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  • 2020-2024  (33)
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  • 1
    Publikationsdatum: 2023-06-12
    Beschreibung: Soils and landscapes are bridges of space and time, as they simultaneously and authentically show essential aspects that were previously separated by time and space (such as cultural and activity-related aspects from past and present) to the trained observer - albeit only in excerpts. Therefore, this article presents a series of impact indicators for soil changes, starting with extreme (anthropogenic) interventions and ending with equally extreme ("natural") events. An essential difference to specifically planning-relevant or human ecological concepts, which, for example, specify land use/load categories, is that the following impact indicators perceive soils as a phenomenon in themselves and do not define them through attributed functions. Particular attention is focused on their changeability and vital development potential, as well as on their property as a sphere of penetration of living and material things, with emphasis on the noetic effect. The intervention or event spaces on the earth's surface can be differentiated quantitatively through the type, strength, and duration of the phenomena. The intensity of all processes can be described by amplitude (the strength of the interventions/events) and frequency (the repetition rate of the interventions/events) and can be specifically identified and quantified by, for example, material inputs or outputs per unit of time. For the first time, there would be a system for measuring the ecological quality of anthropogenic land use, which could serve as an "alert system for the external technological culture," and could help us become aware of our "inner" culture.
    Beschreibung: research
    Schlagwort(e): ddc:550 ; Boden ; Landschaft ; Bodendegeneration ; Deutschland ; Anthropogene Bodenveränderung
    Sprache: Deutsch
    Materialart: doc-type:article
    Format: 9
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  • 2
    Publikationsdatum: 2024-02-14
    Beschreibung: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Aseismic slip may occur during a long preparatory phase preceding earthquakes, and what controls it remains poorly understood. In this study, we explored the role of load point velocity and surface roughness on slow slip during the preparatory stage prior to stick‐slip events. To that end, we conducted displacement‐rate controlled friction experiments by imposing varying load point velocities on sawcut granite samples with different surface roughness at a confining pressure of 35 MPa. We measured the average slip along the fault with the recorded far‐field displacements and strain changes, while acoustic emission sensors and local strain gages were used to capture local slip variations. We found that the average amount of aseismic slip during the preparatory stage increases with roughness, whereas precursory slip duration decreases with increased load point velocity. These results reveal a complex slip pattern on rough faults which leads to dynamic ruptures at high load point velocities.〈/p〉
    Beschreibung: Plain Language Summary: Earthquakes occur mostly along preexisting faults in the earth crust. These faults exhibit various geometrical complexities and are subjected to different strain rates. In the laboratory, we produce earthquake analogs by sliding sawcut granite blocks. We vary the geometrical complexity of the faults by roughening their surfaces and modify the strain rate by displacing the blocks at varying velocities. Under these different conditions, we measure how the forces accumulated by friction are released, by measuring stresses and displacements applied on the block's edges, using local strain deformation sensors, and by recording very small earthquakes occurring during sliding along the sawcut faults. We find that smooth sawcut faults tend to release all the energy accumulated very abruptly, after a very small amount of slip, regardless of the load point velocity applied. The processes leading to failure in the case of a rough fault are much more complex, involving a large amount of slip, and numerous small earthquakes which are distributed heterogeneously in space and time.〈/p〉
    Beschreibung: Key Points: 〈list list-type="bullet"〉 〈list-item〉 〈p xml:lang="en"〉Acoustic emissions highlight the complex preparatory phase prior stick‐slips on rough faults〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Preparatory slip increases with roughness and the duration of the preparatory phase decreases with increasing load point velocity〈/p〉〈/list-item〉 〈list-item〉 〈p xml:lang="en"〉Rough and heterogeneous faults are more stable than smooth faults, but can become unstable with a small increase of load point velocity〈/p〉〈/list-item〉 〈/list〉 〈/p〉
    Beschreibung: SAIDAN
    Beschreibung: https://doi.org/10.5281/zenodo.6411819
    Schlagwort(e): ddc:550.78 ; rock friction ; roughness ; dynamic rupture ; acoustic emissions
    Sprache: Englisch
    Materialart: doc-type:article
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  • 3
    Publikationsdatum: 2023-11-13
    Beschreibung: Rifting and continental break‐up are fundamental tectonic processes, the understanding of which is of prime importance. However, the vast temporal and spatial scales involved pose major limitations to researchers. Analog tectonic modeling represents a great means to mitigate these limitations, but studying the complex internal deformation of lithospheric‐scale models remains a challenge. We therefore present a novel method for lithospheric‐scale rifting models that are uniquely monitored in an X‐ray CT scanner, which combined with digital image correlation (DIC) techniques, provides unparalleled insights into model deformation. Our first models illustrate how the degree of coupling between competent lithospheric layers, which are separated by a weak lower crustal layer, strongly impacts rift system development. Low coupling isolates the upper crust from the upper lithospheric mantle layer below, preventing an efficient transfer of deformation between both layers. By contrast, fast rifting increases coupling, so that deformation in the mantle is efficiently transferred to the upper crust, inducing either a symmetric or asymmetric (double) rift system. Furthermore, oblique divergence may lead to en echelon graben arrangements and delayed exhumation of the lower crustal layer. The successful application of our novel modeling approach, yielding these first‐order insights, provides a clear incentive to continue running lithospheric‐scale rifting models, and to apply advanced monitoring techniques to extract as much information from models as possible. There is indeed a broad range of opportunities for follow‐up studies within (and beyond) the field of rift tectonics.
