APPLICATION OF HF COASTAL OCEAN RADAR TO TSUNAMI OBSERVATIONS
M.L. Heron, A. Prytz AIMS@JCU and Marine Geophysical Laboratory,
James Cook University, Townsville, Australia [email protected]
S.F. Heron PortMap Remote Ocean Sensing Pty Ltd, Townsville,
Australia [email protected] T. Helzel Helzel Messtechnik
GmbH, Kaltenkirken, Germany [email protected] T.Schlick University
of Hamburg, Germany [email protected] ABSTRACT: When
tsunami waves propagate across open ocean they are steered by
Coriolis force and refraction due to gentle gradients in the
bathymetry on scales longer than the wavelength.When the wave
encounters steep gradients at the edges of continental shelves and
at the coast, the wave becomes non-linear and conservation of
momentum produces
squirtsofsurfacecurrentattheheadofsubmergedcanyonsandincoastalbays.HFcoastaloceanradariswell-conditioned
to observe the current bursts at the edge of the continental shelf
and give a warning of 40 minutes to 2 hours
whentheshelfis50-200kmwide.Theperiodoftsunamiwavesisinvariantoverchangesinbathymetryandisinthe
range 2-30 minutes. Wavelengths for tsunamis (in 500-3000 m depth)
are in the range 8.5-to over 200 km and on a shelf where the depth
is about 50 m (as in the Great Barrier Reef) the wavelengths are in
the range 2.5 - 30 km.There is a trade-off between resolution of
surface current speed and time resolution.It is shown that the
phased array HF ocean
surfaceradarbeingdeployedintheGreatBarrierReef(GBR)andoperatinginaroutinewayformappingsurface
currents,canresolvesurfacecurrentsquirtsfromtsunamisinthewaveperiodrange20-30minutesandinthe
wavelengthrangegreaterthanabout6km.Iftheradarisactivelymanagedwithautomaticinterventionduringa
tsunami alert period (triggered from the global seismic network)
then it is estimated that the time resolution of the GBR radar may
be reduced to about 2 minutes, which corresponds to a capability to
detect tsunamis at theshelfedgeinthe
periodrange5-30minutes.Itisestimatedthatthelowerlimitofsquirtvelocitydetectionattheshelfedgewould
correspond to a tsunami with water elevation of less than 5 cm in
the open ocean.This means that the GBR HF radar is
well-conditionedforuseasamonitorofsmallandmediumscaletsunamis,andhasthepotentialtocontributetothe
understanding of tsunami genesis research. KEY WORDS:HF Radar,
Tsunamis, Surface Currents 1.INTRODUCTION 1.1TsunamisTsunamis are
long wavelength, long period ocean gravity
waveswhicharenormallyproducedbyearthquakesor
underwaterslumps.Thewaveperiodoftsunamis depends upon the scale
sizeofthebathymetriceventand
waterdepth.Historicalrecordsofimpactsontheshore
indicatethatthetsunamiwaveperiod(theso-called
tsunamiwindow)isintherange2-30minutes(Bryant,
2001).Theconnectionbetweenearthquakesand tsunamis is not well
understood; some marine earthquakes
donotproducetsunamis,andsometsunamisoccurwith
quitesmallseismicactivity.Oncethepulseofelevated
waterisproduceditcantravellargedistanceswithlittle
attenuationandsomedispersion.Theelevationofthe
tsunamiwavesintheopenoceanisgenerallylessthan
about0.5m.Thisisamplifiedtotensofmetresatthe
shoreandcanproducerun-upofover100m(Bryant, 2001).
TheAcehtsunamionBoxingDay,2005producedwave
heightatthebeachoftheorderof10mandtheopen
oceanelevationwas0.5metresasmeasuredbythe JASON 1 altimeter in the
Indian Ocean some 2 h after the earthquake in the Sumatra-Andaman
Islands. Barrick(1979)showedthatHFradarshavethepotential
toobservetsunamisupto80kmoffshore.Inthispaper
webuildonthatconcepttoevaluatethefeasibilityof operational
long-range HF radars to detect tsunami effects
attheedgeofcontinentalshelves.Herewehavethe
advantageofamplificationofsurfacecurrentsasthe tsunami encounters
the shelf edge, but the challengeisin
theuselongrangeHFradarsystemswheretime resolution is often
sacrificed. 1.2.HF Radar Characteristics
HFoceansurfaceradarobtainshighenergyechoesfrom
theBraggwavesontheoceansurfacewhichhave
wavelengthsequaltohalftheradarwavelengthandare propagating radially
towards and away from the radar
site.Theradialresolutionscaleforradarsissetbythe bandwidth of the
transmitted signal.For HF radars this is
typically50KHzandiscontrolledbyinternational
agreementsontheuseoftheradiospectrum.Some
systemsareworkingwithbandwidthsupto150KHz.TheradarbeinginstalledontheGreatBarrierReefin
Australia has a bandwidth of 50 KHz, which corresponds
toaradialrangeresolutionof3km.Theazimuthal
resolutioniscontrolledbythewidthofthelobeofa beam-forming radar,
and by the accuracy of amplitude or phase measurements for
direction-finding radars. A typical radar spectrum is shown in Fig.
