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POMA - ICA 2013 Montreal
Conference Location: Montreal, Canada Conference Date: 2 - 7 June 2013
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back to top Session 1aAO: Estuarine Acoustics
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The impact of acoustic oceanographic methods on estuarine dynamics research

W Rockwell Geyer, Peter Traykovski, and Andone Lavery

POMA Volume 19, pp. 005001 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Acoustic methods have revolutionized the field of estuarine physical oceanography, providing orders-of-magnitude improvement in the temporal and spatial resolution of velocity, suspended sediment and density structure. That revolution started in the 1970's when some Japanese researchers realized that they could resolve the structure of the estuarine salt wedge with a conventional echo sounder by turning up the gain. David Farmer became a champion of this technique, most notably exemplified by his sensational images of hydraulic jumps in fjords. Refinements to this approach with broadband sonars are producing high-resolution imagery of shear instability and its transition to turbulence in highly stratified estuaries. Acousticians are now going beyond simply imaging the turbulent structure to quantifying the dissipation rate based on the intensity of acoustic backscatter. The acoustic Doppler current profiler has impacted all of physical oceanography, but it is particularly valuable in estuarine settings for resolving the intense vertical shears and complex horizontal velocity gradients in estuaries. Acoustic backscatter also provides a key approach to quantifying suspended sediment. The high spatial resolution afforded by acoustic backscatter sensors has revealed the occurrence of fluid muds in estuaries - extremely high concentrations that alter the dynamics and rheology of the flow.
Show PACS
43.30.Ft Volume scattering
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
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The spatial properties of breaking wave generated and bedload transport generated noise in the sediment layer of a shallow water wave guide.

David R. Barclay, Len Zedel, Alex E. Hay, and Mathew G. Hatcher

POMA Volume 19, pp. 005002 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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In May of 2012, three weeks of ambient noise measurements from a hydrophone buried 30 cm deep in the sediment were recorded at Advocate Beach, a 1:10 sloped beach at the head of the Bay of Fundy, Nova Scotia. While tides varied the mean water depth between 0 and 4 m, 0.8 m surface waves passed overhead, driving sediment bedload transport and creating an ambient noise field in the sediment consisting of two primary components: noise generated by bubbles formed in breaking waves at the surface and noise generated by the collisions of sand, gravel and cobble in the bedload transport along the seabed. Both of these noise sources are stochastic and can be described by their second order statistics: power spectral density, spatial coherence and directional density. In an effort to distinguish these two noise sources, the spatial properties of three full wave models of the noise field in the sediment are compared, using an infinite sheet of sources placed near the surface of a Pekeris waveguide to model breaking wave noise, near the fluid-fluid interface of a Pekeris waveguide to model bedload transport noise, or near the fluid-fluid interface of two infinite half-spaces to model bedload transport noise.
Show PACS
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Nb Noise in water; generation mechanisms and characteristics of the field
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Acoustic measurements of the spatial distribution of suspended sediment at a site on the Lower Mekong River

Stephanie A. Moore, Guillaume Dramais, Philippe Dussouillez, Jérôme Le Coz, Colin Rennie, and Benoît Camenen

POMA Volume 19, pp. 005003 (June 2013); (9 pages)

Online Publication Date: June 02, 2013

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The Mekong River spans thousands of kilometers, flows through six countries and its basin is one of the world's richest in terms of biodiversity. However, land-use changes, dredging of the river bed and the construction of dams are changing its sediment dynamics and morphology, with consequences such as increased bank erosion and reduction in sediment supply to floodplains. In order to monitor these changes, the current conditions must be well understood. Comprehensive measurements of the spatial distribution of sediment (both suspended and bed load) were made at three locations in different physiographic regions of the Lower Mekong at the end of the 2012 rainy season. Data are presented from the Luang Prabang site. Acoustic Doppler Current Profilers and a multi-frequency acoustic backscatter system, the AQUAscat, were used in combination with water sampling to provide high resolution measurements of concentration and grain size. The AQUAscat consisted of four monostatic transducers operating at 0.5, 1, 2.5 and 4 MHz. It was deployed horizontally at four across-stream positions and 4-6 depths per vertical; a 10 m profile was recorded at each point. Particle size and concentration are determined from the multi-frequency attenuation data. This data set provides a baseline for future measurements.
Show PACS
43.20.Fn Scattering of acoustic waves
43.20.Hq Velocity and attenuation of acoustic waves
43.30.Ft Volume scattering
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
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Acoustic propagation characteristics of the estuarine salt wedge

