Browsing by Author "Hole, John A."
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- Analysis and Application of Coal Seam Seismic Waves for detection of Abandoned MinesYancey, Daniel Jackson (Virginia Tech, 2006-05-01)It is not uncommon for underground coal mining to be conducted in the proximity of abandoned underground mines that are prone to accumulate water, methane or other toxic gases, and are often either poorly mapped or without good surface survey control. Mining into such abandoned voids poses a great safety risk to personnel, equipment, and production from inundation or toxic/explosive gas release. Often, surface or underground drilling is employed to detect the mine void and evaluate the hazards, sometimes with disastrous results. The use of guided waves within coal seams can be utilized to locate voids, faults, and abrupt seam thickness changes. The use of seam waves for void detection and mine planning has tremendous value and use. To demonstrate the feasibility of abandoned mine void detection utilizing coal seam seismic waves, two in-seam reflection surveys and a transmission survey were acquired at an abandoned underground mine near Hurley, Virginia. Numerical modeling of the seam waves was examined as well. The Airy phase was observed in the synthetic and real field data. Dispersion analysis of the field data shows reasonable agreement with the dispersion characteristics of the synthetic data. Using standard commonly available seismic reflection processing tools, a known and well-mapped mine was detected and located. Detection of the mine with both surveys indicates that ``exploratory'' drilling can be replaced by noninvasive seismic methods. Location, however, was not good enough to replace drilling entirely. Hence seismic methods can be used for detection, but if a potential void is detected, focused drilling should be applied for accurate mapping and circumvention of potentially hazardous areas.
- Aquifer Characterization in the Blue Ridge Physiographic ProvinceSeaton, William (Virginia Tech, 2002-01-25)Existing models of the hydrogeology in the Blue Ridge Province in the eastern United States generally assume a simplified two-layered system consisting of shallow unconsolidated and relatively homogeneous and porous regolith with a water-table aquifer that slowly supplies water downward to the underlying variably fractured crystalline bedrock. In these models, interconnected fractures in the crystalline bedrock act as conduits for predominantly downward vertical and limited horizontal flow. Fracture density is depthà limited and correlated with proximity to topographic lineaments. Current models consider the porous regolith as the primary water storage reservoir for the entire aquifer system. In this research, detailed hydrogeologic studies in the Blue Ridge Province in Floyd County, Virginia reveal a substantially different framework for groundwater flow. Recent acquisition of two-dimensional surface resistivity profiles collected using a variety of array techniques combined with borehole geophysical logs revealed new insights into this geologically complex province. Dipole-dipole arrays were particularly important in gathering high resolution resistivity profiles that document horizontal and vertical resistivity variation reflecting changes in subsurface geology and anomalous low resistivity areas in crystalline bedrock associated with fault zones. The shallow regolith contains unsaturated areas and also localized sand and clay prone facies with water table and confined aquifer conditions residing locally. Hydraulic heads between the shallow aquifer and the deeper fractured bedrock aquifer can vary by 20 m vertically. Within the crystalline bedrock are anomalous lower resistivity intervals associated with ancient fault shear zones. Brecciated rock adjacent to the shear zones, and the shear zones themselves, can be hydraulically conductive and serve as pathways for groundwater movement. Aquifer testing of the regolith-bedrock fracture system occurred over a 6-day period and produced rapid and relatively uniform drawdowns in surrounding wells completed in the fractured bedrock aquifers. The shallow aquifers experienced minimal drawdowns from the aquifer test indicating low vertical hydraulic conductivity and limited communication between the shallow and deeper bedrock aquifers. Water chemistry and chlorofluorocarbon (CFC) age dating analyses indicated significant differences between water samples from the shallow and deep aquifers. A new conceptual model for Blue Ridge aquifers is proposed based on these research findings.
- An Assessment of Hypocenter Errors Associated with the Seismic Monitoring of Induced Hydro-fracturing in Hydrocarbon ReservoirsGilliland, Ellen (Virginia Tech, 2009-10-14)Expanding the standard, single-well recording geometry used to monitor seismicity during hydro-fracture treatments could provide more accurate hypocenter locations and seismic velocities, improving general reservoir characterization. However, for the real, two-well data set obtained for this project, only S-wave picks were available, and testing resulted in anomalous hypocenter location behavior. This study uses a hypocenter location algorithm and both real and synthetic data sets to investigate how the accuracy of the velocity model, starting hypocenter location, recording geometry, and arrival-time picking error affect final hypocenter locations. Hypocenter locations improved using a velocity model that closely matched the observed sonic log rather than a smoothed version of this model. The starting hypocenter location did not affect the final location solution if both starting and final locations were between the wells. Two solutions were possible when the true solution was not directly between the wells. Adding realistic random picking errors to synthetic data closely modeled the dispersed hypocenter error pattern observed in the real data results. Adding data from a third well to synthetic tests dramatically reduced location error and removed horizontal geometric bias observed in the two-well case. Seismic event data recorded during hydro-fracture treatments could potentially be used for three-dimensional joint hypocenter-velocity tomography. This would require observation wells close enough to earthquakes to record P- and S-wave arrivals or wells at orientations sufficient to properly triangulate hypocenter locations. Simulating results with synthetic tests before drilling could optimize survey design to collect data more effectively and make analysis more useful.
