Thesis (Ph. D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 170-182). Also available on the Internet.

Thesis (Ph. D.)--University of Missouri-Columbia, 2004. / Typescript. Vita. Includes bibliographical references (leaves 170-182). Also available on the Internet.

Thesis (M.S.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2005. / Laurence J. Jacobs, Committee Chair ; Reginald DesRoches, Committee Member ; Jianmin Qu, Committee Member. Includes bibliographical references.

Thesis (M.S.)--Naval Postgraduate School, 1971. / Includes bibliographical references (leaves 141-142).

[pt] Os desvanecimentos por multipercurso atmosférico são a principal causa de degradação do desempenho de enlaces rádio digitais de alta capacidade, acentuando os efeitos do ruído térmico e da interferência intersimbólica na taxa de bits errados. A previsão da distribuição cumulativa de probabilidades destes desvanecimentos em função de parâmetros do enlace e características rádio climáticas da região é de fundamental importância no projeto de sistemas rádio digitais terrestres. Existem na literatura técnica métodos para previsão do desvanecimento em enlaces individuais, mas não para um tronco de microondas formado por vários enlaces. Medidas realizadas no Brasil indicam que a usual suposição de decorrelação total entre desvanecimentos profundos em enlaces consecutivos em um mesmo tronco é pessimista, podendo levar a uma indisponibilidade combinada maior do que a real. Neste trabalho foram analisados dados experimentais de 15 enlaces na região centro-oeste do Brasil e 5 enlaces no Japão e desenvolvido um modelo empírico de previsão da distribuição conjunta do desvanecimento por multipercurso atmosférico conjunta em enlaces adjacentes. O modelo apresenta boa concordância com os resultados experimentais e fornece uma nova expressão para o cálculo da indisponibilidade em troncos de microondas formados por vários enlaces. Os dados experimentais foram utilizados ainda para modelar a duração de eventos de atenuação por multipercurso em enlaces em visibilidade. Mostrou-se que a distribuição de duração de eventos é bem caracterizada por distribuições de Weibull. Esta caracterização permite a distinção entre eventos de perda de qualidade (duração inferior a 10 segundos) e de indisponibilidade (duração superior a 10 segundos). / [en] Multipath fading is the main cause of performance degradation in high capacity line-of-sight microwave links, increasing enhancing the effects of the thermal noise and intersymbol interference in the bit error rate. Tested methods are available in the literature for the prediction of cumulative probability distribution multipath fading in individual link but not in a microwave network formed by several tandem links. Measurements carried out in Brazil indicate that the usual assumption that deep fade events in consecutive links are totally uncorrelated may be pessimistic for tropical regions and may lead to the prediction excessive multihop links unavailability. Experimental data obtained in measurements of multipath fading in 15 pairs of tandem links located in the center- western region of Brazil were analyzed to provide cumulative distributions of attenuation in each pair. Similar results are available in the literature for 5 pairs of links in Japan. Based on these results, an empirical model was developed for the prediction of the joint probability distribution the multipath fading in adjacent links was developed. The attenuation simultaneously exceeded in both links for given percentage of time can be predicted as a function of the equiprobable values of attenuation exceeded in the individual links, path lengths and operation frequency. The predicted distributions show good agreement with the experimental results and provide an expression for the calculation of unavailability of line-of- sight tandem links. The experimental data had been used still to model the duration of events of attenuation for multipath of links in visibility. One revealed that the distribution of duration of events well is characterized by distributions of Weibull. This characterization allows the distinction between events of loss of quality (lowest duration at 10 seconds) and of non-availability (higher duration the 10 seconds).

This thesis designs new methods for Q tomography and Q-compensated prestack depth migration when the recorded seismic data suffer from strong attenuation. A motivation of this work is that the presence of gas clouds or mud channels in overburden structures leads to the distortion of amplitudes and phases in seismic waves propagating inside the earth. If the attenuation parameter Q is very strong, i.e., Q<30, ignoring the anelastic effects in imaging can lead to dimming of migration amplitudes and loss of resolution. This, in turn, adversely affects the ability to accurately predict reservoir properties below such layers. To mitigate this problem, I first develop an anelastic least-squares reverse time migration (Q-LSRTM) technique. I reformulate the conventional acoustic least-squares migration problem as a viscoacoustic linearized inversion problem. Using linearized viscoacoustic modeling and adjoint operators during the least-squares iterations, I show with numerical tests that Q-LSRTM can compensate for the amplitude loss and produce images with better balanced amplitudes than conventional migration. To estimate the background Q model that can be used for any Q-compensating migration algorithm, I then develop a wave-equation based optimization method that inverts for the subsurface Q distribution by minimizing a skeletonized misfit function ε. Here, ε is the sum of the squared differences between the observed and the predicted peak/centroid-frequency shifts of the early-arrivals. Through numerical tests on synthetic and field data, I show that noticeable improvements in the migration image quality can be obtained from Q models inverted using wave-equation Q tomography. A key feature of skeletonized inversion is that it is much less likely to get stuck in a local minimum than a standard waveform inversion method. Finally, I develop a preconditioning technique for least-squares migration using a directional Gabor-based preconditioning approach for isotropic, anisotropic or anelastic least-squares migration. During the least-squares iterations, I impose sparsity constraints on the inverted reflectivity model in the local Radon domain. The forward and the inverse mapping of the reflectivity to the local Radon domain is done through 3D Fourier-based discrete Radon transform operators. Using numerical tests on synthetic and 3D field data, I demonstrate that the proposed preconditioning approach can discriminate against artifacts in the image resulting from irregular or insufficient acquisition and can produce images with improved signal-to-noise ratio when compared with standard migration.