D. B. Reeder

POMA Volume 19, pp. 005004 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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The estuarine environment often hosts a salt wedge-a layer of denser seawater advected by the rising tide under fresh water discharged by the river. The nature of the stratification is a function of the tide's range and speed of advance, river discharge volumetric flow rate and river mouth morphology. The competing effects of temperature and salinity on sound speed present the question: Is the salt wedge acoustically observable? Using temperature and salinity profiles collected in situ, numerical results show that the salt wedge can impact acoustic propagation. Acoustically, this environment can be approximated by two isospeed layers separated by a thin gradient. While this three-layer very shallow water acoustic waveguide is typically dominated by high angle multipath propagation, refraction occurring in the gradient layer allows low-angle energy from near-surface sources to be trapped above the gradient and creates a shadow zone below the gradient. Energy from near-bottom sources is refracted to higher angles and attenuated more quickly. Acoustic fluctuations observed at an upstream/downstream receiver depend upon the presence/absence of bedforms and the interaction between the advancing/receding tide and the river discharge, which can include the presence of internal waves propagating along the top of the salt wedge.
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43.30.Cq Ray propagation of sound in water
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
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Quantification of stratified turbulence using acoustic propagation and broadband scattering techniques

Andone Lavery, W Rockwell Geyer, and Malcolm E. Scully

POMA Volume 19, pp. 005005 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Narrowband acoustical backscattering techniques have been used for decades as a tool for remote imaging of small-scale physical processes in energetic coastal environments, such as oceanic internal waves and microstructure, on spatial and temporal scales difficult to probe with in situ measurements. However, to date, it has been challenging to infer quantitative information about turbulent intensity from the measured backscatter, in part due to uncertainty in the sources of scattering. In contrast to narrowband techniques, emerging broadband techniques result in increased spectral classification and quantification capabilities. Broadband backscattering collected in the Connecticut River Estuary in 2009 in concert with in situ measurements of turbulence have illustrated the potential of these techniques for quantitative remote-sensing of microstructure intensity over relevant spatial and temporal scales. These measurements have resulted in remote quantification of finescale variability of turbulent mixing as well as examination of the mechanisms and structure of shear instability across a broad range of stratification and shear conditions. Recent measurements of high frequency acoustic propagation have been performed in December 2012 at the same location in the CT River estuary, aimed at using reciprocal transmission acoustic scintillation techniques to infer path-averaged turbulent parameters, and the analysis of these data is ongoing.
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43.30.Ft Volume scattering
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Acoustics and estuarine ecology: using active and passive methods to survey the physical environment, vegetation and animals in North Carolina's coastal estuaries

Joseph Luczkovich, Mark Sprague, Cecila S. Krahforst, John Walsh, and Dean Carpenter

POMA Volume 19, pp. 005006 (June 2013); (9 pages)

Online Publication Date: June 02, 2013

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Estuarine systems have complex interactions of physical and biological processes. Regular observations are needed in order to understand their dynamics. Acoustic observation systems (echosounders, acoustic Doppler current profilers (ADCPs), passive acoustic dataloggers) can provide observations on a wide spectrum of processes in estuaries. We have used echosounders to monitor changes in bathymetry, submerged aquatic vegetation, fishes, and invertebrates over time. In addition, sediment changes, resupsension events, turbidity and waves are monitored using ADCPs. The higher trophic level species of fishes and marine mammals that are soniferous have been monitored by passive acoustic methods. We provide examples of each acoustic method used to study the dynamics of seagrasses, fishes, and the physical environment of the Albemarle, Pamlico, Currituck and Core Sounds in North Carolina. While it is possible to combine methods to use acoustics to measure the dynamics of estuarine systems (estuarine observing systems), the challenge we face is to ground-truth these acoustic metrics using traditional sampling methods (e.g., quadrats for plants, trawls for fishes, water samples for sediments) and integrate each of these measures. We could then examine the effect of storms, waves, and resuspension events on estuarine plant and animal distributions and abundances using acoustics metrics.
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43.30.Sf Acoustical detection of marine life; passive and active
43.30.Vh Active sonar systems
43.80.Ka Sound production by animals: mechanisms, characteristics, populations, biosonar
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Investigation of low-frequency acoustic tissue properties of seagrass

