Browsing by Subject "Gravity"
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Item Characterization of the Southern High Plains by seismic, gravity, and topographic analysis(2010-12) Hoemberg, Jeffrey; Gurrola, Harold; Zhou, Hua-wei; Leverington, David; Asquith, George B.A topographic database was downloaded from the USGS and applied to several wavelength filtering processes. Seismic data were collected over a period of nine years at seven seismic stations deployed by Texas Tech University across the Southern High Plains, and all available additional data were downloaded from an addition 12 stations in the US Array. This dataset was processed by deconvolution with a subsequent application of an array processing algorithm and then stacked. The processed topographic and seismic data were analyzed in conjunction with gravity data acquired by Ozyavas to observe the presence of shallow crustal structures, midcrustal discontinuities, and the Moho in an attempt to relate these to subtle modern topographic relief across the southern High Plains. Stacks produced from the receiver functions identified five distinct boundaries at: 10km, 18km, Moho, 85km, and 170km. Each boundary was cataloged and mapped spatially. A hinge line trending southwest-northeast observed in the 18km discontinuity lies collinear and appears to have influence over the expression of the Caprock Escarpment. Similarly, an observed ridge in the Moho topography lies collinear to the Matador Arch and may implicate a trend of crustal weakness. The deeper discontinuities at 85km and 170km have structures that mimic features observed in the filtered gravity datasets. Gravity profiles were then reconstructed from Ozyavas, 2004, along three transects of the South Plains and resulting subsurface structure to 200 kilometers was approximated using the discontinuities from seismic as tie points through each line. These cross sections were useful for verification that the inferred layers from the discontinuities were valid results based on observed gravity data.Item Cosmology and gravity in the brane world(Texas A&M University, 2005-11-01) Dent, James BlackmanThe cosmology in the Hubble expansion era of the Horava-Witten M-theory compactified on a Calabi-Yau threefold is studied in the reduction to five-dimensions where the effects of the Calabi-Yau manifold are summarized by the volume modulus, and all perturbative potentials are included. Matter on the branes are treated as first order perturbations of the static vacuum solution, and all equations in the bulk and all boundary conditions on both end branes are imposed. It is found that for a static volume modulus and a static fifth dimension, y, one can recover the four dimensional Robertson-Friedmann-Walker cosmology for relativistic matter on the branes, but not for non-relativistic matter. For relativistic matter, the Hubble parameter H becomes independent of y to first order in matter density, and if a consistent solution for nonrelativistic matter exists it would require H to be y dependent. These results hold also when an arbitrary number of 5-branes are included in the bulk. The five dimensional Horava-Witten model is compared with the Randall Sundrum phenomenology with a scalar field in the bulk where a bulk and brane potential are used so that the vacuum solutions can be rigorously obtained.(In the Appendix, the difficulty of obtaining approximate vacuum solutions for other potentials is discussed.) In this case nonrelativistic matter is accommodated by allowing the distance between the branes to vary. It is suggested that non-perturbative potentials for the vacuum solution of Horava-Witten theory are needed to remove the inconsistency that non-relativistic matter creates. Also considered is the problem of gravitational forces between point particles on the branes in a Randall-Sundrum (R-S) two brane model with S1/Z2 symmetry. Matter is assumed to produce a perturbation to the R-S vacuum metric and all the 5D Einstein equations are solved to linearized order (for arbitrary matter on both branes). We show that while the gauge condition hi5 = 0, i = 0, 1, 2, 3 can always be achieved without brane bending, the condition h55 = 0 leads to large brane bending. The static potential arising from the zero modes and the corrections due to the Kaluza-Klein (KK) modes are calculated. Gravitational forces on the Planck (y1 = 0) brane recover Newtonian physics with small KK corrections (in accord with other work). However, forces on the TeV (y2) brane due to particles on that brane are strongly distorted by large R-S exponentials, making the model in disagreement with experiment if the TeV brane is the physical brane.Item Dynamical refinement in loop quantum gravity(2015-08) Hassan, Syed Asif; Matzner, Richard A. (Richard Alfred), 1942-; Dicus, Duane A; Freed, Daniel S; Morrison, Philip J; Weinberg, StevenIn Loop Quantum Gravity, a quantum state of the gravitational field has a semiclassical interpretation as a three-dimensional lattice discretization of space. We explore the possibility that the scale of the lattice is only as fine as it needs to be in order to carry the dominant frequency excitations of the auxiliary fields living on the lattice, by considering graph-changing transition amplitudes in the context of a pure gravity quantum theory. We define regular graphs that correspond to closed spatial slices of FLRW spacetime in a novel way, with coherent state labels that correspond to physical observables. This correspondence is obtained using the novel concept of a pseudoregular polyhedron which affords a dimensionless volume to surface area ratio in terms of the number