    Beschreibung: Plain Language Summary: The Earth's surface consists of tectonic plates that are in constant motion, driven by titanic forces deep within the planet. One of the key plate tectonic processes is the stretching (rifting) and eventual break‐up of continents, leading to the opening of oceanic basins. Understanding the mechanisms involved is of great importance. However, studying continental break‐up is challenging due to the vast size of plate tectonic systems, and the extensive timescales over which they evolve: plate tectonic processes can rarely be directly observed. A practical solution to this issue is the use of analog experiments, which reproduce these processes in a matter of hours or days in a modestly sized laboratory. However, a major obstacle that remains is the opacity of these models: similar to tectonic plates, these models are opaque, so that their internal evolution remains hidden. X‐ray CT‐scanning provides an unrivaled means to reveal a model's internal structures during a model run. Here we present the first‐ever application of CT‐scanning to monitor relatively complex lithospheric‐scale models of continental rifting. The CT scans provide unique insights into the internal evolution of such models, and we point out various possibilities for interesting follow‐up studies.
    Beschreibung: Key Points: We present the first‐ever lithospheric‐scale analog models of rifting monitored in a CT scanner, revealing their complex internal deformation. We quantify this deformation via Digital Image Correlation analysis, and show the impact of coupling and oblique rifting on rift evolution. The successful application of our novel modeling approach provides a strong incentive for follow‐up tectonic modeling studies.
    Beschreibung: Schweizerischer Nationalfonds zur Förderung der Wissenschaftlichen Forschung http://dx.doi.org/10.13039/501100001711
    Beschreibung: Helmholtz‐Zentrum Potsdam ‐ Deutsches GeoForschungsZentrum GFZ http://dx.doi.org/10.13039/501100010956
    Beschreibung: https://doi.org/10.5880/fidgeo.2022.030
    Beschreibung: https://doi.org/10.5880/fidgeo.2022.008
    Beschreibung: https://doi.org/10.5880/fidgeo.2023.006
    Beschreibung: https://doi.org/10.5880/fidgeo.2023.005
    Schlagwort(e): ddc:550.78 ; rifting ; analog modeling ; continental break‐up ; X‐ray CT‐scanning ; visualization ; monitoring
    Sprache: Englisch
    Materialart: doc-type:article
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  • 4
    Publikationsdatum: 2023-11-17
    Beschreibung: 〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Laser ablation multi‐collector‐inductively coupled plasma‐mass spectrometry (LA‐MC‐ICP‐MS) has become a valuable tool for the 〈italic toggle="no"〉in situ〈/italic〉 measurement of the boron isotope composition of geological samples at high (tens to hundreds of μm) spatial resolution. That said, this application suffers from significant analytical challenges. We focus in this study on the underlying processes of two of the main causes for inaccuracies using this technique. We provide empirical evidence that not only Ca ions (Sadekov 〈italic〉et al〈/italic〉. 2019, Standish 〈italic〉et al〈/italic〉. 2019, Evans 〈italic〉et al〈/italic〉. 2021) but also Ar ions, that are reflected within the flight tube of the mass spectrometer, are the source for previously reported issues with spectral baselines. We also address the impact of plasma conditions on the instrumental mass fractionation as a source for matrix‐ and mass‐load‐related analytical biases. Comparing experimental data with the results of a dedicated release and diffusion model (RDM) we estimate that a close to complete (~ 97%) release of boron from the sample aerosol is needed to allow for consistently accurate LA boron isotope measurement results without the need for corrections.〈/p〉
    Beschreibung: Key Points: 〈list list-type="bullet" id="ggr12500-list-0101"〉 〈list-item〉〈p〉Two separate main sources for inaccuracy of boron isotope measurements by laser ablation: B isotope fractionation in the ICP and the known scattered ion baseline problem.〈/p〉〈/list-item〉 〈list-item〉〈p〉Boron isotope fractionation in the ICP varies systematically with plasma condition (NAI).〈/p〉〈/list-item〉 〈list-item〉〈p〉Behaviour of B isotopes within the ICP simulated by a release and diffusion model (RDM).〈/p〉〈/list-item〉 〈/list〉 〈boxed-text position="anchor" content-type="graphic" id="ggr12511-blkfxd-0001" xml:lang="en"〉 〈graphic position="anchor" id="jats-graphic-1" xlink:href="urn:x-wiley:16394488:media:ggr12511:ggr12511-toc-0001"〉 〈/graphic〉 〈/boxed-text〉 〈/p〉
    Schlagwort(e): ddc:550.78 ; boron isotopes ; laser ablation ; scattered ions ; mass fractionation ; plasma conditions
    Sprache: Englisch
    Materialart: doc-type:article
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  • 5
    Publikationsdatum: 2023-01-20
    Beschreibung: Understanding conditions in the Earth's interior requires data derived from laboratory experiments. Such experiments provide important insights into the conditions under which mineral reactions take place as well as processes that control the localization of deformation in the deep Earth. We performed Griggs‐type general shear experiments in combination with numerical models, based on continuum mechanics, to quantify the effect of evolving sample geometry of the experimental assembly. The investigated system is constituted by CaCO3 and the experimental conditions are near the calcite‐aragonite phase transition. All experimental samples show a heterogeneous distribution of the two CaCO3 polymorphs after deformation. This distribution is interpreted to result from local stress variations. These variations are in agreement with the observed phase‐transition patterns and grain‐size gradients across the experimental sample. The comparison of the mechanical models with the sample provides insights into the distribution of local mechanical parameters during deformation. Our results show that, despite the use of homogeneous sample material (here calcite), stress variations develop due to the experimental geometry. The comparison of experiments and numerical models indicates that aragonite formation is primarily controlled by the spatial distribution of mechanical parameters. Furthermore, we monitor the maximum pressure and σ1 that is experienced in every part of our model domain for a given amount of time. We document that local pressure (mean stress) values are responsible for the transformation. Therefore, if the role of stress as a thermodynamic potential is investigated in similar experiments, an accurate description of the state of stress is required.