Gregory Enenstein, Craig Dolder, Preston S. Wilson, and Jean-Pierre Hermand

POMA Volume 19, pp. 005007 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Understanding the acoustic properties of seagrass is important for applications in mine hunting, shallow water sonar performance, and acoustic remote sensing for ecological surveys. Previous laboratory and field investigations have shown that the plant biomass and tissue structure of seagrass, rather than just the overall gas content, play a determinant role in its acoustic behavior. Hence, effective medium models of propagation through seagrass meadows have been ruled out, and a complete description of both tissue structure and tissue elastic properties is required to describe the acoustic response of seagrass meadows. To begin to address these deficiencies, a resonance tube experiment was set up to determine the low-frequency acoustic response of multiple species of seagrass in relation to leaf biomass and tissue acoustic compliance independent of tissue structure. Responses to frequency-modulated signals in the range from 0.5~kHz to 10~kHz were obtained for Thalassia testudinum (turtle grass) and Halodule wrightii (shoal grass), two species with well-differentiated morphological features. An elastic waveguide model was used to account for the minor effect of the tube walls on the resonance characteristics. Initial measurements of tissue compliance will be presented. [Work supported by ONR and ARL:UT.]
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43.80.Cs Acoustical characteristics of biological media: molecular species, cellular level tissues
back to top Session 2aAO: Seismic Oceanography
FREE

Uncertainty of transmission loss due to small scale fluctuations of sound speed in two environments

J. P. Fabre and Warren T. Wood

POMA Volume 19, pp. 005008 (June 2013); (11 pages)

Online Publication Date: June 02, 2013

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Seismic oceanography techniques reveal detection of small scale variations in sound speed not detectable via conventional oceanographic means, i.e. frequent XBT or CTD casts). Due to computational and practical limitations, such small scale spatial and temporal detail that exists in a real ocean environment is not typically included in acoustic ocean models. However, such measurements can provide insight to the small scale variability (uncertainty) that exists in the ocean but is not predicted by mesoscale ocean models. We show acoustic predictions made with the Range Dependent Acoustic Model (RAM) using measured seismic oceanography and CTD data at two locations in significantly different environments. Additionally, the CTD measurements are smoothed to a resolution comparable to that provided by a dynamic ocean model and acoustic predictions are computed. The Uncertainty Band (UBAND) algorithm [Zingarelli, R. A., "A mode-based technique for estimating uncertainty in range-averaged transmission loss results from underwater acoustic calculations" J. Acoust. Soc. Am. 124 (4), October 2008] is applied to the smoothed oceanographic data using estimates of sound speed uncertainty calculated from the high resolution measurements. We find reasonable estimates of uncertainty due to the small scale oceanography that is not characterized by mesoscale ocean models.
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43.30.Zk Experimental modeling
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Inversion of density in the ocean from seismic reflection data

Berta Biescas Gorriz, Barry R. Ruddick, and Valenti Sallares

POMA Volume 19, pp. 005009 (June 2013); (9 pages)

Online Publication Date: June 02, 2013

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Previous works in seismic oceanography explain that acoustic reflections are primarily associated with temperature vertical variations, so seismic images in the ocean can be interpreted as thermal contrasts maps. Temperature and salinity are the physical properties that describe structures in the ocean. However, the main physical parameter that controls oceans dynamics is the density and, since the ocean is a compressible fluid, potential density is the property that determines the stability, mixing and mesoscale motions of the particles. We have inverted oceanic impedance from seismic data and then derived density and potential density surfaces from the oceanic impedance. Results of the inverted potential density have been compared with seismic reflectors to show the relation between isopycnals and reflectors. We have also compare the seismic profiles of the GO Survey with the space-coincident CTDs and space and time-coincident XBTs to understand the nature of the reflectivity and its relation with the potential density in the ocean.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Pt Signal processing techniques for acoustic inverse problems
43.60.Rw Remote sensing methods, acoustic tomography
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Seismic oceanography imaging of thermal intrusions in strong frontal regions

Jeffrey W. Book, Warren T. Wood, Ana E. Rice, Sandro Carniel, Richard W. Hobbs, Isabelle Ansorge, Tim Fischer, and Hartmut Prandke

POMA Volume 19, pp. 005010 (June 2013); (5 pages)

