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An alternative road traffic noise barrier using an array of periodically arranged vertical cylinders known as a Sonic Crystal (SC) is investigated. As a result of multiple (Bragg) scattering, SCs exhibit a selective sound attenuation in frequency bands called band gaps or stop bands related to the spacing and size of the cylinders. Theoretical studies using Plane Wave Expansion (PWE), Multiple Scattering Theory (MST) and Finite Element Method (FEM) have enabled study of the performance of SC barriers. Strategies for improving the band gaps by employing the intrinsic acoustic properties of the scatterer are considered. The use of the tube cavity (Helmholtz type) resonances in Split Ring Resonator (SRR) or the breathing mode resonances observed in thin elastic shells is shown to increase Insertion loss (IL) in the low-frequency range below the first Bragg stop band. Subsequently, a novel design of composite scatterer uses these 2 types of cylindrical scatterer in a concentric configuration with multiple symmetrical slits on the outer rigid shell. An array of composite scatterers forms a system of coupled resonators and gives rise to multiple low-frequency resonances. Measurements have been made in an anechoic chamber and also on a full-scale prototypes outdoors under various meteorological conditions. The experimental results are found to confirm the existence of the Bragg band gaps for SC barriers and the predicted significant improvements when locally resonant scatterers are used. The resonant arrays are found to give rise to relatively angle-independent stop bands in a useful range of frequencies. Good agreement between computational modelling and experimental work is obtained. Studies have been made also of the acoustical performances of regular arrays of cylindrical elements, with their axes aligned and parallel to a ground plane including predictions and laboratory experiment.

<p>The resolution of near-surface seismic reflection data is often limited by attenuation and scattering in the shallow subsurface which reduces the high frequencies in the data. Compensating for attenuation and scattering, as well as removing the propagating source wavelet in a time-variant manner can be used to improve the resolution. Here we investigate continuous non-stationary iterative time-domain deconvolution (CNS-ITD), where the seismic wavelet is allowed to vary along the seismic trace. The propagating seismic wavelet is then a combination of the source wavelet and the effects of attenuation and scattering effects, and can be estimated in a data-driven manner by performing a Gabor decomposition of the data. For each Gabor window, the autocorrelation is estimated and windowed about zero lag to estimate the propagating wavelet. Using the matrix-vector equations, the estimated propagating wavelets are assigned to the related columns of a seismic wavelet matrix, and these are then interpolated to the time location where the maximum of the envelope of the trace occurs within the iterative time-domain deconvolution. Advantages of using this data-driven, time-varying approach include not requiring prior knowledge of the attenuation and scattering structure and allowing for the sparse estimation of the reflectivity within the iterative deconvolution. We first apply CNS-ITD to synthetic data with a time-varying attenuation, where the method successfully identified the reflectors and increased the resolution of the data. We then applied CNS-ITD to two observed shallow seismic reflection datasets where improved resolution was obtained. </p>

by Wing-keung Mak. / Thesis (M.Phil.)--Chinese University of Hong Kong, 1995. / Includes bibliographical references (leaves 51-53). / by Wing-keung Mak. / Acknowledgments --- p.i / Abstract --- p.ii / Table of Content --- p.iii / Chapter Chapter One --- Introduction --- p.1 / Chapter Chapter Two --- Theory / Chapter 2.1 --- Theory of Two-wave Coupling --- p.3 / Chapter 2.2 --- Theory of Fanning --- p.8 / Chapter 2.3 --- Theory of Time Dependent Absorption Coefficients of Photorefractive Crystals --- p.10 / Chapter Chapter Three --- Photorefractive Fabry-Perot Etalon / Chapter 3.1 --- Theory and Numerical Computation --- p.12 / Chapter 3.2 --- Summary of Crystal Parameters --- p.15 / Chapter 3.3 --- Experimental Studies of Interference Patterns --- p.16 / Chapter 3.4 --- Intensity Dependence of Fabry-Perot Etalon Interference Patterns --- p.25 / Chapter 3.5 --- Fanning --- p.29 / Chapter 3.6 --- Bright Spots on Interference Patterns --- p.31 / Chapter Chapter Four --- Time Dependent Absorption Coefficients of Photorefractive Crystals / Chapter 4.1 --- Experiments Using Laser Light --- p.34 / Chapter 4.2 --- Experiments Using White Light --- p.44 / Chapter Chapter Five --- Conclusion and Future Outlook --- p.49 / References --- p.51