of faces of the polyhedron. We normalize these regular graph states using a new method, employing a saddle point approximation based on the valence of the nodes rather than the large-scale semiclassical limit to obtain a result that holds in the quantum limit. Finally we employ the EPRL spin foam model to obtain a transition amplitude between single-node graphs of arbitrary valence that is valid in both the semiclassical and quantum regimes, using an improved method of normalizing the amplitude. We find that if we fix the scale factor and the fiducial volume of space the amplitude favors final states with infinitely large valence.Item An ensemble solution for the Earth's time-varying gravitational field from the NASA/DLR GRACE mission(2013-08) Sakumura, Carly Frances; Bettadpur, Srinivas Viswanath, 1963-Several groups produce estimates of the Earth's time-varying gravitational field with data provided by the NASA/DLR Gravity Recovery and Climate Experiment (GRACE) mission. These unprecedented highly accurate global data sets track the time-variable transport of mass across and underneath the surface of the Earth and give insight into secular, seasonal, and sub seasonal variations in the global water supply. Knowledge gained from these products can inform and be incorporated into ocean and hydrological models and advise environmental policy planning. Therefore, a complete understanding of the accuracy and variations between these different fields is necessary, and the most accurate possible solutions desired. While the various gravity fields are similar, differences in processing strategies and tuning parameters result in solutions with regionally specific variations and error patterns. This study analyzed the spatial, temporal, and spectral variations between four different gravity field products. The knowledge gained in this analysis was used to develop an ensemble solution that harnesses the best characteristics of each individual field to create an optimal model. Multiple methods were used to combine and analyze the individual and ensemble solutions. First a simple mean model was created; then the different solutions were weighted based on the formal error estimates as well as the monthly deviation from the arithmetic mean ensemble. These ensemble models as well as the four individual data center solutions were analyzed for bias, long term trend, and regional variations between the solutions, evaluated statistically to assess the noise and scatter within the solutions, and compared to independent hydrological models. Therefore, the form and cause of the deviations between the models, as well as the impact of these variations, is characterized. The three ensemble solutions constructed in this analysis were all effective at reducing noise in the models and better correlate to hydrological processes than any individual solution. However, the scale of these improvements is constrained by the relative variation between the individual solutions as the deviation of these individual data products from the hydrological model output is much larger than the variations between the individual and ensemble solutions.Item Essays on international trade(2010-05) French, Scott Thomas; Corbae, Dean; Abrevaya, Jason; Freitas, Kripa; Ramondo, Natalia; Ruhl, KimThis dissertation consists of three essays pertaining to the causes of the levels and composition of the international trade flows of nations, and the consequential implications for the levels of per capita income and welfare of their populations. The first of these documents a pattern of comparative advantage in product level, bilateral trade data that conventional quantitative trade models have difficulty explaining. It goes on to develop a theory of product level productivity differences based on endogenous differences in the allocation of research and development into product and process innovation across countries over time, and it shows that, when fitted to cross-country manufacturing wage data, the predicted product level technology distribution is consistent with the observed trade pattern. The second essay shows that the distribution of technology levels inferred in the first essay can help explain the inability of both ad-hoc and theoretically based gravity models of trade to account for the observed positive correlation between the percentage of manufacturing output that is traded and countries' per capita income. It derives a modified gravity equation based on a Ricardian model of trade with deterministic product level technology differences across countries. It then uses estimates from a product level gravity estimation to compute the component of this equation that differs from a conventional gravity equation in order to determine the extent to which the observed concentration of comparative advantage in a common set of products for low-income countries explains the small percentage of their output that is exported. The final essay shows that a simple model of firm profit maximization in the presence of sunk costs of entering the export market is broadly consistent with the observed persistence of exporting behavior in firm level data. It uses this simple model and moments from data on US manufacturing firms to estimate the value of the sunk export entry costs faced by these firms using an indirect inference strategy. These costs are shown to be substantial relative the revenue stream of a typical firm.Item A generalized flow rate model for primary production and an analysis of gravity drainage through numerical simulation(2014-08) Vitter, Cameron Artigues; Balhoff, Matthew T.; Lake, Larry W.The age of “easy” oil has steadily