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Ian Baker - One of the best experts on this subject based on the ideXlab platform.
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Impact of physical properties and accumulation rate on Pore close-off in layered firn
The Cryosphere, 2014Co-Authors: Stephanie Gregory, Mary R. Albert, Ian BakerAbstract:Investigations into the physical characteristics of deep firn near the lock-in zone through Pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age–gas age difference. The purpose of this study is to examine the nature of Pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open Pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining Pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the Pore Space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of Interconnected Pore Space. The low-accumulation sites, with generally coarse grains, close off at lower open porosities (~ 10%) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on Pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and Pore structure, rather than the density variability of the layers.
C. Pellicioli - One of the best experts on this subject based on the ideXlab platform.
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STUDY OF THE GEOLOGICAL EVOLUTION AND STRUCTURAL PATTERNS AT DIFFERENT SCALES CHARACTERISING THE TRES VIRGENES REGION (MEXICO) TO EVALUATE THE GEOTHERMAL ENERGY POTENTIAL.
'Universita degli Studi di Milano-Bicocca Symphonya Emerging Issues in Management', 2020Co-Authors: C. PellicioliAbstract:Faults play a fundamental role in sustaining fluid circulation across volcanic-geothermal regions, where volcanic rocks commonly exhibit low porosity and permeability. In fact, most of geothermal resources production is often connected to faults and fault zones. Thus, a precise knowledge of deformational patterns is of paramount importance to extend the assessment of the geothermal potential to areas surrounding a producing field, and to find strategies to enhance energy production. Increasing electrical energy production and using geothermal energy to desalinate seawater have recently become main concerns of the Mexican authorities with regards to the Tres Vírgenes region (Baja California, Mexico), where the Las Tres Vírgenes geothermal field represents the only producing zone across the Baja California peninsula, which nowadays struggles with energy and water supplies due to the recent growth of tourism. The herein presented PhD research places into this scenario, by means of the CeMIE Geo project P15 of SENER-CONACyT, which was appointed by the Mexican authorities to UNAM (Universidad Nacional Autónoma de México) and sponsored this Thesis. In order to evaluate the geothermal prospectivity of areas surrounding the currently producing Las Tres Vírgenes field and the role of structural patterns affecting the Tres Vírgenes region in sustaining hydrothermal fluid migration a multi-disciplinary approach has been deployed, including a wide range of methodologies and receiving contributions from several branches of the Earth Sciences. Five studies have been performed, whose results are presented in this Thesis under the form of papers published or submitted to dedicated ISI-Journals: 1) A geological study on La Reforma caldera complex, resulting in a 1:50,000 scale geological map produced with modern survey methodologies and representing an important contribution concerning the geological evolution of the Tres Vírgenes region and the Quaternary volcanic processes linked to the exploitation of geothermal resources in the Las Tres Vírgenes field. 2) The investigation of the structural patterns affecting the Tres Vírgenes region, by coupling observations at the meso- (field data) and micro-scales (image analysis of fault-related micro-fractures and veins), resulting in a structural model for the Tres Vírgenes region. This structural model highlights the strong control of regional-scale strike-slip oblique structures linked to the Gulf of California on Quaternary volcanic processes and the interplay between regional and volcanic structures in exhuming the Cretaceous basement throughout the region. 3) The assessment at the micro-scale of geometrical and chronological relationships between faulting and multiple fluid circulation events depositing mineralization along fault planes, providing further support to the proposed structural model and resulting in a deeper understanding of fault-related mechanisms across the Tres Vírgenes region. Fractal statistics applied to fault-related deformation patterns indicates and quantifies the self-similarity (fractal charcter) of regional structures at the scale of the Tres Vírgenes region, possibly extending to this sector of the Gulf of California. 