Geology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2774
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Item USING BAYESIAN ELECTRICAL RESISTIVITY INVERSION TO REVEAL HILLSLOPE DRY-UP PROCESS IN A MEDITERRANEAN CLIMATE(2024) Shahid, Saffat; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Hydrologic dynamics in hillslopes is essential for comprehending the processes that shape landscape evolution and sustain the Earth’s critical zone. Electrical resistivity (ER) is considered as one of the best geophysical methods to observe these dynamics due to its sensitivity to subsurface water content. To understand hillslope water dynamics and mitigating the risks of slope instability caused by extreme weather events, we studied how subsurface hydrological processes are being influenced by variations in vegetation type across different aspects of hillslopes. Thus, how accurately ER can quantify the dry-up process during the growing season on hillslopes becomes critical, particularly in regions with distinct dry summers and wet winters (i.e. Mediterranean climates). The Blue Oaks Ranch Reserve (BORR) in Central California provides an ideal location for this study due to its consistent ridge-valley systems, which well represents the regional climatic and topographic conditions. Previous work at BORR used active source seismic refraction (SR) to constrain subsurface structure. To additionally investigate moisture content in regolith, we conduct ER surveys with Schlumberger and Dipole-Dipole configurations to invert for resistivity using Transdimensional Hierarchical Bayesian (THB) inversion framework with reversible-jump Markov Chain Monte Carlo (THB rj-MCMC). We also performed 2D synthetic tests to evaluate how well THB can recover a synthetic model with imposed data uncertainty. The results indicate that Schlumberger outperforms Dipole-Dipole in the THB rj-MCMC inversion. However, these results also reveal limited depth resolution to ~10 m depth using current ERT configurations. Finally, we adopt the THB approach for a series of ER surveys at BORR between June and September 2023. The findings suggest a distinct increase in resistivity on the North-facing slope during growing seasons, indicating reduced moisture content particularly in areas with presences of oak trees as they draw water from deep regolith. On the South-facing slope, resistivity remained stable due to the dominance of grass that lacks deep roots for consuming deep moisture. Our resistivity results show that vegetation type particularly trees play a critical role in regolith moisture distribution. To compare and correlate changes in resistivity over dry periods, we analyzed soil probe data previously collected at the site. The correlation suggested that increases in resistivity are related to decreases in volumetric moisture content. Additionally, we compared ERT data with seismic survey data to better understand changes in subsurface properties like porosity and saturation along depth, as ERT and seismic velocity is sensitive to moisture content and material porosity.Item PREDICTING THE SEISMIC SIGNATURE OF LAVA TUBES FOR THE EARTH, THE MOON, AND MARS(2024) Wike, Linden; Schmerr, Nicholas; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lava tubes are a type of volcanically-generated subsurface void structure found on Earth, the Moon, and Mars and hold the potential to serve as shelters for crew members, preservation sites of pristine geological samples, and locations of in situ resources. A key question for lava tube science is how to locate them through geophysical methods. Here, we create a workflow that locates and characterizes the geometry of subsurface voids. We build a suite of subsurface synthetic seismic wavefield models that contain lava tube structures and investigate which seismic method best images them. Our models show that the more readily detectable lava tube simulations have a geophone spacing of 0.5 m, variable diameter, and shallow ceiling depth; and reverse-time migration and phase-shift-plus-interpolation migration techniques produce more accurate lava tube reconstructions than the Kirchhoff method. The synthetic modeling serves as a benchmark for understanding seismic wave propagation around lava tubes and helps answer how voids on the Moon and Mars would be imaged through seismology.Item Safer Grounds: A Study of Landmine Detection using UAV- and Ground-Based Multi-Modal Geophysics(2024) Myers, Heidi Patricia; Lekic, Vedran; Lathrop, Daniel; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)This dissertation addresses the urgent global crisis of landmines, unexploded ordnance (UXO), and explosive remnants of war (ERW) through the lens of multimodal geophysics. Chapter 1 sets the stage by highlighting the humanitarian imperative while underscoring the broader applicability of the developed methods and instruments for shallow critical zone exploration. Unlike conventional engineering-centric approaches, our geoscience-centered methodology offers promising avenues for effectively detecting and characterizing buried hazards. Chapter 2 meticulously examines various geophysical sensors, identifying limitations and proposing innovative solutions. Notably, TetraMag, a novel triaxial magnetic gradiometer, overcomes the deficiencies of single-sensor systems, demonstrating superior sensitivity to small-scale variations in the magnetic field. Chapter 3 delves into the intricate symmetries and invariants of the finite-difference magnetic gradient tensor (FDMGT), elucidating its pivotal role in precise target localization and parameter