declined through the years as many conventional land-based fields have been depleted to residual levels. Novel technologies, however, have reawakened old fields, allowing incremental oil to be added to their recoverable oil in place (ROIP). Underground Gravity Drainage (UGD), an example of one of these technologies, combines improved horizontal and deviated drilling technologies with the longstanding concept of gravity drainage. In this work, a better understanding of gravity drainage has been gained through (1) development of a numerical, three-dimensional, three-phase reservoir simulator (UT-EMPRES), (2) development of a universal, semi-empirical model of production rates through primary depletion, and (3) analysis of the important aspects of gravity drainage through simulation. UT-EMPRES is a new three-phase, finite-difference reservoir simulator, which utilizes a simple, easy-to-use Microsoft Excel interface to access MATLAB-programmed simulation code. This simulator produces nearly identical results to other well-established simulators, including UTCHEM and CMG. UT-EMPRES has some unique features, allows for easy post-processing in MATLAB, and has been utilized extensively in the other two areas of this thesis. The generalized flow rate model (GFRM) is a semi-empirical equation that is used to forecast the dynamic primary production rate of a reservoir with an arbitrary number of wells all operating at the same constant pressure condition. The model is an extension of the classic tank model, which is inherently a single flowing phase development. With the ability to make a priori predictions of production figures, users can screen various prospect assets on the basis of economic potential through optimization routines on the GFRM. Gravity drainage and its approximation through numerical simulation are analyzed. A sensitivity study was conducted on three-phase gravity drainage, leading to the conclusion that small changes in vertical permeability and portions of the relative permeability-saturation relationships can greatly affect production rates. Finally, two-phase (oil and air) and regions of three-phase (water, oil, air) flow simulations were found to exhibit exponential decline in phase production rates, which may enable the GFRM to be applicable to UGD-type processes.Item Improving the observation of time-variable gravity using GRACE RL04 data(2010-12) Bonin, Jennifer Anne; Tapley, Byron D.The Gravity Recovery and Climate Experiment (GRACE) project has two primary goals: to determine the Earth’s mean gravitational field over the lifetime of the mission and to observe the time-variable nature of the gravitational field. The Center for Space Research's (CSR) Release 4 (RL04) GRACE solutions are currently created via a least-squares process that assimilates data collected over a month using a simple boxcar window and determines a spherical harmonic representation of the monthly gravitational field. The nature of this technique obscures the time-variable gravity field on time scales shorter than one month and spatial scales shorter than a few hundred kilometers. A computational algorithm is developed here that allows increased temporal resolution of the GRACE gravity information, thus allowing the Earth's time-variable gravity to be more clearly observed. The primary technique used is a sliding-window algorithm attached to a weighted version of batch least squares estimation. A number of different temporal windowing functions are evaluated. Their results are investigated via both spectral and spatial analyses, and globally as well as in localized regions. In addition to being compared to each other, the solutions are also compared to external models and data sets, as well as to other high-frequency GRACE solutions made outside CSR. The results demonstrate that a GRACE solution made from at least eight days of data will provide a well-conditioned solution. A series of solutions made with windows of at least that length is capable of observing the expected near-annual signal. The results also indicate that the signals at frequencies greater than 3 cycles/year are often smaller than the GRACE errors, making detection unreliable. Altering the windowing technique does not noticeably improve the resolution, since the spectra of the expected errors and the expected non-annual signals are very similar, leading any window to affect them in the same manner.Item Interpreting the Earth's time varying geopotential as observed from space and comparisons to global models of hydrologic transport(2004) Thompson, Paul Frank; Tapley, Byron D.Measurements of temporal changes in Earthís gravitational field were measured using six years of satellite laser ranging (SLR) to Lageos-1 and Lagoes-2 and the results were compared to geophysical models of mass variability for the atmosphere, ocean, and continental hydrology. Annual estimates of spherical harmonic gravity coefficients (degree and order four expansion) derived from the SLR observations when compared to combinations of the mass models had degree correlations that generally exceeded the 90% confidence limit and agreed to about the 1 mm level in terms of geoid height anomaly. The Gravity Recovery and Climate Experiment (GRACE) is measuring Earthís gravitational field approximately every month at spatial scales of a few hundred kilometers. In order to achieve smaller temporal and spatial scales, it is necessary to account for the effects of short period, non-tidal, mass variability which was not previously included in other gravity determinations. Orbital simulations of GRACE showed that the highest degrees were impacted the most by unmodeled