Regulatory acceptance of monitored natural attenuation (MNA) requires demonstration that natural processes, such as sorption and biodegradation, attenuate specific contaminants of concern on a time scale that is comparable to other remediation options while concurrently preventing contaminant migration to site-specific points of contact. Two of the tools used to demonstrate the efficacy of MNA, microcosm experiments and numerical fate and transport modeling, were examined in this study. In the first phase of this work, laboratory microcosm studies were initiated as part of an overall MNA site assessment to determine whether a native microbial consortia collected with a soil sample from a petroleum-hydrocarbon contaminated site was capable of biodegrading specific polynuclear aromatic hydrocarbon (PAH) compounds. Results indicated that selected PAH compounds were biodegraded under simulated natural conditions using oxygen and sulfate as electron acceptors. In the second phase of this study, a numerical experiment was conducted using the three-dimensional, multiple substrate, multiple electron acceptor fate and transport model SEAM3D (Waddill and Widdowson, 1997) to evaluate the impact of including iron(III)-reducing conditions during numerical simulations of natural attenuation. Results for this phase of the study indicated that the mass of hydrocarbon simulated as biodegraded by the iron(III)-reducing population was significantly larger than hydrocarbon biodegradation under aerobic conditions. The final component of research used the SEAM3D model to interpret field observations recorded during a natural attenuation experiment where the fate and transport of selected hydrocarbon contaminants (BTEX and naphthalene) was tracked through an extremely heterogeneous, but well-instrumented test aquifer. Results from the calibrated model for the NATS experiment indicated that the majority of the contaminant remained in the non-aqueous phase during the first year of the experiment, and that aerobic biodegradation was the dominant natural attenuation process. Model results were particularly sensitive to the rate of contaminant release and the starting mass of electron acceptor. / Ph. D.

In the ocean, natural and artificial processes generate clouds of bubbles which scatter and attenuate sound. Measurements have shown that at the individual bubble resonance frequency, sound propagation in this medium is highly attenuated and dispersive. Theory to explain this behavior exists in the literature, and is adequate away from resonance. However, due to excessive attenuation near resonance, little experimental data exists for comparison. An impedance tube was developed specifically for exploring this regime. Using the instrument, unique phase speed and attenuation measurements were made for void fractions ranging from 6.2 × 10^−5 to 2.7 × 10^−3 and bubble sizes centered around 0.62 mm in radius. Improved measurement speed, accuracy and precision is possible with the new instrument, and both instantaneous and time-averaged measurements were obtained. Behavior at resonance was observed to be sensitive to the bubble population statistics and agreed with existing theory, within the uncertainty of the bubble population parameters. Scattering from acoustically compact bubble clouds can be predicted from classical scattering theory by using an effective medium description of the bubbly fluid interior. Experimental verification was previously obtained up to the lowest resonance frequency. A novel bubble production technique has been employed to obtain unique scattering measurements with a bubbly-liquid-filled latex tube in a large indoor tank. The effective scattering model described these measurements up to three times the lowest resonance frequency of the structure. / United States Navy Office of Naval Research Ocean Acoustics Program

Pyrotechnic shock events used during stage separation on rocket vehicles produce high amplitude short duration structural response that can lead to malfunction or degradation of electronic components, cracks and fractures in brittle materials, local plastic deformation, and can cause materials to experience accelerated fatigue life. These transient loads propagate as waves through the structural media losing energy as they travel outward from the source. This work assessed available test data in an effort to better understand attenuation characteristics associated with wave propagation and attempted to update a historical standard defined by the Martin Marietta Corporation in the late 1960's using out of date data acquisition systems. Two data sets were available for consideration. The first data set came from a test that used a flight like cylinder used in NASA's Ares I-X program, and the second from a test conducted with a at plate. Both data sets suggested that the historical standard was not a conservative estimate of shock attenuation with distance, however, the variation in the test data did not lend to recommending an update to the standard. Beyond considering attenuation with distance an effort was made to model the at plate configuration using finite element analysis. The available at plate data consisted of three groups of tests, each with a unique charge density linear shape charge (LSC) used to cut an aluminum plate. The model was tuned to a representative test using the lowest charge density LSC as input. The correlated model was then used to predict the other two cases by linearly scaling the input load based on the relative difference in charge density. The resulting model predictions were then compared with available empirical data. Aside from differences in amplitude due to nonlinearities associated with scaling the charge density of the LSC, the model predictions matched the available test data reasonably well. Finally, modeling best practices were recommended when using industry standard software to predict shock response on structures. As part of the best practices documented, a frequency dependent damping schedule that can be used in model development when no data is available is provided.

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