 Pa@ 1m. These levelswere verified
92Hz(PFM350inTableIIIandforthe800-msourcedepth,the by receptions on a calibrated hydrophone suspended from the
bandwasfrom45to105Hz(PFMinTableIII).Foralldepths, ship.Afulldescriptionoftheplannedtransmissionsignalsand
theperiodofthesweepwas30sandtheperiodwasrepeated40 the duty cycleswere included inan environmental assessment
timestoproduce20-mintransmissions. thatledtotheapprovalforLOAPEX.
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MERCERetal.:LOAPEX:THELONG-RANGEOCEANACOUSTICPROPAGATIONEXPERIMENT 5
Fig.5. DepthsofthefiveVLAsections,thesuspendedsourcedepths,anda
typicalsound-speedprofile.
Fig. 4. Schematic history of the suspended source transmissions during
LOAPEX.Thetransmittedsignalsaregivenasfollows:“M”isanM-sequence,
“P”isaprescriptionFM,and“5”isaPentalinetransmission.Thetransmitter
depthisindicatedbythetypeoffont:350-mdepthsareinitalics,500-mdepths
areinroman,and800-mdepthsareboldroman.Thelengthofthetransmission
isshownbythefontsize:20-mintransmissionsarethesmallersizeand80-min
transmissionsarethelargersize.(Notethat2004wasaleapyear.)
C. TheAcousticReceivers
1) TheVerticalLineArrays: TheVLAswereinstalledbythe
SIO and their locations are represented by a single yellowdot
inFig.1;exactcoordinatesoftheiranchorsaregiveninTableI.
Fig.5providesanillustrationoftheVLAs.Becauseofthecom-
binedweightoftheacousticarrays,itwasnecessarytodeploy
twoseparatemoorings.TheshallowVLA(SVLA)waslocated
5kmduewestofthedeepVLA(DVLA).TheDVLAconsisted
of three acoustic subsections, upper, middle, and lower, each
containing a 20-element array with nominal 35-m spacing be-
Fig.6. IntendedcoveragedepthsofthefiveSIOVLAsubsectionsareindicated
tweenhydrophones.ThetotallengthoftheDVLAwas2100m, bythewhiteregionsofthefigure.Eachregionislabeled;forexample,SVLAU
anditextendedfromanominaldepthof2150downto4270m, meansshallowVLAuppersubsection.Apredictedtimefrontfromtransmit
stationT1000,generatedbyamodifiedMonterey–MiamiParabolicEquation
with one 20-mgap forfloatation. The SVLAconsisted oftwo
(MMPE)code,isoverlaidtoindicatetheintendedcoverageoftheVLAsubsec-
sections, upper and lower, also each containing a 20-element tions.Thetimefrontwascomputedwithoutincludingscatteringphenomena.
arraywithnominal35-mhydrophonespacing.Thetotallength AlthoughtheDVLAMsubsectionwasinoperative,thescatteredextensionof
thethirdarrivingpairofdeepcuspsshouldbeobservableintheDVLALsub-
oftheSVLAwas1400m,anditextendedfromanominaldepth
section.
of 350 m down to 1750 m. The SVLA was positioned about
thesoundchannelaxistooptimizeresolutionofacousticmodes
1–10at75Hz.TheDVLAwaspositionedtospanmanyofthe microsecondlevel.Theacousticreceiverswereremotelysched-
lowercausticsinthepredictedtime-frontarrivalpatternasillus- uledto“turnon”justbeforethereceptionsfromthesuspended
tratedinFig.6.ThepositionsofbothVLAsweretrackedwitha source. In addition, just as they have for almost ten years, the
surveyedsetofsixbottom-mountedacoustictranspondersthat receiverswerescheduledtoreceivetheKauaibottom-mounted
were interrogated by transducers mounted on the arrays. The sourcetransmissionsandperiodicsamplesofambientnoise.
offsets of the rubidium/crystal internal clocks associated with 3) Ocean Bottom Seismometer/Hydrophone Assemblies:
each array section were determined following the recovery of The idea to deploy OBS/Hs for LOAPEX originated at a
thearraysandsubsequentcorrectionsprovidedacousticarrival APL-UW/WHOIworkshop [19].FourOBS/Hunitseachcon-
times accurate to 1 ms in absolute time. Unfortunately, all of taining a vertical geophone and a hydrophone were deployed
thedatafromthemiddlesectionoftheDVLAwerelostdueto totheoceanbottom atabout5000mina 4-kmsquarepattern
awaterleakintoitspressurecase. about the DVLA (Fig. 7). Even though the critical depth, the
2) The Bottom-Mounted Hydrophone Arrays: The approx- deepest depth predicted for purely refracted acoustic arrivals
imate locationsof the bottom-mounted horizontalhydrophone by deterministic models, was roughly 4200 m, the OBS/H
arrays are shown as open circles in Fig. 1. All of these arrays packagesat5000mreceivedtheLOAPEXtransmissions.