4) Multi-scale petrographic and geochemical investigations, determining the composition, origin and temperature of fluids circulating and depositing calcite along regional faults. The integration of different techniques working at different scales defines a novel methodology, which was for the first time applied in a volcanic-geothermal setting in this Thesis. Fluids circulating along regional faults at different temperatures and redox conditions display a geothermal-hydrothermal component and reach maximum temperatures (100°C) in proximity of the Tres Vírgenes feeding system, acting as the heating source for the hydrothermal fluids and defining the areas where fault-sustained hydrothermal circulation is more likely to occur. 5) The quantification of physical properties of undeformed and faulted volcanic rocks through triaxial tests, X-ray computerized tomography (CT) and coupled permeability simulations, suggesting the highly unlikeliness of vertical and lateral fluid migration through the exposed undeformed Quaternary volcanic rocks in the areas surrounding the Las Tres Vírgenes field. The estimate of along- and across-fault fluid flow potential, and of changes in the Interconnected Pore Space and fault-permeability once these volcanic rocks are involved within regional damage zones, confirms the role of regional structures in focusing hydrothermal fluid circulation across the Tres Vírgenes region. The novel and quantitative findings of the several studies constituting this Thesis bear multifaceted implications: of regional and industry-related importance concerning modelling of fluid flow through the Tres Vírgenes and other geothermal regions worldwide, and more far-reaching concerning upper-crustal deformation processes in volcanic and non-volcanic settings
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STUDY OF THE GEOLOGICAL EVOLUTION AND STRUCTURAL PATTERNS AT DIFFERENT SCALES CHARACTERISING THE TRES VIRGENES REGION (MEXICO) TO EVALUATE THE GEOTHERMAL ENERGY POTENTIAL.
Università degli Studi di Milano, 2020Co-Authors: C. PellicioliAbstract:Faults play a fundamental role in sustaining fluid circulation across volcanic-geothermal regions, where volcanic rocks commonly exhibit low porosity and permeability. In fact, most of geothermal resources production is often connected to faults and fault zones. Thus, a precise knowledge of deformational patterns is of paramount importance to extend the assessment of the geothermal potential to areas surrounding a producing field, and to find strategies to enhance energy production. Increasing electrical energy production and using geothermal energy to desalinate seawater have recently become main concerns of the Mexican authorities with regards to the Tres V\uedrgenes region (Baja California, Mexico), where the Las Tres V\uedrgenes geothermal field represents the only producing zone across the Baja California peninsula, which nowadays struggles with energy and water supplies due to the recent growth of tourism. The herein presented PhD research places into this scenario, by means of the CeMIE Geo project P15 of SENER-CONACyT, which was appointed by the Mexican authorities to UNAM (Universidad Nacional Aut\uf3noma de M\ue9xico) and sponsored this Thesis. In order to evaluate the geothermal prospectivity of areas surrounding the currently producing Las Tres V\uedrgenes field and the role of structural patterns affecting the Tres V\uedrgenes region in sustaining hydrothermal fluid migration a multi-disciplinary approach has been deployed, including a wide range of methodologies and receiving contributions from several branches of the Earth Sciences. Five studies have been performed, whose results are presented in this Thesis under the form of papers published or submitted to dedicated ISI-Journals: 1) A geological study on La Reforma caldera complex, resulting in a 1:50,000 scale geological map produced with modern survey methodologies and representing an important contribution concerning the geological evolution of the Tres V\uedrgenes region and the Quaternary volcanic processes linked to the exploitation of geothermal resources in the Las Tres V\uedrgenes field. 2) The investigation of the structural patterns affecting the Tres V\uedrgenes region, by coupling observations at the meso- (field data) and micro-scales (image analysis of fault-related micro-fractures and veins), resulting in a structural model for the Tres V\uedrgenes region. This structural model highlights the strong control of regional-scale strike-slip oblique structures linked to the Gulf of California on Quaternary volcanic processes and the interplay between regional and volcanic structures in exhuming the Cretaceous basement throughout the region. 3) The assessment at the micro-scale of geometrical and chronological relationships between faulting and multiple fluid circulation events depositing mineralization along fault planes, providing further support to the proposed structural model and resulting in a deeper understanding of fault-related mechanisms across the Tres V\uedrgenes region. Fractal statistics applied to fault-related deformation patterns indicates and quantifies the self-similarity (fractal charcter) of regional structures at the scale of the Tres V\uedrgenes region, possibly extending to this sector of the Gulf of California. 