470 research outputs found
Crustal wave speed structure of North Texas and Oklahoma based on ambient noise cross-correlation functions and adjoint tomography
No full textFull text access from Treasures at UT Dallas is restricted to current UTD affiliates (use the provided Link to Article). Non UTD affiliates will find the web address for this item by clicking the "Show full item record" link, copying the "dc.relation.uri" metadata and pasting it into a browser.Recently, seismologists observed increasing seismicity in NorthTexas and Oklahoma. Based on seismic observations and other geophysical measurements, numerous studies have suggested links between the increasing seismicity andwastewater injection during unconventional oil and gas exploration. To better monitor seismic events and investigate their triggering mechanisms, we need an accurate 3-D crustalwave speed model for the study region. Considering the uneven distribution of earthquakes in this area, seismic tomography with local earthquake records has difficulties achieving even illumination. To overcome this limitation, in this study, ambient noise cross-correlation functions are used to constrain subsurface variations in wave speeds. I use adjoint tomography to iteratively fit frequency-dependent phase differences between observed and predicted band-limited Green's functions. The spectral element method is used to numerically calculate the band-limited Green's functions and the adjoint method is used to calculate misfit gradients with respect to wave speeds. A total of 25 preconditioned conjugate gradient iterations is used to updatemodel parameters and minimize datamisfits. Features in the new crustal model TO25 correlate well with geological provinces in the study region, including the Llano uplift, the Anadarko basin, the Ouachita orogenic front, etc. In addition, there are relatively good correlations between seismic results with gravity and magnetic observations. This new crustal model can be used to better constrain earthquake source parameters in North Texas and Oklahoma, such as epicentre location as well as moment tensor solutions, which are important for investigating triggering mechanisms between these induced earthquakes and unconventional oil and gas exploration activities. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.School of Natural Sciences and Mathematic
In-situ TiC particle reinforced TiCuZrNi brazing alloy for joining C/C composites to Ti6Al4V
No full textChromium carbide modified C/C and Ti6Al4V were successfully joined using a
TiCuZrNi brazing alloy in powder form. The braze/composite interface and the
mechanical strength of C/C-Ti6Al4V joints were evaluated. The apparent shear
strength of chromium carbide modified C/C joined to Ti6Al4V, measured by single
lap test in compression, was 52 ± 6 MPa, which was highest among that
without chromium carbide modification (15 ± 2 MPa) and the intrinsic C/C shear
strength. The fractography of joints without chromium carbide modification
indicated that failure mainly occurred at the TiC layer formed at the composite/
braze interface while the joints with chromium carbide modification failed within
the C/C
Microstructure and mechanical properties of C/C composite/Ti6Al4V joints with aCu/TiCuZrNi composite brazing alloy
No full textC/C compositesandTi6Al4V sheetswerejoinedusinganovelCu/TiCuZrNicompositebrazeandtheeffectofCufoilthicknessonthe
microstructure andmechanicalpropertiesofthejointswereinvestigated.Acompositejoiningmaterial,consistingofaTiCparticlereinforced
brazing alloyformedinsituduringthebrazingprocesswasobtained.Beforethejoiningprocess,theC/Ccompositesurfacewasmodified by
solid statereactionwithchromiumtoformachromiumcarbidecoating.ThiscarbidereactionlayercanreactwiththeTi-basedbrazingalloyand
form evenlydistributedTiCparticles.Themaximumapparentshearstrengthofas-receivedjointwas3978.5 MPaobtainedbyusinga40 μm
thick CufoiltogetherwiththeTiCuZrNialloy;thisvaluewas70%higherthanthemechanicalstrengthobtainedforjointsbrazedwiththe
TiCuZrNi alloyonly
Elastic Wavefield Separation in Anisotropic Media Based on Eigenform Analysis and Its Application in Reverse-Time Migration