1.The offset of theBraggpeaksfromthereference(whichisthe
transmitterfrequency)isduetoDopplershift,
predominantlycausedbythephasevelocityoftheBragg waves (which offset
is indicated by the dashed lines in Fig. 1) but also caused by
radial surface current which is
Figure1.Typicalspectrumforaphasedarrayocean
surfaceradar.ThepeakslabelledAandBarethefirst-order Bragg
scatter.Thesidebandpeaksoneachsideof the Bragg lines are produced
by wind waves.When swell
ispresentitappearsassidebandsontheBragglinesand
separatedfromthembyafrequencyequaltothe
frequencyoftheswell;inthisrecordtheswelllinesare
barelysignificantcomparedwithfluctuationsinthe spectrum. indicated
by df in Fig. 1. The question arises here about
thedifferentiationbetweenswell ( whichshowsup as
sidebandsonthespectrum)andtideswhichare
effectivelyverylongwaves.Westeparoundtsunamis
herebecausetheyfallinbetweenthetwoextremesand
wewilldiscussthemlater.Tideshavereturnperiods
muchgreaterthanthetimeittakestorecordthetime series (which is
thebasis of the spectrum shown in Fig. 1), and the pixel size.
fortheradarismuchsmallerthanthelengthscaleofthe
tidalcycles.Thereforethetidalcurrentsareresolvedin time and space
by the HF radars.Swell is not resolved in
timeorspaceandismanifestinthespectraassidebands
onthefirst-orderBragglines.Inthispaperwe investigate the time and
space scales for tsunamis. 2.TSUNAMI CHARACTERISTICS 2.1Tsunami
Period and Wavelength Tsunamis are shallow water gravity waves and
we can use linearwavetheoryatleastwhilethewavesareinopen
waterwithgentlegradientsinthebathymetry.The expression for wave
celerity, c, for gravity waves is( )2tanhgc kHk=(1)
(Kinsman,1965)wherekisthewavenumberandHis
thewaterdepth.Inshallowwateragoodapproximation is c gH = (2) which
is illustrated in Fig. 2 by a family of curves with H
varyingfrom500to3000m.Theflatsectionsforwave
periodsaboveabout300secondsconfirmthe
applicabilityof(2),whileforsmallerwaveperiods(1) becomes more
appropriate.One important indication
Figure2.Propertiesofgravitywavesderivedfrom(1)
fortsunamiconditions.Celerityversuswaveperiod
showsthetransitionfromdispersiveconditionsatshort
waveperiodstonodispersionatlongperiods.The vertical marker at 120
seconds marks the shortest tsunami period (Bryant, 2001).
fromFig.2isthatforapulseofwaterelevationatthe
source,thesubsequentpropagationwillbesubjectto
somedispersionforthehigherfrequencycomponents (wave periods less
than about300seconds)whichmeans
thatfordistantsourcesthefirstoceanwaveswillhave periods longer than
about 300 seconds. 2.3. Tsunami propagation
Inopenwater,withgentlegradientsinbathymetry,the
wavedirectioniscontrolledbyrefractionandCoriolis
rotationoffgreatcirclepaths.Whentsunamisapproach
steepgradientsinthebathymetry(andatthecoast)the scale of the
physical features is usually much less than the
wavelengthofthetsunamiandwegetnon-lineargrowth
inwaterelevationandsurfacecurrents.Thisisdifferent
fromswellwaveswhicharerefractedastheyapproach
thecoast.Themaindifferencebetweenswelland
tsunamisisthatswellfocusesontoheadlandsawayfrom
coastalbays,whereastsunamishavethebiggesteffectin the bays and at
the head of submerged canyons.
Non-lineareffectsarisefromtheconservationof
momentuminthewatercolumnwhichtendstodrive
waterparalleltothedepthcontourstoproduce
enhancementofsurfaceelevationattheheadof
submergedcanyons,whichinturnproducesthegreatest
effectatthecoastinbaysandestuariesratherthanat headlands.If the
bathymetry changes significantly within
onewavelengththenmomentumeffectsdominateover
refraction.Wavelengthsfortsunamisintheopenocean (500-3000 m depth)
are in therange8.5-toover200km,
andbecausebathymetricgradientsaregentle,wemay
assumethatbathymetricsteeringintheopenoceanis
controlledbyrefraction.Asatsunamipropagates across a basin it will
normally be refracted so that itapproaches
thecoastorthogonallytothelarge-scalebathymetric contours.Near the
coast however, where the scale of the
spatialfeaturesislessthanthewavelength,refraction
doesnotworkbecausethewaveischanging(non-linearly) within the space
of a wavelength.Here the water
elevation(andrun-up)iscontrolledbymomentum transfer as the water
becomes shallower. .Themomentumtransferisaresultofnon-linear
processesandisbettermodelledbynumerical
calculationsthanbyananalyticwavepropagation
approach.AnexampleofsuchamodelisshowninFig
4bwherethesurfacecurrentsattheedgeofthe
continentalshelfneartheSeychellesareshownforthe
firstwaveoftheBoxingDaytsunamiwhichoriginated
nearAcehinIndonesia.Therelevantbathymetryis
showninFig4a.Itcanbeseenthatthehighercurrent
surgesareproducedattheheadsofsubmergedcanyons while the submerged
ridges experience smaller surges. 3. TIME AND SPACE SCALES 3.1
Space scales for radar observation (a) (b) (c)
Fig.3.ModelcalculationsfortheimpactoftheBoxing
DayTsunamiontheSeychelles.(a)Thebathymetry around the Seychelles
shows a platform in otherwise deep
water;(b)thelargescaleamplificationofsurfacecurrent
whenthefirsttsunamiwaveencounterstheedgeofthe
continentalshelf;and(c)asectionoftheshelf-edge which could be
monitored by an HF ocean radar.