Online Publication Date: June 02, 2013

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The Naval Research Laboratory and collaborating partners carried out two dedicated seismic oceanography field experiments in two very different strong frontal regions. ADRIASEISMIC took seismic oceanography measurements at the confluence of North Adriatic Dense Water advected along the Western Adriatic Current and Modified Levantine Intermediate Water advected around the topographic rim of the Southern Adriatic basin. ARC12 took seismic oceanography measurements in and around the Agulhas Return Current as it curved northwards past the Agulhas Plateau and interacted with a large anticyclone that had collided with the current. Despite one study focused on coastal boundary currents and the other focused on a major Western Boundary Current extension, the complex horizontal structures seen through seismic imaging are tied to the processes of thermal intrusions and interleaving in both systems. Seismic Oceanography provides a unique capability of tracking the fine-scale horizontal extent of these intrusions.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
FREE

Exploring the shelf-slope dynamics in the Adriatic Sea using numerical models and seismic oceanography (SO)

Andrea Bergamasco, Francesco M. Falcieri, Jeffrey W. Book, Sandro Carniel, Warren T. Wood, Mauro Sclavo, and Richard W. Hobbs

POMA Volume 19, pp. 005011 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Dense shelf waters are formed and spread in the Adriatic Sea during winter periods, which dynamics are usually investigated by means of sea truth campaigns and modeling efforts. The former are either based on observational approaches (moored instruments, CTD, current meters, etc.) or on more innovative techniques, e.g. employing Seismic Oceanography. Recent studies have shown that SO techniques can produce maps of vertical transects along the survey lines with horizontal and vertical resolution of, respectively, 10 and 100 m, suitable to explore the horizontal structures of BBL dynamics. Elaborating on these considerations, a novel approach combining the SO dataset collected during the ADRIASEISMIC cruise and high-resolution numerical model (ROMS) results was performed in two restricted areas of the Adriatic Sea: off the Gargano promontory and off the Bari shelf break. We present the first steps along the definition of a novel methodology. On one hand SO can help to image the existing dynamical structures and their spatial/temporal resolution; on the other, the numerical model can quantify these acoustic snapshots in terms of temperature, salinity and density, integrating the XBTs that are acquired during SO lines, and help identifying the nature of other processes (e.g. turbulence, internal waves, etc.).
Show PACS
43.30.Lz Underwater applications of nonlinear acoustics; explosions
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.40.Ph Seismology and geophysical prospecting; seismographs
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Mapping turbidity layers using a combination of high resolution seismic oceanographic and physical oceanographic data

Ekaterina A. Vsemirnova and Richard W. Hobbs

POMA Volume 19, pp. 005012 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Synchronised seismic and oceanographic data were acquired during the GO (Geophysical Oceanography) project cruise in the Gulf of Cadiz in April - May 2007. The small volume (117 cu-inch) mini GI-gun seismic source used during the GO experiment provided high resolution seismic data which revealed features of the internal structure of the ocean. The seismic acquisition design gave a usable bandwidth of 50-250 Hz with a vertical resolution of 1.25 m which is similar to that achieved by co-located CTD casts. We focus on the reflections observed on seismic data covering an area to the east of the Portimao canyon in water depths of 600-800 m. To test the hypothesis that measurable reflections can be generated by suspended sediment we perform forward modelling of seismic response based on the temperature, salinity and sediment concentration derived from light attenuation measurements, available from CTD casts. Modelling based solely on temperature and salinity data show that thermohaline structure does not always explain reflections in water column, but these discrepancies are resolved when the contribution from suspended sediment is included.
Show PACS
43.58.Bh Acoustic impedance measurement
43.58.Dj Sound velocity
43.60.Lq Acoustic imaging, displays, pattern recognition, feature extraction
43.60.Rw Remote sensing methods, acoustic tomography
FREE

Characterization of thermohaline staircases in the Tyrrhenian Sea using stochastic heterogeneity mapping

Grant G. Buffett, Richard W. Hobbs, Ekaterina A. Vsemirnova, Dirk Klaeschen, Charles A. Hurich, César Ranero, and Valentí Sallarès

POMA Volume 19, pp. 005013 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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Processed multi-channel seismic (MCS) data acquired in the Tyrrhenian Sea in April-May 2010 provide images of oceanic thermohaline staircases. Using Stochastic Heterogeneity Mapping we characterize spatial reflector variations. This method is based on the band-limited von Kármán function. For scale sizes smaller than the correlation length, the von Kármán model describes a power law (fractal) process. We are most interested in the extraction of the exponent in the power law (The Hurst exponent) because it allows us to characterize the richness of scales present in the data set. Lower Hurst exponents represent a richer range of wavenumbers and therefore correspond to a broader range of heterogeneity in the observed seismic reflection events. The Hurst exponent is related to the fractal dimension and to the slope in the Garrett-Munk wavenumber spectrum. We interpret a richer range of heterogeneity as indicative of a greater degree of turbulent mixing. Data are presented alongside benchmark calibrations of synthetic seismic data generated from random fractal surfaces. We observe an oscillation in the Hurst exponent spectra as a function of frequency that is interpreted to represent a preferential coupling of energy across different spatial scales.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Re Signal coherence or fluctuation due to sound propagation/scattering in the ocean
43.60.Pt Signal processing techniques for acoustic inverse problems
FREE

