Influence of positional correlations on the propagation of waves in a complex medium with polydisperse resonant scatterers

We present experimental results on a model system for studying wave propagation in a complex medium exhibiting low frequency resonances. These experiments enable us to investigate a fundamental question that is relevant for many materials, such as metamaterials, where low-frequency scattering resonances strongly influence the effective medium properties. This question concerns the effect of correlations in the positions of the scatterers on the coupling between their resonances, and hence on wave transport through the medium. To examine this question experimentally, we measure the effective medium wave number of acoustic waves in a sample made of bubbles embedded in an elastic matrix over a frequency range that includes the resonance frequency of the bubbles. The effective medium is highly dispersive, showing peaks in the attenuation and the phase velocity as functions of the frequency, which cannot be accurately described using the Independent Scattering Approximation (ISA). This discrepancy may be explained by the effects of the positional correlations of the scatterers, which we show to be dependent on the size of the scatterers. We propose a self-consistent approach for taking this "polydisperse correlation" into account and show that our model better describes the experimental results than the ISA

A method for estimating sound speed and the void fraction of bubbles from sub-bottom sonar images of gassy seabeds

There is increasing interest in the effect of bubbles in gassy sediment. This is, first, because ofthe impact those bubbles have on the structural integrity and load-bearing capabilities of thesediment; second, because the presence of bubbles can be indicative of a range of biological,chemical or geophysical processes (such as the climatologically-important flux of methanefrom the seabed to the atmosphere); and third, because of the effect which the bubbles haveon any acoustic systems used to characterise the sediment. For this reason, a range of methodshave been investigated for their ability to estimate the bubble population in the seabed. Withinsuch a range, there will a mix of advantages and limitation to given techniques. This reportoutlines a very basic method by which an observations which have already been taken forother purposes (sub-bottom profiles) may be subjected to a rapid analysis to obtain anestimate of the effect of bubbles on the sound speed in the sediment, and from there toprovide a rapid preliminary estimate of the void fraction of bubbles present (assuming quasistaticbubble dynamics). This approach is not meant to compete with large-scale field trialswhich deploy specialist equipment to monitor gas bubbles in sediment, but rather to provide amethod to exploit archived sub-bottom profiles, or to survey a large area rapidly withcommercial equipment from a small vessel, in order to obtain an estimation of the local voidfractions present, and their location and extent in three dimensions

Split-beam echosounder observations of natural methane seep variability in the northern Gulf of Mexico

A method for positioning and characterizing plumes of bubbles from marine gas seeps using an 18 kHz scientific split-beam echo sounder (SBES) was developed and applied to acoustic observations of plumes of presumed methane gas bubbles originating at approximately 1400 m depth in the northern Gulf of Mexico. A total of 161 plume observations from 27 repeat surveys were grouped by proximity into 35 clusters of gas vent positions on the seafloor. Profiles of acoustic target strength per vertical meter of plume height were calculated with compensation for both the SBES beam pattern and the geometry of plume ensonification. These profiles were used as indicators of the relative fluxes and fates of gas bubbles acoustically observable at 18 kHz and showed significant variability between repeat observations at time intervals of 1 h–7.5 months. Active gas venting was observed during approximately one third of the survey passes at each cluster. While gas flux is not estimated directly in this study owing to lack of bubble size distribution data, repeat surveys at active seep sites showed variations in acoustic response that suggest relative changes in gas flux of up to 1 order of magnitude over time scales of hours. The minimum depths of acoustic plume observations at 18 kHz averaged 875 m and frequently coincided with increased amplitudes of acoustic returns in layers of biological scatterers, suggesting acoustic masking of the gas bubble plumes in these layers. Minimum plume depth estimates were limited by the SBES field of view in only five instances

Price Bubbles in Chinese Agricultural Commodity Market

This cumulative dissertation presents four contributions that attempt to shed light on the issues regarding price bubbles in Chinese agricultural commodity market. Given that the public and policymakers show their concern on the price bubbles in Chinese agricultural commodity market, chapter 2 and 3 investigate the origin of price bubbles in futures and spot markets, respectively. In particular, after accurately identifying the bubble dates in agricultural futures market and fixing the estimation bias of rare events models, our empirical results in chapter 2 indicate that bubble episodes only account for a very limited proportion of the sample period, meanwhile, China’s corn and soybeans markets respond differently to the speculative activity and external shocks from international markets. Price bubbles are more likely to be associated with strong economic activity, high interest rates and low inflation levels. Furthermore, by gauging the synchronization level of bubble occurrences between futures and spot markets in chapter 3, we find that even cointegrated futures and spot prices for agricultural commodities seldom bubble together. Further analysis through a regime-switching approach of price transmission reveals that the adjustment effect of futures prices on spot prices is the lowest during the regime where bubbles occur the most frequently for spot prices, while the spot price returns are more likely to be affected by its own lagged terms. All these results challenge the idea that bubbles are originated from over-financialization in futures markets and are then transmitted to spot markets. Therefore, we conclude that futures price bubbles are more sensitive to fundamental factors, while spot price bubbles are more likely to be affected by their own market features. Apart from empirical analyses on the origin of price bubbles, it is widely believed that bubbles could distort resource allocation and a recession usually follows the collapse of bubbles. Inspired by the findings from chapter 2 and 3, chapter 4 attempts to build a systematic theoretical framework that explains the observed economic process with bubbles. From a new perspective of firm growth, we construct a theoretical model to describe the evolvement of bubbles, including their origin, development, collapse, and their effect on the output of economy. Following our research topic, chapter 5 tends to investigate the effects of the newly established futures contract for apples in China. The results of various tests suggest that the apple futures market does not serve well for the price discovery and may reduce the spot price volatility to some extent. In order to improve the efficiency of the apple futures market, the regulators should consider effective measures to attract more commercial traders from different regions in China into the futures market

