Browsing by Subject "Geophysics"

Now showing 1 - 8 of 8
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    Coupling geothermal energy capture with carbon dioxide sequestration in naturally permeable, porous geologic formations -- a novel approach for expanding geothermal energy utilization.
    (2011-08) Randolph, Jimmy Bryan
    This thesis research presents a new method to harness geothermal energy by combining it with geologic carbon dioxide (CO2) sequestration. CO2 is injected into deep, naturally porous and permeable geologic formations. The geothermally heated CO2 is piped to the surface, used to produce electricity, and then returned to the subsurface. This new approach represents a radical shift in electric/heat power generation as it not only utilizes a renewable energy source but has a negative carbon footprint. This research explores the potential and applicability of the approach and related aspects of geologic fluid and heat flow.
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    Dislocations in Magnetite: Experimental Observations of their Structural, Magnetic, and Low-temperature Effects
    (2013-09) Lindquist, Anna K.
    Magnetite (Fe3O4) is the most important mineral to the rock magnetic and paleomagnetic communities and is ubiquitous in igneous, sedimentary, and metamorphic rocks. Larger multidomain (MD) magnetite grains are more common than single domain grains, so understanding how they record paleomagnetic fields would be a boon to paleomagnetists. MD magnetite grains are divided into multiple domains, regions with uniform magnetization, separated by domain walls. Domain walls sweep through magnetite grains easily, so slight changes in ambient magnetic fields can alter the magnetization of MD magnetite. Because of this, MD magnetite is not considered reliable for paleomagnetic studies, and the mechanisms by which MD grains may record past magnetic fields are not well understood. Dislocations, linear crystallographic defects, may increase magnetic coercivity by pinning domain walls in place. This study, for the first time, experimentally investigates this pinning behavior by using a transmission electron microscope (TEM) to simultaneously image magnetic domain walls, dislocations, and low-temperature twin structures. Magnetite grains were deformed in the dislocation glide regime, which is active in natural magnetite grains. Dislocations were not uniformly distributed throughout the sample, but regions with more and longer dislocations pinned domain walls more strongly. First-order reversal curve diagrams demonstrate the presence of regions with pinning strengths of over 125 mT. The strength of domain wall pinning at dislocations was found experimentally and theoretically to be proportional to dislocation length, with longer dislocations pinning more strongly. Average pinning fields were around 0.2 mT. Magnetite grains with more uniformly distributed dislocations would likely have coercivities that were high enough to enable MD magnetite to record geomagnetic fields over geologic timescales. Further, low-temperature TEM and magnetic studies demonstrated that dislocations can affect twin growth in magnetite below the Verwey transition. Deformed magnetite samples had more soft-shouldered Verwey transitions and were able to retain more remanence after low-temperature demagnetization (LTD). Therefore, MD magnetite grains may be able to retain relevant magnetizations, even after LTD. Dislocation length, density, and distribution are then all important considerations when investigating the ways in which MD magnetite may retain a stable record of paleomagnetic field characteristics, even after LTD.