Mapping Non-Local Turbulent Breakdown of Oceanic Lee Waves Offshore Costa Rica Through Seismic Oceanography

Will F. Fortin, W. Steven Holbrook, Raymond Schmitt, and Scott Smith

POMA Volume 19, pp. 005014 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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Data analysis techniques in Seismic Oceanography are rapidly becoming more complex. Beyond first-order observation of oceanic structures it is possible to extract quantifiable information about internal wave energies and turbulent dissipation rates. We use two co-located seismic surveys taken one day apart to estimate turbulent diffusivities of lee wave breakdown with emphasis on the mid-water turbulence generated. Through a horizontal wavenumber spectral analysis of our seismic images we estimate turbulent dissipation rates throughout the cross-section. By integrating horizontal seismic slope spectra in the wavenumber domain over the turbulent subrange we obtain relative turbulent energies across the survey. To resolve absolute turbulent diffusivities we scale the relative measures to known absolute energies from tracked seismic reflectors (isopycnals). The analysis section spans 22km laterally and full ocean depth with a resolution on turbulent diffusivity of 10m vertically by 400m laterally. We focus on the region of elevated turbulent diffusivity caused by the breakdown of the lee wave above its generating site. We find the turbulent diffusivities related to the lee wave breakdown to be about 5 times greater than surrounding waters and 15 times greater than average open ocean diffusivities. We also see increased turbulent diffusivity around the rough bathymetry.
Show PACS
43.30.Ma Acoustics of sediments; ice covers, viscoelastic media; seismic underwater acoustics
43.60.Lq Acoustic imaging, displays, pattern recognition, feature extraction
FREE

Current-eddy interaction in the Agulhas Return Current region from the seismic oceanography perspective

Ana E. Rice, Jeffrey W. Book, Warren T. Wood, and Tim Fischer

POMA Volume 19, pp. 005015 (June 2013); (5 pages)

Online Publication Date: June 02, 2013

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Interleaving in the Agulhas Return Current (ARC) frontal region is commonly manifested in the form of thermohaline intrusions, as sub-tropical and sub-polar water masses of similar density meet. In Jan/Feb 2012, the Naval Research Laboratory and collaborators carried out a field experiment in which seismic and traditional hydrographic observations were acquired to examine frontal zone mixing processes. The high lateral resolution (10 m) of the seismic observations allowed fine-scale lateral tracking of thermal intrusions, which were corroborated with simultaneous XBT casts. Between seismic deployments both salinity and temperature data were acquired via CTD, Underway-CTD and microstructure profiles. This study focuses on analyzing seismic reflection data in a particular E-W transect where the northward flowing ARC interacted with the southward flowing portion of a large anticyclonic eddy. Strong reflectors were most prominent at the edge of a hyperbolic zone formed between the eddy and ARC, where sub-polar waters interacted with waters of sub-tropical origin on either side. Reflectors were shallow within the hyperbolic zone and extended to 1200 m below the ARC. The nature of the observed reflectors will be determined from comparison of seismic reflection and derived ∂T/∂z fields, and XBT and TS profiles from the available hydrographic data.
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43.58.Bh Acoustic impedance measurement
43.58.Dj Sound velocity
back to top Session 3aAO: Acoustical Oceanography Prize Lecture
FREE

Acoustical tomography in the shallow water ocean: Dream or reality?