Attenuating random and coherent noise is an important part of seismic data processing. Successful removal results in an enhanced image of the subsurface geology, which facilitate economical decisions in hydrocarbon exploration. This motivates the search for new and more efficient techniques for noise removal. The main goal of this thesis is to present an overview of the Singular Spectrum Analysis (SSA) technique, studying its potential application to seismic data processing. An overview of the application of SSA for time series analysis is presented. Subsequently, its applications for random and coherent noise attenuation, expansion to multiple dimensions, and for the recovery of unrecorded seismograms are described. To improve the performance of SSA, a faster implementation via a randomized singular value decomposition is proposed. Results obtained in this work show that SSA is a versatile method for both random and coherent noise attenuation, as well as for the recovery of missing traces. / Geophysics

Engineering industries usually require nondestructive evaluation (NDE) methods to ensure quality control, safety, and optimized use of resources. Among potential NDE techniques, ultrasonic wave methods are widely used because of their versatility and affordability. For applications to layered structures, ultrasonic guided waves are naturally excited and detected, so these guided waves are the preferred choice when compared to conventional bulk waves. The main advantage of guided waves over bulk waves for layered structures is that these guided waves can propagate a much farther distance, and thus they enable long range inspection. It is important to note that guided waves are multi-mode, so a preferred mode can be selectively used, although it is sometimes more efficient to use multiple wave modes. The characteristics of guided waves, namely dispersive propagation and attenuation, are directly related to the properties of the system in which they are propagating, so the measurement of these wave characteristics can be used for material characterization and condition monitoring. Despite a number of successful techniques to experimentally measure propagation characteristics of guided waves, there is a lack of a standard procedure to obtain attenuation characteristics. This research develops such a quantitative and systematic procedure to extract attenuation characteristics from real guided wave time-domain signals. This research considers multiple wave-modes, and focuses on broadband attenuation measurements with laser ultrasonic techniques. The analytical model of guided waves with attenuation is studied in general cases, and a numerical simulation is developed to model the point source/receiver laser measurement system. The attenuation extraction technique is developed using synthetic signals generated by the simulation. Finally, this research demonstrates the use of experimentally-measured attenuation data for material characterization and condition monitoring by developing an inversion scheme to back-calculate material properties for a number of practical cases.

Abstract This thesis studies the practical appraisal for pipeline inspection using the guided wave T(0,1) mode. The characteristic of reflected signals from the features of pipeline for various coated materials and fluid-filled pipes are also evaluated. The attenuation and the traveling distance of the guided wave are then calculated from the above-mentioned data for pipeline inspection in petro-chemical industries. In the experimental setup, the torsional mode is excited at one axial location using an array of transducers distributed around the circumference of the 6-inch test pipe. The reflected signals from various features, such as flanges, welds, supports, bends, defects and patches are analyzed at first at specific frequencies in the experiments. The effect of various coated material such as bitumen, PE and insulated material are also evaluated for the propagating torsional mode T(0,1) in the pipe. The results show that the attenuation of reflected signal is heavy for the bitumen-coated case because its viscosity is much higher than the other cases. Furthermore, the effect of pipe contents for defect detection using T(0,1) mode is investigated in this thesis. Various pipe contents, such as water, diesel oil, lubricant and fuel oil are deposit into the test pipe, respectively, to evaluate the influence to T(0,1). For the attenuation evaluation of reflected signal from flange in pipe, the reflected signal from an air-content pipe is measured for reference to compare with the measurements of other pipe contents in the experiments. The results show that the low viscosity liquid deposit in the pipe, such as water, diesel oil and lubricant, has no effect on the torsional mode; while the high viscous of the fuel oil deposit in the pipe attenuates the reflected signal heavily. It became evident that the torsional mode T(0,1) is most suitable for use in fluid-filled pipeline inspection.