    Beschreibung: Plain Language Summary: To understand processes in the Earth's interior, we can simulate the extreme conditions via laboratory experiments by compressing and heating millimeter‐sized samples. Such experiments provide important insights into mineral reactions and processes that control deformation in the Earth. We performed rock deformation experiments close to calcite‐aragonite phase (CaCO3) transition. Deforming the sample leads to stress variations due to the experimental geometry. These variations are documented by locally occurring phase transition and variation in the grain‐size. We performed computer simulations of the deforming sample to quantify, for the first time, the effect of sample geometry on the distribution of mechanical variables, such as stress, pressure, or deformation, inside the sample. The new findings document that any mechanical variable cannot be treated as homogeneous within the sample because the variations can be significant. Deforming the sample leads to stress concentrations. By comparing the experimental observations and simulation results, we show that locally high pressure triggers the phase transition to aragonite, the high‐pressure polymorph. This has important consequences for further thermodynamic interpretations of systems under stress, where the role of deformation, pressure, or maximum principal stress on mineral reactions is investigated.
    Beschreibung: Key Points: Heterogeneous stress distribution in deformation experiments is investigated by numerical models, locally resolving mechanical variables. Resolving the mechanical variables in experiments suggests a link between local pressure (mean stress) variations and phase transition. Thermodynamic interpretations of deformed samples require a detailed understanding of local mechanical parameters.
    Beschreibung: ETH Zürich Foundation http://dx.doi.org/10.13039/501100012652
    Beschreibung: https://doi.org/10.5281/zenodo.6974768
    Schlagwort(e): ddc:550.78 ; deformation experiments ; numerical modeling ; phase transition ; rock deformation ; localization ; stress variations
    Sprache: Englisch
    Materialart: doc-type:article
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  • 6
    Publikationsdatum: 2024-03-25
    Beschreibung: This study aims to experimentally investigate the possibility of combining two extended continuum theories for two‐phase flow. One of these theories considers interfacial area as a separate state variable, and the other explicitly discriminates between connected and disconnected phases. This combination enhances our potential to effectively model the apparent hysteresis, which generally dominates two‐phase flow. Using optical microscopy, we perform microfluidic experiments in quasi‐2D artificial porous media for various cyclic displacement processes and boundary conditions. Specifically for a number of sequential drainage processes, with detailed image (post‐)processing, pore‐scale parameters such as the interfacial area between the phases (wetting, non‐wetting, and solid), and local capillary pressure, as well as macroscopic parameters like saturation, are estimated. We show that discriminating between connected and disconnected clusters and the concept of the interfacial area as a separate state variable can be an appropriate way of modeling hysteresis in a two‐phase flow scheme. The drainage datasets of capillary pressure, saturation, and specific interfacial area, are plotted as a surface, given by f (P〈sup〉c〈/sup〉, s〈sup〉w〈/sup〉, a〈sup〉wn〈/sup〉) = 0. These surfaces accommodate all data points within a reasonable experimental error, irrespective of the boundary conditions, as long as the corresponding liquid is connected to its inlet. However, this concept also shows signs of reduced efficiency as a modeling approach in datasets gathered through combining experiments with higher volumetric fluxes. We attribute this observation to the effect of the porous medium geometry on the phase distribution. This yields further elaboration, in which this speculation is thoroughly studied and analyzed.
    Beschreibung: Key Points: A large number of two‐phase displacement microfluidic experiments under different boundary conditions are performed. A code has been developed in order to spatially and temporally resolve the experiments with high accuracy. Including both connectivity and interfacial area as state variables in two‐phase flow continuum theories improves modeling hysteresis.
    Beschreibung: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Beschreibung: Stuttgart Center for Simulation Science, Universität Stuttgart http://dx.doi.org/10.13039/501100022175
    Beschreibung: https://doi.org/10.18419/darus-2250
    Schlagwort(e): ddc:550.78 ; microfluidic experiments ; optical microscopy ; two‐phase flow in porous medium ; image processing ; extended theory of porous medium ; phase connectivity
    Sprache: Englisch
    Materialart: doc-type:article
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  • 7
    Publikationsdatum: 2024-04-03
    Beschreibung: To complement the information provided by deterministic seismic imaging at length scales above a certain resolution limit we present the first application of adjoint envelope tomography (AET) to experimental data. AET uses the full envelopes of seismic records including scattered coda waves to obtain information about the distribution of absorption and small‐scale heterogeneity which provide complementary information about the investigated medium. Being below the resolution limit this small‐scale structure cannot be resolved by conventional tomography but still affects wave propagation by attenuating ballistic waves and generating scattered waves. Using ultrasound data from embedded sensors in a meter‐sized concrete specimen we image the distribution of absorption and heterogeneity expressed by the intrinsic quality factor Q〈sup〉−1〈/sup〉 and the fluctuation strength ɛ that characterizes the strength of the heterogeneity. The forward problem is solved by modeling the 2‐D multiple nonisotropic scattering in an acoustic medium with spatially variable heterogeneity and attenuation using the Monte‐Carlo method. Gradients for the model updates are obtained by convolution with the back‐propagated envelope misfit using the adjoint formalism in analogy to full waveform inversion. We use a late coda time window to invert for absorption and an earlier time window to infer the distribution of heterogeneity. The results successfully locate an area of salt concrete with increased scattering and concentric anomalies of intrinsic attenuation. The resolution test shows that the recovered anomalies constitute reasonable representations of internal structure of the specimen.