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    High-rayleigh-number thermal convection of compressed gases in inclined rectangular enclosures of varied aspect ratios
    (2019-08) Madanan, Umesh
    In the present study, heat transfer measurements and flow visualization are carried out for high-Rayleigh-number thermal convection in horizontal and tilted rectangular enclosures. The heat transfer measurements are performed for a wide range of Rayleigh numbers (1.85×10^6 ≤ Ra ≤ 1.04×10^11) using enclosures of different aspect ratios (AR = 1, 3, 6, and 10) and angles of inclination (ϕ = 0°, 30°, 60°, 90°, 120°, and 150°). Here, high Rayleigh numbers are achieved using compressed nitrogen and argon. Another set of experiments is conducted using sidewalls made of three materials having different thermal conductivities (Styrofoam, Plexiglas, and high-density polyethylene) to assess the effect of sidewall conductance heat loss on the horizontal enclosure free convection. Additionally, a z-type shadowgraph visualization is employed at angles of inclination between 0° and 90° to characterize the buoyant flow and validate the observed heat transfer trends. Firstly, the effect of sidewall conductance heat loss on Nusselt number is examined by performing nearly identical sets of experiments using horizontal cubical enclosures with sidewalls made of three different materials. The results from these experiments reveal a higher difference (∆Nu) between the sidewall-uncorrected Nusselt number (Nu_net) and the sidewall-corrected Nusselt number (Nu_c) than that obtained when using a traditional empty-cell gradient assumption. Thus, a semi-analytical model is proposed to estimate the sidewall-corrected Nusselt number, given the corresponding uncorrected values, which is found to predict this experimentally observed difference in Nusselt numbers to within 11% (when Wn ≥ O(1)). Another empirical model is also proposed to estimate the sidewall-corrected Nusselt numbers for smaller wall numbers (or, Wn → 0) and the predicted ∆Nu values for this case are found to be within 1.5% of the corresponding experimental data. Additionally, the Nusselt numbers for an ideal zero-thermal-conductivity sidewalls case are also estimated by extrapolating the corresponding Nu_net values obtained from the experiments. Further experiments in the present study are conducted after taking into consideration this effect of the sidewall conductance heat loss and using a sidewall material of low wall number (Styrofoam). Another set of experiments is carried out to determine the correlating equations for Nusselt number, computed from steady-state electrical power input and temperature measurements, in terms of the studied variables for the horizontal and tilted enclosures. For the horizontal enclosure problem, this correlation is found to closely follow the classical 1/3^rd scaling relation between Nusselt number and Rayleigh number. For the tilted enclosure problem, a set of single-parameter (Nu = f(Ra)) and two-parameter (Nu = f(Ra,AR)) correlating equations are proposed to estimate the average Nusselt number at any of the investigated angles of inclination. The proposed correlations are found to predict the experimental values with reasonable accuracy. The effect of aspect ratio on Nusselt number is assessed by performing experiments with varied aspect ratios at a fixed Rayleigh number. For inclined enclosures, at any angle of inclination and a given Rayleigh number, Nusselt number is observed to follow a decreasing trend with an increase in the aspect ratio. Moreover, this decreasing trend is observed to gradually amplify as the angle of inclination is increased, with a negligible effect at an angle of inclination of 0° (or, for the horizontal enclosure problem) and a prominent effect at an angle of inclination of 90° (or, for the vertical enclosure problem). The effect of angle of inclination on Nusselt number is also examined by performing experiments with varied aspect ratios and Rayleigh numbers. Nusselt number is found to decrease with an increase in the angle of inclination and this decreasing trend remains qualitatively the same for all the studied aspect ratios and Rayleigh numbers. There is a substantial drop observed in the Nu values between the angles of inclination 0° and 90°, whereas, in general, this drop is found to be minimal between the angles of inclination 90° and 150°. For any given aspect ratio, this variation in the Nu values is observed to become more prominent as the Rayleigh number increases. The flow visualization studies for the horizontal enclosure problem indicate the presence of thermal plumes together with a large scale flow. These thermal plume eruptions are found to move across the central region (or, core) of the enclosure toward the opposing active wall. The frequency of the thermal plume eruptions and the velocity of the large scale flow are observed to increase with an increase in the Rayleigh number. For tilted enclosures, mixing within the core region is found to decrease as the angle of inclination is increased. Thus, for the vertical enclosure problem, the most prominent feature is an unperturbed core, with traveling wave-like structures over the boundary layers on the hot and cold vertical walls. In addition, the buoyant flow velocity is observed to decrease with an increase in the angle of inclination.
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    Magnetic and physical characteristics of magnetite associated with deformation and exsolution.