- Body and surface wave ambient noise seismic interferometry across the Salton Sea Geothermal Field, CaliforniaSabey, Lindsay Erin (Virginia Tech, 2015-01-13)Virtual source gathers were generated using the principles of seismic interferometry from 135 hours of ambient noise recorded during a controlled-source survey across the Salton Sea Geothermal Field in southern California. The non-uniform nature of the noise sources violated a primary assumption of the method and generated artifacts in the data. The artifacts generated by the high-energy impulsive sources (e.g. earthquakes, shots) were removable using traditional methods of amplitude normalization prior to cross-correlation. The continuous source artifacts generated by the geothermal wells and highways required an unconventional approach of utilizing only normalized impulsive sources to successfully reduce the artifacts. Virtual source gathers were produced successfully that contained strong surface waves at 0.4-2.5 Hz, an order of magnitude below the corner frequency of the geophones, and modest body waves at 22-30 Hz, which are generally more difficult to obtain due to the need for many large, well-distributed subsurface sources. The virtual source gathers compare well to nearby explosive shots and are more densely spaced, but have a much lower signal-to-noise ratio. Analysis of the surface waves was complicated by strong higher-order modes. Spectral analysis of virtual source gathers required utilization of the geothermal plant energy, which produced usable signal at offsets required for mode separation. The virtual source dispersion curve compared well to a dispersion curve from a nearby explosive shot. P-waves were observed on the virtual source gathers. Creation of a low-quality multichannel reflection stack revealed two weak reflectors in the upper 2 km.
- Building Velocity Models for Steep-Dip Prestack Depth Migration through First Arrival Traveltime TomographyCarney, Brooke J. (Virginia Tech, 2000-11-20)Although the petroleum industry has imaged reflections from the sides of salt domes, steeply dipping structures have not been imaged as reflectors outside of sedimentary basins; to do so requires appropriate data acquisition, prestack depth migration, and an excellent seismic velocity model. Poststack time migrated seismic images, normal moveout velocity analysis, well logs, and other geologic information are used to build the velocity model. In regions of interest outside of sedimentary basins, such as major strike-slip faults, seismic reflectivity is often sparse and little is known of detailed subsurface geology. Alternate methods of velocity model construction must be used. First arrival (refraction and turning ray) traveltime tomography is proposed to construct the preliminary velocity model for steep-dip prestack depth migration in settings with little a priori subsurface information. A densely spaced synthetic seismic data set with long-offset recording, modeled after a real survey across the San Andreas Fault, was constructed using a finite-difference algorithm. First arrival traveltimes were picked from the data and a velocity model was constructed using tomography. The velocity model was used to perform a Kirchhoff prestack depth migration of the synthetic shot gathers. The subsurface structure was sufficiently reconstructed that the velocity model could be refined through migration velocity analysis. A series of tomography tests was used to determine the spatial resolution limits of the velocity model. Isolated erroneous anomalies with sizes near the resolution limits were added to the velocity model derived from tomography and used as input for migration. This pessimistic test provided an adequate image and identifiable arrivals in migrated common image gathers, allowing the velocity model to be improved through migration moveout analysis. Data acquisition requirements for tomography include long recording offsets and times, larger sources, and dense spacings, very similar to the requirements for steep-dip reflection imaging.