    Beschreibung: Plain Language Summary; No matter how small the structures are that a seismic imaging method is able to resolve, there is structure with smaller length scale. On the one hand this small‐scale structure causes unwanted signals for conventional imaging approaches. But on the other hand it provides complementary information about the investigated medium. To turn this to our advantage we, for the first time, apply a new imaging method that uses the waves which are caused by the small‐scale structure. Using data of an experiment in a concrete block we demonstrate that we can identify areas of anomalous small‐scale structure. The results may help in the future to locate minute perturbations in the medium as they occur in the advent of volcanic eruptions or after earthquakes and to obtain new information about the geologic history of subsurface materials. The approach can be transferred to investigate man‐made materials and structures, such as deteriorating concrete constructions.
    Beschreibung: Key Points: Adjoint envelope tomography is for the first time applied to image the scattering and absorption structure with the real data. The iterative inversion uses envelopes of ultrasound signals from embedded transducers in a meter‐sized reinforced concrete specimen. We successfully image the distribution of statistical parameters characterizing the small‐scale heterogeneity and attenuation.
    Beschreibung: China Scholarship Council http://dx.doi.org/10.13039/501100004543
    Beschreibung: https://doi.org/10.5281/zenodo.7152278
    Schlagwort(e): ddc:550.78 ; seismic tomography ; adjoint method ; small‐scale heterogeneity ; attenuation ; ultrasonic experiment ; reinforced concrete
    Sprache: Englisch
    Materialart: doc-type:article
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  • 8
    Publikationsdatum: 2023-08-08
    Beschreibung: The unknown cooling-rate history of natural silicate melts can be investigated using differential scanning heat capacity measurements together with the limiting fictive temperature analysis calculation. There are a range of processes occurring during cooling and re-heating of natural samples which influence the calculation of the limiting fictive temperature and, therefore, the calculated cooling-rate of the sample. These processes occur at the extremes of slow cooling and fast quenching. The annealing of a sample at a temperature below the glass transition temperature upon cooling results in the subsequent determination of cooling-rates which are up to orders of magnitude too low. In contrast, the internal stresses associated with the faster cooling of obsidian in air result in an added exothermic signal in the heat capacity trace which results in an overestimation of cooling-rate. To calculate cooling-rate of glass using the fictive temperature method, it is necessary to create a calibration curve determined using known cooling- and heating-rates. The calculated unknown cooling-rate of the sample is affected by the magnitude of mismatch between the original cooling-rate and the laboratory heating-rate when using the matched cooling-/heating-rate method to derive a fictive temperature/cooling-rate calibration curve. Cooling-rates slower than the laboratory heating-rate will be overestimated, while cooling-rates faster than the laboratory heating-rate are underestimated. Each of these sources of error in the calculation of cooling-rate of glass materials—annealing, stress release and matched cooling/heating-rate calibration—can affect the calculated cooling-rate by factor of 10 or more.
    Beschreibung: Georg-August-Universität Göttingen (1018)
    Schlagwort(e): ddc:550.78 ; Fictive temperature ; Annealing ; Thermal stress ; Cooling-rate ; Calorimetry
    Sprache: Englisch
    Materialart: doc-type:article
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  • 9
    Publikationsdatum: 2023-08-08
    Beschreibung: In this study, the influence of CO〈sub〉2〈/sub〉 on the rheology of silica poor and K-rich melts from highly explosive eruptions from the Colli Albani Volcanic District (Italy) (CAVD) is measured for the first time. The investigated melts range from foidite to tephri-phonolite to tephrite from the CAVD to a phonolite from the Vesuvius (Italy) with CO〈sub〉2〈/sub〉 concentrations up to 0.50 wt%. Viscosity and calorimetric measurements are performed in the glass transition range Tg between 600 and 780 °C. Although nominally anhydrous, the investigated melts contain H〈sub〉2〈/sub〉O concentrations up to 0.23 wt%. The data exhibit a decrease in viscosity of approx. 100.40 Pa s for the phonolitic composition with ~ 0.07 wt% CO〈sub〉2〈/sub〉 and a Tg reduced by approx. 14 °C. For the tephritic composition, Tg is approx. 5 °C lower and has a viscosity reduced by 100.25 Pa s for the sample containing ~ 0.5 wt% CO〈sub〉2〈/sub〉. Calorimetric measurements of the tephri-phonolite show lowered onset of Tg by approx. 6 °C for the melt with ~ 0.11 wt% CO2 and Tg of the foidite appears not to be influenced by a CO〈sub〉2〈/sub〉 concentration of ~ 0.37 wt% CO〈sub〉2〈/sub〉. However, these tephri-phonolitic and foiditic melts foamed during calorimetric measurements preventing a reliable measurement. It would appear that most of this overall drop in viscosity is caused by the small amounts of H〈sub〉2〈/sub〉O in the melts with CO〈sub〉2〈/sub〉 slightly reducing the viscosity or having no effect on viscosity. Additionally, it is shown that the reduction in viscosity decreases with an increasing degree of the depolymerisation for the investigated melts. Consequently, the explosive style of the CAVD eruptions is mainly caused by crystals and bubbles which form and rise during magma storage and ascent which increases the magma viscosity whereas the CO〈sub〉2〈/sub〉 in the melt slightly reduces the viscosity.