Cosmology at Low Frequencies: The 21 cm Transition and the High-Redshift Universe

Observations of the high-redshift Universe with the 21 cm hyperfine line of neutral hydrogen promise to open an entirely new window onto the early phases of cosmic structure formation. Here we review the physics of the 21 cm transition, focusing on processes relevant at high redshifts, and describe the insights to be gained from such observations. These include measuring the matter power spectrum at z~50, observing the formation of the cosmic web and the first luminous sources, and mapping the reionization of the intergalactic medium. The epoch of reionization is of particular interest, because large HII regions will seed substantial fluctuations in the 21 cm background. We also discuss the experimental challenges involved in detecting this signal, with an emphasis on the Galactic and extragalactic foregrounds. These increase rapidly toward low frequencies and are especially severe for the highest redshift applications. Assuming that these difficulties can be overcome, the redshifted 21 cm line will offer unique insight into the high-redshift Universe, complementing other probes but providing the only direct, three-dimensional view of structure formation from z~200 to z~6.Comment: extended review accepted by Physics Reports, 207 pages, 44 figures (some low resolution); version with high resolution figures available at http://pantheon.yale.edu/~srf28/21cm/index.htm; minor changes to match published versio

Non Linear Ultrasound Doppler and the Detection of Targeted Contrast Agents

One of the main challenges in molecular imaging with targeted contrast agents is the detection and discrimination of attached agents from the rest of the signals originating from freely flowing agents and tissue. The aim of this thesis was to develop methods for the detection of targeted microbubbles. One approach consisted of investigating the use of nonlinear Doppler for this purpose. Nonlinear Doppler enables the differentiation of moving from non-moving and linear from nonlinear scattering. Targeted microbubbles are static and nonlinear scatterers and they should be detected using this technique. A novel nonlinear Doppler technique: Pulse subtraction Doppler, was developed and compared to pulse inversion Doppler. It is shown that both techniques lead to similar Doppler spectra and depending on the medical applications and the equipment limitations, both techniques have benefits. This served as a starting point for the derivation of a generalised nonlinear Doppler technique, based on combined linear pulse pair sequences and tested in a simulation study. The response from a single microbubble was simulated for different pulse combinations and the pulse sequences were compared with regards to criteria specific to imaging requirements. It was shown that depending on initially set criteria, such as transmitted energy, mechanical index or scanner characteristics, certain pulse combinations offer alternatives to the current imaging modalities and allow to take into account specific constrains due to the targeted application/equipment. Furthermore, the proposed approach is directly applicable in a strict non linear imaging approach, without Doppler processing. An in vitro phantom was designed in order to assess pulse subtraction Doppler for the detection and discrimination of static nonlinear microbubbles in the presence of free flowing ones. It was shown that pulse subtraction Doppler enables such discrimination and the practicability for in vivo situations is discussed. The pulse subtraction Doppler sequences were also tested on a phantom containing magnetic bubbles. It was shown that the magnetic bubbles can be immobilised through a magnetic field to a specific region of interest under flow conditions. The bubbles also showed to be acoustically detectable and to scatter linearly at diagnostic driving pressures. Preliminary work regarding experimental biotinylated microbubbles and their attachment to streptavidin coated surfaces is also presented. Due to their proximity to a wall, researchers have found that targeted microbubbles exhibit different acoustic signatures compared to free ones and this knowledge can improve their detection techniques. The behaviour of microbubbles against a membrane of varying stiffness was also studied through high speed camera observations. It was found both experimentally and by comparison to theoretical modelling that within the stiffness range of human blood vessels the change in acoustical behaviour of microbubbles is negligible. This thesis has taken two complementary research approaches which have shown to constitute advancements for the detection and discrimination of targeted microbubbles

A Cavitation Susceptability Meter with Optical Cavitation Monitoring-Part One: Design Concepts

This work is concerned with the design of a Cavitation Susceptibility Meter based on the use of a venturi tube for the measurement of the active cavitation nuclei concentration in water samples as a function of the applied tension. The operation of the Cavitation Susceptibility Meter is analyzed and the main considerations leading to the proposed design are illustrated and critically discussed. The results of this analysis indicate that the operational range is mainly limited by nuclei interference, flow separation and saturation (choking), and suggest to develop a Cavitation Susceptibility Meter where; (1) the flow possesses a laminar potential core throughout the venturi throat section in all operational conditions; (b) the pressure at the venturi throat is determined from the upstream pressure and the local flow velocity; (c) the detection of cavitation and the measurement of the flow velocity are carried out optically by means of a Laser Doppler Velocimeter; (d) a custom-made electronic Signal Processor incorporating a frequency counter is used for real time data generation and temporary storage; (e) a computerized system performs the final acquisition and reduction of the data

An acoustic water tank disdrometer

Microwave engineers and geomorphologists require rainfall data with a much greater temporal resolution and a better representation of the numbers of large raindrops than is available from current commercial instruments. This thesis describes the development of an acoustic instrument that determines rain parameters from the sound of raindrops falling into a tank of water. It is known as the acoustic water tank disdrometer (AWTD).There is a direct relationship between the kinetic energy of a raindrop and the acoustic energy generated upon impact. Rain kinetic energy flux density (KE) is estimated from measurements of the sound field in the tank and these have been compared to measurements from a co-sited commercial disdrometer.Furthermore, using an array of hydrophones it is possible to determine the drop size and impact position of each raindrop falling into the tank. Accumulating the information from many impacts allows a drop size distribution (DSD) to be calculated.Eight months of data have been collected in the eastern UK. The two methods of parameter estimation are developed and analysed to show that the acoustic instrument can produce rain KE measurements with a one-second integration times and DSDs with accurate large drop-size tails