- Comparison of 4.5 Hz Geophones and a Broadband Seismometer in a Real Field DeploymentRasmussen, Tyler Wyatt (Virginia Tech, 2019-06-18)An analysis of waveforms, power spectral density and array responses was performed using geophones and broadband seismometers, co-deployed as part of a geologically motivated study. Broadband seismometers record excellent waveforms but, due to cost and deployment effort, wavefields are usually spatially aliased above ~0.1 Hz. Industry rapidly deploys many thousands of inexpensive, passive geophones to record full, unaliased seismic wavefields; however, waveform quality is limited below the instrument's natural frequency of ≥2 Hz. In 2012, coincident passive and controlled-source seismic surveys were deployed to investigate tectonics in Idaho and Oregon. Broadband stations were deployed at quiet sites every 15 km, taking experienced professionals >1 person-days per station. Fifty 4.5 Hz geophones and "Texan" seismographs at 200-m spacing were deployed per person-day by inexperienced students. Geophone data were continuously recorded for 3 nights and 1 day, while broadband seismometers were deployed for ~2 years. The spectral and array responses of these real deployments were compared. For a M7.7 teleseismic event, the broadband seismometer and geophone recorded nearly identical waveforms down to <0.03 Hz (32 s) and matching power spectral density down to 0.02 Hz (50 s). For quiet ambient noise, the waveforms strongly correlate down to <0.25 Hz (4 s) and the power spectral density match to the low-frequency side of the microseismic peak at ~0.15 Hz (~7 s). By deploying a much larger number of geophones, waveforms can be stacked to reduce instrument self-noise and beamforming can be used to identify wavefield azimuth and apparent velocity. Geophones can be an effective tool in ambient noise seismology down to ~7 seconds and can be used to record large seismic events effectively down to tens of seconds, well below the natural frequency of the instruments. A well-designed deployment of broadbands and geophones can enable full wavefield studies from long period to short period. Scientific and societal applications that could benefit from the improved unaliased wavefield bandwidth include local to regional seismicity, strong ground motion, magma migration, nuclear source discrimination, and crustal studies.
- Compressible Convection and Subduction: Kinematic and Dynamic ModelingLee, Changyeol (Virginia Tech, 2010-10-05)Subduction is a dynamic and time-dependent process which requires time-dependent models for its study. In addition, due to the very high pressures within the Earth's interior, an evaluation of the role of compressibility in subduction studies should be undertaken. However, most subduction studies have been conducted by using kinematic, steady-state, and/or incompressible mantle convection models; these simplifications may miss important elements of the subduction process. In this dissertation, I evaluate the effects of time-dependence and compressibility on the evolution of subduction by using 2-D Cartesian numerical models. The effect of compressibility on the thermal and flow structures of subduction zones is evaluated by using kinematically prescribed slab and steady-state models. The effect of compressibility is primarily expressed as an additional heat source created by viscous dissipation. The heat results in thinner thermal boundary layer on the subducting slab and increases slab temperatures. With that exception, the effect of compressibility is relatively small compared with, for example, the effect of the mantle rheology on the thermal and flow structures of the mantle wedge. Plate reconstruction models show that the convergence rate and age of the incoming plate to trench vary with time, which poses a problem for steady-state subduction models. Thus, I consider the time-dependent convergence rate and age of the incoming plate in the kinematic-dynamic subduction models in order to understand the localization of high-Mg# andesites in the western Aleutians. The results show that the localization of high-Mg# andesites is a consequence of the time-dependent convergence rate and slab age along the Aleutian arc. The influence of mantle and slab parameters as well as compressibility on the slab dynamics is evaluated by using 2-D dynamic subduction models. The results demonstrate that periodic slab buckling in the mantle results in periodic convergence rate and dip of the subducting slab; time-dependence is a natural expression of subduction. The effect of compressibility on the slab dynamics is not significant. The periodic convergence rate and dip of the subducting slab explain time-dependent seafloor spreading at the mid-ocean ridge, convergence rate of the oceanic plate at trench and arc-normal migration of arc volcanoes.
- Continental Arc Processes in British Columbia and Earthquake Processes in Virginia: Insights from Seismic ImagingWang, Kai (Virginia Tech, 2014-02-07)Travel times from a refraction and wide-angle reflection seismic survey across the Coast Plutonic Complex and Stikine terrane of British Columbia were inverted to derive two dimensional P and S-wave seismic velocity models of the crust and uppermost mantle. A felsic upper crust and a felsic to intermediate middle crust are observed in both the batholith complex and the accreted Stikine island arc terrane. The P and S wave models demonstrate a high-velocity (P 7.0 km/s, S 3.8 km/s) layer in the lower crust beneath the youngest (late Cretaceous to Eocene) portion of the continental arc complex. In contrast, the lower crust under the Stikine terrane has lower velocities consistent with amphibolite or other hydrated mafic rocks. The Moho is at ~35 km depth under the Stikine terrane, deepens to ~38 km beneath the youngest portion of the arc, then shallows towards the coast. The high velocity zone under the younger portion of the Coast Plutonic Complex has a 1.81 Vp/Vs ratio and is interpreted to have a bulk composition of mafic garnet granulite. This garnet granulite and large volumes of granodiorite-dominated melt were created by arc dehydration melting of amphibolite (or hydrated gabbro) in the pre-existing lower crust Reverse time migration method was applied to image aftershocks recorded by a dense array deployed after the 2011 Virginia earthquake. Events as tiny as magnitude -2 were successfully imaged as point sources. The propagation of energy release as a function of time and space was observed for events larger than magnitude 2.5. Spatial resolution of the images was ~200 m, which synthetic data tests show was primarily limited by the temporal sampling rate. Improved temporal and spatial sampling could produce images with sharper resolution.