    Beschreibung: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Beschreibung: Georg-August-Universität Göttingen (1018)
    Schlagwort(e): ddc:550.78 ; Viscosity ; Calorimetry ; Colli Albani ; Carbon dioxide ; Glass transition ; Foidite
    Sprache: Englisch
    Materialart: doc-type:article
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  • 10
    Publikationsdatum: 2023-08-08
    Beschreibung: Equilibria between Ti oxides and silicate melt lead to Ti isotope fractionation in terrestrial samples, with isotopically light Ti oxides and isotopically heavy coexisting melt. However, while Ti is mostly tetravalent in terrestrial samples, around 10% of the overall Ti is trivalent at f〈sub〉O2〈/sub〉 relevant to lunar magmatism (~ IW-1). The different valences of Ti in lunar samples, could additionally influence Ti stable isotope fractionation during petrogenesis of lunar basalts to an unknown extent. We performed an experimental approach using gas mixing furnaces to investigate the effect of Ti oxide formation at different f〈sub〉O2〈/sub〉 on Ti stable isotope fractionation during mare basalt petrogenesis. Two identical bulk compositions were equilibrated simultaneously during each experiment to guarantee comparability. One experiment was investigated with the EPMA to characterize the petrology of experimental run products, whereas the second experiment was crushed, and fabricated phases (i.e., oxides, silicates and glass) were handpicked, separated and digested. An aliquot of each sample was mixed with a Ti double-spike, before Ti was separated from matrix and interfering elements using a modified HFSE chemistry. Our study shows f〈sub〉O2〈/sub〉-dependent fractionation within seven samples from air to IW-1, especially ∆49Ti〈subs〈armalcolite〈/sub〉-melt and ∆49Ti〈sub〉armalcolite-orthopyroxene〈/sub〉 become more fractionated from oxidized to reduced conditions (− 0.092 ± 0.028-  − 0.200 ± 0.033 ‰ and  − 0.089 ± 0.027- − 0.250 ± 0.049 ‰, respectively), whereas ∆49Ti〈sub〉orthopyroxene〈/sub〉-melt shows only a minor fractionation (− 0.002 ± 0.017-0.050 ± 0.025 ‰). The results of this study show that Ti isotope fractionation during mare basalt petrogenesis is expected to be redox dependent and mineral-melt fractionation as commonly determined for terrestrial fO2 may not be directly applied to a lunar setting. This is important for the evaluation of Ti isotope fractionation resulting from lunar magmatism, which takes place under more reducing conditions compared to the more oxidized terrestrial magmatism.
    Beschreibung: Deutsche Forschungsgemeinschaft
    Beschreibung: Projekt DEAL
    Schlagwort(e): ddc:550.78 ; Titanium isotopes ; Lunar magma ocean ; Fractionation ; Experiments ; Ti oxides ; Armalcolite
    Sprache: Englisch
    Materialart: doc-type:article
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  • 11
    Publikationsdatum: 2023-08-08
    Beschreibung: Aquifer storage and recovery systems using multiple partially penetrating wells (MPPW-ASR) can form a viable solution to the problem of freshwater buoyancy when using brackish aquifers for freshwater storage. This study presents the result of a series of laboratory experiments that aimed at visualizing the shape of freshwater bodies injected into a brackish aquifer and determining the effect on the recovery efficiency (RE) of several MPPW-ASR operational variables. A model aquifer was built in a Plexiglas tank using glass beads and water was injected and abstracted through point and vertical wells, which were operated in various combinations. Numerical models were used to support the interpretation of the time-lapse photographs, and showed that three-dimensional flow effects had to be considered for a correct interpretation of the visible dye patterns. Upward migration of both fresh (during injection) and brackish water (during recovery) along the vertical wells was observed, indicating that the role of well infrastructure as conduits is a critical design criterion for real-world systems. Gravitational instabilities formed when freshwater did not extend all the way to the top of the aquifer, and this negatively impacted the RE by causing greater mixing. The positive freshwater buoyancy led to freshwater bodies that became narrower with depth, and the formation of thin, elongated buffer zones along the aquifer top in multicycle experiments. Up-coning below abstraction wells resulted in lower RE values, reinforcing the potential of scavenger wells to enhance MPPW-ASR system performance.
