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Browsing by Author "Holbrook, W. Steven"

Now showing 1 - 10 of 10
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    Anisotropic Turbulence Models for Wakes in an Active Ocean Environment
    Wall, Dylan Joseph (Virginia Tech, 2021-07-13)
    A set of second-moment closure turbulence models are implemented for the study of wake evolution in an oceanic environment. The effects of density stratification are considered, and the models are validated against laboratory experiments mimicking the stratified ocean environment, and against previous experimental study of wakes subjected to a density stratification. The turbulence models are found to reproduce a number of important behaviors which differentiate stratified wakes from those in a homogeneous environment, including the appropriate decay rates in turbulence quantities, buoyant suppression of turbulence length scales, and canonical stages in wake evolution. The existence of background turbulence is considered both through the introduction of production terms to the turbulence model equations and the replication of scale-resolved simulations of wakes embedded in turbulence. It is found that the freestream turbulence causes accelerated wake growth and faster decay of wake momentum. Wakes are then simulated at a variety of Re and Fr representative of full-scale vehicles operating in an ocean environment, to downstream distances several orders of magnitude greater than existing RANS studies. The models are used to make some general predictions concerning the dependence of late-wake behavior on these parameters, and specific insights into expected behavior are gained. The wake turbulence is classified using "fossil turbulence" and stratification strength criteria from the literature. In keeping with experimentally observed behavior, the stratification is predicted to increase wake persistence. It is also predicted that, regardless of initial Re or F r, the wake turbulence quickly becomes a mixture of overturning eddies and internal waves. It is found that the high Re wakes eventually become strongly affected by the stratification, and enter the strongly-stratified or LAST regime. Additional model improvements are proposed based on the predicted late wake behavior.
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    Geophysical imaging of the Yellowstone hydrothermal plumbing system
    Finn, Carol A.; Bedrosian, Paul A.; Holbrook, W. Steven; Auken, Esben; Bloss, Benjamin R.; Crosbie, Jade (Springer Nature, 2022-03-23)
    The nature of Yellowstone National Park’s plumbing system linking deep thermal fluids to its legendary thermal features is virtually unknown. The prevailing concepts of Yellowstone hydrology and chemistry are that fluids reside in reservoirs with unknown geometries, flow laterally from distal sources and emerge at the edges of lava flows. Here we present a high-resolution synoptic view of pathways of the Yellowstone hydrothermal system derived from electrical resistivity and magnetic susceptibility models of airborne geophysical data. Groundwater and thermal fluids containing appreciable total dissolved solids significantly reduce resistivities of porous volcanic rocks and are differentiated by their resistivity signatures. Clay sequences mapped in thermal areas and boreholes typically form at depths of less than 1,000  metres over fault-controlled thermal fluid and/or gas conduits. We show that most thermal features are located above high-flux conduits along buried faults capped with clay that has low resistivity and low susceptibility. Shallow subhorizontal pathways feed groundwater into basins that mixes with thermal fluids from vertical conduits. These mixed fluids emerge at the surface, controlled by surficial permeability, and flow outwards along deeper brecciated layers. These outflows, continuing between the geyser basins, mix with local groundwater and thermal fluids to produce the observed geochemical signatures. Our high-fidelity images inform geochemical and groundwater models for hydrothermal systems worldwide.
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    Geophysical Investigation of the Yellowstone Hydrothermal System
    Dickey, Kira Ann (Virginia Tech, 2018-08-27)
    Yellowstone National Park hosts over 10,000 thermal features (e.g. geysers, fumaroles, mud pots, and hot springs), yet little is known about the hydrothermally active zones hundreds of meters beneath the features. Transient electromagnetic (TEM) soundings and 2D direct current (DC) resistivity profiles show that hydrothermal alteration at active sites have a higher electrical conductivity than the surrounding hydrothermally inactive areas. For that reason, airborne TEM is an effective method to characterize large areas and identify hydrothermally active and inactive zones using electrical conductivity. Here we present results from an airborne TEM survey acquired jointly by the U.S. Geological Survey and the University of Wyoming in November, 2016. We integrate resistivity from the airborne electromagnetic (EM) survey with research drillhole data and rock physics models to investigate the controls on electrical conductivity in the upper few hundreds of meters of the Yellowstone hydrothermal system. Resistivities in Yellowstone are the product of complex variations of lithology, temperature, salinity, clay content, and hydrothermal fluids. Results show that the main drivers in lowering the high resistivitiy of volcanic rocks are water saturation and hydrothermal alteration. Salinities are not significantly elevated in Yellowstone and temperature is not a first order affect.