have an undersea cable to shore, and their receivers are au- 4) Signal Processing: In general, signal processing for all
tonomous with remote command and control from APL-UW. receptionsisbaseduponreplicacorrelation.Thefirststepisse-
AccuratetimingwasprovidedbyTrueTimeGPSreceiversatthe quence summing in which consecutive sequences, the stan-
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6 IEEEJOURNALOFOCEANICENGINEERING,VOL.34,NO.1,JANUARY2009
Fig. 7. Deployment locations of four OBS/hydrophone assemblies near the
DVLAandtheSVLA.
Fig.8. Sourcelocationandvelocityweredeterminedfromanumericaldy-
namiccablemodelforcedbydatafromaC-NavGPSreceiverandtheship’s
dardM-sequences,forexample,areaddedtogethercoherently ADCP.Datafromapressuresensor,currentmeter,andanacoustictransponder
inthetimedomain.Tooptimizeprocessing, isbasedonthe wereusedtoverifythemodeloutput.
coherence time of the received signal and the resulting pro-
cessinggainis .Thesecondstepisbeamforming.If
thesignaliscoherentacrossthearrayandthenoiseisisotropic, D. SourceandReceiverNavigation
the array gain is , where is the number of hy-
drophoneelementsinthearray.Thenextstepispulsecompres- 1) SourceNavigation: Asignificanteffortwasmadetocol-
sioninwhichthe recordeddataarecomplexdemodulatedand lectdatathatwouldallowtheprecisedeterminationandverifica-
correlatedwithastoredreplicaofthetransmission.Thisprocess tionofthesuspendedsourcelocationandvelocityduringtrans-
producesatriangular-shapedpulsewithatimeresolutionof1-b missions.Thethermographicsnapshotoftheoceanrequirespre-
length,or27ms,andadditionalprocessinggainof , cise source localization and the estimates of signal coherence
where isthenumberofbitsinthesequence.Asafinalstep, requireprecisedeterminationofsourcevelocities.Althoughthe
individual coherent results can be grouped and summed inco- R/V Melville maintained a relatively constant position at each
herently. station using its dynamic positioning system, the great depths
The processing described aboveassumes that the signal co- towhichtheacousticsourcewasdeployedrequiredanovelap-
herencetimeisaknownquantity,whereinrealitydetermining proach for source position and velocity measurements. Fig. 8
coherencetimewasaprinciplegoaloftheexperiment.Twopri- provides an illustration of the various navigation instruments
maryfactorsdeterminethereceivedcoherencetime:1)themag- thatwereused. Agoalwasset tomeasuretheabsolute source
nitudeandextentofvariabilityintheinterveningocean;and2) positionat1-sintervalstoone-tenthoftheacousticwavelength,
themotionofthesourceandthereceiver.Thefirstitemisdeter- or about 2 m, and relative velocities to 0.2 cm/s. The primary
minedbythelocationsofthesourceandreceivers.Sourceand methodofachievingthisgoalwastoapplyanumericalfinite-
receivermotionisnotanissueforthebottom-mountedsource differencedynamiccablepredictionmodel[20].Inadditionto
near Kauai and receptions on the bottom-mounted horizontal thestaticinputparameterslistedinTableIV,themodelforcing
hydrophone arrays or the OBSs, but it is an issue for the sus- dataconsistedofthe3-Dpositionofthesourcesuspensionpoint
pendedsourceandtheVLAs.Tobetterunderstandthisissue,a ontheR/VMelville’sA-frameasdeterminedbyaC-NavGPS
DopplersimulationstudyforthesuspendedsourceandVLAre- (C-NavGPSismarketedbyC&CTechnologies,Lafayette,LA),
ceiverwascompleted.Simulatedsourcemotionsofafewmeters andthewatercurrentprofileasdeterminedbyanRDI(Poway,
per20minandVLAhydrophonemotionsofhundredsofmeters CA)acousticDopplercurrentprofiler(ADCP).
per M2 tidal cycle were used. The resulting de-coherence due TheC-NavGPSpackageprovidesdual-frequencyworldwide
to motion was compared to that resulting from internal waves corrected position and velocity estimates. The dual frequency
withaGarret–Munkspectrum(GM)strengthequaltoone.The corrects for ionospheric errors, while data from globally dis-
LOAPEX study concludes: 1) Doppler-induced intensity fluc- tributed ground stations are used to correct for GPS satellite
tuationsareafewtenthsofadecibel,rarelymorethan0.5dB; ephemeris errors, GPS clock error, and other atmospheric ef-
2)Doppler-inducedtraveltimefluctuationsareafewmillisec- fects in real time via a geostationary satellite downlink. The
onds,rarelymorethan5ms(cf.,1bofa75-HzM-sequenceis C-Nav data were generally very good during the experiment.