- Continental Tectonics from Dense Array Seismic Imaging: Intraplate Seismicity in Virginia and a Steep Cratonic Margin in IdahoDavenport, Kathy (Virginia Tech, 2016-09-21)Dense array seismic techniques can be applied to multiple types of seismic data to understand regional tectonic processes via analysis of crustal velocity structure, imaging reflection surfaces, and calculating high-resolution hypocenter locations. The two regions presented here include an intraplate seismogenic fault zone in Virginia and a steep cratonic margin in eastern Oregon and Idaho. The intraplate seismicity study in Virginia consisted of using 201 short-period vertical-component seismographs, which recorded events as low as magnitude -2 during a period of 12 days. Dense array analysis revealed almost no variation in the seismic velocity within the hypocentral zone, indicating that the aftershock zone is confined to a single crystalline-rock terrane. The 1-2 km wide cloud of hypocenters is characterized by a 29° strike and 53° dip consistent with the focal mechanism of the main shock. A 5° bend along strike and a shallower dip angle below 6 km points toward a more complex concave shaped fault zone. The seismic study in Idaho and Oregon was centered on the inversion of controlled-source wide-angle reflection and refraction seismic P- and S-wave traveltimes to determine a seismic velocity model of the crust beneath this part of the U.S. Cordillera. We imaged a narrow, steep velocity boundary within the crust that juxtaposes the Blue Mountains accreted terranes and the North American craton at the western Idaho shear zone. We found a 7 km offset in Moho depth, separating crust with different seismic velocities and Poisson's ratios. The crust beneath the Blue Mountains terranes is consistent with an intermediate lithology dominated by diorite. In the lower crust there is evidence of magmatic underplating which is consistent with the location of the feeder system of the Columbia River Basalts. The cratonic crust east of the WISZ is thicker and characterized by a felsic composition dominated by granite through most of the crust, with an intermediate composition layer in the lower crust. This sharp lithologic and rheologic boundary strongly influenced subsequent deformation and magmatic events in the region.
- Crustal Structure in a Mesozoic Extensional Terrane: The South Georgia Rift and the Epicentral Area of the 1886 Charleston, South Carolina, EarthquakeBuckner, Jesse Conard (Virginia Tech, 2011-01-28)On August 31, 1886 a large scale earthquake occurred in Summerville, S.C. causing severe damage in the coastal city of Charleston. Although intensive geological and geophysical studies have been conducted in the area, uncertainty remains about the details of the event. Recently evidence from seismic reflection profiles have shed light on the tectonic environment of the area. The epicentral area of the 1886 event lies within the South Georgia Rift, a Mesozoic rift terrane. Previous studies have revealed clues to the geologic structure and evolution of this feature. SEISDATA4 is the largest seismic reflection profile recorded in the area. By re-processing the line, information about the tectonic structure of the area was revealed. The early Mesozoic extensional basin that hosted the 1886 earthquake and is host to the modern seismicity recorded in the area, extends several kilometers to the south and west of Charleston, along SEISDATA4. Cenozoic and Mesozoic faults were resolved within the basin and along its northwestern boundary that is distinguished by a strong gradient in the magnetic field. However, the question as to which fault was responsible for the rupture of 1886 still remains. The refraction analysis provides better resolution of the lithology in Lower Mesozoic section. The termination of the strong reflection at the base of the Atlantic Coastal Plain occurs in a section of the profile that shows major disruption of the underlying reflections, and suggests that the termination of a lower Mesozoic basalt flow responsible for the reflection may be related to tectonic deformation.