4) Multi-scale petrographic and geochemical investigations, determining the composition, origin and temperature of fluids circulating and depositing calcite along regional faults. The integration of different techniques working at different scales defines a novel methodology, which was for the first time applied in a volcanic-geothermal setting in this Thesis. Fluids circulating along regional faults at different temperatures and redox conditions display a geothermal-hydrothermal component and reach maximum temperatures (100\ub0C) in proximity of the Tres V\uedrgenes feeding system, acting as the heating source for the hydrothermal fluids and defining the areas where fault-sustained hydrothermal circulation is more likely to occur. 5) The quantification of physical properties of undeformed and faulted volcanic rocks through triaxial tests, X-ray computerized tomography (CT) and coupled permeability simulations, suggesting the highly unlikeliness of vertical and lateral fluid migration through the exposed undeformed Quaternary volcanic rocks in the areas surrounding the Las Tres V\uedrgenes field. The estimate of along- and across-fault fluid flow potential, and of changes in the Interconnected Pore Space and fault-permeability once these volcanic rocks are involved within regional damage zones, confirms the role of regional structures in focusing hydrothermal fluid circulation across the Tres V\uedrgenes region. The novel and quantitative findings of the several studies constituting this Thesis bear multifaceted implications: of regional and industry-related importance concerning modelling of fluid flow through the Tres V\uedrgenes and other geothermal regions worldwide, and more far-reaching concerning upper-crustal deformation processes in volcanic and non-volcanic settings
Clemens Muller - One of the best experts on this subject based on the ideXlab platform.
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particle rearrangement and Pore Space coarsening during solid state sintering
Journal of the American Ceramic Society, 2009Co-Authors: Hans Eckart Exner, Clemens MullerAbstract:Coarsening of porosity during sintering has been observed in powder compacts of metallic, ceramic, and amorphous materials. Monitoring and modelling of the growth of individual (closed) Pores in the late sintering stages are well established. Porosity is Interconnected up to very high densities. Coarsening of the continuous Pore Space takes place during the initial and intermediate sintering stages. This coarsening is caused by localized transport of atoms or molecules (diffusion or viscous flow) as well as by bulk particle movement (rearrangement). Its quantitative exploration poses problems both experimentally and theoretically. Ways to characterize the geometry of the Interconnected Pore Space and of closed Pores are discussed with emphasis on stereological parameters. Recent and classical approaches, experimental findings with 2D model arrangements (as the formation and opening up of particle contacts, Pore coarsening, and particle rearrangement) and some advances of computer simulations are discussed together with open questions.
K. Soga - One of the best experts on this subject based on the ideXlab platform.
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Mechanics of granular column collapse in fluid at varying slope angles
Journal of Hydrodynamics, 2017Co-Authors: K. Kumar, J.-y. Delenne, K. SogaAbstract:This paper investigates the effect of initial volume fraction on the runout characteristics of collapse of granular columns on slopes in fluid. 2-D sub-grain scale numerical simulations are performed to understand the flow dynamics of granular collapse in fluid. The discrete element method (DEM) technique is coupled with the lattice Boltzmann method (LBM), for fluid-grain interactions, to understand the evolution of submerged granular flows. The fluid phase is simulated using multiple-relaxation-time LBM (LBM-MRT) for numerical stability. In order to simulate Interconnected Pore Space in 2-D, a reduction in the radius of the grains (hydrodynamic radius) is assumed during LBM computations. The collapse of granular column in fluid is compared with the dry cases to understand the effect of fluid on the runout behaviour. A parametric analysis is performed to assess the influence of the granular characteristics (initial packing) on the evolution of flow and run-out distances for slope angles of 0°, 2.5°, 5° and 7.5°. The granular flow dynamics is investigated by analysing the effect of hydroplaning, water entrainment and viscous drag on the granular mass. The mechanism of energy dissipation, shape of the flow front, water entrainment and evolution of packing density is used to explain the difference in the flow characteristics of loose and dense granular column collapse in fluid.