26.7ms);and3)Dopplerprocessingisnecessaryforcoherence WhenthenumberofavailableGPSsatellites“inview”dropped
studies to separate incoherence due to source–receiver motion belowfive,someoutlierswereobserved,butthisoccurredless
fromthatduetooceanvariability.SectionII-Ddescribestheef- than 2% of the time; when it did occur, the duration was less
forttotrackthepositionsandvelocitiesofthesuspendedsource thanthetimeconstant ofthe suspendedsource pendulummo-
andtheVLAhydrophones. tionsothattheerrorswereeasilyaddressed.
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MERCERetal.:LOAPEX:THELONG-RANGEOCEANACOUSTICPROPAGATIONEXPERIMENT 7
TheC-Navsystemwasreportedtohavedecimeteraccuracy ceivedoneachinterrogatorandonsixhydrophoneswithineach
and this was verified, at least while the ship was at the pier 20-element array section. Because of the depth of the VLAs,
in the San Diego harbor. However, to provide validation of the high tension in the array cables, and the lack of a surface
the position data and model output while at sea, additional forcingcomponent,themotionoftheVLAswasrelativelyslow
measurements were acquired.As the R/V Melvilleapproached andcorrespondedprimarilytothetidalforcing.Fig.10provides
each of the transmit stations (approximately 5 km before) an anexampleoftheDVLAnavigation.Notetheexaggeratedap-
acoustic transponder was dropped to the ocean floor. Once pearanceoftiltduetotheaxes’values.Dataforthemiddlesec-
the LOAPEX source was deployed, interrogations from an tion of the DVLA were lost due to a water leak into the elec-
acoustictransmitterattachedtothecable6mabovethe75-Hz tronicspressurecase.Positionsfor the remaining14ofthe 20
acousticsourceprovideda1-Dcomparison alongtheacoustic hydrophoneelementsineachsectionwereinterpolatedfromthe
path to the VLAs (approximately east–west) of the source sixhydrophoneinterrogatorreceptionswithinthesection.Due
position with the output of the dynamic cable model. Even to infrequent VLA navigation during the LOAPEX transmis-
though transponder receptions were relatively noisy, the root sions,ithasprovendifficulttopositionthe VLAhydrophones
mean square difference between the transponder data and the with an accuracy better than 5 m. Many analytical techniques
dynamic cable model estimate of position in the east–west wereemployedincludingtheincorporationofatidalmodelbut
directionrangedbetween0.6and2.2m. improvementwasnotpossible.Thiswilllimittheeventualcon-
A Seabird MicroCAT (Sea-Bird Electronics, Inc., Bellevue, clusionsregardingcoherence.
WA)wasattachedtothecable20mabovethe sourcetomea-
E. EnvironmentalMeasurements
suredepth(pressure)andtoallowacomparisonwiththevertical
positionestimatedbythedynamiccablemodel.BecausetheMi- EnvironmentaldatawerecollectedduringLOAPEXtosup-
croCATlogged6-sdepthaveragesevery15s(nottheidealsam- porttheeventualacousticnumericalmodelingeffortandthere-
plingforanominal10-ssurfacewave/shipheaveperiod)adirect sulting comparisons with actual acoustic data. At each of the
comparisonwiththe1-soutputfromthedynamiccablemodel eightacousticstations,afull-ocean-depthconductivity–temper-
wasproblematic.However,theresultsofthecomparisonnever ature–depth(CTD)profilewastaken.AstheR/VMelvilletran-
appearedinconsistent.Forexample,thecablemodeloutputfor sited between each of the stations from T50 to T2300, an un-
stationT250whilethesourcewasatadepthof800mshowed derway CTD (UCTD) [22] was deployed. This novel device
typicalvariationsindepthofroughly1mwithafewexcursions consists of a CTD probe that was dropped off the fantail as
of 2 m. The data logged by the MicroCAT followed the same the ship transited; as it fell, a Kevlar line spooled off from a
patternexcepttheamplitudeswereapproximately60%ofthat reel on the fantail and from a spool in the probe. The double
predictedbythemodel,whichiswhatonewouldexpectgiven spoolingallowedtheprobetofallfreelyuntilallofthelinewas
thelowsamplerateoftheMicroCAT. removedfrom the spool in the probe. Typical depths achieved
The ship’s ADCP was able to make 3-D current estimates while transiting at 6.2–6.7 m s were 300–400 m. When all
down to 800 m, the deepest of the source deployment depths. of the line was spooled off the probe, the probe was reeled in
Absolutecurrentmeasurementswereobtainedbyremovingthe on a powered reel and taken into the lab for data transfer. As
shipmotionasdeterminedbytheship’sAshtechP-codeGPS. the data were being transferred the probe spool was rewound
ADCPdepthbinsof16mwereaveragedover5-minintervals andmadereadyforanotherdeployment.Atotalof156UCTD
toreducemeasurementuncertaintyandrandomerrors.Finally, casts were completed at approximately 15-km intervals. Two
anInterOceanSystemsS4currentmeter(InterOceanSystems, UCTDprobeswereavailabletous,andafterlosingoneprobe
Inc.,SanDiego,CA)wasinstalledonthecable6mbelowthe duetofrayingoftheloweringline,westoppedtheUCTDoper-
acoustic source to measure the 3-D velocity of the water rel- ationafter2300kmforfearoflosingthelastprobe.TheUCTD
ative to the source. By combining these data with the ADCP probeswerecalibratedbeforeandafterthecruise.