estimation within the shallow critical zone. The methodology outlined streamlines data processing and interpretation, laying a robust foundation for UAV-based detection systems. Chapter 4 introduces machine learning techniques, particularly convolutional neural networks (CNNs), as robust target detection and parameter estimation tools. By synergizing multiple geophysical modalities, these methods enhance our ability to discern subtle anomalies with high accuracy. Chapter 5 proposes a method to mitigate magnetic self-noise in UAV-mounted gradiometers, enhancing data fidelity and spatial coherence. This approach, applicable to various vehicle platforms, further extends the reach of our detection capabilities. In Chapter 6, we integrate and apply these methodologies to a real-world minefield scenario, successfully detecting and localizing buried targets. While acknowledging limitations such as payload constraints and computational demands, our findings underscore the versatility and robustness of the developed techniques. This dissertation addresses the pressing humanitarian challenge of landmine detection and advances the broader field of shallow critical zone geophysics. The methodologies and technologies presented here hold promise for diverse applications beyond military contexts, ranging from infrastructure mapping to hydrogeological studies.Item Thermal Control on the Location of the Volcanic Arc at Subduction Zones(2023) Ha, Goeun; Montesi, Laurent G.J.; Zhu, Wenlu; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)At subduction zones, where oceanic plates are recycled into the Earth’s interior, water released by the subducting plate initiates partial melts that form volcanic arcs. Partial melts can be present in a broad melting zone below a narrow volcanic arc. The second melting zone can be formed by mantle upwelling induced by active extension behind the arc and subsequent decompression melting. In this dissertation, I explain the locations of the arc in global using a temperature-dependent melt focusing mechanism. I present a simple geometrical model to explain the observed correlation between the location of the arc and the back-arc spreading center (BASC) at five subduction zones. Lastly, I discuss the thermal influence of the BASC on the arc location. The melts rise vertically through the pore spaces in the mantle rock until they encounter a low permeability barrier formed at a temperature where the crystallization rate is maximum. As the melt trajectory is deflected laterally, the melts are focused at the apex of the permeability barrier and the volcano is more likely to form immediately above the magma pool. In the subduction zones without back-arc spreading, the projection of the apex of the barrier-forming isotherm shows good agreement with the observed arc locations. The arc and the BASC location are negatively correlated with the slab dip at five subduction zones. The decoupling depth between the slab and the overlying mantle defines the closest approach of the nose of the isotherm. The horizontal distance from the trench to the decoupling depth is controlled by the slab dip, which produces the negative correlation. The back-arc extension is related to the trench retreat and the slab anchoring at 660 km discontinuity, which results in a decrease in the slab dip. The relation between the slab anchoring depth and the slab dip generates the observed negative correlation. When the BASC develops near the trench, the thermal structure is disrupted by the mantle upwelling and thereby the predicted arc location moves toward the spreading center.Item ASSESSING FAULT SLIP HAZARD IN TAIWAN USING SPACE GEODESY(2022) Robbins, Kathryn Rose; Huang, Mong-Han; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Taiwan is a geologically complex region due to the continuous collision of the Eurasian Plate and the Philippine Sea Plate. This study aimed to quantify the interseismic crustal deformation of Taiwan and detail the island’s seismic hazard potential using space geodesy. Data were collected between 2016 and 2021 through C-band Copernicus Sentinel-1 synthetic aperture radar imagery and continuous GNSS data from Academia Sinica, Taiwan. I excluded major earthquake events within this time period and generated a dataset consisting of interferometric synthetic aperture radar ground motion velocities with GNSS corrections and interpolated GNSS ground motion velocities. Then, utilizing this dataset, I performed a deformation rate analysis and error analysis. Next, I explored block modeling and used a total variation regularization approach to determine the reference block model that best reduced velocity residuals and minimized the number of independently rotating blocks. Results suggested that the Taipei Basin, Ilan Basin, Western Foothills, and Longitudinal Valley were experiencing increased total strain rate accumulation and, therefore, posed increased seismic hazard.Item Earth's Radiogenic Heat Production and the Composition of the Deep Continental Crust(2022) Sammon, Laura Grace; McDonough, William F; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Much of the continental crust, the 40+ km thick plates of rock that make up the outer shell of our planet, is inaccessible to us living on its surface. Thus its composition is a mystery. We lack the technology to sample it directly at depths past 5 km, aside from a few deep (expensive) drill holes, so we must come up with a clever alternative for establishing its composition. The deep crust, the lower two-thirds of the continent, serves as a supporting root. When