Preliminary results are presented from an analysis of mid-frequency acoustic transmission data collected at range 550m during the Shallow Water 2006 Experiment. The acoustic data were collected on a vertical array immediately before, during, and after the passage of a nonlinear internal wave on 18 August, 2006. Using oceanographic data collected at a nearby location, a plane-wave model for the nonlinear internal wave’s position as a function of time is developed. Experimental results show a new acoustic path is generated as the internal wave passes above the acoustic source

Moum, James N.

Rouseff, Daniel

Tang, Dajun

Wang, Zhongkang

Williams, Kevin L.

Undetermined

ScholarsArchive@OSU

Rouseff et al.: JASA Express Letters DOI: 10.1121/1.2963097 Published Online 28 August 2008J. Acoust. SocDownloaded 20 Jul 2010 to 1Mid-frequency sound propagation throughinternal waves at short range withsynoptic oceanographic observationsDaniel Rouseff, Dajun Tang, Kevin L. Williams, and Zhongkang Wanga)Applied Physics Laboratory, College of Ocean and Fishery Sciences, University of Washington,1013 NE 40th Street, Seattle, Washington 98105-6698rouseff@apl.washington.edu, djtang@apl.washington.edu, williams@apl.washington.edu;zkwang@apl.washington.eduJames N. MoumCollege of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administration Building,Corvallis, Oregon 97331-5503jmoum@coas.oregonstate.eduAbstract: Preliminary results are presented from an analysis of mid-frequency acoustic transmission data collected at range 550 m during theShallow Water 2006 Experiment. The acoustic data were collected on a ver-tical array immediately before, during, and after the passage of a nonlinearinternal wave on 18 August, 2006. Using oceanographic data collected at anearby location, a plane-wave model for the nonlinear internal wave’s posi-tion as a function of time is developed. Experimental results show a newacoustic path is generated as the internal wave passes above the acousticsource.© 2008 Acoustical Society of AmericaPACS numbers: 43.30.Ft, 43.30.Re, 43.30.Xm [WC]Date Received: February 23, 2008 Date Accepted: June 18, 20081. IntroductionIn shallow water, there is extensive ongoing research into the impact of nonlinear internal waveson low-frequency 1 kHz sound propagation.1–4 Issues that have been studied include acous-tic mode coupling and horizontal refraction of ray paths. By comparison, less attention has beendevoted to the effect of internal waves in the mid-frequency 1–10 Hz band.During the Shallow Water 2006 Experiment (SW06), mid-frequency acoustic trans-mission data were collected over a continuous 7 h period at range 550 m. The relatively shortrange was deemed desirable for isolating the effects of shallow water internal waves on acousticpropagation. At the SW06 site, both linear and nonlinear internal waves were potentially impor-tant. Linear internal waves often are modeled as a background random process introducingsmall changes in the sound speed that cause fluctuations in the acoustic field. At range 550 m,mid-frequency transmissions between 1 and 10 kHz were thought to span the transition be-tween the regimes where classical weak- and strong-scattering theories for random mediawould apply.5 Nonlinear internal waves are often modeled as a more event-like process causingstrong, localized changes in the sound speed. Packets of nonlinear internal waves are not un-usual and it was anticipated that a 550 m acoustic path might permit individual waves in thepacket to be isolated.This paper describes an initial analysis of acoustic data collected immediately before,during, and after the passage of a nonlinear internal wave. The results show that new acousticpaths are generated and that these new paths are particularly strong as the nonlinear internalwave passes above the acoustic source.aPermanent affiliation: Hangzhou Applied Acoustics Research Institute, 96 Huaxing Road, Hangzhou, China.. Am., Vol. 124, No. 3, Pt. 2, September 2008 © 2008 Acoustical Society of America EL7328.193.70.43. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jspRouseff et al.: JASA Express Letters DOI: 10.1121/1.2963097 Published Online 28 August 2008EL74 J. AcouDownloaded 20 Jul 2010 to 12. Configuration of experimentThe SW06 experiment was performed in summer 2006 over the continental shelf of New Jersey,USA. The present analysis concentrates on data collected on 18 August in water nominally80 m deep. When nonlinear internal waves were absent, the surface mixed layer extended tonominal depth 20 m.The acoustic transmitter, an ITC-2015 (International Transducer Corporation) trans-ducer, was positioned at 39 01.3152 N, 73 01.9364 W off the stern of the R/V KNORR. Deploy-ment depth was 40 m to keep the transmitter below a thin layer of warm, salty water that waspresent that day at depth 30 m. Of present interest are linear frequency-modulated (LFM) chirpsignals 20 ms in duration. The chirps spanned the 1.5–10.5 kHz frequency band with a raisedcosine window and 10 percent taper. Transmissions were repeated approximately every 19 s.The signals were recorded at range 550 m using the MORAY moored receivingsystem.6 The system had two four-element vertical subarrays with the top four elements atdepths 25.0, 25.2, 25.5, and 26.4 m, and the bottom four at 50.0, 50.2, 50.5, and 51.4 m. Thedata were stored at the array but a two-way radio frequency (rf) link permitted communicationbetween MORAY and the R/V KNORR. The rf link was useful for adjusting amplifier gains,evaluating samples of the collected data, and assessing overall data quality.While the acoustic data were being collected, investigators onboard the R/V OCEANUScollected oceanographic data (Fig. 1). At 20:53 Coordinated Universal Time (UTC), the R/VOCEANUS was positioned 250 m east of the R/V KNORR as a nonlinear internal wave first ap-proached and then passed the two ships. The wave subsequently passed MORAY. X-band radarmeasurements estimated the bearing of the wave. Additional measurements included the water’stemperature, salinity, density, and turbulence dissipation rate using the Chameleon turbulenceprofiler.7 Two acoustic Doppler current profilers (ADCPs) were deployed to obtain vertical pro-files of currents. A 120 kHz echosounder acoustically imaged the flow field.3. ResultsTo understand the sequence of acoustic arrivals during quiescent periods when nonlinear inter-nal waves were absent, the eigenrays from the source to different elements in the receiving arraywere calculated. The simulations assumed a range-independent environment and used threesound speed profiles collected from the R/V KNORR over a quiescent 4 h period. The simula-tions suggested that the direct acoustic path to the receiver at a depth of 50 m was the mostsensitive to the details of the sound speed profile; depending on the particular profile, the directpath might or might not fragment into multiple direct arrivals. The fragmented direct path mightarrive before or after the path that bounced off the sea surface. Furthermore, the direct pathmight be stronger or weaker than the surface bounce path. The acoustic ray that was launched ata downward angle and bounced once off the seabed, however, was strong and stable and insen-sitive to the particular sound speed profile used in the simulation.Figure 2 shows a typical result from an acoustic transmission during a quiescent pe-Fig. 1. Color online Experiment geometry. Acoustic source deployed off the stern of the R/V KNORR with trans-mitted signals measured 550 m away on MORAY vertical array. Oceanographic data were collected on R/VOCEANUS while a nonlinear internal wave passed.riod. An LFM chirp signal measured at 50 m depth and 550 m range was matched filteredst. Soc. Am., Vol. 124, No. 3, Pt. 2, September 2008 Rouseff et al.: Short range propagation through internal waves28.193.70.43. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jspRouseff et al.: JASA Express Letters DOI: 10.1121/1.2963097 Published Online 28 August 2008J. Acoust. SocDownloaded 20 Jul 2010 to 1yielding the arrival pattern for the different acoustic paths. The figure identifies the strongestdirect path as well as four others: the sea surface bounce path, the bottom bounce path thatreflected off the seabed, the surface-bottom bounce path and the bottom-surface path.Figure 3 shows the fluid velocity measured from the R/V OCEANUS immediately be-fore, during, and after the passage of nonlinear internal wave. Specifically, the vertical compo-nent of fluid velocity is mapped as a function of