velocities at the nominal source depth, an estimate of the ab- DuringthetransitintervalsinwhichUCTDcastsweremade,
solutevelocityofthesourcewasobtainedforcomparisonwith expendablebathythermograph(XBT)dropsweremadeat50km
thedynamiccablemodel.Again,samplingmismatcheswerea intervals, and after the UCTD casts were terminated, the dis-
minor problem. The ADCP was averaged over 5 min and the tancebetweenXBTdropswasreducedto25km.ADCPmea-
S4 provided data at 30-s intervals. Nevertheless, the compar- surementsandbathymetricmeasurementswiththeship’smulti-
isons were rather good. Fig. 9 provides a comparison of the beamsonarweremadeatalltimesduringtransit.
east–west velocities as estimated by the dynamic cable model Another novel measurement approach was the use of au-
andtheADCP/S4methodwhileatadepthof800matstation tonomous vehicles to collect CTD data. In this case, two
T250.Detailedanalysisofthesourcemotionisthesubjectofa Seagliders [23] manufactured at the APL-UW, were deployed
thesisbyZarnetske[21]. nearstationT50.Thesevehiclesarenotpoweredbyapropeller,
2) VLANavigation: TheVLAs shared one transponder net but rather by buoyancy control; a hydraulic system moves oil
madeupofsixtransponders.EachoftheVLAsections(twoin inandoutofanexternalbladdertoforcethegliderupordown
theSVLAandthreeintheDVLA)hadanelectronicspackage through the ocean. In addition, the location of the glider’s
that was capable of interrogating the transponder net. The in- batterypackcanbeadjustedtocausetheglider’snosetopitch
terrogators transmitted once per hour, but sequentially so that up or down, or to roll its wings to change compass heading.
a total of 400 s was required for all five of them to transmit. The LOAPEX Seagliders measured temperature, pressure,
Thesixtransponderrepliesfromallfiveinterrogatorswerere- salinity, oxygen, and RAFOS long-range acoustic navigation
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8 IEEEJOURNALOFOCEANICENGINEERING,VOL.34,NO.1,JANUARY2009
TABLEIV
INPUTDATAFORTHENUMERICALDYNAMICCABLEMODEL
Fig.9. Comparisonofthesuspendedsourcevelocityintheeast–westdirectionasdeterminedbythedynamiccablemodelandtheS4currentmetercombined
withtheADCP.Note:thehorizontalscaleisinyear-daysfromJanuary1,2004butrepresentsonlyabout30minoftimeduringatransmissionfromsiteT250.
time. From T50, the first Seaglider was directed back to the
DVLA before returning on the main LOAPEX path to station
T1000 where it was turned toward the island of Kauai. The
second Seaglider was directed to intersect the path between
the DVLA and the Kauai acoustic source. Once this path was
intersected,itfollowedthepathtoKauai.After191daysatsea,
theSeagliderswerefinallysteeredtotheleewardsideofKauai
for pickup on March 24, 2005. Fig. 11 illustrates the paths of
the two Seagliders along the specified acoustic transmission
pathsduringtheirrecordsettingdeployment.
III. ACOUSTICDATASAMPLES
A. Bottom-MountedHydrophoneArray
SectionII-C4describedthebasicstepsinsignalprocessing.
