Sound channel formation in the Strait of Juan de Fuca

Sound channel formation in the Strait of Juan de Fuca David G. Browning, and J. W. Powell Citation: The Journal of the Acoustical Society of America 77, S14 (1985); doi: 10.1121/1.2022186 View online: https://doi.org/10.1121/1.2022186 View Table of Contents: https://asa.scitation.org/toc/jas/77/S1 Published by the Acoustical Society of America not be uniquelyrelated(theWKB resultis fortuitous).Instead,someauxilliary condition,suchasTindle'sidentificationof bottomlossper bounce and mode attenuationper cycle distance,must be invoked to uniquely definecycledistance{asopposedto theperiodicityof interference between two modes)as a singlemode quantity. As a bonus,a straightforward eigenvalueestimatoris suggested for numericaleigenmodecalculations. [Work supportedby ONR.] of the wedgefrom 0ø--10ø.A verticalline array of sevenelementsis usedas a sourceto preferentiallyexcitelow orderedmodesin the waveguide.The fieldof propagatingmodesis studiedwith a probehydrophoneasa function of rangeand depthin both the water columnand the sandsediment. Measurementson modeconversionin propagationout of the wedgeare discussed andcompared totl•eory. [Worksupported bytheOffice ofNavalResearch.] alOn leavefromDepartment of Physics, BergenUniversity, Norway. 10:05 F6. Sound channel formation in the Strait of Juan de Fuca. David G. Browning(New LondonLaboratory,Naval UnderwaterSystems Center,New London,CT 06320)and J. W. Powell{DefenceResearch Establishment Pacific, FMO, Victoria, British Columbia V0S lB0, Canada) Twodistinctoceanographic layersexistin theStraitofJuandeFuca:a reduced salinitysurface layer,approximately 100m thick,whichcontains seawardflowingfreshwater runoff;anda deeperlayerwhichhasaccess to the North Pacificbut is blockedto landwardby a sill at the headof the Strait.The annualtemperature cyclein eachlayeris distinctlydifferent, forexample, thesurface layerattainsitshighest temperature in Julywhen thedeeperlayerreaches itslowest.Thiscontributes to a complexevolution of the soundchannelduringtheyear,whichwedescribe.In general, wefindthe soundchannelaxisto belocatedin the shallowlayerduringthe winterandin thedeeperlayerduringthe summer.[Work supported by NUSC and DREP.] 10:20 F7. Measurementof soundpropagation,down-slopeto a bottom-limited soundchannel.William M. Carey (NORDA, 800 North Quincy Street, Arlington, VA 22217-5000),Estvan Gereben (TRW, McLean, VA), 10:50 F9. Propagation loss measurementsin a region of complexbathymetry overthe continentalslope.J. Syckand R. Chapman(DefenceResearch EstablishmentPacific,FMO, Victoria, BC V0S lB0, Canada) Measurementsof propagationlossover a'slopingbottom have been obtainedin experimentson the continentalslopeoff the west coastof VancouverIsland.Shotrunswerecarriedout to rangesof 100km using18 m and 180m SUSchargesin upslopeanddownslopegeometries. The data wereprocessed in 1/3-oct bandsfrom 12.5-630Hz. The 18-mshotswere bottom-limitedin theseexperiments,and the effectof the interactionwith the seafloorwasobservedfor thesechargesin both experimentalgeometries. An enhancementin the propagationloss was observedfor the downsloperun, with the lossdecreasingby up to l0 dB for shotsat the crestof the slope,whereasthe lossincreasedwith rangefasterthan cylindricalspreadingfor theupsloperun. Also,an optimumfrequencyof propagationwasobservedat 50 Hz for bothgeometries.In contrast,thepropa- gationlossincreased withbothrangeandfrequency for thedeepershots. The measurements havebeenmodelcAusinga wide-angleparabolicequation methodwhich is capableof accountingfor the interactionwith the slopingbottom.The modeledresultsprovidean accuratedescriptionof the features observed in the measurements. 