Philippe Roux

POMA Volume 19, pp. 005016 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Acoustical tomography in shallow waters relies on the identification and tracking of stable ray arrivals. The variation of the arrival time of these rays is used to solve the inverse problem and to estimate the physical properties as sound speed variations associated to internal waves or currents. In practice, however, technical difficulties appear (1) when the number of resolved rays in this multipath environment is too small using a set of point-to-point recordings or (2) when the travel-time fluctuations are dominated by the fast-evolving surface that blur the slower internal-wave driven perturbations around the thermocline. Recent experimental studies based on laboratory-scaled demonstrators showed the efficiency of array processing algorithms in combination with source-receiver arrays to tackle issue (1) above. Depending on the waveguide geometry, the acquisition procedure can also be adjusted to provide a separation between the slow and fast travel-time fluctuations. Inversion results based on laboratory experiments are presented that focus on the dynamics of (1) a thermal plume in the water column and (2) a gravity wave at the air-water interface. Finally, these results are discussed in the framework of ocean acoustic research.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Fg Acoustic array systems and processing, beam-forming
43.60.Pt Signal processing techniques for acoustic inverse problems
back to top Session 3pAO: Ocean Acoustical Tomography
FREE

Influence on sound spread considering the flow velocity in a horizontal layer media

Yang Song and Zhen-Qi Zhao

POMA Volume 19, pp. 005017 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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The marine currents could often influence sound propagation underwater. The traveling time of sound ray is distinctly effected by current velocity in some conditions. So it will be made certain errors in seeking eigenrays and inversing sound speed profile if current velocity is ignored. In order to improve the computation accuracy of sound ray model, a sound ray model of horizontal layer is induced in which the media flow is considered. Eigen rays are searched and their traveling time is calculated by the ray model. It is also discussed that the velocity of flow media influences ray trace and traveling time. An average sound speed profile measured under some shallow water is cited to calculate the eigenrays. The differences of sound ray are given under two kinds of condition which the velocity of current is considered and not considered. The computation results show that the sound ray trace is changed indistinctively under small Mach number condition, but the traveling time of eigenrays is fluctuated obviously. The fluctuation of ray traveling time is bigger according to the larger Mach number and the longer spread distance. The results of study will provide some help in the inversion of ocean acoustic tomography.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
FREE

Time-angle ocean acoustic tomography using sensitivity kernels: The forward problem

Florian Aulanier, Barbara Nicolas, Philippe Roux, and Jérôme I. Mars

POMA Volume 19, pp. 005018 (June 2013); (8 pages)

Online Publication Date: June 02, 2013

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Broadband acoustic signals around 1 kHz propagate through shallow water oceanic waveguides of ~100 m in depth and ~2 km in range as multiple ray-like wavefronts. These acoustic arrivals can be characterized by the following observables: travel-time (TT), direction-of-arrival (DOA) and direction-of-departure (DOD). By applying double-beamforming on the point-to-point signals recorded between two source-receiver arrays, the acoustic contribution of each arrival can be separated from the multi-reverberated data and the TT, DOA and DOD observable variations are accurately measured. This study deals with the use of time-angle sensitivity kernels (TASK) to estimate the observable variations induced by sound speed perturbations in the waveguide. This approach is based on the first order Born approximation and takes into account the finite-frequency effects associated with wave propagation. The robustness the TASK approach is analyzed and compared to numerical parabolic equation simulations involving different sound speed perturbations. For example, parameters such as the perturbation location, the value and shape of the perturbation in the waveguide are modified. The combination of several perturbations and the influence of the source-receiver array apertures on the TT, DOA and DOD estimates are also studied.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Fg Acoustic array systems and processing, beam-forming
43.60.Pt Signal processing techniques for acoustic inverse problems
FREE

Time-angle ocean acoustic tomography using sensitivity kernels: Numerical and experimental inversion results

Florian Aulanier, Barbara Nicolas, Philippe Roux, Romain Brossier, and Jérôme I. Mars

POMA Volume 19, pp. 005019 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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In shallow water acoustic tomography, broadband mid-frequency acoustic waves (1 to 5 kHz) follow multiple ray-like paths to travel through the ocean. Travel-time (TT) variations associated to these raypaths are classically used to estimate sound speed perturbations of the water column using the ray theory. In this shallow water environment, source and receiver arrays, combined with adapted array processing, provide the measurement of directions-of-arrival (DOA) and directions-of-departure (DOD) of each acoustic path as new additional observables to perform ocean acoustic tomography. To this aim, the double-beamforming technique is used to extract the TT, DOA and DOD variations from the array-to-array acoustic records. Besides, based on the first order Born approximation, we introduce the time-angle sensitivity kernels to link sound speed perturbations to the three observable variations. This forward problem is then inverted with the maximum a posteriori method using both the extracted-observable variations and the proposed sensitivity kernels. Inversion results obtained on numerical data, simulated with a parabolic equation code, are presented. The inversion algorithm is performed with the three observables separately, namely TT, DOA and DOD. The three observables are then used jointly in the inversion process. The results are discussed in the context on ocean acoustic tomography.
Show PACS
43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.60.Fg Acoustic array systems and processing, beam-forming
43.60.Pt Signal processing techniques for acoustic inverse problems
FREE

