Viscoelastic wave propagation along a borehole using squirt flow and Biot poroelastic theory

Observations of seismic waves provide valuable understanding of Earth subsurface properties. These measurements are used to study large-scale subsurface features, kilometers in width, borehole-scale situations, meters of interest, and with core samples, a few centimeters in length. A common practice is to assume that the elastic rock-properties (P- and S-wave velocities) are the same for all frequencies. This is why sonic logs without corrections, for example, are used to constrain velocity models that transform seismic data from time to depth and to calibrate rock physics models used in seismic inversion to link elastic properties to reservoir properties. However, when seismic waves propagate in Earth materials, they are subject to different dispersion mechanisms, which makes the velocities frequency dependent. Understanding these effects on acoustic wave propagation can improve our models that constrain the subsurface and ultimately give us better hydrocarbon predictability. The main objective of this dissertation is to contribute to the understanding of how fluid in the pore space affects acoustic wave propagation. To achieve this goal, I first developed a frequency-dependent wave equation that accounts for local (squirt) and global (Biot) flow. The new model is tested against other squirt-Biot flow theories for both synthetic cases and utrasonic velocity data. I find the developed model to be consistent with the compared models in the synthetic cases. For the utrasonic velocity data, I find predictions from the new model to be closest to the measured data. In the second part of the dissertation, I use the developed squirt-Biot flow wave equation to simulate wave propagation in fluid-filled boreholes containing formations with different quantities of compliant pores. These are compared with formations where no compliant pores are present. I use the discrete wavenumber summation method with both a monopole and a dipole source to generate the wave fields. I find that fluid-saturated compliant pores can significantly affect the effective formation P- and S-wave velocities. This in turn affects the various acoustic wave modes causing increasing dispersion and attenuation. Thus, knowledge of the micro-scale structure of the fluid-saturated rock is of importance for understanding the acoustic waveforms and the dispersive behavior of the various modes. Depending on the locations where the critical frequencies for the different dispersion mechanisms occurs, acoustic velocity estimates can differ from the seismic-frequency velocities. Having a frequency dependent model accounting for the various dispersion mechanisms can help better connect the various velocity measurements and ultimately serve to give us an even more realistic picture of the subsurface.Geological Science

Pseudomonas aeruginosa defense systems against microbicidal oxidants

Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/139121/1/mmi13768_am.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139121/2/mmi13768-sup-0004-suppinfo4.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139121/3/mmi13768.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/139121/4/mmi13768-sup-0001-suppinfo1.pd

Spatial coherence in human tissue: implications for imaging and measurement

The spatial coherence properties of the signal backscattered by human tissue and measured by an ultrasound transducer array are investigated. Fourier acoustics are used to describe the propagation of ultrasound through a model of tissue that includes reverberation and random scatterering in the imaging plane. The theoretical development describes how the near-field tissue layer, transducer aperture properties, and reflectivity function at the focus reduce the spatial coherence of the imaging wave measured at the transducer surface. Simulations are used to propagate the acoustic field through a histologically characterized sample of the human abdomen and to validate the theoretical predictions. In vivo measurements performed with a diagnostic ultrasound scanner demonstrate that simulations and theory closely match the measured spatial coherence characteristics in the human body across the transducer array’s entire spatial extent. The theoretical framework and simulations are then used to describe the physics of spatial coherence imaging, a type of ultrasound imaging that measures coherence properties instead of echo brightness. The same echo data from an F/2 transducer was used to generate B-mode and short lag spatial coherence images. For an anechoic lesion at the focus the contrast to noise ratio is 1.21 for conventional B-mode imaging and 1.95 for spatial coherence imaging. It is shown that the contrast in spatial coherence imaging depends on the properties of the near-field tissue layer and the backscattering function in the focal plane

Sound Speed Estimation for Distributed Aberration Correction in Laterally Varying Media

Spatial variation in sound speed causes aberration in medical ultrasound imaging. Although our previous work has examined aberration correction in the presence of a spatially varying sound speed, practical implementations were limited to layered media due to the sound speed estimation process involved. Unfortunately, most models of layered media do not capture the lateral variations in sound speed that have the greatest aberrative effect on the image. Building upon a Fourier split-step migration technique from geophysics, this work introduces an iterative sound speed estimation and distributed aberration correction technique that can model and correct for aberrations resulting from laterally varying media. We first characterize our approach in simulations where the scattering in the media is known a-priori. Phantom and in-vivo experiments further demonstrate the capabilities of the iterative correction technique. As a result of the iterative correction scheme, point target resolution improves by up to a factor of 4 and lesion contrast improves by up to 10.0 dB in the phantom experiments presented

Hybrid Group IV Nanophotonic Structures Incorporating Diamond Silicon-Vacancy Color Centers