variability in the atmosphere, oceans, and continental hydrology (a factor of ~20 increase in degree error in the case of the atmosphere). The use of approximate models gave the greatest reduction in aliasing error for the mid-degrees and higher; however, the lowest degrees (~2-5) were dominated by the sensitivity of the GRACE processing system to systematic error. GRACE data processing that used a combined atmosphere-ocean de-aliasing (AOD) model showed improvement in the gravity estimates consistent with the simulations: the shorter spatial wavelengths (higher degrees) were improved while the longest spatial wavelengths (particularly important for time-variable gravity studies) were relatively unaffected. Monthly gravity solutions from GRACE resolved features on the order of 2-3 mm geoid height anomaly when smoothed to 400-km spatial scales. Comparisons with the Global Land Data Assimilation System (GLDAS) terrestrial water storage model indicated a high degree of correlation up to spatial wavelengths of 600 km or larger; a significant improvement over the spatial and temporal scales obtained with SLR observations. However, temporal variability in the degree 2 coefficients, particularly the zonal, seemed to be better resolved by SLR observations than by GRACE observations.Item Investigation of the role of extracellular nucleotide gradients in plant gravity responses(2016-12) Cannon, Ashley Elisabeth; Roux, Stanley J.; Browning, Karen S; Huq, Enamul; Levin, Donald A; Mehdy, MonaExtracellular ATP (eATP) was first identified as a neurotransmitter in animal systems decades ago, but has only recently been classified as a signaling molecule in plants. Previous studies have shown that exogenously applied ATP can disrupt gravitropism in roots, depolarize root hairs, and alter auxin distribution. These results support a clear role for this molecule as a regulatory signal in plants. To further define eATP as a signal in plants, Ceratopteris spores, a model system, were used to study gravity-directed cell polarization. This polarization begins with the uptake of Ca2+ through channels at the bottom of the spore, a process required for the cell’s gravity response. Previous data showing that mechanosensitive channels can release ATP and that eATP can induce the opening of Ca2+ channels led to the hypothesis that eATP could play a role in the gravity-directed polarization. Data described in this dissertation show that an eATP gradient, with significantly higher [ATP] outside the bottom of the cell, is present during and promotes gravity-directed polarization. To explore the link between eATP and Ca2+ in gravity-directed polarization of spores, microparticle bombardment was used to transform Ceratopteris cells with a FRET-based Ca2+ sensor, Yellow Cameleon 3.60. The success of this effort has generated a uniquely valuable tool that can be used to analyze intracellular Ca2+ dynamics and rapidly screen transformants in Ceratopteris, a primitive plant system. In addition to studying the role of eATP signaling in the gravity response of single cells, an assessment of its role in the gravity response of a multicellular system, primary roots of Arabidopsis, was carried out. By using ecto-luciferase-expressing seedlings, a gradient of eATP, with the highest concentration being along the bottom of the root, was visualized within 30 min of gravistimulation. When this gradient was disrupted by excess ATP or an eATP receptor antagonist, the gravity response was attenuated. These results characterize the role of eATP gradients in the gravity responses of single spore cells of ferns and multicellular primary roots of a flowering plant. They suggest that the preferential accumulation of eATP along the bottom of gravity-responding cells is an evolutionarily conserved mechanism for promoting gravity-directed development.Item Preliminary design of spacecraft trajectories for missions to outer planets and small bodies(2015-08) Lantukh, Demyan Vasilyevich; Russell, Ryan Paul, 1976-; Fowler, Wallace; Bettadpur, Srinivas; Guo, Yanping; Broschart, StephenMultiple gravity assist (MGA) spacecraft trajectories can be difficult to find, an intractable problem to solve completely. However, these trajectories have enormous benefits for missions to challenging destinations such as outer planets and primitive bodies. Techniques are presented to aid in solving this problem with a global search tool and additional investigation into one particular proximity operations option is discussed. Explore is a global grid-search MGA trajectory pathsolving tool. An efficient sequential tree search eliminates v∞ discontinuities and prunes trajectories. Performance indices may be applied to further prune the search, with multiple objectives handled by allowing these indices to change between trajectory segments and by pruning with a Pareto-optimality ranking. The MGA search is extended to include deep space maneuvers (DSM), v∞ leveraging transfers (VILT) and low-thrust (LT) transfers. In addition, rendezvous or nπ sequences can patch the transfers together, enabling automatic augmentation of the MGA sequence. Details of VILT segments and nπ sequences are presented: A boundaryvalue problem (BVP) VILT formulation using a one-dimensional root-solve enables inclusion of an efficient class of maneuvers with runtime comparable to solving ballistic transfers. Importantly, the BVP VILT also allows the calculation of velocity-aligned apsidal maneuvers (VAM), including inter-body transfers and orbit insertion maneuvers. A method for automated inclusion of nπ transfers such as resonant returns and back-flip trajectories is introduced: a BVP is