    Beschreibung: Bundesanstalt für Geowissenschaften und Rohstoffe (BGR) (4230)
    Schlagwort(e): ddc:550.78 ; Artificial recharge ; Laboratory experiments/measurements ; Numerical modelling ; Multiple partially penetrating wells ; Salt-water/fresh-water relations
    Sprache: Englisch
    Materialart: doc-type:article
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  • 12
    Publikationsdatum: 2023-08-08
    Beschreibung: The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with D〈sup〉biotite/melt〈/sup〉 of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with D〈sup〉biotite/melt〈/sup〉 〈 0.9. This is likely one of the reasons why Cu is enriched together with Mo (D〈sup〉biotite/melt〈/sup〉 = 0.29) in porphyry deposits associated with intermediate to felsic plutons, while the other transition metals are not. Both Nb and Ta are mildly compatible in biotite with D〈sup〉biotite/melt〈/sup〉 being larger for Nb (3.69) than for Ta (1.89). Moderate (15—30%) biotite fractionation would be sufficient to reduce the Nb/Ta ratio from the chondritic value to the range observed in the continental crust. Moreover, the strong partitioning of Ti into biotite implies that already modest biotite fractionation suppresses the saturation of Ti-oxide phases and thereby indirectly facilitates the enrichment of Ta over Nb in the residual melt. The heavy alkalis, alkaline earths, and Pb are only mildly fractionated between biotite and melt (D〈sup〉biotite/melt〈/sup〉 = 3.8 for Rb, 0.6 for Cs, 0.6 for Sr, 1.8 for Ba, 0.7 for Pb). The rare earth elements are generally incompatible in biotite, with a minimum for Dbiotite/melt of 0.03–0.06 at Gd, Tb, and Dy, while both the light and heavy rare earths are less incompatible (e.g. D〈sup〉biotite/melt〈/sup〉 = 0.6 for La and 0.3 for Yb). This behavior probably reflects a partitioning into two sites, the K site for the light rare earths and the octahedral Mg site for the heavy rare earths. There is no obvious dependence of the rare earth partition coefficients on tetrahedral Al in the biotite, presumably because charge balancing by cation vacancies is possible. Allanite was found as run product in some experiments. For the light rare earths, D〈sup〉allanite/melt〈/sup〉 is very high (e.g. 385 to 963 for Ce and Nd) and appears to increase with decreasing temperatures. However, the rather high solubility of allanite in the melts implies that it likely only crystallizes during the last stages of cooling of most magmas, except if the source magma is unusually enriched in rare earths.
    Beschreibung: Deutsche Forschungsgemeinschaft
    Beschreibung: Universität Bayreuth (3145)
    Schlagwort(e): ddc:550.78 ; Biotite ; Allanite ; Granite ; Partitioning ; Solubility ; Rare earths ; Niobium ; Tantalum ; Nb/Ta ratio ; Porphyry copper deposits
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 13
    Publikationsdatum: 2023-07-27
    Beschreibung: Fracture toughness is one of the key parameters for the characterization of brittle rock fracturing. Yet, constraints on it mainly rest on measurements performed at ambient pressure, although rock fracturing frequently occurs at elevated pressures even in geotechnical applications. To address the lack of a generally accepted evaluation procedure for tests at elevated pressure we explored the conditions for initiation and propagation of mode I fractures in samples subjected to bending at elevated pressure by numerical modeling and analytical considerations of the involved angular moments. We derived an evaluation procedure and applied it to experimental observations for specimens with either a chevron or a single-edge notch of four different rocks (a granite, a limestone, a marble and a sandstone) subjected to three-point bending at confining pressures up to 30 MPa. Two sealing methods were considered. Specimens were either varnished or jacketed by a rubber tube, differing in whether pressure is allowed to build up inside the pre-fabricated notch or not, respectively. Irrespective of notch geometry and sealing method, the determined toughness values increase significantly with confining pressure. The apparent toughness determined for jacketed specimens is, however, larger than that for varnished specimens, for which toughness seems to reach a plateau with increasing pressure. The similarity of the pressure dependence of the toughness determined for varnished, i.e., uniformly pressurized, samples with that of other physical properties suggests that it is controlled by the closure of pre-existing micro-cracks; the absence of pressure dependence beyond some tens of MPa suggests that non-linearity effects may not be as severe at depths beyond a few kilometers as previously discussed. Our study points to the necessity of resolving numerical issues associated with compressed fractures and of further improving experimental facilities for the determination of fracture toughness at elevated pressure.
    Beschreibung: German Federal Ministry of Economy
    Beschreibung: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Beschreibung: Ruhr-Universität Bochum (1007)
    Schlagwort(e): ddc:550.78 ; Toughness ; Confining pressure ; Three-point bending tests ; Chevron-edge notch
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 14
    Publikationsdatum: 2023-07-27
    Beschreibung: Along with the advance of the working face, coal experiences different loading stages. Laboratory tests and numerical simulations of fracture and damage evolution aim to better understand the structural stability of coal layers. Three-dimensional lab tests are performed and coal samples are reconstructed using X-ray computer tomography (CT) technique to get detailed information about damage and deformation state. Three-dimensional discrete element method (DEM)-based numerical models are generated. All models are calibrated against the results obtained from uniaxial compressive strength (UCS) tests and triaxial compression (TRX) tests performed in the laboratory. A new approach to simulate triaxial compression tests is established in this work with significant improved handling of the confinement to get realistic simulation results. Triaxial tests are simulated in 3D with the particle-based code PFC3D using a newly developed flexible wall (FW) approach. This new numerical simulation approach is validated by comparison with laboratory tests on coal samples. This approach involves an updating of the applied force on each wall element based on the flexible nature of a rubber sleeve. With the new FW approach, the influence of the composition (matrix and inclusions) of the samples on the peak strength is verified. Force chain development and crack distributions are also affected by the spatial distribution of inclusions inside the sample. Fractures propagate through the samples easily at low confining pressures. On the contrary, at high confining pressure, only a few main fractures are generated with orientation towards the side surfaces. The evolution of the internal fracture network is investigated. The development of microcracks is quantified by considering loading, confinement, and structural character of the rock samples. The majority of fractures are initiated at the boundary between matrix and inclusions, and propagate along their boundaries. The internal structure, especially the distribution of inclusions has significant influence on strength, deformation, and damage pattern.