continents collide, they make mountain ranges, or when pulled apart they make rift valleys and basins. The composition of the deep crust, and specifically its silica, molecular water, and heat producing element (HPE: K, Th, U) contents, directly influence the crust's rheology during tectonic events and its potential for deadly earthquakes. Its chemical makeup is the sum of 4.5 billion years of crustal evolutionary processes that continuously shape and reshape the platform upon which society sits. An accurate description of the deep crust, however, requires careful integration of many different data sources. My research combines geochemistry with thermodynamics, geophysics, mineral-physics, seismology, and even particle physics to produce self-consistent models for the crust’s composition. Using thermodynamic calculations, I generate densities and seismic sound wave speeds from a range of chemical compositions. Matching these forecasted models to Earth’s seismic and gravity data allows me to translate the deep crust's physical properties into chemical compositions on both the regional and the global scale. Importantly, by quantifying not only the compositions, but also the uncertainties and the misfit in these results, I can better define the differences between competing models for crust deformation and evolution. Charting the distribution of Earth's geochemical resources has led to our collaborations with particle physicists, who need our expertise to determine the frequency of radioactive decay and therefore the amount of HPE decay emissions (known as geoneutrinos) in the crust; this geoneutrino flux is the background signal in their nuclear physics experiments. Their global flux measurements constrain our models for heat production and the amount of radiogenic energy that heats the Earth – which provides power to mantle convection, plate tectonics, and the destruction and creation of more continental crust.Our main sources of data are threefold. First, we have critically compiled geochemical analyses of >10,000 rock samples from pre-existing literature (Earthchem.org and affiliates). Second, we use geophysical data provided by sources such as the United States Geological Survey, the Earthscope USArray, and others to determine which of our geochemical samples could produce Earth’s observed seismic and density signals. Third, we partner with particle physicists in the United States, Canada, Italy, Japan, and China to jointly interpret data from three international geoneutrino detectors. By focusing on Earth as a whole system we seek a comprehensive understanding of its natural hazards and resources. Using multidisciplinary constraints, my goal is to build compositional models of the continental crust, with quantifiable uncertainties, that can be applied regionally and at larger scales. These findings will provide predictive insights on the strength and response of the continents when subjected to the dynamic processes of plate tectonics.Item ANALYZING TIME-VARYING SEISMICITY AND AFTERSHOCK BEHAVIOR IN CENTRAL AND EASTERN UNITED STATES(2021) Pearson, Karen M; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Central and Eastern United States (CEUS) earthquakes are far less common than those in the tectonically active west coast, but the significance is elevated for a few reasons. Due to older, harder, and often denser rocks making up the bedrock geology east of the Rockies, seismic waves can travel much further without losing energy. Poor construction, efficient transmission of seismic waves, and site amplification effects can make even light to moderate earthquakes pose high risk within the CEUS. The CEUS has significant aging infrastructure and some of the highest population densities in the country, which would lead to great economic losses and even the potential for human injury if hazards are not properly identified and communicated. Aftershock sequences are governed by several descriptive statistical “laws,” each with one or more characteristic parameters. These parameters are used to illustrate factors such as the overall productivity, the rate of decay, and the relative frequency of larger and smaller magnitude aftershocks. Variations in these parameters can relate to the geologic region being studied, the tectonic environment, the driving force of seismicity (i.e. induced earthquakes, volcanic, or geothermal-related), and more. This work discusses the aftershock sequences of two unusual CEUS earthquakes from the past five years. The first earthquake I study is a M4.2 earthquake that occurred east of Dover, DE, in late 2017. I continue by studying the aftershocks in the six weeks following a M5.8 earthquake that occurred near Pawnee, OK, in autumn 2016. Both of these earthquakes experienced below-normal aftershock productivities. I explore the challenges of analysis when station coverage is heterogeneous for the period of aftershock analysis. From there, I discuss the limitations of some statistical tests for special cases such as aftershock decay. The work concludes by highlighting additional CEUS earthquakes exceeding M4 that have occurred in the past 20 years and discussing some preliminary analytical findings.Item DEFORMATION AND FAILURE OF LUNAR LAVA TUBES(2021) Williams, Edward Alexander; Montesi, Laurent G. J.; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Lava tubes are long void spaces left by lava flows. Terrestrial tubes have been extensively studied. Evidence indicates lunar tubes also exist, potentially ideal environments for lunar exploration, habitation, and study. This thesis presents models