    (2011-10) Till, Jessica Lynn
    This thesis contains a collection of laboratory-based studies designed to characterize the magnetic properties and physical aspects of magnetite that result from deformation or high temperature growth. In Chapter 2, a detailed rock magnetic characterization of rocks containing nanoscale magnetite exsolved from volcanic glass identifies the location of domain-state thresholds through distinct transitions in remanence and susceptibility properties. This unique material is an excellent candidate for standard material to be used in studies of magnetite granulometry. In Chapter 3, theoretical timescales for the growth of sub-microscopic magnetite needles during exsolution from plagioclase are calculated using results of diffusion experiments. Measured diffusivities are modeled to calculate the amount of diffusion-limited growth possible under different conditions of nucleation temperature and cooling rate. In Chapters 4 and 5, the development and evolution of magnetic fabrics are investigated through deformation experiments on synthetic rock-analogues at high temperatures and ductile conditions. Stress-induced changes in rock magnetic properties after deformation are significant. Examination of deformation-induced remagnetization demonstrates that a primary remanence can survive conditions equivalent to moderate metamorphism in certain cases and that petrofabric can play an important role in determining the remanence stability. High-temperature deformation experiments result in a pattern of anisotropy development that indicates plastic deformation of magnetic grains, which is distinct from anisotropy development resulting from different magnetite strain responses. Experimental data are combined with theoretical magnetic anisotropy models and used to estimate effective magnetite strains and strain partitioning from magnetic fabric data in deformed samples. Finally, observations of strong shape-preferred orientation and deformation-induced microstructures in magnetite grains from high-temperature shear experiments indicate plastic deformation of magnetite. Microstructural observations place constraints on the rheological behavior of magnetite and the conditions in which dislocation creep is dominant. These observations prompt a re-examination of the previously established magnetite flow laws which are modified and used to construct new deformation mechanism maps.
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    Mantle dynamics, composition, and state in regions associated with active and ancient subduction.
    (2008-05) Courtier, Anna Mahr
    Abstract not available.
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    Seismic structure of the mantle beneath the Pacific Hemisphere
    (2011-06) Bagley, Brian C.
    Aside from xenoliths, the Earth's mantle is a region that is inaccessible directly, leaving us with limited tools to investigate its characteristics indirectly. Seismology is a tool well-suited for this purpose, and has provided valuable insight regarding many fundamental processes occurring within the mantle. It is fortuitous that the mantle is layered, and that these layers are often punctuated by distinct changes in density and/or velocity that are seismically detectable. By investigating the seismic structure of the mantle we are able to infer properties such as composition, temperature, anisotropy, and water content. Seismic tomography has informed our understanding of subduction and the fate of slabs, and we are beginning to realize that the lower mantle might also be rich with heterogeneity. Our picture of the Earth's mantle is becoming clearer, however, there is much that we do not understand. Receiver function studies of the oceans are fewer and suffer the common malady of looking beneath oceanic islands, not generic oceanic crust. Most of the detailed information regarding the seismic discontinuity structure of open ocean mantle comes from bottom-side reflections that are precursors to SS phases (a shear wave that has traveled from source to receiver with one bottom-side surface bounce in between). SS and PP (a compressional wave with a path analogous to SS) precursors offer extensive geographic coverage and good sensitivity to small velocity contrasts and reasonable localization. They do not perform well for shallow reflectors, or reflectors near the larger transition zone discontinuities. In our studies we use multiple ScS reverberations to gain better resolution of these features. The primary goals of this research are to study mantle discontinuities, and fill in some of the missing detail regarding mantle heterogeneity. We do this by examining the Pacific ocean, beginning with the open ocean mantle, then moving to the subduction zones in the west Pacific. This region, containing the Boso Triple Junction, is one of the most complex subduction zones on the planet. Finally we continue west beneath the Sea of Japan, the Sea of Okhotsk, and the northeast Chinese craton. The changes in mantle structure across the Pacific reveal many interesting differences between the open ocean mantle and the mantle in regions of subduction.
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    Stress-driven melt segregation and reactive melt in ltration in partially molten rocks deformed in torsion with applications to melt extraction from Earth's mantle.