depth and time as measured by the ADCP.Isopycnals are superimposed. The maximum internal wave displacement is approximately10 m and occurs at 21:14 UTC.The oceanographic data collected on the R/V OCEANUS can be used to construct asimple model for the internal wave. When the internal wave was in the vicinity of the R/VOCEANUS, X-band radar measurements indicated the wave’s bearing as 288 deg (Fig. 1) and itsspeed as 0.89 m/s. As an initial model, wavefront curvature is neglected and the internal waveis treated locally as a plane wave. The plane wave assumption, together with the known speedand direction of the wave, permits measurements made at the R/V OCEANUS to be propagated toother locations. Referring to the geometry in Fig. 1, the peak internal wave displacement ob-served on the R/V OCEANUS at 21:14 UTC will have propagated to the R/V KNORR at 21:18:30UTC and to the MORAY at 21:28:31. The peak internal wave displacement, consequently, willlie somewhere between the acoustic transmitter and receiver for 10 min.Figure 4 is a waterfall plot showing how the acoustic arrival structure measured at50 m depth changes in time. The matched filter output is plotted for 100 consecutive LFMchirps with a gap of 18–20 s between transmissions. The 32 min of data include the periodsimmediately before, during, and after the passage of the internal wave. The superimposed hori-zontal lines bracket the time when the peak internal wave displacement, as calculated in theprevious paragraph, is expected to pass between the acoustic source and receiver. The grossshifts in arrival time are due to relative motion between the source and the moored receiver. Thebottom, bottom-surface, and surface-bottom bounce paths are labeled.Fig. 2. Color online Typical acoustic arrival structure during quiescent period. Data collected on an array elementat 50 m depth and 550 m range. Shown is the matched filter output where the various arrival paths are indicated.Fig. 3. Color online Sample oceanographic result showing vertical component of fluid velocity as measured on R/VOCEANUS. Contours of isopycnals are plotted over the color image. Peak isopycnal displacement from a nonlinearinternal wave occurs at 21:14 UTC.. Am., Vol. 124, No. 3, Pt. 2, September 2008 Rouseff et al.: Short range propagation through internal waves EL7528.193.70.43. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jspRouseff et al.: JASA Express Letters DOI: 10.1121/1.2963097 Published Online 28 August 2008EL76 J. AcouDownloaded 20 Jul 2010 to 1Similar arrival structures were observed on the other three elements in the bottom subarray. On each element, the bottom, bottom-surface, and surface-bottom bounce paths werestrong and stable. Using the arrival times for a particular transmission as measured on each ofthe four array elements as input to a least-squares estimator, the arrival angle for a particularacoustic path could be estimated. Without correcting for array motion, the bottom bounce pathhad an average arrival angle of 8.4 deg relative to horizontal.The most striking feature of Fig. 4 is the apparent generation of a new acoustic path asthe internal wave passes above the acoustic source. The new path, circled in the figure as itappears, splits from the bottom bounce. At 21:21:13 UTC, approximately 3 min after the peakinternal wave displacement has passed the acoustic source, the new path is at its strongest andits intensity exceeds the bottom bounce. Its arrival angle at 21:21:13 UTC is 12° and so steeperthan the bottom-bounce path that precedes it. As the internal wave continues to move away fromthe acoustic source, the arrival angle for the new path continues to steepen but its intensityfades. It should be noted that a similar analysis was performed using the four elements in theupper sub array. On the upper sub array, whose shallowest element was at a depth of 25.0 m, thesplitting of the bottom-bounce path was not observed as the internal wave passed.A hypothesis is developed for the observed new ray. The passing internal wave de-presses the mixed layer and hence the sound speed profile. Since the acoustic source is belowthe mixed layer, an upward-launched acoustic ray will be refracted downward by the passinginternal wave. The refracted ray passes through an