- Discrimination and Enhancement of Fracture Signals on Surface Seismic DataBansal, Reeshidev (Virginia Tech, 2003-05-05)Fracture patterns control flow and transport properties in a tight gas reservoir and therefore play a great role in siting the production wells. Hence, it is very important that the exact location and orientation of fractures or fracture swarms is known. Numerical models show that the fractures may be manifested on seismograms as discrete events.A number of data processing workflows were designed and examined to enhance these fracture signals and to suppress the reflections in seismic data. The workflows were first tested on a 2D synthetic data set, and then applied to 3D field data from the San Juan Basin in New Mexico. All these workflows combine conventional processing tools which makes them easily applicable. Use of conventional P-wave data may also make this approach to locate fractures more economical than other currently available technology which often requires S-wave survey or computationally intensive inversion of data. Diode filtering and dip-filtering in the common-offset domain yield good results and work very well in the presence of flat reflectors. NMO-Dip filter depends on the NMO velocity of the subsurface, but removes both flat and slightly dipping reflectors without affecting the fracture signals. Prior application of dip-moveout correction (DMO) did not make any difference on reflections, but included some incoherent noise to the data. The Eigenvector filter performed very well on flat or near-flat reflectors and left the fracture signals almost intact, but introduced some incoherent noise in the presence of steeply dipping reflectors. Harlan's scheme and Radon filtering are very sensitive with regard to parameters selection, but perform exceptionally well on flat or near-flat reflectors. Dip-filter, Eigenvector filter, and Radon filter were also tested on 3D land data. Dip-filter and Eigenvector filter suppressed strong reflections with slight perturbations to the fracture signals. Radon filter did not produce satisfactory result due to small residual moveout difference between reflectors and fracture signals.
- Evaluation of Coupled Erosional Processes and Landscape Evolution in the Teton Range, WyomingTranel, Lisa Marie (Virginia Tech, 2010-06-17)The evolution of mountain landscapes is controlled by complex interactions between large-scale tectonic, surficial and climate conditions. Dominant processes are attributed to creating characteristic features of the landscape, but topographic features are the cumulative result of coupled surficial processes, each locally effective in a different climate or elevation regime. The focus of erosion by glacial, fluvial, or mass wasting processes is highly sensitive to small changes in boundary conditions, therefore spatial and temporal variability can be high when observed over short time scales. This work evaluated methods for dissecting the history of complex alpine landscapes to understand the role of individual processes influenced by changing climate and underlying bedrock. It also investigated how individual and combined mechanisms of surficial processes influenced the evolution of topography in the Teton Range in Wyoming. Detrital apatite (U-Th)/He thermochronology and cosmogenic radionuclide erosion rates were applied to determine spatial and temporal variability of erosion in the central catchments of the range. Spatial variability existed between the glacial and fluvial systems, indicating that sediment erosion and deposition by these processes was controlled by short-term variability in climate conditions. Effective glacial incision also controlled other processes, specifically enhancing rock fall activity and inhibiting fluvial incision. Short-term erosion rates were highly variable and were controlled by stochastic processes, particularly hillslope failures in response to slope oversteepening due to glacial incision and orientation and spacing of bedrock fractures. Erosion rates averaged over 10 ky time scales were comparable to long-term exhumation rates measured in the Teton Range. The similarity of spatial erosion patterns to predicted uniform erosion and the balance between intermediate and long-term erosion rates suggests the landscape of the Teton Range is approaching steady-state, but frequent stochastic processes, short-term erosional variability and coupled processes maintain rugged topographic relief.
- Evaluation of Spring Discharge for Characterization of Groundwater Flow in Fractured Rock Aquifers: A Case Study from the Blue Ridge Province, VAGentry, William Miles (Virginia Tech, 2003-01-10)Recent models of groundwater flow in the Blue Ridge Province suggest multiple aquifers and flow paths may be responsible for springs and seeps appearing throughout the region. Deep confined aquifers and shallow variably confined aquifers may contribute water to spring outlets, resulting in vastly different water quality and suitability for potable water supplies and stock watering. A new Low Flow Recording System (LoFRS) was developed to measure the discharge of these springs that are so ubiquitous throughout the Blue Ridge Province. Analysis of spring discharge, combined with electrical resistivity surveying, aquifer tests, and water chemistry data reveal mixed shallow and deep aquifer sources for some springs, while other springs and artesian wells are sourced only in the deep aquifer. The technique is suitable for rapid characterization of flow paths leading to spring outlets. Rapid characterization is important for evaluation of potential water quality problems arising from contamination of shallow and deep aquifers, and for evaluation of water resource susceptibility to drought. The spring discharge technique is also suitable for use in other locations where fractured rock and crystalline rock aquifers are common.
