304 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
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
Numerical simulation of slider air bearings in head-disk interface system based on a mesh-free method
No full textIn this thesis, a mesh-free methodology is developed for modeling and simulating air bearing sliders in hard disk drives. The developed CMMS Air Bearing Design Program implements mesh-free method to simulate complex air bearing sliders with shaped rails, multiple etch depths and highly recessed regions.DOCTOR OF PHILOSOPHY (MAE
Design of battery-free real-time pulse detection system based on near field communication
No full textWith great flexibility, high sensitivity, and low power consumption properties, wearable sensors can be widely used in many aspects, such as intelligent robots, human physiological signal detections, etc. Apart from fabrication and performance testing of wearable sensors, building efficient and reliable electronic system for sensors to broaden their application areas is also very important. With characteristics of extremely low power consumption, simple energy harvesting process and quickly contactless data acquisition, near-field communication (NFC) technology can be applied with specific wearable sensors to create miniaturized battery-free wireless devices and systems for signal detection and sample collection. This dissertation focuses on the application of a new type wearable pressure sensor in human healthcare and designs the related wearable devices based on NFC technology to detect the variation of human pulse. Firstly, previous work related to the design and application of wearable devices in human health monitoring and detection will be reviewed. Secondly, some critical design methods and procedures of the human pulse detection system will be described and shown thoroughly, including the fabrication of wearable pressure sensors, the layout of functional components in the detection system based on NFC technology, the pulse data acquisition and display facilitated by a demo app written by using Android Studio. According to the finished real-time pulse detection system, data acquisition process can be displayed by means of plotting in the NFC-enabled smartphone. Finally, the detected pulse information will be analyzed and some conclusions related to the detection system will be drawn. Future work of the detection system will be given in the end.Master of Science (Smart Product Design
Study on Performances of Two New Types of Gasoline Detergents and Their Effects on Fuel Properties
No full textEffects of Di-(2-ethoxyethyl) Carbonate as an Oxygenated Fuel on Diesel Fuel Properties and Engine Performances
No full textP-SVC-01 AN EFFICIENT ACTIVE VIBRATION SUPPRESSION MODEL FOR HDD TRACK SERVO(Servo Control,Technical Program of Poster Session)
No full textTemperature control using a microcontroller
No full textFor over 40 years, oven industries have specialized in the design and manufacture of custom temperature controllers, electronic control systems and temperature sensors. Temperature control has become a relatively mature technology. This project aimed to develop a temperature control system which will be used to control the temperature inside a small oven case for some special applications based on microchip PIC microcontroller. Through reviewing past and current temperature control studies, the author finally established a control system with heating and cooling functions according to the ambient temperature it sensed. In this report, the design process of the temperature control system and how it works to achieve the goal of temperature controlling are elaborated.Bachelor of Engineerin
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Constraining East Asia tectonics from full waveform seismic tomography
No full textEast Asia is a unique place to study plate tectonics due to its complicated tectonic history. In this dissertation, I use full waveform seismic tomography to study the seismic structure of the crust and mantle beneath East Asia. The high-resolution 3-D seismic model provides new insights into tectonic processes including oceanic subduction, continental collision, lithospheric evolution, and intraplate volcanism. The model reveals narrow and strong linear fast anomalies associated with subduction zones in the west Pacific and Southeast Asia. These subducting slabs exhibit varying depth extents and geometries. Notably, different mantle structures beneath the India-Asia continental collision zone are observed, compared to oceanic slabs. My analysis suggests that cratonic Indian lithosphere collided with Eurasian plate around 25 Ma, subsequently northward underthrusting beneath Tibet horizontally. Consequently, rheologically weaker and less buoyant mantle lithosphere beneath north-central Tibet has undergone thickening, destabilization, and delamination. The model further reveals isolated lithospheric fragments within the mantle transition zone and lower mantle beneath the collision zone. These observations suggest that for non-cratonic continental collision, discontinuous blobs of lithosphere sinking into the deeper mantle is a common phenomenon, as opposed to the classical oceanic subduction. My results emphasize the important role of rheological properties of continental lithospheres in shaping the dynamics of continental collision. Furthermore, my model shows extensive slow anomalies beneath Southeast Asia, with their complex geometry challenging a simple plume interpretation. Instead, these anomalies are more likely linked to deep subduction-induced mantle upwelling, as subducted slabs are imaged in the mantle transition zone within this region. I next use full waveform inversion to investigate 3-D seismic radial and azimuthal anisotropy structures beneath East Asia. The model shows a decrease in the amplitude of anisotropy with increasing depth, indicative of a transition in the deformation mechanism from dislocation creep in the shallow upper mantle to diffusion creep in the deeper upper mantle. My model also suggests subduction-induced rotational corner flows and trench-parallel sub-slab flows around the Ryukyu slabs. Moreover, a significant change in the direction of azimuthal anisotropy beneath the Japan subduction zone is observed around 400 km depth. Finally, I discuss possible future research opportunities to improve the current seismic model.Earth and Planetary Science
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