    (2010-10) King, Daniel S. H.
    Melt extraction from Earth's upper mantle requires transport of magma from regions of partial melting at depth to the Earth's surface. During its ascent, melt interacts chemically and mechanically with the rock matrix. Melt reduces the viscosity of the partially molten rock compared to that of a melt-free rock. This weakening is a potential mechanism of strain localization that could have significant geodynamical implications. Magma interacts chemically with mineral phases during its ascent, dissolving phases in which it is undersaturated and precipitating phases in which it is oversaturated. Such melt-rock reaction can be a driving force for melt migration. Water and other volatiles also partition into the melt from minerals and are then expelled to Earth's oceans or atmosphere. This process leaves behind stronger dehydrated rocks, and it could be the mechanism by which the oceanic lithosphere (mechanical boundary layer) is formed. The work presented here is an experimental investigation of several mechanisms that influence the distribution of melt within a viscously deformable partially molten rock. Three mechanisms are considered, either alone or in various combinations. (1) An applied shear stress causes melt to align and segregate into melt-rich bands with a consistent geometrical relationship to the shear geometry. In Chapter 2, we investigate possible means of scaling the bands that form in experimental samples to Earth's mantle and explore the evolution of melt-rich bands at high shear strain. (2) Interfacial tension driven flow acts to homogenize the distribution of melt within a partially molten sample. In Chapter 3, we investigate the evolution of melt distribution during static annealing of a sample containing melt-rich bands. We compare the experimental results with models of interfacial tension driven flow to determine which mechanisms control the rate of melt redistribution. (3) A melt source that is undersaturated in some component, when coupled with a sink that is rich in that component, will infiltrate into the sink through reactive flow. This reactive flow can develop into an instability in which fingers of high melt fraction propagate into the sink. In Chapter 4 we investigate this process both under static conditions and in combination with stress-driven melt segregation.
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    Suitability of chondrules for studying the magnetic field of the early solar system: an examination of synthetically produced dusty olivine
    (2009-12) Hu, Yifan
    Chondritic meteorites are rare, yet incredibly valuable windows into the geophysical and geochemical environment of the early solar system. Dusty olivine grains containing exsolved nanometer scale iron nickel alloy inclusions are present in many chondritic meteorites and their remanent magnetization may give insight into the strength of the solar dynamo at the time of chondrule formation. Laboratory methods for determining the paleointensity of these rare materials must be optimized prior to conducting experiments on actual meteorite samples. To this end, we have used high temperature recrystallization techniques to produce synthetic dusty olivine samples with textures remarkably similar to those observed in chondritic meteorites. The olivine grains used in these annealing experiments are from the 13 kya Haleyjabunga picritic basalt flow in Iceland and have compositions of Fo90, which closely resembles the olivine composition observed in chondritic meteorites. Samples were annealed at 1350 ˚C, 1315 ˚C and 1425 ˚C either under vacuum in the presence of graphite or under controlled oxygen fugacity using pure CO gas. The laboratory produced magnetic mineral assemblages in 4 different types of samples as well as the starting material have been characterized using low and high-temperature magnetic measurements, hysteresis loops, FORC diagrams, and scanning electron microscopy. The room temperature remanence properties of these materials have been explored using stepwise IRM and ARM acquisition and alternating field demagnetization. These synthesis techniques allow us to produce a wide range of iron nickel grain sizes with correspondingly large variations in coercivity (between 0 and 500 mT). High-temperature measurements of saturation magnetization show that all the samples reach their Curie temperatures at ~760 ˚C, consistent with kamacite, a low Ni high Fe metal alloy. Multiple experiments have shown that care must be taken to rigorously control the atmosphere in which the samples are heated and cooled in order to avoid forming trace amounts of magnetite on the surface of the samples. Future research will explore the feasibility of using modified Thellier protocols or the Shaw method to determine the paleointensity of laboratory induced thermoremanent magnetizations.