- Evolution of Deformation Along Restraining Bends Based on Case Studies of Different Scale and ComplexityCochran, William Joseph (Virginia Tech, 2018-06-25)Globally, deformation along obliquely converging plate margins produce a wide variety of complex fault patterns, including crustal pop-ups, fault duplex structures, restraining bends, and flower structures. Depending on the plate velocity, plate obliquity, crustal rheology, length-scale, and climate, the evolution of faulting into translational and vertical strain can range in complexity and fault slip partitioning (i.e. vertical vs. horizontal strain). In this dissertation I studied two restraining bends to understand how these factors influence patterns of deformation along two major plate boundaries: The North American-Caribbean and the North AmericanPacific plate boundaries. First, I estimate the exhumation and cooling history along the Blue Mountains restraining bend in Jamaica using multiple thermochronometers. Three phases of cooling have occurred within Jamaica: 1) initial rock crystallization and rapid emplacement of plutons from 75-68 Ma, 2) slow cooling from 68-20 Ma, and 3) two-stage exhumation from 20 Ma – Present. During the most recent phase of Jamaica’s cooling history, two stages of exhumation have been identified at 0.2 mm/yr (20 – 5 Ma) and ~1 mm/yr (5 Ma – Present). Given the plate velocity to exhumation rate ratio during the most recent phase, we suggest that the climate of Jamaica increases the erosivity of the Blue Mountain suite, whereby the Blue Mountains may be in an erosional stead-state. Second, I studied the long-term evolution of a restraining bend at San Gorgonio Pass in southern California by relating fault kinematics within the uplifted San Bernardino Mountains to the nearby Eastern California shear zone. Using highresolution topography (i.e. UAV and lidar surveys), I studied the plausibility of faulting along two potentially nascent faults within the San Bernardino Mountains, namely the Lone Valley and Lake Peak faults. We found that while both faults display evidence for Quaternary faulting, deciphering true fault slip rates was challenging due to the erosive nature of the mountainous landscape. Coupled with evidence of Quaternary faulting along other faults within the San Bernardino Mountains, we suggest a western migration of the Eastern California shear zone.
- An experimental study on characterization of physical properties of ultramafic rocks and controls on evolution of fracture permeability during serpentinization at hydrothermal conditionsFarough, Aida (Virginia Tech, 2015-09-28)Serpentinization is a complex set of hydration reactions, where olivine and pyroxene are replaced by serpentine, magnetite, brucite, talc and carbonate minerals. Serpentinization reactions alter chemical, mechanical, magnetic, seismic, and hydraulic properties of the crust. To understand the complicated nature of serpentinization and the linkages between physical and chemical changes during the reactions, I performed flow-through laboratory experiments on cylindrically cored samples of ultramafic rocks. Each core had a well-mated through-going tensile fracture, to investigate evolution of fracture permeability during serpentinization. The samples were tested in a triaxial loading machine at an effective pressure of 30 MPa, and temperature of 260°C, simulating a depth of 2 km under hydrostatic conditions. Fracture permeability decreased by one to two orders of magnitude during the 200 to 340 hour experiments. Electron microprobe and SEM data indicated the formation of needle-shaped crystals of serpentine composition along the walls of the fracture, and chemical analyses of sampled pore fluids were consistent with dissolution of ferro-magnesian minerals. The rate of transformation of olivine to serpentine in a tensile fracture is calculated using the data on evolution of fracture permeability assuming the fracture permeability could be represented by parallel plates. Assuming the dissolution and precipitation reactions occur simultaneously; the rate of transformation at the beginning of the experiments was ~ 10-8-10-9 (mol/m2s) and decreased monotonically by about an order of magnitude towards the end of the experiment. Results show that dissolution and precipitation is the main mechanism contributing to the reduction in fracture aperture. The experimental results suggest that the fracture network in long-lived hydrothermal circulation systems may be sealed rapidly as a result of mineral precipitation, and generation of new permeability resulting from a combination of tectonic and crystallization-induced stresses may be required to maintain fluid circulation. Another set of flow through experiments were performed on intact samples of ultramafic rocks at room temperature and effective pressures of 10, 20 and 30 MPa to estimate the pressure dependency of intact permeability. Porosity and density measurements were also performed with the purpose of characterizing these properties of ultramafic rocks. The pressure dependency of the coefficient of matrix permeability of the ultramafic rock samples fell in the range of 0.05-0.14 MPa-1. Using porosity and permeability measurements, the ratio of interconnected porosity to total porosity was estimated to be small and the permeability of the samples was dominantly controlled by microcracks. Using the density and porosity measurements, the degree of alteration of samples was estimated. Samples with high density and pressure dependent permeability had a smaller degree of alteration than those with lower density and pressure dependency.