    Beschreibung: Technische Universität Bergakademie Freiberg (3135)
    Schlagwort(e): ddc:550.78 ; Coal sample ; X-ray CT ; Sample reconstruction ; Triaxial compression test ; Discrete element modeling ; Flexible wall approach
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 15
    Publikationsdatum: 2023-07-29
    Beschreibung: It is commonly assumed that dry and saturated sands exhibit similar cone resistance–relative density relationships. Some studies pointed out that partial saturation and calcareous sands with considerable fines content are potential factors affecting these relationships. However, there is experimental evidence in Shaqour Bull Eng Geol Environ 66:59-70, (2006) that clean uncemented quartz sand may exhibit lower cone resistance in saturated conditions. The present study aims on contributing towards better understanding the effect of water saturation on cone resistance in sand. For this purpose, Ticino sand samples were prepared dry and saturated in a calibration chamber and cone penetration tests were performed over a wide range of relative densities and at two consolidation stresses. Overall, it was observed that dry and saturated samples exhibited similar cone resistances. Only slightly higher cone resistances were observed for dry samples at the lower consolidation stress. Two anomalous samples, which were tested dry at medium relative density, were found to exhibit way higher cone resistances than expected from published cone resistance–relative density relationships. The Young's modulus was observed to be proportional to cone resistance and independent of whether a sample was tested dry or saturated, being therefore considered as more robust soil property for cone resistance relationships.
    Beschreibung: Bundesministerium für Wirtschaft und Energie http://dx.doi.org/10.13039/501100006360
    Schlagwort(e): ddc:550.78 ; Cone penetration test ; Water saturation ; Dry sand ; Calibration chamber ; Relative density ; Young’s modulus
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 16
    Publikationsdatum: 2023-08-02
    Beschreibung: Predicting the onset, style and duration of explosive volcanic eruptions remains a great challenge. While the fundamental underlying processes are thought to be known, a clear correlation between eruptive features observable above Earth’s surface and conditions and properties in the immediate subsurface is far from complete. Furthermore, the highly dynamic nature and inaccessibility of explosive events means that progress in the field investigation of such events remains slow. Scaled experimental investigations represent an opportunity to study individual volcanic processes separately and, despite their highly dynamic nature, to quantify them systematically. Here, impulsively generated vertical gas-particle jets were generated using rapid decompression shock-tube experiments. The angular deviation from the vertical, defined as the “spreading angle”, has been quantified for gas and particles on both sides of the jets at different time steps using high-speed video analysis. The experimental variables investigated are 1) vent geometry, 2) tube length, 3) particle load, 4) particle size, and 5) temperature. Immediately prior to the first above-vent observations, gas expansion accommodates the initial gas overpressure. All experimental jets inevitably start with a particle-free gas phase (gas-only), which is typically clearly visible due to expansion-induced cooling and condensation. We record that the gas spreading angle is directly influenced by 1) vent geometry and 2) the duration of the initial gas-only phase. After some delay, whose length depends on the experimental conditions, the jet incorporates particles becoming a gas-particle jet. Below we quantify how our experimental conditions affect the temporal evolution of these two phases (gas-only and gas-particle) of each jet. As expected, the gas spreading angle is always at least as large as the particle spreading angle. The latter is positively correlated with particle load and negatively correlated with particle size. Such empirical experimentally derived relationships between the observable features of the gas-particle jets and known initial conditions can serve as input for the parameterisation of equivalent observations at active volcanoes, alleviating the circumstances where an a priori knowledge of magma textures and ascent rate, temperature and gas overpressure and/or the geometry of the shallow plumbing system is typically chronically lacking. The generation of experimental parameterisations raises the possibility that detailed field investigations on gas-particle jets at frequently erupting volcanoes might be used for elucidating subsurface parameters and their temporal variability, with all the implications that may have for better defining hazard assessment.
    Beschreibung: Seventh Framework Programme http://dx.doi.org/10.13039/501100004963
    Beschreibung: Deutsche Forschungsgemeinschaft
    Beschreibung: European Research Council http://dx.doi.org/10.13039/501100000781
    Beschreibung: https://doi.org/10.5880/fidgeo.2020.030
    Schlagwort(e): ddc:550.78 ; Explosive volcanism, ; Experimental volcanology, ; Spreading angle, ; Shock-tube
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 17
    Publikationsdatum: 2023-11-13
    Beschreibung: The aim of this work is to provide a complete data set of direct shear tests and to propose a corresponding simulation approach. Tests have been conducted on crystalline rock samples applying constant normal load (CNL) and constant normal stiffness (CNS) boundary conditions. A physical consistent algorithm which explicitly calculates the forces acting on the fracture surface (FFS) has been developed. This FFS approach can explain the occurrence of surface degradation and shows the main shear characteristics. After all, shearing of rough rock joints remains a complex process and the differences between laboratory and simulation results are still significant in some cases. All data and input files are provided free for download and testing.