We demonstrate a new approach for engineering group IV semiconductor-based quantum photonic structures containing negatively charged silicon-vacancy (SiV$^-$) color centers in diamond as quantum emitters. Hybrid SiC/diamond structures are realized by combining the growth of nanoand micro-diamonds on silicon carbide (3C or 4H polytype) substrates, with the subsequent use of these diamond crystals as a hard mask for pattern transfer. SiV$^-$ color centers are incorporated in diamond during its synthesis from molecular diamond seeds (diamondoids), with no need for ionimplantation or annealing. We show that the same growth technique can be used to grow a diamond layer controllably doped with SiV$^-$ on top of a high purity bulk diamond, in which we subsequently fabricate nanopillar arrays containing high quality SiV$^-$ centers. Scanning confocal photoluminescence measurements reveal optically active SiV$^-$ lines both at room temperature and low temperature (5 K) from all fabricated structures, and, in particular, very narrow linewidths and small inhomogeneous broadening of SiV$^-$ lines from all-diamond nano-pillar arrays, which is a critical requirement for quantum computation. At low temperatures (5 K) we observe in these structures the signature typical of SiV$^-$ centers in bulk diamond, consistent with a double lambda. These results indicate that high quality color centers can be incorporated into nanophotonic structures synthetically with properties equivalent to those in bulk diamond, thereby opening opportunities for applications in classical and quantum information processing

Spatially-resolved electronic and vibronic properties of single diamondoid molecules

Diamondoids are a unique form of carbon nanostructure best described as hydrogen-terminated diamond molecules. Their diamond-cage structures and tetrahedral sp3 hybrid bonding create new possibilities for tuning electronic band gaps, optical properties, thermal transport, and mechanical strength at the nanoscale. The recently-discovered higher diamondoids (each containing more than three diamond cells) have thus generated much excitement in regards to their potential versatility as nanoscale devices. Despite this excitement, however, very little is known about the properties of isolated diamondoids on metal surfaces, a very relevant system for molecular electronics. Here we report the first molecular scale study of individual tetramantane diamondoids on Au(111) using scanning tunneling microscopy and spectroscopy. We find that both the diamondoid electronic structure and electron-vibrational coupling exhibit unique spatial distributions characterized by pronounced line nodes across the molecular surfaces. Ab-initio pseudopotential density functional calculations reveal that the observed dominant electronic and vibronic properties of diamondoids are determined by surface hydrogen terminations, a feature having important implications for designing diamondoid-based molecular devices.Comment: 16 pages, 4 figures. to appear in Nature Material

Blocked Elements in 1-D and 2-D Arrays—Part I: Detection and Basic Compensation on Simulated and <italic>In Vivo</italic> Targets

During a transcostal ultrasound scan, ribs and other highly attenuating and/or reflective tissue structures can block parts of the array. Blocked elements tend to limit the acoustic window and impede visualization of structures of interest. Here, we demonstrate a method to detect blocked elements and we measure the loss of image quality they introduce in simulation and in vivo. We utilize a fullwave simulation tool and a clinical ultrasound scanner to obtain element signals from fully sampled matrix arrays during simulated and in vivo transcostal liver scans, respectively. The elements that were blocked by a rib showed lower average signal amplitude and lower average nearest-neighbor cross correlation than the elements in the remainder of the 2-D aperture. The growing receive-aperture B-mode images created from the element data indicate that the signals on blocked elements are dominated by noise and that turning them OFF has a potential to improve visibility of liver vasculature. Adding blocked elements to the growing receive apertures for five in vivo transcostal acquisitions resulted in average decrease in vessel contrast and contrast to noise ratio of 19% and 10%, respectively

Laser-induced fluorescence of free diamondoid molecules

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.We observe the fluorescence of pristine diamondoids in the gas phase, excited using narrow band ultraviolet laser light. The emission spectra show well- defined features, which can be attributed to transitions from the excited electronic state into different vibrational modes of the electronic ground state. We assign the normal modes responsible for the vibrational bands, and determine the geometry of the excited states. Calculations indicate that for large diamondoids, the spectral bands do not result from progressions of single modes, but rather from combination bands composed of a large number of Delta v = 1 transitions. The vibrational modes determining the spectral envelope can mainly be assigned to wagging and twisting modes of the surface atoms. We conclude that our theoretical approach accurately describes the photophysics in diamondoids and possibly other hydrocarbons in general.DFG, FOR 1282, Controlling the electronic structure of semiconductor nanoparticles by doping and hybrid formatio

Photocathode device using diamondoid and cesium bromide films

A photocathode structure is presented that shows promise for use in high brightness electron sources. The structure consists of a metal substrate, a monolayer of a diamondoid derivative, and a thin film of cesium bromide. Diamondoid monolayers reduce the energy spread of electron emitters, while cesium bromide increases the yield and stability of cathodes. We demonstrate that the combined structure retains these properties, producing an emitter with lower energy spread than the corresponding cesium bromide emitter (1.06?eV versus 1.45?eV) and higher yield and stability than un-coated diamondoid emitters