ThespatialresolutionofHFradarsintheradial
dimensiondependsonthebandwidthoftheradarinthe radio spectrum.The
range is determined by a time delay and the phase velocity of the
electromagnetic wave as r = ct /2(3)
wherec=3x108ms-1,andtistheoutandbacktime
delaybetweenthetransmitterandthereceiver.For resolution we have Dr
= (c /2)Dt(4) whereDtistheresolutionintime.Thisrequiresa bandwidth
of D fwhere D f = c /(2D r).(5) Most HF ocean radars have
bandwidths between 50 KHz
and150KHzwhichcorrespondstoarangeresolutionof
3to1kmrespectively.Thelongrangeradarstendto have narrower
bandwidths because of the lower operating
frequency.TheGBRradarhasabandwidthof50KHz. This means that the
smallest tsunami wave length that can be observed is 6 km. 3.2 Time
scales for radar observation
ThetimeresolutionforHFradarsdependsonthe
observationtimerequiredtomakeonesurfacecurrent determination. For
the GBR radar, the routine operation
istosamplesurfacecurrentsevery10minutes,because
eachofthetwostationsoperatesfor5minutesandthen
waitsfor5minutes.Somesystemsoperatewithsurface currents produced
routinely every 3 hours. To reduce the
samplingtimewehavetotradeoffaccuracy.Forthe GBR radar,
whichisaphasedarraysystem,thetrade-off
isbetweentimeresolutionandaccuracyofthesurface current values.Fig.
5 shows the relationship where at a
radaroperatingfrequencyof8MHzandatime-series length of 5 minutes
the surface current resolution is about 7 cm/s. 4. CONCLUSION: AN
OPERATIONAL TSUNAMI WARNING SYSTEM
Theparametersdiscussedabovecanbere-arrangedto
formanoptimisedtsunamiwarningsystem.Firstly,one
ofthetworadarstationswillalwaysbebetterpositioned
toobservesurfacecurrentburstsorthogonaltothe bathymetry contours
along the edge of the shelf.During a
tsunamialert(triggeredbyaresponseontheglobal
seismicnetwork)wewouldturnofftheradarstationthat
waslesseffectiveandruntheotheronerepeatedlyfor time series lengths
of, say, 2 minutes.According to Fig. 5 this would provide
surfacecurrentresolutionofabout12 cm/sec. This is a worst-case
estimate because there
areFig.5.Theresolutionforsurfacecurrentisdegradedas the radar
transmitter frequency decreases.The parameter
forthefamilyofcurvesisradarfrequency.Toachieve resolution of 15
cm/s at an operating frequency of 5 MHz
onewouldneedtointegrateforaperiodofnotlessthan 200 seconds.
sophisticated analyses that can be used to combine spatial data to
improve the detection resolution. For the Boxing Day tsunami, the
scale size of the surface
currentsatashelfedgeattheSeychelleswereupto150 cm/sec,and this
corresponded to an open water
elevationofabout50cmandanelevationofabout10matthe
shore.Assumingthatwecanscalelinearly,thepresent
indicationisthattheGBRphasedarrayradarcandetect
tsunamisoflessthanone-tenththemagnitudeofthe Boxing Day tsunami
with wave periods greater than about
4minutes;underactivemanagement.Underroutine operation (which means
a huge archive of data) the GBR
radarwilldetecttsunamislessthanone-tenththe magnitude of the Boxing
Day event butforwaveperiods longer than 20 minutes. This means that
we would have accesstomanytsunamiswhicharenotdestructiveatthe
coast,butwhichprovidesausefulresearcharchivefor studying tsunami
occurrence and genesis. ACKNOWLEDGEMENTS The HF radar deployed in
the Great Barrier Reef region was funded under Linkage
Infrastructure Equipment and Facilities GrantLE0560892 from the
Australian Research Council. REFERENCES
Barrick,D.E,Acoastalradarsystemfortsunamiwarning, Remote Sensing of
the Environment, 8, 353-358, 1979.
Bryant,E.,Tsunami:theunderratedHazard,Cambridge University Press,
320pp, 2001 Kinsman,B.,WindWaves,Prentice-Hall,EnglewoodCliffs,
N.J., 676pp, 1965