- Exploring the relationship between crustal permeability and hydrothermal venting at mid-ocean ridges using numerical modelsSingh, Shreya (Virginia Tech, 2015-06-16)Hydrothermal systems associated with oceanic spreading centers account for a quarter of Earth's total heat flux and one third of the heat flux through the ocean floor. Circulation of seawater through these systems alters both the crust and the circulating fluid, impacting global geochemical cycles. The warm vent fluids rich in nutrients support a wide variety of unique biological communities. Thus, understanding hydrothermal processes at oceanic spreading centers is important to provide insight into thermal and biogeochemical processes. In this dissertation I present the results of numerical modeling efforts for mid-ocean ridge hydrothermal systems. In the three manuscripts presented, permeability emerges as a key controlling factor for hydrothermal venting. In the first manuscript, I use 2-D numerical models to find that the distribution of permeability in the crust controls fluid velocity as well as the amount of mixing between hot hydrothermal fluids and cold seawater. This, in turn, effects the temperature and composition of fluids emerging on the surface. For the second manuscript, I construct single-pass 1-D models to show that a sudden increase in permeability caused due to magmatic or seismic events in the seafloor causes a sharp rise in the fluid output of the system. This, in conjunction with steep thermal gradients close to the surface, results in a rapid increase of venting temperatures. In the third manuscript, I develop a particle tracking model to study fluid trajectories in the subsurface. The results show that permeability distribution in the subsurface governs fluid paths and consequently, the residence time of fluids in the crust. Based on the work presented in this document, I conclude that permeability distribution, both local and field scale, exerts a major control on hydrothermal circulation in the subsurface and on the temperature and composition of venting fluids on the surface.
- Exploring two-phase hydrothermal circulation at a seafloor pressure of 25 MPa: Application for EPR 9°50′NHan, Liang (Virginia Tech, 2011-10-21)We present 2-D numerical simulations of two phase flow in seafloor hydrothermal systems using the finite control volume numerical scheme FISHES. The FISHES code solves the coupled non-linear equations for mass, momentum, energy, and salt conservation in a NaCl-H2O fluid to model the seafloor hydrothermal processes. These simulations use homogeneous box geometries at a fixed seafloor pressure of 25 MPa with constant bottom temperature boundary conditions that represent a sub-axial magma chamber to explore the effects of permeability, maximum bottom temperature and system depth on the evolution of vent fluid temperature and salinity, and heat output. We also study the temporal and spatial variability in hydrothermal circulation. The two-phase simulation results show that permeability plays an important role in plume structure and heat output of hydrothermal systems, but it has little effect on vent fluid temperature and salinity, given the same bottom temperature. For some permeability values, multiple plumes can vent at the seafloor above the simulated magma chamber. Temporal variability of vent fluid temperature and salinity and the complexity of phase separation suggest that pressure and temperature conditions at the top of the axial magma chamber cannot be easily inferred from vent fluid temperature and salinity alone. Vapor and brine derived fluids can vent at the seafloor simultaneously, even from neighboring locations that are fed by the same plume.
- Fluids in Planetary SystemsElwood Madden, Megan Erica (Virginia Tech, 2005-05-17)From the early stages of planetary accretion and differentiation to the geomorphology of planetary surfaces and the evolution of life, fluids play an integral role in shaping planetary bodies. Fluid properties and processes were investigated under a range of planetary conditions through (1) experimental simulations of impact events and petrographic analysis of terrestrial impactites to determine the effects of shock metamorphism on fluid inclusion properties; and (2) numerical thermodynamic equilibrium modeling of aqueous alteration processes on Mars. Results of impact experiments and analyses of fluid inclusions in rocks from the Ries Crater and Meteor Crater indicate that fluid inclusions reequilibrate systematically with increasing shock pressure: stretching and decrepitating under low shock pressure conditions and collapsing at moderate shock pressures. Above the Hugenoit Elastic Limit, fluid inclusion vesicles are destroyed due to plastic deformation and phase transitions within the host mineral. This suggests that impact processing may result in the destruction of fluid inclusions, leading to shock devolatilization of target rocks. In addition, the absence of fluid inclusions in planetary materials does not preclude the presence of fluids on the meteorite's parent body. Thermodynamic modeling of aqueous alteration of basalt under Mars-relevant conditions provides constraints on the conditions under which secondary sulfates are likely to have formed. The results of this study indicate that jarosite is likely to form as a result of water-limited chemical weathering of basalts. Magnesium sulfates are only predicted to form as a result of evaporation. This suggests that in order to form the alteration assemblages recently observed by the Mars Exploration Rover Opportunity at Meridiani Planum, water must have been removed from the system after a geologically short period of time, before fluids came into equilibrium with mafic surface materials and became alkaline.