posed on the v∞ sphere and solved with one or more nπ transfers – which may additionally fulfill specified science objectives. The nπ sequence BVP is implemented within the broader search, combining nπ and other transfers in the same trajectory. To aid proximity operations around small bodies, analytical methods are used to investigate stability regions in the presence of significant solar radiation pressure (SRP) and body oblateness perturbations. The interactions of these perturbations allow for heliotropic orbits, a stable family of low-altitude orbits investigated in detail. A novel constrained double-averaging technique analytically determines inclined heliotropic orbits. This type of knowledge is uniquely valuable for small body missions where SRP and irregular body shape are very important and where target selection is often a part of the mission design.Item Simulation study of surfactant transport mechanisms in naturally fractured reservoirs(2010-08) Abbasi Asl, Yousef; Pope, Gary A.; Mohanty, Kishore K.Surfactants both change the wettability and lower the interfacial tension by various degrees depending on the type of surfactant and how it interacts with the specific oil. Ultra low IFT means almost zero capillary pressure, which in turn indicates little oil should be produced from capillary imbibition when the surfactant reduces the IFT in naturally fractured oil reservoirs that are mixed-wet or oil-wet. What is the transport mechanism for the surfactant to get far into the matrix and how does it scale? Molecular diffusion and capillary pressure are much too slow to explain the experimental data. Recent dynamic laboratory data suggest that the process is faster when a pressure gradient is applied compared to static tests. A mechanistic chemical compositional simulator was used to study the effect of pressure gradient on chemical oil recovery from naturally fractured oil reservoirs for several different chemical processes (polymer, surfactant, surfactant-polymer, alkali-surfactant-polymer flooding). The fractures were simulated explicitly by using small gridblocks with fracture properties. Both homogeneous and heterogeneous matrix blocks were simulated. Microemulsion phase behavior and related chemistry and physics were modeled in a manner similar to single porosity reservoirs. The simulations indicate that even very small pressure gradients (transverse to the flow in the fractures) are highly significant in terms of the chemical transport into the matrix and that increasing the injected fluid viscosity greatly improves the oil recovery. Field scale simulations show that the transverse pressure gradients promote transport of the surfactant into the matrix at a feasible rate even when there is a high contrast between the permeability of the fractures and the matrix. These simulations indicate that injecting a chemical solution that is viscous (because of polymer or foam or microemulsion) and lowers the IFT as well as alters the wettability from mixed-wet to water-wet, produces more oil and produces it faster than static chemical processes. These findings have significant implications for enhanced oil recovery from naturally fractured oil reservoirs and how these processes should be optimized and scaled up from the laboratory to the field.Item Static and normal mode computer analysis of solar bowl receiver support systems(Texas Tech University, 1985-12) Lao, Hei-kuanNot availableItem The gravitational attraction of certain two-dimensional bodies(Texas Tech University, 1957-08) Shurbet, Gerald LynnNot availableItem Toward a theory of observation(2014-08) Carney, Daniel Joseph, Jr.; Fischler, Willy; Paban, SoniaQuantum mechanics is usually formulated in terms of a single Hilbert space and observables are defined as operators on this space. Attempts to describe entire spacetimes and their resident matter in this way often encounter paradoxes. For example, it has been argued that an observer falling into a black hole may be able to witness deviations from unitary, violations of semi-classical quantum field theory, and the like. This thesis argues that the essential problem is the insistence on the use of a single, global Hilbert space, because in general it may be that a physical observer cannot causally probe all of the information described by this space due to the presence of horizons. Instead, one could try to define unitary quantum physics directly in terms of the information causally accessible to particular observers. This thesis makes steps toward a systematization of this idea. Given an observer on a timelike worldline, I construct coordinates which (in good cases) cover precisely the set of events to which she can send and then receive a signal. These coordinates have spatial sections parametrized by her proper time, and the metric manifestly encodes the equivalence principle in the sense that it is flat along her worldline. To describe the quantum theory of fields according to these observers, I define Hilbert spaces in terms of field configurations on these spatial sections and show how to implement unitary time-evolution along proper time. I explain how to compare the observations of a pair of observers, and how to obtain the description according to some particular observer given some a priori global description. In this sense, the program outlined here constructs a manifestly unitary description of the events which the observer can causally probe. I give a number of explicit examples of the coordinates, and show how the quantum theory works for a uniformly accelerated observer in flat spacetime and for an inertial (co-moving) observer in an inflating universe.