Towards subsurface positioning of gliders using fixed acoustic tomography sources

Lora Van Uffelen, Eva-Marie Nosal, Bruce M. Howe, Glenn Carter, Peter Worcester, Matthew A. Dzieciuch, Kevin Heaney, Richard Campbell, and Patrick Cross

POMA Volume 19, pp. 005020 (June 2013); (4 pages)

Online Publication Date: June 02, 2013

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Acoustic Seagliders can be positioned precisely using GPS at the surface, but are underwater and unable to utilize GPS for up to 9 hours at a time as they dive to depths of up to 1000 m. During this time, a kinematic model estimates the position of the glider. Four acoustic Seagliders were deployed in the Philippine Sea November 2010 - April 2011, and received transmissions from five broadband acoustic tomography sources moored in the region. Over 2000 acoustic receptions were recorded at ranges up to 700 km from the moored sources. Measured acoustic arrival peaks were unambiguously associated with ray arrivals predicted using the model-estimated glider position at the time of reception and a mean sound-speed profile. Estimates of source-receiver range uncertainty were calculated from statistics of travel-time offsets between the measured arrivals and the eigenray dispersion patterns. The uncertainty in range between the source and the modeled glider position during a dive is estimated to be 639 m (426 ms) rms disregarding the effects of ocean sound-speed variability, which are anticipated to be on the order of 70 ms rms. The range uncertainty is attributed primarily to advection of the glider by unobserved ocean currents.
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43.30.Pc Ocean parameter estimation by acoustical methods; remote sensing; imaging, inversion, acoustic tomography
43.30.Tg Navigational instruments using underwater sound
back to top Session 4pAO: Biologic and Non-Biologic Scatterers
FREE

Acoustic scattering from a water-filled cylindrical shell: Mode identification and interpretation via finite element and analytical models

Aubrey Espana, Kevin L. Williams, Daniel S. Plotnick, and Philip L. Marston

POMA Volume 19, pp. 005021 (June 2013); (9 pages)

Online Publication Date: June 02, 2013

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Understanding the physics governing the interaction of sound with targets in an underwater environment is essential to improving upon existing target detection and classification algorithms. Simple models are viable tools for meaningful interpretation of scattering results. To illustrate this, two modeling techniques are employed to study the acoustic scattering from a water-filled cylindrical shell. The first model is a hybrid 2-D/3-D finite element (FE) model, whereby the scattering in close proximity to the target is handled via a 2-D axisymmetric FE model, and the subsequent 3-D propagation to the farfield is determined via a Helmholtz integral. This model is characterized by the decomposition of the fluid pressure and its derivative in a series of azimuthal Fourier modes, a technique that has previously facilitated mode identification [A. L. Espana et al., J. Acoust. Soc. Am. 130, 2332 (2011)]. The second is an analytical solution for an infinitely long cylindrical shell, coupled with a simple approximation that converts the results to an analogous finite length form function. These two model results, when examined together on a mode-by-mode basis, offer visualization of the mode dynamics and the ability to distinguish the different physics driving the target response (i.e. structural modes versus water-waveguide modes).
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43.30.Jx Radiation from objects vibrating under water, acoustic and mechanical impedance
43.40.Ey Vibrations of shells
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Testing of an extended target for use in high frequency sonar calibration

John L. Heaton, Thomas C. Weber, Glen Rice, and Xavier Lurton

POMA Volume 19, pp. 005022 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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Acoustic backscatter tests were performed in a tank with a 200-kHz, 7°, SIMRAD EK60 Single-Beam Echo-Sounder, and a 256-beam RESON SeaBat 7125 Multi-Beam Echo-sounder. Tests were done in order to investigate the angular and range dependency of the scattering strength of a new test target in order to validate its use in sonar testing. This target was constructed of small chain links arranged in a 'curtain' simulating an extended scattering surface, such as the seafloor. Target strength for individual links was collected as the links were rotated 360°. The links are combined into an extended surface target, spacing between scatterers being approximately 1 cm. The scattering network irregularity is enough to ensure phase randomicity at the wavelength considered. The target scattering strength was measured as a function of grazing angle and range, hence varying the number of scatterers within the beam footprint. These tests suggest that the amplitude envelope of the scattered signals is Rayleigh distributed and that the backscatter strength depends linearly on the number of active scatterers, all desirable features for calibrating sonars used to make measurements of similarly random surfaces such as the seafloor. Results show a promising in-tank calibration technique alternative to the classical sphere-target measurements.
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43.30.Yj Transducers and transducer arrays for underwater sound; transducer calibration
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Observing natural methane seep variability in the northern Gulf of Mexico with an 18-kilohertz split-beam scientific echosounder