Modern seismic imaging tools for oil and gas exploration and production (E&P) assume that seismic data contain responses only of waves that bounce (e.g., reflect, diffract) only once at each interface in the subsurface. This type of response is called a primary. Unfortunately, actual seismic data also contain responses of waves that bounce at several interfaces in the subsurface. This type of response is called a multiple. In general, multiples in seismic data fall into two categories: (1) events that bounce at least once at the free surface in addition to any other bounce in the sub- surface and (2) events that do not bounce at the free surface but instead inside the subsurface, at two or more interfaces. The first category has the greater amount of energy; therefore most of the research and development efforts in E&P have so far focused on attenuating this category of multiples accurately. At present, more knowledge of the subsurface is expected from seismic imaging. To avoid any misinterpretation of these details, there is a growing need in the E&P industry to also attenuate the second category of multiples, known as internal multiples. In this work I describe a new method of attenuation of internal multiples. The method consists of predicting the internal multiples and then subtracting them from the data. The prediction of internal multiples from seismic data is made possible by the discovery of a new type of seismic scattering event known as a virtual event. Seismic virtual events constitute a calculational device, which is becoming an important part of seismic data processing. Virtual events combine forward and back- ward wave propagation in such a way that their convolution with real events allows us to predict internal multiples. In addition to showing how virtual events can be constructed from real seismic events, I also show that virtual events obey physical laws, despite their counterintuitive wavepath. I have illustrated the findings in this thesis with synthetic examples. In particu- lar, I have shown the effectiveness of my internal-multiple-attenuation method for a 1D data set, which includes several primaries and internal multiple interferences.

Modern seismic data acquisition and processing methods now enable scientists to extract information on both the stratigraphy and the physical properties of subsurface rocks. Laboratory acoustic measurementsa llow the physical conditions to be precisely measured and controlled. In the present study, P- and S-wave velocities (Vs, VS) and attenuations (1000/Qp, 1000/Qs) were measured in a range of sandstones using the ultrasonic pulse-echo technique, at effective pressures of 5 MPa to 60 MPa. The measurement accuracy is ±0.3 % for velocity and ±0.1 dB/cm for attenuation using this method. Velocities and quality factors( Q) fall with decreasinge ffectivep ressure,a nd the relationships are described by the empirical equationsV =A+KP-B C71' and Q=A-B e7DP , where P is the effective pressure and A, K, B, and D are the regression coefficients (D=0.115±0.016 and 0.048±0.010 for V and Q, respectively). Velocity and Q can therefore be extrapolated to pressures beyond the experimental range. The Biot, Gassmann, and unrelaxed pore-fluid models of seismic wave propagation in porous media fail to explain the pressure-dependenceo f the velocities. The difference between the experimental and Biot model predictions of the rate of change in P-wave velocity with pore fluid salinity (dVýdM) increases with percentage clay content (C) of the rock at the approximately linear rate of 0.95 m/s/mol. There is no clear relationship for dVs/dM. In clean sandstones there is a close agreement between the experimental results and Biot model predictions for dVP/dM, but the agreement breaks down when C>5%. This suggests that changes in the pore-fluid salinity alter the frame bulk and shear moduli of sandstones. Attenuation is generally independent of pore-fluid salinity. Attenuation and velocity are often strongly dependent on the degree of pore-fluid saturation. A study of nine samples shows that 1000/Qp exhibits a resonance peak at midrange saturations (SW av 30 % to SW = 70 %) in most samples, and 1000/Qs shows similar behaviour in several of these. For porosities greater than 13%, the normalised amplitudes of the peaks in P-wave and bulk attenuation are correlated to porosity; the latter increases at a linear rate of 0.98 per percentage increase in porosity. These data suggest that attenuation reaches a maximum when the gas/water mixture is neither too compressible nor too incompressible. The Biot/squirt (BISQ) theory inadequately models the saturation dependence of 1000/Qp and Vp in a sample at low confining pressure. Vp falls with decreasing saturation between SW =100 % to SW - 50 %; below SW = 50 %, the behaviour of Vp is dependent on the confining pressure. Vs generally increases with decreasing saturation over the entire saturation range in all samples. The unrelaxed pore-fluid model of Mavko and Nolen-Hoeksema (1994) describes the Vp data reasonably well in most samples using low wetting fractions (< 15 %), which indicates that the pore fluid is unrelaxed at both the grain and sample scales. The wetting pore fluid becomes unrelaxed at high frequencies and/or low permeabilities. The V. data are poorly described by the model, possibly due to matrix softening by the wetting fluid. The experimental data have indicated significant shortcomings in the mathematical models of seismic wave propagation in reservoir rocks. The data highlight important aspects of wave propagation that must be addressed in revised theories.

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