Global ETD Search

1	Mean-field reflection of omni-directional acoustic wave from rough seabed with non-uniform sediment layers Wu, Yung-Hong 23 June 2004 (has links) Omni-directional acoustic wave source interactions with a rough seabed with a continuously varying density and sound speed in a fluid-like sediment layer. The acoustic properties in the sediment layer possess an exponential type of variation in density and one of the three classes of sound speed profiles, which are constant,~$k^2$-linear, or inverse-square variations. Analytical solution of mean field. The mean field reflection coefficients corresponding to the aforementioned density and sound speed profiles for various frequencies, roughness parameters, are numberically generated and analyzed. Physical interpretations are provided for various results. This simple model characterizes two important features of sea floor, including seabed roughness, sediment inhomogenieties, therefore, provide a canonical analysis in seabed acoustics. rough seabed Line source non-uniform sediment layers Point source scattering Mean-field reflection acoustic wave
2	Development of a Sediment Sampling Free Fall Penetrometer Add-on Unit for Geotechnical Characterization of Seabed Surface Layers Bilici, Cagdas 27 June 2018 (has links) In-situ geotechnical testing of surficial sediment layers in areas of active sediment dynamics can provide essential information about physical and geotechnical variations of sediment properties with regards to active sediment remobilization processes. For example, portable free fall penetrometers (PFFPs) can assist with the detection of mobile sediment layers. They are easy to deploy, and can provide a large spatial coverage in a time- and cost-effective manner. However, they often struggle to provide more detailed information about the properties of mobile sediment layers due to a lack of calibration and validation in existing data sets. Currently, existing sediment samplers often disturb, or ignore the uppermost sediment layers. Simultaneous sediment sampling and geotechnical profiling is needed to fill this gap, and to drive data interpretation forward. A field investigation of surficial sediments was conducted in the wetland waterways of coastal Louisiana in 2014. In-situ tests were conducted using PFFP, and disturbed sediment samples were collected in selected locations. The results allowed us to map changes in sediment strength and stratification, and correlate the geotechnical results to local site characteristics. However, the need for high quality sediment samples for calibration and validation was emphasized by the results. Three different sediment sampler add-on units targeting mobile layers were designed and manufactured based on lessons-learned from the literature. The designs were tested in the laboratory and in the field (Yakutat, Alaska and York River, Virginia) in 2017. The samples were analyzed to understand the influence of different sampler characteristics on collected sample quality, and, to define mobile layer sampler characteristics that enable simultaneous geotechnical testing and the collection of high quality samples. Following field survey campaigns in the York River, Virginia in 2016 allowed to assess surficial sediment layer characteristics and behavior based on a coupled analysis of geotechnical data from in-situ PFFP tests and the sedimentological data collected using box cores and the novel sediment sampler. In summary, novel strategies and instrumentation to carry out simultaneous sediment sampling and geotechnical profiling of seabed surface layers were tested, and new pathways for geotechnical data analysis for the investigation of mobile seabed layers were presented. / PHD Free-fall penetrometer mobile layer sediment sampler surficial sediment layers geotechnical site characterization
3	Numerical simulation of acoustic wave propagation with a focus on modeling sediment layers and large domains Estensen, Elias January 2022 (has links) In this report, we study how finite differences can be used to simulate acoustic wave propagation originating from a point source in the ocean using the Helmholtz equation. How to model sediment layers and the vast size of the ocean is studied in particular. The finite differences are implemented with summation by parts operators with boundary conditions enforced with simultaneous approximation terms and projection. The numerical solver is combined with the WaveHoltz method to improve the performance. Sediment layers are handled with interface conditions and the domain is artificially expanded using absorbing layers. The absorbing layer is implemented with an alternative approach to the super-grid method where the domain expansion is accomplished by altering the wave speed rather than with coordinate transformations. To isolate these issues, other parameters such as variations in the ocean floor are neglected. With this simplification, cylindrical coordinates are used and the angular variation is assumed to be zero. This reduces the problem to a quasi-three-dimensional system. We study how the parameters of the alternative absorbing layer approach affect its quality. The numerical solver is verified on several test cases and appears to work according to theory. Finally, a semi-realistic simulation is carried out and the solution seems correct in this setting. Helmholtz wave equation acoustics ocean acoustics numerical analysis computational science finite differences summation by parts sbp waveholtz cylindrical coordinates sediment layers interface super-grid super grid absorbing layer Helmholtz vågekvationen akustik undervattensakustik numerisk analys beräkningsvetenskap finita differenser summation by parts sbp cylindriska koordinater waveholtz sedimentskikt interface super-grid absorberande lager Computational Mathematics Beräkningsmatematik

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