Kevin Jerram, Thomas C. Weber, and Jonathan Beaudoin

POMA Volume 19, pp. 005023 (June 2013); (5 pages)

Online Publication Date: June 02, 2013

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Underwater methane seeps support diverse biological communities on the seafloor and, in cases of bubble survival to the surface, contribute to the quantity of atmospheric methane. The National Oceanic and Atmospheric Administration (NOAA) ship Okeanos Explorer completed two research cruises for seep mapping and characterization in the northern Gulf of Mexico during August and September of 2011 and April of 2012. Seeps originating at depths of approximately 1500 m were observed during multiple transects with a 30-kHz Kongsberg EM 302 multibeam echosounder (MBES) and an 18-kHz Simrad EK60 split-beam scientific echosounder calibrated for backscatter. A methodology for determining vessel offsets for the EK60 using MBES seep observations as benchmarks is discussed as part of a larger framework for transformation of seep targets from the split-beam echosounder reference frame to the geographical reference frame. Utilizing sound speed and attitude data collected for the MBES, several EK60 observations of strong individual seeps are scrutinized for variability of seep position and target strength between 2011 and 2012.
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43.30.Ft Volume scattering
43.30.Vh Active sonar systems
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In situ measurement of the individual target strength of crustacean zooplankton with concurrent optical identification

Christian Briseño-Avena, Jules Jaffe, Paul L. Roberts, and Peter J. Franks

POMA Volume 19, pp. 005024 (June 2013); (7 pages)

Online Publication Date: June 02, 2013

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Acoustic methods are common tools in the study of zooplankton distributions and behaviors. However, relating acoustic scattering to zooplankton abundance is difficult because zooplankton acoustic properties have been challenging to obtain in situ. Most of the acoustic measurements of zooplankton come from either preserved or recently dead organisms, or are derived from computer models. Other attempts to measure in situ acoustic target strength (TS) of zooplankton have targeted larger taxa such as euphausiids and amphipods. In situ measurements of copepods and other small crustaceans are extremely desirable as these taxa can be dominant in marine ecosystems. Here we present in situ measurement of TS using frequencies of 1.5−2.5 MHz with concurrent stereoscopic imaging. The concurrent calibration of the optics and acoustics permits the quantification of individual acoustic TS associated with the individual organism that gave rise to the echo. Furthermore, new technological advances have allowed us to measure organisms with TS as small as -125 dB. The results of this work will permit improvements in extant acoustic models, and enhance our interpretation of acoustic data collected in the field.
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43.30.Sf Acoustical detection of marine life; passive and active
43.30.Vh Active sonar systems
43.30.Xm Underwater measurement and calibration instrumentation and procedures
43.80.Ev Acoustical measurement methods in biological systems and media
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Complimentary ultrasound methods for the estimation of sound speed in macroalgae.

Jo Randall, Jean-Pierre Hermand, Marie-Elise Arnould, Jeff Ross, and Craig Johnson

POMA Volume 19, pp. 005025 (June 2013); (6 pages)

Online Publication Date: June 02, 2013

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Temperate kelp forests are amongst the most productive ecosystems in the world. However, there is mounting evidence that these habitats are in decline, both in range and productivity. Acoustic propagation modelling has been used to identify primary productivity in seagrass beds, and work is ongoing in development as a method of providing large scale measurements of productivity in macroalgae forests. Acoustic predictive models require knowledge of the material properties of interest, yet little is known about the acoustic properties of seaweed species. As a preliminary step towards acoustic modelling of seaweed systems, this study investigates the acoustic properties of Ecklonia radiata, a key species in temperate Australian marine systems. Measuring sound speed in macroalgae, as with other biological material, provides unique challenges due to their intrinsic morphological and anatomical characteristics. Using a range of frequencies between 2-10 MHz different methods are proposed to measure sound speed both directly and indirectly. The measurements show a consistent result, with variation according to tissue type. This research provides an important first step towards the development of acoustic propagation models in kelp forest ecosystems.
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43.80.Ev Acoustical measurement methods in biological systems and media
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