- Geophysical Imaging of Earth Processes: Electromagnetic Induction in Rough Geologic Media, and Back-Projection Imaging of Earthquake AftershocksBeskardes, Gungor Didem (Virginia Tech, 2017-06-04)This dissertation focuses on two different types of responses of Earth; that is, seismic and electromagnetic, and aims to better understand Earth processes at a wider range of scales than those conventional approaches offer. Electromagnetic responses resulting from the subsurface diffusion of applied electromagnetic fields through heterogeneous geoelectrical structures are utilized to characterize the underlying geology. Geology exhibits multiscale hierarchical structure which brought about by almost all geological processes operating across multiple length scales and the relationship between multiscale electrical properties of underlying geology and the observed electromagnetic response has not yet been fully understood. To quantify this relationship, the electromagnetic responses of textured and spatially correlated, stochastic geologic media are herein presented. The modelling results demonstrate that the resulting electromagnetic responses present a power law distribution, rather than a smooth response polluted with random, incoherent noise as commonly assumed; moreover, they are examples of fractional Brownian motion. Furthermore, the results indicate that the fractal behavior of electromagnetic responses is correlated with the degree of the spatial correlation, the contrasts in ground electrical conductivity, and the preferred orientation of small-scale heterogeneity. In addition, these inferences are also supported by the observed electromagnetic responses from a fault zone comprising different lithological units and varying wavelengths of geologic heterogeneity. Seismic signals generated by aftershocks are generally recorded by local aftershock networks consisted of insufficient number of stations which result in strongly spatially-aliased aftershock data. This limits aftershock detections and locations at smaller magnitudes. Following the 23 August 2011 Mineral, Virginia earthquake, to drastically reduce spatial aliasing, a temporary dense array (AIDA) consisting of ~200 stations at 200-400 m spacing was deployed near the epicenter to record the 12 days of the aftershocks. The backprojection imaging method is applied to the entire AIDA dataset to detect and locate aftershocks. The method takes advantage of staking of many seismograms and improves the signal-to-noise ratio for detection. The catalog obtained from the co-deployed, unusually large temporal traditional network of 36 stations enabled a quantitative comparison. The aftershock catalog derived from the dense AIDA array and the backprojection indicates event detection an order of magnitude smaller including events as small as M–1.8. The catalog is complete to magnitude –1.0 while the traditional network catalog was complete to M–0.27 for the same time period. The AIDA backprojection catalog indicate the same major patterns of seismicity in the epicentral region, but additional details are revealed indicating a more complex fault zone and a new shallow cluster. The b-value or the temporal decay constant were not changed by inclusion of the small events; however, they are different for two completeness periods and are different at shallow depth than greater depth.
- Global Structure of the Mantle Transition Zone Discontinuities and Site Response Effects in the Atlantic and Gulf Coastal PlainGuo, Zhen (Virginia Tech, 2019-09-03)This thesis focuses on two different topics in seismology: imaging the global structures of the mantle transition zone discontinuities and studying the site response effects in the Atlantic and Gulf Coastal Plain. Global structures of the mantle transition zone discontinuities provide important constraints on thermal structures and dynamic processes in the mid mantle. In this thesis, global topographic structures of the 410- and 660-km discontinuities are obtained from finite-frequency tomography of SS precursors. The finite-frequency sensitivities of SS waves and precursors are calculated based on a single-scattering (Born) approximation and can be used for data selection. The new global models show a number of smaller-scale features that were absent in back-projection models. Good correlation between the mantle transition zone thickness and wave speed variations suggests dominantly thermal origins for the lateral variations in the transition zone. The high-resolution global models of the 410- and 660-km discontinuities in this thesis show strong positive correlation beneath western North America and eastern Asia subduction zones with both discontinuities occurring at greater depths. Wavespeed and anisotropy models support vertical variations in thermal structure in the mid mantle, suggesting return flows from the lower mantle occur predominantly in the vicinity of stagnant slabs and the region overlying the stagnant slabs. In oceanic regions, the two discontinuities show a weak anti-correlation, indicating the existence of a secondary global far-field return flow. The Atlantic and Gulf Coastal Plain is covered by extensive Cretaceous and Cenozoic marine sediments. In this thesis, the site response effects of sediments in the Coastal Plain region relative to the reference condition outside that region are investigated using Lg and coda spectral ratios. The high-frequency attenuation factors (kappa) in the Coastal Plain are strongly correlated with the sediment thickness. At frequencies between 0.1-2.86 Hz, the Lg spectral ratio amplitudes are modeled as functions of frequency and thickness of the sediments in the Coastal Plain. Analysis of the residuals from the stochastic ground motion prediction method suggests that incorporating the site response effects as functions of sediment thickness may improve ground motion prediction models for the Coastal Plain region.
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