Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 99 for “"Sound speed"”.
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Inverse methods for sound speed estimation in the ocean
… and computational approaches for estimating sound-speed in the ocean under different conditions. The first part of the dissertation discusses a fast approach for solving the inverse problem of estimating sediment sound-speed based on the Deift-Trubowitz trace formula. Under certain …
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Reconstruction of spatially varying sound speed distributions from pulse-echo data
… modes, such as tomographic reconstruction of sound speed distributions. Historically, sound speed tomography has typically been performed in transmission-mode, which requires specialized hardware configurations and is limited to acoustically transparent tissue such as the female breast. For …
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A tomographic ocean sound speed profile from a long veritcal acoustic array
An average sound speed profile over a 1000 km section of the northeast Pacific ocean is obtained using Ocean Acoustic Tomography, from data acquired during the 1987 SVLA experiment on a long (900 m) 120 hydrophone vertical acoustic array. In particular, we pulse compress the received signal with a …
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The Role Of Tissue Sound Speed As A Surrogate Marker Of Breast Density
… including the use of ionizing radiation. Ultrasound tomography (UST) does not use ionizing radiation and can create tomographic breast sound speed images. These sound speed images are useful because breast density is proportional to sound speed. The purpose of this work was to assess the …
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A modal/WKB inversion method for determining sound speed profiles in the ocean and ocean bottom
Two approaches to determining the ocean sound speed profile using measured acoustic modal eigenvalues are examined. Both methods use measured eigenvalues and mode dependent assumed values of the WKB phase integral as input data and use the WKB phase integral as a starting point for relating the …
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Spatiotemporal excitation sequence manipulation and processing for Doppler ultrasound and pulse-echo sound speed imaging applications
… detectable velocity in pulsed-wave Doppler ultrasound. This is achieved by a reordering of the frames to minimize the relative phase difference between them while preserving spatial resolution that could be expected using a traditional sequence. The second application, and arguably the more …
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Use of historical salinity data in the computation of depth correction due to variations in sound speed
… in the determination of depth corrections due to sound speed variations. The averaging techniques range in complexity from the use of a single value of salinity throughout the entire water column to the use of a historical salinity average profile corrected for the surface salinity. Results show …
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An inverse method for obtaining the attenuation profile and small variations in the sound speed and density profiles of the ocean bottom
… have a direct effect on the propagation of sound in the ocean. In the frequency range of interest (50 - 500 Hz) the sediment can be modelled as a fluid. Assuming horizontal stratification of the ocean bottom, the acoustic parameters of interest are the compressional wave speed, the …
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An inverse method for obtaining the attenuation profile and small variations in the sound speed and density profiles of the ocean bottom
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Ocean Engineering, 1985.
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Frequency Domain Ultrasound Waveform Tomography Breast Imaging
<p>Ultrasound tomography is an emerging modality for imaging breast tissue for the detection of disease. Using the principles of full waveform inversion, high-resolution quantitative sound speed and attenuation maps of the breast can be created. In this thesis, we introduce some basic principles of …
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Environmental analysis and prediction of transmission loss in the region of the New England Shelfbreak
… water column, particularly the temperature and sound speed fields. Empirical orthogonal function (EOF) analysis of the ocean sound speed field defined a set of perturbations to the background sound speed field for each of the NEST Scanfish surveys. Attenuation due to bottom sediments is the …
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Environmental analysis and prediction of transmission loss in the region of the New England Shelfbreak
… water column, particularly the temperature and sound speed fields. Empirical orthogonal function (EOF) analysis of the ocean sound speed field defined a set of perturbations to the background sound speed field for each of the NEST Scanfish surveys. Attenuation due to bottom sediments is the …
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A modal analysis of acoustic propagation in the changing Arctic environment
… look at acoustic propagation through double duct sound speed profiles in the Arctic. While the traditional Arctic sound speed profile has a single surface duct, some portions of the Arctic have a sound speed profile which includes a second, lower duct. These double duct systems are seen through …
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Diagonalization of 1-D Schrodinger Operators with Piecewise Constant Potentials
… on a specific equation that is used to model sound speed in the ocean. As depth increases, the sound speed changes. This means the variable related to the sound speed is not constant. We will modify this variable so that it is piecewise constant. The specific operator in this equation also …
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Geographic Variations of Sound Channel Axis in the Global Ocean
Sound signals are used for imaging and communication in the ocean. Underwater sound speed has a minimum at a certain depth that forms a sound channel for long-range underwater sound propagation. The geographic variation of sound channel is crucial in uses of underwater sound in various environments …
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Innovative Acoustic Reflection Imaging Techniques And Application To Clinical Breast Tomography
<p>Conventional ultrasound techniques use beam-formed, constant sound speed ray models for fast image reconstruction. However, these techniques are inadequate for the emerging new field of ultrasound tomography (UST). We present a new technique for reconstruction of reflection images from UST data. …
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A matched field processing approach to long range acoustic navigation
… The system uses a ray tracing model of sound propagation in a horizontally stratified ocean to associate each multipath arrival with a particular ray path through the ocean. This ray path identification allows an inversion for both vehicle position and sound speed profile parameters. …
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Inverse methods and results from the 1981 Ocean Acoustic Tomography Experiment
… Stream recirculation. The propagation of 224 Hz sound energy in the ocean can be described as a set of rays traveling from source to receiver, with each ray taking a different path through the ocean in a vertical plane connecting the source and receiver. The sources transmitted a phase-coded …
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An experimental study of air entrainment by breaking waves
… is developed. Simultaneous measurements of the sound-speed (and thus the void-fraction) at several depths are conducted during two field experiments. Time-series of sound-speed and attenuation show dramatic fluctuations over time periods on the order of minutes or less. These are attributed to …
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Antipodal receptions in global acoustics
… of the earth and the range dependence of the sound speed field within the ocean can not be ignored. The purpose of this thesis is to investigate the nature of antipodal receptions with reference to the form of the earth and horizontal sound speed variations within the ocean and thereby …
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