upper turning point and then strikes thebottom further down range than the original bottom-bounce path. After reflection, this new patharrives at the receiving array at a steeper angle than the unperturbed original bottom bounce.4. Concluding remarksFuture work will include a more extensive data analysis together with numerical modeling. Acomplicating factor in developing a model is that the experiment was performed in a regionwhere new individual internal waves were spawned rapidly.8 Possible evidence for new internalwaves is in Fig. 4 where a possible new acoustic arrival is apparent after the internal wave haspassed the acoustic receiver at 21:28:31 UTC. Consequently, the simple plane-wave model forthe internal waves used in the present work may prove inadequate for some calculations. Todevelop a more complete oceanographic model, temperature data collected on the MORAYacoustic array will be integrated into the analysis. A previously developed9 ray tracing codevalid for range-dependent media will then be applied in an attempt to reproduce and betterunderstand the ray path generation observed in the data.AcknowledgmentsFig. 4. Color online Time evolving acoustic arrival structure as the nonlinear internal wave in Fig. 3 enters theacoustic propagation path. The bottom, bottom-surface, and surface-bottom bounce paths are labeled. New acousticpath circled is generated as the internal wave passes above the acoustic source. Red-green-blue color scale has50 dB dynamic range.This work was supported by the Office of Naval Research.st. Soc. Am., Vol. 124, No. 3, Pt. 2, September 2008 Rouseff et al.: Short range propagation through internal waves28.193.70.43. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jspRouseff et al.: JASA Express Letters DOI: 10.1121/1.2963097 Published Online 28 August 2008J. Acoust. SocDownloaded 20 Jul 2010 to 1References and links1J. X. Zhou, X. Z. Zhang, and P. H. Rogers, “Resonant interaction of sound waves with internal solitons in thecoastal zone,” J. Acoust. Soc. Am. 90, 2042–2054 (1991).2R. H. Headrick, J. F. Lynch, J. N. Kemp, A. E. Newhall, K. von der Heydt, J. Apel, M. Badiey, C.-S. Chiu, S.Finette, M. Orr, B. Pasewark, A. Turgot, S. Wolf, and D. Tielbuerger, “Acoustic normal mode fluctuationstatistics in the 1995 SWARM internal wave scattering experiment,” J. Acoust. Soc. Am. 107, 201–220 (2000).3M. Badiey, Y. Mu, J. Lynch, J. Apel, and S. Wolf, “Temporal and azimuthal dependence of soundpropagation in shallow water with internal waves,” IEEE J. Ocean. Eng. 27, 117–129 (2002).4M. Badiey, B. G. Katsnelson, J. F. Lynch, S. Pereselkov, and W. L. Siegmann, “Measurement and modeling ofthree-dimensional sound intensity variations due to shallow-water internal waves,” J. Acoust. Soc. Am.117, 613–625 (2005).5B. J. Uscinski, Elements of Wave Propagation in Random Media (McGraw–Hill, New York, 1977), pp. 69–77.6P. H. Dahl, J. W. Choi, N. J. Williams, and H. C. Graber, “Field measurements and modeling of attenuationfrom near-surface bubbles for frequencies 1–20 kHz,” J. Acoust. Soc. Am. 124, EL163–EL169 (2008).7J. N. Moum, M. C. Gregg, R. C. Lien, and M. E. Carr, “Comparison of turbulence kinetic-energy dissipationrate estimates from two ocean microstructure profilers,” J. Atmos. Ocean. Technol. 12, 346–366 (1995).8E. L. Shroyer, J. N. Moum, and J. D. Nash, “Observations of polarity reversal in shoaling non-linear internalwaves,” J. Phys. Oceanogr. , in press (2008).9F. S. Henyey, D. Tang, K. L. Williams, R.-C. Lien, K. M. Becker, R. L. Culver, P. C. Gabel, J. E. Lyons, and T.C. Weber, “Effect of non-linear internal waves on mid-frequency acoustic propagation on the continentalshelf,” J. Acoust. Soc. Am. 119, 3345 (2006).. Am., Vol. 124, No. 3, Pt. 2, September 2008 Rouseff et al.: Short range propagation through internal waves EL7728.193.70.43. Redistribution subject to ASA license or copyright; see http://asadl.org/journals/doc/ASALIB-home/info/terms.jsp

Mid-frequency sound propagation through internal waves at short range with synoptic oceanographic observations

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Mid-frequency sound propagation through internal waves at short range with synoptic oceanographic observations

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