ThedataexampleshowninFig.12isabeamformedreception
onthebottom-mountedhorizontalarrayindicatedbytheletter
Fig.10. ExampleofacousticnavigationoftheDVLA.Allthreeaxesarein
meters,but thescalechangeexaggeratestheapparenttiltof thearrays.The “R”inFig.1.Thisarrayisatadepthof1309m.Thesuspended
middlesectionismissingduetoaleakintheelectronicscase. source transmitter was located at station T250; the source–re-
ceiver range was 848.571 km. In this example, the coherence
timehasnotyetbeendeterminedsotheprocessedreceptionin-
datawhiletravelingabout3000km,andmakingapproximately cludes only one of the 27.28-s M-sequences. In addition, the
600 dives to a depth of 1000 m. The Seagliders contacted a source and receiver motions have not yet been removed. The
pilot at APL-UW by Iridium modem each time they surfaced, vertical axis is the usual conical arrival angle associated with
so their position, status, and data were available in near real horizontal line arrays. Because the array is not normal to the
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MERCERetal.:LOAPEX:THELONG-RANGEOCEANACOUSTICPROPAGATIONEXPERIMENT 9
Fig.11. PathsofthetwoLOAPEXSeaglidersfromtheirdeploymentatT50to
Kauaiwheretheywererecoveredafternearly600divesto1000mandajourney
ofapproximately3000kmover190days.
Fig.13. Multiplereceptionsonthebottomarrayindicatedbytheletter“R”
(Fig.1)whilethesuspendedsourcewasatstationT50andstationT1600.While
atstationT50,transmissionsweremadefromtwosourcedepths,800and350
m.Theseparationintimebetweencloselyarrivingpairsdecreaseswiththeshal-
lowerdepth.WhileatstationT1600,thesourcedepthwas500m.
figure,theverticalaxisisthereducedtraveltimewindow,and
thehorizontalaxisistheyear-day.Averticallineofreception
dots is typically separated by 1 h from the adjacent vertical
line of dots. In this presentation, the horizontal alignment of
dots quickly reveals those individual acoustic path receptions
that are consistent. Each dot represents the arrival time of a
signal-to-noiseratio(SNR)peakafterprocessing.Thediameter
ofthedotindicatestheSNRofthearrival.SNRslessthan8dB
were eliminated from this plot. In the earliest seconds of the
reduced arrival time window, horizontal pairs of arrivals are
seen.Thepairsofarrivalscorrespondtoequal,butpositiveand
negative vertical source angles, and therefore, the ray paths
Fig.12. Beamformedreceptionshowing“ray-like”arrivalswithrelativein-
experienced approximately the same average sound speed. As
tensitiesasmeasuredonthebottom-mountedhorizontalhydrophonearrayindi-
catedbytheletter“R”inFig.1.ThesuspendedsourcewasatstationT250and predictedbeforetheexperiment,thesepairsarrivemuchcloser
adepthof800m.Onlyone27.28-sM-sequencewasprocessedforthisfigure. together in time when the source is at a shallower depth as
duringthesecondgroupfromT50whenthesourcedepthwas
350 m, and the receptions from T1600 when the source depth
direction of propagation and because the multipaths have dif- was500m.
ferent verticalarrival angles, the apparent angle of arrival(the
conical angle)varies during the reception. Thehorizontal axis
B. VerticalLineArray
is a reduced travel time window that captures several of the
early “ray-like” arrivals. Consecutive processed M-sequences Although a portion of the VLA was lost, the data that were
revealfadinginamplitudeandsplittingintimeofdeterministic recorded should be very useful. Fig. 14 is an example of
ray-likearrivals(notshown). processed receptions on the upper 20-element subarray of the
Because the suspended source was typically transmitting DVLAwhentheLOAPEXsourcewassuspendedtoadepthof
every hour while onstation it is possible to “stack” the recep- 350matarangeof3200km.Fifteentransmissionswereinco-
tionsintoa“dotplot”tobettervisualizetheconsistentarrivals. herently averaged together. In this figure, each hydrophone of
Fig. 13 is a dot plot comparing receptions on the horizontal thesubarraywasprocessedindependentlyandthe“accordion”
array located at position “R” in Fig. 1, while the suspended time-frontstructureisevident.Thetailofthe“accordion”isnot
sourcewaslocatedatstationT50atadepthof800m,andwhile present due to the shallow source depth. There have been no
located at station T1600 at a depth of 500 m; source–receiver correctionsforsourcemotionorVLAmotioninthesedata.The
ranges were 1004.399 and 926.205 km, respectively. In this initial effort will be to quantify fourth-moment statistics, for
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