A one dimensional normalized model for the frequency response of the acoustic power transmitted through nonuniform materials is developed. Using the ideal mixture model to relate acoustic velocity and impedance, this normalized model demonstrates that the power transmission characteristics are completely determined using only a composition profile and the parameters defining percent variation in acoustic velocity and impedance. For purposes of comparison, an analytically exact solution for exponential tapers is obtained. / Master of Science

A simple dual interferometer which uses two pairs of orthogonally polarized optical beams to measure both the amplitude and direction of propagation of broadband ultrasonic surface waves is described. Each pair of focused laser probe beams is used in a separate wideband differential interferometer to independently detect the component of surface wave motion along one direction of the surface. By combining the two output signals corresponding to both components, the two-dimensional surface profile and its variation as a function of time may be determined. Although the system has an optically adjustable -3db acoustic bandwidth of more than two decades (eg. 30kHz to 3MHz for acoustic emission measurements) and may detect peak displacements in the sub-Angstrom range, it is designed to be insensitive to low-frequency specimen translation. Potential applications in nondestructive evaluation are discussed. / Master of Science

The purpose of this thesis is to investigate a novel method to develop broadband microstrip (patch) antennas using substrates containing photonic crystals. Photonic crystals are a class of periodic dielectric, metallic, or composite structures that when introduced to an electromagnetic signal can exhibit a forbidden band of frequencies (or bandgap) in which the incident signal destructively interferes and thus is unable to propagate. It is proposed that such photonic crystals will reduce surface waves and prohibit the formation of substrate modes, which are commonly known inhibitors of patch antenna designs. By reducing or eliminating the effects of these electromagnetic inhibitors with photonic crystals, a broadband response can be obtained from inherently narrowband antennas. In addition, it is also proposed that the behavior of the photonic crystals will lead to a reduction in pattern sidelobes resulting in improvements in radiation pattern front-to-back ratio and overall antenna efficiency. This research is verified through analytical simulations and experimental investigations in the Virginia Tech anaechoic chamber. / Master of Science

The scattering of acoustic waves by boundary discontinuities in waveguides is analyzed using the Method of Matched Asymptotic Expansions (MAE). Existing theories are accurate only for very low frequencies. In contrast, the theory developed in this thesis is valid over the entire range of frequencies up to the first cutoff frequency. The key to this improvement lies in recognizing the important physical role of the cutoff cross-modes of the waveguide, which are usually overlooked. Although these modes are evanescent, they contain information about the interaction between the local field near the discontinuity and the far-field. This interaction has a profound effect on the far-field amplitudes and becomes increasingly important with frequency. The cutoff modes also present novel mathematical problems in that current asymptotic techniques do not offer a rational means of incorporating them into a mathematical description. This difficulty arises from the non-Poincare form of the cross-modes, and its resolution constitutes the second new result of this thesis. We develop a matching scheme based on block matching intermediate expansions in a transform domain. The new technique permits the matching of expansions of a more general nature than previously possible, and may well have useful applications in other physical situations where evanescent terms are important. We show that the resulting theory leads to significant improvements with just a few cross-mode terms included, and also that there is an intimate connection with classical integral methods. Finally, the theory is extended to waveguides with slowly varying shape. We show that the usual regular perturbation analysis of the wave regions must be completely abandoned. This is due to the evanescent nature of the cross-modes, which must be described by a WKB approximation. The pressure field we so obtain includes older results. The new terms account for the cutoff cross-modes of the variable waveguide, which play a central role in extending the dynamic range of the theory. / Ph. D.

This study seeks to prove the usefulness of the Multichannel Analysis of Surface Waves (MASW) method as complementary data to conventional geotechnical and geological data in the characterization of areas of landslide risk. The setting is located in a low income housing neighborhood in the city of Campos do Jordão, state of São Paulo, Brazil. The area was devastated by numerous landslides between December 1999 to January 2000 after heavy rainfall in the area. The landslides in this area are known to be shallow. Refraction and MASW surveys were performed in this research and then processed to obtain seismic velocity profiles of the subsurface. In order to better identify the fundamental mode of the Rayleigh wave, separate surveys isolating the vertical and radial components of the Rayleigh wave were performed. By comparing the inverted shear wave ( ) and compressional wave ( ) profiles with the already known geotechnical and geological data of the site, a better understanding of the geological interfaces that constitute the landslide prone area is obtained. / Este estudo busca demonstrar a aplicabilidade do método de análise multicanal de ondas superficiais (MASW) e a importância da integração de seus resultados com os da investigação geológica-geotécnica para a caracterização de áreas sujeitas a deslizamentos de terra. O local de estudo situa-se em uma área residencial com precárias condições de infraestrutura urbana na cidade de Campos do Jordão, estado de São Paulo, Brasil. Na área houve vários episódios de escorregamento entre dezembro de 1999 e janeiro de 2000 após a ocorrência de chuvas intensas. Os escorregamentos nesta área são classificados como rasos. Nesta pesquisa foram realizados ensaios de sísmica de refração e MASW e gerados perfis e seções de velocidade de propagação das ondas sísmicas. Para obter uma melhor identificação do modo fundamental da onda Rayleigh foram realizadas diferentes aquisições visando ao registro das componentes vertical e radial do movimento da onda. A integração dos perfis de velocidades da onda cisalhante (VS) e da onda compressional (VP) com os dados geológicos e geotécnicos obtidos anteriormente permitiu aprimorar o mapeamento das interfaces geológicas importantes para o estudo dos processos de escorregamento do local.

