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Influence of Statistical Surface Models on Dynamic Scattering of High-Frequency Signals from the Ocean Surface (A)

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Influence of Statistical Surface Models on Dynamic Scattering of High-Frequency Signals from the Ocean Surface (A) Downloaded from orbit.dtu.dk on: Dec 17, 2017 Influence of statistical surface models on dynamic scattering of high-frequency signals from the ocean surface (A) Bjerrum-Niese, Christian; Jensen, Leif Bjørnø Published in: Acoustical Society of America. Journal Link to article, DOI: 10.1121/1.411137 Publication date: 1994 Document Version Publisher's PDF, also known as Version of record Link back to DTU Orbit Citation (APA): Bjerrum-Niese, C., & Jensen, L. B. (1994). Influence of statistical surface models on dynamic scattering of high- frequency signals from the ocean surface (A). Acoustical Society of America. Journal, 96(5), 3232-3232. DOI: 10.1121/1.411137 General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain • You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. PROGRAM OF The 128th Meeting of the AcousticalSociety of America Stouffer Austin Hotel ß Austin, Texas ß 28 November-2 December 1994 NOTE: All Journalarticles and Lettersto the Editor are peer reviewedbefore publication. Program abstracts,however, are not reviewedbefore publication, since we areprohibited by time andschedule. MONDAY MORNING, 28 NOVEMBER 1994 SABINE ROOM, 8:00 A.M. TO 12:15 P.M. Session laAO AcousticalOceanography: Acoustic Inversions for Propertiesof GaseousSediments Michael D. Richardson,Chair Naval ResearchLaboratory, Stennis Space Center, Mississippi 39529-5004 Chair's Introduction---8:00 Invited Papers 8:05 laAO1. Biogeochemicalprocesses controlling gas bubble production and distribution in organic-rich sediments. ChristopherS. Martens(Marine Sci., CB-3300, Univ. of NorthCarolina, Chapel Hill, NC 27599-3300),Daniel B. Albert (Univ. of NorthCarolina, Chapel Hill, NC), HanneloreFiedler (ForschungsanstaltderBundeswehr fur Wasserschallund Geophysik, 2300 Kiel, Germany),and FriedrichAbegg (Geologisch-PalaontologischesInstitut und Museumder UniversitatKiel, 2300 Kiel, Germany) Biogeochemicalprocesses in organic-rich, muddy sediments often result in thenet production of biogenicgases including methane. In coastalsediments, methane production ultimately leads to nearsaturation gas concentrations and bubble formation. Rates of production,oxidation, and transport processes, together with in situ temperature and pressure (depth), combine todetermine the actual sedimentcolumn depth of methanebubble occurrence. Recent studies along North Carolina's Outer Banks and Eckemfoerde Bay in theBaltic Sea reveal how these processes combine to controlsaturation gas concentrations and bubble distributions in the upper few metersof coastalsediments. At the NorthCarolina site, gas production depths vary seasonally, resulting in a bubblelayer whose shallowestdepth oscillates between 10- and30-cm depth from summer to winter,respectively. Large quantities of gasescape the sedimentsvia diffusionand bubble ebullition during the warm months. Similar oscillations in the depthof the bubble(acoustic absorption)layer appear to occurin thesediments of EckernfoerdeBay; however, competing microbial processes prevent saturation methaneconcentrations at depths above approximately 50 cm.Stable isotope measurements reveal that microbial methane oxidation consumesmethane transported above the bubble layer, resulting in littlerelease of gasinto the water column. 8:25 laAO2. Predictionsof the acousticresponse of free-methanebubbles in muddysediments. Anthony P. Lyons,Michael E. Duncan(Dept. of Oceanogr.,Texas A&M Univ.,College Station, TX 77843-3146),James A. Hawkins,Jr. (NavalRes. Lab., StennisSpace Center, MS 39529-5004), and AubreyL. Anderson(Texas A&M Univ.,College Station, TX 77843-3146) Theresponse of the sediments of Eckemfoerde Bay, Germany toacoustic remote sensing has been attributed togas features found withinthe sediment.The existenceof featuresas smallas 0.5 mm equivalentspherical radius has been confirmed by x-ray computed tomographyof cores taken and scanned under in situpressures. The interaction of an acoustic pulse from the Acoustic Sediment ClassificationSystem (ASCS) with this type of gassysediment was modeled. The bubble scattering response included the effects of shearmodulus and nonspherical bubbles. Model predictions made using the observed gas feature distribution and normal incidence ASCSdata agree and show extended returns (greater than a pulselength) from the seafloor bubble layers as well as high attenuation 3217 J. Acoust.Soc. Am., Vol. 96, No.5, Pt. 2, November1994 128thMeeting: Acoustical Society of America 3217 Downloaded 27 Jun 2010 to 192.38.67.112. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp withinthe bubble layers. These results show that the acoustic rentms from the gas bubble layers are probably due to scatteringand that differences in level of return can be attributed to random variations in the distributions of scatterers. 8:4• laAO3. The effectsof free-methanebubbles on the propagationand scatteringof compressionaland shear wave energyin muddy sediments. MichaelD. Richardson,Scan R. Griffin, Kevin B. Briggs (Naval Res. Lab., $tcnnisSpace Center, MS 39529-5004), AubreyL. Anderson,and Anthony P. Lyons (TexasA&M Univ.,College Station, TX 77843-3146) Free-methanebubbles cause significant scattering of acousticenergy in thesoft sediments of EckernfoerdcBay, Baltic Sea. In situ and laboratorymeasurement of sedimentgeoacoustic and physicalproperties were made in an attemptto understandthe physical mechanismsresponsible for thisscattering. In situshear wave velocities (at 100-500 Hz) increasedfrom 5-7 nearthe surface to 15-20 m s-• at2 m intothe seafloor, whereas in situcompressional wave velocities (at 38 and58 kHz) varied litfie (•1425 ra s- •) withdepth. Methane bubblesapparently caused significant attenuation of compressionalwaves at depthsbelow 1 m, whereasshear wave attenuationwas unaffected by gasand decreased with depth.Compressional waves (at 400 kHz) in cores(1%-5% freegas} maintained at in situpressures were highly attenuated but showlittle evidenceof velocitydispersion. Comparison of gcoacousticdata with theory suggestthat primarycontrol of the interactionof acousticprofilers with thisgassy seafloor is by bubbleslarger than about 1 mm diameterand that the observedhigh-frequency scattering can be describedby modelsof the seafloorbubbles as individualscarers of acousticenergy. Response of theseafloor as a mediumwith modifiedbulk propagation properties would occur at lowerfrequencies thanthose used in the Eckemfoerdeexperiments. 9:05 1aAO4.Modeling of high-frequencyacoustic wave scattering by sedimentgas voids. DajunTang (Dept.of Appl.Ocean Phys. andEng., Woods Hole Oceanographic Inst., Woods Hole, MA 02543) Scatteringby volumetricinhomogeneities of marinesediments comes in manyforms. It is foundin a 1993CBBL-SRP experiment conductedin EckernfocrdeBay, Germany,that high-frequency backscattering is causedby gasvoids buried, at abouta meterbeneath theseafloor [Tang et el., I. Acoust.Soc. Am. {to be publishcd)].The backscatteringstrength at 40 kHz is estimatedto be -10 dB. Assumingthe gas voids do notresonate, a simplescattering model is developedbased on theKirchhoff approximation to calmlatethe backscatteringand bistatic scattering strength. Acoustic ray bendingdue to thesound-speed discontinuity at thewater-bottom interface as well as sedimentattenuation are takeninto account.We find that at 40 kHz, only thosegas voids whose exposed cross section is largerthan the acoustic wavelength contribute to backscatteringsignificantly. The gasvoid distributionis estimatedbased on datafrom the few coresobtained in situ. The modelresults are comparedwith backscatteringdata, and it is intendedthat this modelbe usedto comparewith bistatic scattering data in thefuture. [Work supported by ONR throughNRL.] 9:25-9:40 Break ContributedPapers attenuation,sediment density values from diver-collected cores, and values for interfaceroughness parameters from underwater photogrammetry were laAO5. Geophysicalground-truthing experiments in Eckernfoerde usedto calculatethe parametersfor the compositeroughness model. The Bay. AngelaDavis, DCI Huws, and Run Haynes (Schoolof OceanSci., modelprediction for scatteringstrength from the sedhncnt-water interface Univ.College of NorthWales, Manai Bridge, Gwynedd LL59 5EY, U.K.) fell well belowthe measuredvalue, indicating that volume scattering from During the 1994 CoastalBenthie BoundaryLayer SpecialResearch within the sedimentwas the dominantprocess. Data obtainedby other Program's(CBBLSRP) experiment in EckemfoerdeBay, multichannel investigatorsreveal a layer of free methanelocated approximately 1 m digitalseismic and electricalresistivity data were collectedusing surface below the sediment-waterinterface. These gas bubbleswere evidently and bottomtowed arrays. Profiling with a bottomtowed sledge yielded responsiblefor the high level of backscatteringobserved. A fit of the shearwave velocity and electricalresistivity
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