No full textSeparating compressional and shear wavefields is an important step in elastic reverse-time migration, which can remove wave-mode crosstalk artefacts and improve imaging quality. In vertical (VTI) and titled (TTI) transversely isotropic media, the state-of-the-art techniques for wavefield separation are based on either non-stationary filter or low-rank approximation. They both require intensive Fourier transforms for models with strong heterogeneity. Based on the eigenform analysis, we develop an efficient pseudo-Helmholtz decomposition method for the VTI and TTI media, which produces vector P and S wavefields with the same amplitudes, phases and physical units as the input elastic wavefields. Starting from the elastic VTI wave equations, we first derive the analytical eigenvalues and eigenvectors, then use the Taylor expansion to approximate the square-root term in the eigenvalues, and finally obtain a zero-order and a first-order pseudo-Helmholtz decomposition operator. Because the zero-order operator is the true solution for the case of ϵ = δ, it produces accurate wavefield separation results for elliptical anisotropic media. The first-order separation operator is more accurate for non-elliptical anisotropy. Since the proposed pseudo-Helmholtz decomposition requires solving an anisotropic Poisson's equation, we propose two fast numerical solvers. One is based on the sparse lower-upper (LU) factorization, which can be repeatedly applied to the input elastic wavefields once computing the lower and upper triangular matrices. The second solver assumes the model parameters are laterally homogeneous within a given migration aperture. This assumption allows us to efficiently solve the anisotropic Poisson's equation in the z k x domain, where k x and z denote the horizontal wavenumber and depth, respectively. Using the coordinate transform, we extend the pseudo-Helmholtz decomposition to the TTI media. The separated vector wavefields are used to produce PP and PS images by applying a dot-product imaging condition. Several numerical examples demonstrate the feasibility and applicability of the proposed methods. © The Author(s) 2019. Published by Oxford University Press on behalf of The Royal Astronomical Society.Natural Sciences and Mathematic
Time-domain least-squares migration using the Gaussian beam summation method
No full textArticle is freely available on publisher's website. Use the Link to ArticleWith a finite recording aperture, a limited source spectrum and unbalanced illumination, traditional imaging methods are insufficient to generate satisfactory depth profiles with high resolution and high amplitude fidelity. This is because traditional migration uses the adjoint operator of the forward modelling rather than the inverse operator.We propose a least-squares migration approach based on the time-domain Gaussian beam summation, which helps to balance subsurface illumination and improve image resolution. Based on the Born approximation for the isotropic acoustic wave equation, we derive a linear time-domain Gaussian beam modelling operator, which significantly reduces computational costs in comparison with the spectral method. Then, we formulate the corresponding adjoint Gaussian beam migration, as the gradient of an L2-norm waveform misfit function. An L1-norm regularization is introduced to the inversion to enhance the robustness of least-squares migration, and an approximated diagonal Hessian is used as a pre-conditioner to speed convergence. Synthetic and field data examples demonstrate that the proposed approach improves imaging resolution and amplitude fidelity in comparison with traditional Gaussian beam migration. © The Author(s) 2018. Published by Oxford University Press on behalf of The Royal Astronomical Society.School of Natural Sciences and Mathematic
Optimization of Metal Ratio and CO Desorption Behavior of Rh-Mn-Li/SiO2 Catalyst for CO Hydrogenation
No full textA highly efficient Rh-Mn-Li/SiO2 catalyst for CO hydrogenation to C-2 oxygenates was developed by the optimization of metal ratio, and the effects of Mn and Li loadings on the CO desorption behavior of the catalyst were studied by temperature-programmed desorption of adsorbed CO (CO-TPD) and temperature-programmed surface reaction (TPSR) of adsorbed CO with H-2. At 543-573 K, the Rh efficiency (E-Rh) reached the maximum when the Rh loading was 1.5%. The space time yield of C-2 oxygenates (STYC2-oxy), E-Rh, and the selectivity for C-2 oxygenates (SC2-oxy) were dramatically increased when a small amount of Mn was added to the Rh/SiO2 catalyst. STYC2-oxy and E-Rh reached the maximum when the Mn loading was 0.53%, while the SC2-oxy increased slowly with increasing Mn loading. SC2-oxy increased remarkably with increasing Li loading, while STYC2-oxy and E-Rh obviously decreased. The results of CO-TPD and TPSR showed that the addition of Mn to the Rh-based catalyst increased the number of active sites responsible for CO dissociation and their ability to dissociate CO and therefore increased the activity of the Rh-based catalyst. However, the number of active sites and their CO dissociation ability decreased with the addition of Li. On the other hand, the addition of Mn and Li increased the proportion of the active sites for nondissociative CO adsorption and therefore increased the SC2-oxy