11:05 BurlieA. Brunson (PSI,McLean,VA 22102), andMarshall R. Bradley (PSI, Slidell,LA 70458) Signaltransmission lossandspatialcoherence datafor source-receiver separationsbetween100 and 250 km were acquiredin the Gulf of Mexico with a calibratedseismicmeasurementsystem(400 m deep),a towedprojector(100m deep)which emitted67 and 173-Hz tones,and a moorcAWebb soundsourceat 988-m depthdrivenat 175 Hz. Environmentaldata suchasthe rangedependentbathymetryand soundvelocity profilesweremeasured.The 67-Hz data showeda persistentsoundtransmissionlossof approximately90 dB whereasthe 173Hz showedseveral pronounced lossminimabetween100-90dB. Slopeenhancements were found to be on the order 2-4 dB at 67 Hz and 6 dB at 173 Hz when comparedto fiat bottomcalculations.Pairwisecoherencedata showthe combinedeffectsof multipathinterferenceand signal-to-noise ratio. Estimatesof signalcoherence lengthfrom thecoherentsummationof streamer hydrophones yieldcoherence lengthsbetween70-300 m at a frequency of 173Hz. Fastasymptoticcoherentandnormalmodetransmission loss calculations producedresultsconsistent with measureddatafor the deep fiat portionof the measurement trackswhenmeasuredgeoacoustic profilesor relatedbottom losscurveswereused.The implicit finite difference parabolicequationcalculations wereconsistent with range-averaged data for thefiat portionof thetrackaswellason theslope.Theseresultsshow that if properqualitativedescriptionof the sub-bottomvelocityprofiles are used,then computationswith either a parobolicequationor normal modetechniqueare consistentwith experimentalresults. 10:35 FS. Model exi•riments on modepropagationin a shallowwater wedge. H. Hobaek, a) J. Lindberg,and T. G. Muir {Applied Research Laboratories,The Universityof Texasat Austin, P.O. Box 8029, Austin, TX 78713-8029) Guided modepropagationin a wedgeboundedby the oceansurface and a slopedbottomis modeledin a 80-kHz experimentconductedin an indoor tank, under controlledconditions.The bottom is simulatedby a sandfillcAtray, l0 m long, 1m wide,and20 cm deep.The tray issuspendCAbeneaththe water surface,with oneend pivotedsoasto vary the angle S14 J. Acoust.Soc.Am.Suppl.1, Vol.77, Spring1985 F10. Shallow water acoustic modeling over a sloping bottom. C.T. Tindle and G. B. Deane (PhysicsDepartment,The Universityof Auckland,PrivateBag,Auckland,New Zealand) Ray theorywith beamdisplacementgivesan approximatemethodof findingthe acousticfieldin shallowwater. It wasshownto be quiteaccurate for a horizontallystratifiedtwo fluid (Pekeris)modelby comparison with normal moderesults.[C. T. Tindle, J. Acoust.Soc.Am. 73, 15811586{1983)].The methodis extendedto the slopingbottomsituationby simplegeometricargumentsand without further approximation.Results showthat evensmallbottom slopeshave a dramaticeffecton the sound field.The resultsare comparedwith thosefor the adiabaticnormalmode approximationand agreementis good for higher frequencies.At lower frequencies differences are attributedto normal modespassingthrough cutoff,a processwhich is ignoredin simpleadiabaticmodetheory. 11:20 FII. Shallow water propagationwith variable depth. Daniel N. Dixon and Stephen K. Mitchell (Applied Research Laboratories, The University of Texasat Austin, Austin, TX 78713-8029) This paperpresentscalculations of acousticpropagationin a shallow depth-varyingoceanenvironmentusingboththesimpleadiabaticandthe uniformlyvalid adiabatic{UVA) normalmodetheoryof Desaubies[J. Acoust.Soc.Am. 76, 624-626{1984)].In theUVA approximation, modal couplingeffectsareaccountcA for througha secondordercorrectionterm in the phaseof the soundfield;only the phaserelationships betweenthe modesare affected,andthe modalamplitudesremainunchanged. Consequently,the approximationis adiabaticin that no energyis exchanged betweenmodes.This expansiontechnique,whichDesaubiesappliedto a range-varyingsound speedprofile, is generalizedhere to the variable depth waveguideproblem.In the shallowwater exampleconsidered,it wasfound that the phasingeffectscan significantlychangethe interferencepatternsof propagation losscurves.In general,thephasechanges are found to becomemore significantwith greaterbottom slopesand higher frequencies. Comparisons of calculationsand data are presentcA. [This work wassupportedby the ARL:UT IR&D Program.] 109thMeeting:Acoustical Societyof America S14