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    Limited Mantle Hydration by Bending Faults at the Middle America Trench
    Miller, Nathaniel C.; Lizarralde, Daniel; Collins, John A.; Holbrook, W. Steven; Van Avendonk, Harm J. A. (2021-01)
    Seismic anisotropy measurements show that upper mantle hydration at the Middle America Trench (MAT) is limited to serpentinization and/or water in fault zones, rather than distributed uniformly. Subduction of hydrated oceanic lithosphere recycles water back into the deep mantle, drives arc volcanism, and affects seismicity at subduction zones. Constraining the extent of upper mantle hydration is an important part of understanding many fundamental processes on Earth. Substantially reduced seismic velocities in tomography suggest that outer rise plate-bending faults provide a pathway for seawater to rehydrate the slab mantle just prior to subduction. Estimates of outer-rise hydration based on tomograms vary significantly, with some large enough to imply that, globally, subduction has consumed more than two oceans worth of water during the Phanerozoic. We found that, while the mean upper mantle wavespeed is reduced at the MAT outer rise, the amplitude and orientation of inherited anisotropy are preserved at depths >1 km below the Moho. At shallower depths, relict anisotropy is replaced by slowing in the fault-normal direction. These observations are incompatible with pervasive hydration but consistent with models of wave propagation through serpentinized fault zones that thin to 1 km below Moho. Confining hydration to fault zones reduces water storage estimates for the MAT upper mantle from similar to 3.5 wt% to <0.9 wt% H(2)0. Since the intermediate thermal structure in the similar to 24 Myr-old MAT slab favors serpentinization, limited hydration suggests that fault mechanics are the limiting factor, not temperatures. Subducting mantle may be similarly dry globally.
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    Links between physical and chemical weathering inferred from a 65-m-deep borehole through Earth's critical zone
    Holbrook, W. Steven; Marcon, Virginia; Bacon, Allan R.; Brantley, Susan L.; Carr, Bradley J.; Flinchum, Brady A.; Richter, Daniel D.; Riebe, Clifford S. (Springer Nature, 2019-03-14)
    As bedrock weathers to regolith -defined here as weathered rock, saprolite, and soil - porosity grows, guides fluid flow, and liberates nutrients from minerals. Though vital to terrestrial life, the processes that transform bedrock into soil are poorly understood, especially in deep regolith, where direct observations are difficult. A 65-m-deep borehole in the Calhoun Critical Zone Observatory, South Carolina, provides unusual access to a complete weathering profile in an Appalachian granitoid. Colocated geophysical and geochemical datasets in the borehole show a remarkably consistent picture of linked chemical and physical weathering processes, acting over a 38-m-thick regolith divided into three layers: soil; porous, highly weathered saprolite; and weathered, fractured bedrock. The data document that major minerals (plagioclase and biotite) commence to weather at 38 m depth, 20 m below the base of saprolite, in deep, weathered rock where physical, chemical and optical properties abruptly change. The transition from saprolite to weathered bedrock is more gradational, over a depth range of 11-18 m. Chemical weathering increases steadily upward in the weathered bedrock, with intervals of more intense weathering along fractures, documenting the combined influence of time, reactive fluid transport, and the opening of fractures as rock is exhumed and transformed near Earth's surface.
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    Measuring snow water equivalent from common-offset GPR records through migration velocity analysis
    St. Clair, James; Holbrook, W. Steven (Copernicus Publications, 2017-12-19)
    Many mountainous regions depend on seasonal snowfall for their water resources. Current methods of predicting the availability of water resources rely on long-term relationships between stream discharge and snowpack monitoring at isolated locations, which are less reliable during abnormal snow years. Ground-penetrating radar (GPR) has been shown to be an effective tool for measuring snow water equivalent (SWE) because of the close relationship between snow density and radar velocity. However, the standard methods of measuring radar velocity can be time-consuming. Here we apply a migration focusing method originally developed for extracting velocity information from diffracted energy observed in zero-offset seismic sections to the problem of estimating radar velocities in seasonal snow from common-offset GPR data. Diffractions are isolated by planewave- destruction (PWD) filtering and the optimal migration velocity is chosen based on the varimax norm of the migrated image. We then use the radar velocity to estimate snow density, depth, and SWE. The GPR-derived SWE estimates are within 6% of manual SWE measurements when the GPR antenna is coupled to the snow surface and 3–21% of the manual measurements when the antenna is mounted on the front of a snowmobile ~ 0.5m above the snow surface.