Realtime correction of probe pulse in subpicosecond time-resolved transient absorption spectroscopy
No full textDue to its breakthrough the electro-optical time response limitation for picosecond dynamics of processes, many research groups around the world operating in different fields from physics and chemistry to biology, medicine and material science, utilize laser setups capable of providing subpicosecond pulses and femtosecond-level time delay lines. However the signal amount of Femto-picosecond Dynamics is about at an altitude of 1%, but the fluctuation of femtosecond probe pulse at its best can only reach about 5%. Real-time correct the probe pulse is the only effective way to realize subpicosecond time-resolved detection precision of transient absorption spectroscopy. So in this paper, reference pulse was drawn into the measurement equipment through different methods to dispel the fluctuation of probe pulse. Firstly, in the case of Subpicosecond time-resolved transient absorption spectroscopy, probe and reference beams have the same spectral distribution and derived from the same source to measure the variation of transmittance in the excited volume. Secondly probe pulse is spatially and temporally overlapped to the excitation pulse, reference pulse spatially overlapped but temporally anticipated in respect to the excitation pulse. Finally reference pulse passes through the sample in a different position. The detector can be a CCD camera or a double photodiode. The results shown that when reference pulse passes through the sample in a different position and detected by CCD, the correction results can reach to 1%. Which meet the femto-picosecond dynamics precise requirement
Study of miniaturized timing device
No full textOver decades, there are evolutions on miniaturized precision timing devices, not only to enhance their efficiency, but also to create them in a better aesthetic design. Engineers always seek ways to improve their capabilities and compatibilities as they are common electrical appliances that may be used in daily life. This project focuses only on one type of atomic clock which is the Chip-Scale Atomic Clock (SA.45s).
In this report, the author aims to study all the different aspects of the Chip-Scale Atomic Clock and comprehends the characteristics of each individual mode found in the Chip-Scale Atomic Clock. A literature review is given to provide readers with some background knowledge of the miniaturized precision timing devices in the market. Measurement procedures and equipment used for testing are also included in this report.
Findings and analysis are included and it is concluded that Chip-Scale Atomic Clock has better performance than the traditional quartz oscillators. Recommendations for future possible work and testing are included at the end of the report.Bachelor of Engineering (Mechanical Engineering
China's New Energy Revolution: How the World Super Power is Fostering Economic Development and Sustainable Growth through Thin-Film Solar Technology
No full textAl-based MOF derived self-assembled carbon nanosheets as innovative anodes for Li-and Na-ion batteries
Get PDFFunctional modification and structural design of carbon electrode materials are considered as a cost-effective method to improve their electrochemical performance. In this study, a solvothermal method is applied to realize self-assembly of the metal-organic framework. After simple carbonization and acid treatment, carbon nanosheets with 2D adjustable defective sub-units are successfully prepared for the first time. It is found that carbonization temperature has a significant effect on the carbon skeleton structure. The optimal nanostructures with large specific surface area and appropriate pore size distribution make self-assembled carbon nanosheets having excellent Li/Na- ion storage properties. In addition, the adjustable carbon skeleton structure can effectively avoid irreversible damage when charge-discharge cycles. For Li-ion batteries, a specific capacity of 825 mAh g−1 is achieved after 100 cycles at 0.1 C, while for Na-ion batteries a specific capacity of 193 mAh g−1 is observed after 100 cycles at 0.5 C. Moreover, for Na-ion batteries, even at a high rate of 5 C the material delivers a specific capacity of 109.5 mAh g−1 after 3500 cycles
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