of elastic deformation around lunar lava tubes to determine the expected stresses, failure regions, and surface features associated with tubes and their dimensions. Failure on internal surfaces is extensive in the modeled tubes, leaving only small usable floor regions (maximum 500 m) near tube edges. Cracks and debris may be problematic for utilizing tubes. Shape influences failure extents; narrower, semicircular-floored tubes suffer less floor failure. Linear surface cracks and bulges are expected, at distances from the tube that depend on tube dimensions, and might be used to locate tubes and determine their dimensions without entering. Such cracking seems to be present near the Southern end of Rima Mairan, suggesting the rille continues into a tube.Item Geophysical Exploration of Terrestrial and Lunar Volcanic Fields(2021) Bell, Jr., Ernest Robert; Schmerr, Nicholas; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Planetary analogs are environments representative of current or past conditions on other planetary bodies. My research uses terrestrial volcanic fields as lunar analogs to conduct geophysical studies on the subsurface structure of cinder cones, lava flows, and lava tubes, as well as understand terrestrial field methods for application to lunar surface exploration. As part of this research, I relate the magnetic anomalies produced by lava tubes to their location and geomorphology. By comparison of magnetic anomalies against synthetic predictions, I derive a relationship between the terrestrial magnetic anomalies and underlying tube geometry. The model is shown to predict terrestrial lava tube magnetic anomalies, and adjusting for the lunar magnetic environment, anomalies resulting from tubes on the Moon. Active source seismic experiments performed by Apollo astronauts were used to determine lunar tectonic and volcanic structure at depth. Terrestrial geophysical analogs are useful for understanding the Apollo results, and for improving the quality of future lunar seismic studies. I use seismic refraction to attempt to identify subsurface continuation of locally mapped faults beneath lava flows and cinder deposits to examine their association to cinder cone vent chains. However, due to high seismic energy attenuation, my analysis was unable to resolve displacement of stratigraphic layers indicative of fault locations. The seismic attenuation properties of the field area were able to be characterized. I then analyze the Apollo 17 Lunar Seismic Profiling Experiment (LSPE) data and an Apollo LSPE equivalent terrestrial data set to provide insights into the subsurface imaging potential for a terrestrial equivalent array in the Taurus-Littrow Valley on the Moon. Finally, I insert active seismic refraction into a previously executed simulated human lunar rover mission where the traverse route and associated science station locations omitted geophysical studies. Data from these lines are used to create 1-D seismic velocity profiles to examine subsurface structural trends and geophysical features of the field area. The seismic fieldwork and analysis are related to similar activities performed by the Apollo 14 & 16 crews to highlight similarities in issues encountered with both terrestrial and lunar field operations, and discuss considerations for future human lunar surface science.Item Inference of Mass Anomalies in Planetary Interiors Using a Bayesian Global Gravity Field Inversion(2020) Izquierdo Gonzalez, Kristel Del Carmen; Montesi, Laurent G. J.; Lekic, Vedran; Geology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Knowledge about the interior density distribution of a planetary body can constraingeophysical processes and reveal information about the origin and evolution of the body. Properties of this interior distribution can be inferred by analyzing gravity acceleration data sampled by orbiting satellites. Usually, the gravity data is complemented with additional laser ranging or seismic data in order to reduce the range of possible density models of the interior. However, additional data might not be available and tight prior constraints on model parameters might not be justified. In this case, the flexibility of using non-informative priors and the ability to quantify the non-uniqueness of the gravity inversions are of even greater importance. In this work, we present a gravity inversion algorithm, THeBOOGIe, thatsamples the posterior distribution of density in the interior of a planet or moon according to Bayes theorem, following a Metropolis-Hastings iterative algorithm. It uses non-informative priors on the number, location, shape and magnitude of density anomalies. Different samples of the posterior show different density models of the interior consistent with the observed gravity data. Inversions of synthetic gravity data are ran using point masses, spherical caps and Voronoi regions (VRs) to parametrize density anomalies. THeBOOGIe is able to retrieve the lateral location of shallow density anomalies and the shape, depth and magnitude of a mid-mantle anomaly. The uncertainty of the model parameters increases with depth, as expected. Bouguer gravity data of the Moon obtained by the GRAIL mission was invertedusing a VR parametrization. Shallow anomalies related to the SPA basin, crustal dichotomy and near side basins were found in the correct latitude and longitude and a trade-off in their thickness and magnitude. Positive and negative density anomalies were found in the depth range 500-1141 km. The location of deep moonquakes do not have a clear relation to the location of these density anomalies.