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    Porosity production in weathered rock: Where volumetric strain dominates over chemical mass loss
    Hayes, Jorden L.; Riebe, Clifford S.; Holbrook, W. Steven; Flinchum, Brady A.; Hartsough, Peter C. (AAAS, 2019)
    Weathering in the critical zone causes volumetric strain and mass loss, thereby creating subsurface porosity that is vital to overlying ecosystems. We used geochemical and geophysical measurements to quantify the relative importance of volumetric strain and mass loss—the physical and chemical components of porosity—in weathering of granitic saprolite of the southern Sierra Nevada, California, USA. Porosity and strain decrease with depth and imply that saprolite more than doubles in volume during exhumation to the surface by erosion. Chemical depletion is relatively uniform, indicating that changes in porosity are dominated by processes that cause strain with little mass loss. Strain-induced porosity production at our site may arise from root wedging, biotite weathering, frost cracking, and the opening of fractures under ambient topographic stresses. Our analysis challenges the conventional view that volumetric strain can be assumed to be negligible as a porosity-producing mechanism in saprolite.
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    Reynolds Creek Experimental Watershed and Critical Zone Observatory
    Seyfried, Mark S.; Lohse, Kathleen A.; Marks, Danny; Flerchinger, Gerald; Pierson, Fred; Holbrook, W. Steven (2018-12-13)
    The Reynolds Creek Experimental Watershed (RCEW) was established in 1960 as an "outdoor hydrological laboratory" to investigate hydrological processes of interest in the interior northwestern part of the United States. Initial emphasis was on installing and testing instrumentation and data collection and dissemination. The initial instrumentation network sampled the climatic gradient within the 239-km(2) watershed and focused on specific subwatersheds for intensive instrumentation. This network has expanded and supported ad hoc research and provides a stable platform for the development of long-term programs supporting research and model development in snow hydrology, climate change, water and energy balance, land management, carbon cycling, and critical zone hydrology. Recently, the challenge taken up at the RCEW is to integrate different processes over space for applications to larger areas outside the watershed. The presence of steep local environmental gradients associated with topography in addition to more gradual, elevational gradients requires high-resolution modeling. The snow hydrology program has demonstrated the potential for high-resolution, process-based modeling across large landscapes. The direct linkage of biogeochemical processes with hydrological processes ultimately requires a multidisciplinary approach that has been adopted at the RCEW since inclusion in the Critical Zone Observatory program. We think that coupling of these processes will lead to a better understanding and management of natural resources on the landscape.
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    Spatiotemporal Heterogeneity of Water Flowpaths Controls Dissolved Organic Carbon Sourcing in a Snow-Dominated, Headwater Catchment
    Radke, Anna G.; Godsey, Sarah E.; Lohse, Kathleen A.; McCorkle, Emma P.; Perdrial, Julia; Seyfried, Mark S.; Holbrook, W. Steven (Frontiers, 2019-02-27)
    The non-uniform distribution of water in snowdrift-driven systems can lead to spatial heterogeneity in vegetative communities and soil development, as snowdrifts may locally increase weathering. The focus of this study is to understand the coupled hydrological and biogeochemical dynamics in a heterogeneous, snowdrift-dominated headwater catchment (Reynolds Mountain East, Reynolds Creek Critical Zone Observatory, Idaho, USA). We determine the sources and fluxes of stream water and dissolved organic carbon (DOC) at this site, deducing likely flowpaths from hydrometric and hydrochemical signals of soil water, saprolite water, and groundwater measured through the snowmelt period and summer recession. We then interpret flowpaths using end-member mixing analysis in light of inferred subsurface structure derived from electrical resistivity and seismic velocity transects. Streamwater is sourced primarily from groundwater (averaging 25% of annual streamflow), snowmelt (50%), and water traveling along the saprolite/bedrock boundary (25%). The latter is comprised of the prior year's soil water, which accumulates DOC in the soil matrix through the summer before flushing to the saprolite during snowmelt. DOC indices suggest that it is sourced from terrestrial carbon, and derives originally from soil organic carbon (SOC) before flushing to the saprolite/bedrock boundary. Multiple subsurface regions in the catchment appear to contribute differentially to streamflow as the season progresses; sources shift from the saprolite/bedrock interface to deeper bedrock aquifers from the snowmelt period into summer. Unlike most studied catchments, lateral flow of soil water during the study year is not a primary source of streamflow. Instead, saprolite and groundwater act as integrators of soil water that flows vertically in this system. Our results do not support the flushing hypothesis as observed in similar systems and instead indicate that temporal variation in connectivity may cause the unexpected dilution behavior displayed by DOC in this catchment.
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    What Do P-Wave Velocities Tell Us About the Critical Zone?
    Flinchum, Brady A.; Holbrook, W. Steven; Carr, Bradley J. (Frontiers, 2022-01-10)
    Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information-the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.