This study seeks to prove the usefulness of the Multichannel Analysis of Surface Waves (MASW) method as complementary data to conventional geotechnical and geological data in the characterization of areas of landslide risk. The setting is located in a low income housing neighborhood in the city of Campos do Jordão, state of São Paulo, Brazil. The area was devastated by numerous landslides between December 1999 to January 2000 after heavy rainfall in the area. The landslides in this area are known to be shallow. Refraction and MASW surveys were performed in this research and then processed to obtain seismic velocity profiles of the subsurface. In order to better identify the fundamental mode of the Rayleigh wave, separate surveys isolating the vertical and radial components of the Rayleigh wave were performed. By comparing the inverted shear wave ( ) and compressional wave ( ) profiles with the already known geotechnical and geological data of the site, a better understanding of the geological interfaces that constitute the landslide prone area is obtained. / Este estudo busca demonstrar a aplicabilidade do método de análise multicanal de ondas superficiais (MASW) e a importância da integração de seus resultados com os da investigação geológica-geotécnica para a caracterização de áreas sujeitas a deslizamentos de terra. O local de estudo situa-se em uma área residencial com precárias condições de infraestrutura urbana na cidade de Campos do Jordão, estado de São Paulo, Brasil. Na área houve vários episódios de escorregamento entre dezembro de 1999 e janeiro de 2000 após a ocorrência de chuvas intensas. Os escorregamentos nesta área são classificados como rasos. Nesta pesquisa foram realizados ensaios de sísmica de refração e MASW e gerados perfis e seções de velocidade de propagação das ondas sísmicas. Para obter uma melhor identificação do modo fundamental da onda Rayleigh foram realizadas diferentes aquisições visando ao registro das componentes vertical e radial do movimento da onda. A integração dos perfis de velocidades da onda cisalhante (VS) e da onda compressional (VP) com os dados geológicos e geotécnicos obtidos anteriormente permitiu aprimorar o mapeamento das interfaces geológicas importantes para o estudo dos processos de escorregamento do local.

Thesis (Ph. D.)--University of Rhode Island, 1993. / Includes bibliographical references (leaves 173-181).

Comparative studies of nondestructive evaluation methods have shown that nonlinear ultrasonic techniques are more sensitive than conventional linear methods to changes in material microstructure and the associated small-scale damage. Many of the material degradation processes such as carbonation in concrete, corrosion in metals, etc., begin at the surface. In such cases, ultrasonic Rayleigh surface waves are especially appropriate for detection and characterization of damage since their energy is concentrated in the top layer of the test object. For the civil engineering infrastructure, only a limited number of field applicable nonlinear ultrasonic techniques have been introduced. In this paper a nonlinear ultrasonic measurement technique based on the use of Rayleigh waves is developed and used to characterize carbonation in concrete samples. Wedge transducer is used for the generation and an accelerometer for detection of the fundamental and modulated ultrasonic signal components. The measurements are made by varying the input voltage and along the propagation distance. The slope of the normalized modulated amplitudes is taken as the respective nonlinearity parameter. Concrete samples with two different levels of damage are examined, and the difference of the two fundamental frequencies is used to quantify damage state.

Surface wave methods have been used to determine dynamic properties of near-surface soils in geotechnical engineering for the past 50 years. Although the capabilities of engineering surface wave methods have improved in recent years due to several advances, several issues including (1) near-field effects, (2) combined active and passive measurements, and (3) accurate measurements of surface wave attenuation still require study to further improve the capabilities of modern surface wave methods. Near-field effects have been studied for traditional surface wave methods with two receivers and several filtering criteria to mitigate the effects have been recommended. However, these filtering criteria are not applicable to surface wave methods with multiple receivers. Moreover, the criteria are not quantitatively based and do not account for different types of soil profiles, which strongly influence near-field effects. A new study of near-field effects on surface wave methods with multiple receivers was conducted with numerical and experimental methods. Two normalized parameters were developed to capture near-field effects. Quantitatively based near-field effect criteria for an ideal homogeneous half-space and three typical soil profiles are presented. Combining active and passive surface wave measurements allows developing a shear wave velocity profile to greater depth without sacrificing the near-surface resolution offered by active measurements. Generally, active and passive measurements overlap in the frequency range from approximately 4 to 10 Hz, and there are often systematic differences between the two measurements. The systematic errors in active and passive surface wave methods were explored to explain and resolve the differences, allowing for a more accurate composite dispersion curve. The accuracy of measured surface wave attenuation is improved by properly accounting for (1) geometric spreading, (2) near-field effects, and (3) ambient noise. In this study, a traditional estimation method and a frequency-wavenumber method utilizing sub-arrays were investigated using displacement data from numerical simulations, focusing on near-field and ambient noise effects. Detailed procedures for the frequency-wavenumber estimation method are developed based on a study of the primary factors affecting attenuation estimates. The two methods are also evaluated using experimental displacement data obtained from surface wave field measurements with three different arrays.

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