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Underwater granular flows down inclined planes
2014Co-Authors: K. Kumar, K. Soga, J.-y. DelenneAbstract:In this study, two-dimensional sub-grain scale numerical simulations are performed to understand the local rheology of dense granular flows in fluid. The Discrete Element (DEM) technique is coupled with the Lattice Boltzmann Method (LBM), for fluid-grain interactions, to understand the evolution of immersed granular flows. The fluid phase is simulated using Multiple-Relaxation-Time LBM (LBM-MRT) for numerical stability. The Eulerian nature of the LBM formulation, together with the common explicit time step scheme of both LBM and DEM makes this coupling strategy an efficient numerical procedure for systems dominated by both grain-grain and grain-fluid interactions. In order to simulate Interconnected Pore Space in 2D, a reduction in the radius of the grains (hydrodynamic radius) is assumed during LBM computations. By varying the hydrodynamic radius of the grains, granular materials of different permeabilities can be simulated. A parametric analysis is performed to assess the influence of the granular characteristics (initial packing, permeability, slope of the inclined plane) on the evolution of flow and run-out distances. The effect of hydrodynamic forces and hydroplaning on the run-out evolution is analysed by comparing the mechanism of energy dissipation and flow evolution in dry and immersed granular flows. Voronoi tesselation was used to study the evolution of local density and water entrainment at the flow front.
Stephanie Gregory - One of the best experts on this subject based on the ideXlab platform.
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Impact of physical properties and accumulation rate on Pore close-off in layered firn
The Cryosphere, 2014Co-Authors: Stephanie Gregory, Mary R. Albert, Ian BakerAbstract:Investigations into the physical characteristics of deep firn near the lock-in zone through Pore close-off are needed to improve understanding of ice core records of past atmospheric composition. Specifically, the permeability and microstructure profiles of the firn through the diffusive column influence the entrapment of air into bubbles and thus the ice age–gas age difference. The purpose of this study is to examine the nature of Pore closure processes at two polar sites with very different local temperatures and accumulation rates. Density, permeability, and microstructure measurements were made on firn cores from the West Antarctic Ice Sheet (WAIS) Divide, a site that has moderate accumulation rates with a seasonal climate archive, and Megadunes in East Antarctica, a site that is a natural laboratory for accumulation rate effects in the cold low-accumulation desert. We found that the open Pore structure plays a more important role than density in predicting gas transport properties, throughout the porous firn matrix. For firn below 50 m depth at both WAIS Divide and Megadunes, finer-grained layers experience close-off shallower in the firn column than do coarser-grained layers, regardless of which grain size layer is the denser layer at depth. Pore close-off occurs at a critical open porosity that is accumulation rate dependent. Defining Pore close-off at a critical open porosity for a given accumulation rate as opposed to a critical total porosity accounts for the Pore Space available for gas transport. Below the critical open porosity, the firn becomes impermeable despite having small amounts of Interconnected Pore Space. The low-accumulation sites, with generally coarse grains, close off at lower open porosities (~ 10%) of high-accumulation sites that have generally finer grains. The microstructure and permeability even near the bottom of the firn column are relic indicators of the nature of accumulation when that firn was at the surface. The physical structure and layering are the primary controlling factors on Pore close-off. In contrast to current assumptions for polar firn, the depth and length of the lock-in zone is primarily dependent upon accumulation rate and microstructural variability due to differences in grain size and Pore structure, rather than the density variability of the layers.