    Beschreibung: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Beschreibung: Technische Universität Bergakademie Freiberg (3135)
    Beschreibung: http://www.ufz.de/record/dmp/archive/7925
    Beschreibung: http://www.ufz.de/record/dmp/archive/7924
    Beschreibung: https://github.com/Poetschke/Ecodist
    Schlagwort(e): ddc:550.78 ; GeomInt project ; Direct shear test ; Rock surface scanning ; Rock joint ; Joint constitutive model
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 18
    Publikationsdatum: 2023-11-15
    Beschreibung: The hydromechanical properties of single self-propping fractures under stress are of fundamental interest for fractured-rock hydrology and a large number of geotechnical applications. This experimental study investigates fracture closure and hydraulic aperture changes of displaced tensile fractures, aligned tensile fractures, and saw-cut fractures for two types of sandstone (i.e., Flechtinger and Fontainebleau) with contrasting mechanical properties, cycling confining pressure between 5 and 30 MPa. Emphasis is placed on how surface roughness, fracture wall offset, and the mechanical properties of the contact asperities affect the self-propping potential of these fractures under normal stress. A relative fracture wall displacement can significantly increase fracture aperture and hydraulic conductivity, but the degree of increase strongly depends on the fracture surface roughness. For smooth fractures, surface roughness remains scale-independent as long as the fracture area is larger than a roll-off wavelength and thus any further displacement does not affect fracture aperture. For rough tensile fractures, these are self-affine over a larger scale so that an incremental fracture wall offset likely leads to an increase in fracture aperture. X-ray microtomography of the fractures indicates that the contact area ratio of the tensile fractures after the confining pressure cycle inversely correlates with the fracture wall offset yielding values in the range of about 3–25%, depending, first, on the respective surface roughness and, second, on the strength of the asperities in contact. Moreover, the contact asperities mainly occur isolated and tend to be preferentially oriented in the direction perpendicular to the fracture wall displacement which, in turn, may induce flow anisotropy. This, overall, implies that relatively harder sedimentary rocks have a higher self-propping potential for sustainable fluid flow through fractures in comparison to relatively soft rocks when specific conditions regarding surface roughness and fracture wall offset are met.
    Beschreibung: Bundesministerium für Wirtschaft und Energie (DE)
    Beschreibung: China Scholarship Council http://dx.doi.org/10.13039/501100004543
    Beschreibung: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217)
    Schlagwort(e): ddc:550.78 ; Self-propping fracture ; Mechanical aperture ; Hydraulic aperture ; Normal stress ; Fracture wall offset ; Surface roughness
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 19
    Publikationsdatum: 2023-11-15
    Beschreibung: Using an innovative experimental set-up (Punch-Through Shear test), we initiated a shear zone (microfault) in Flechtingen sandstone and Odenwald granite under in situ reservoir conditions while monitoring permeability and fracture dilation evolution. The shear zone, which has a cylindrical geometry, is produced by a self-designed piston assembly that punches down the inner part of the sample. Permeability and fracture dilation were measured for the entire duration of the experiment. After the shear zone generation, the imposed shear displacement was increased to 1.2 mm and pore pressure changes of ±5 or ±10 MPa were applied cyclically to simulate injection and production scenarios. Thin sections and image analysis tools were used to identify microstructural features of the shear zone. The geometry of the shear zone is shown to follow a self-affine scaling invariance, similar to the fracture surface roughness. The permeability evolution related to the onset of the fracture zone is different for both rocks: almost no enhancement for the Flechtingen sandstone and an increase of more than 2 orders of magnitude for the Odenwald granite. Further shear displacement resulted in a slight increase in permeability. A fault compaction is observed after shear relaxation which is associated to a permeability decrease by a factor more than 3. Permeability changes during pressure cycling are reversible when varying the effective pressure. The difference in permeability enhancement between the sandstone and the granite is related to the larger width of the shear zones.
    Beschreibung: ReSalt Project
    Beschreibung: H2020 European Research Council http://dx.doi.org/10.13039/100010663
    Beschreibung: Projekt DEAL
    Schlagwort(e): ddc:550.78 ; Microfault ; Fracture permeability ; Microstructure ; Fault architecture ; Roughness ; Sandstone ; Granite
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 20
    Publikationsdatum: 2023-11-15
    Beschreibung: In this work, we aim to verify the predictions of the numerical simulators, which are used for designing field-scale hydraulic stimulation experiments. Although a strong theoretical understanding of this process has been gained over the past few decades, numerical predictions of fracture propagation in low-permeability rocks still remains a challenge. Against this background, we performed controlled laboratory-scale hydraulic fracturing experiments in granite samples, which not only provides high-quality experimental data but also a well-characterized experimental set-up. Using the experimental pressure responses and the final fracture sizes as benchmark, we compared the numerical predictions of two coupled hydraulic fracturing simulators—CSMP and GEOS. Both the simulators reproduced the experimental pressure behavior by implementing the physics of Linear Elastic Fracture Mechanics (LEFM) and lubrication theory within a reasonable degree of accuracy. The simulation results indicate that even in the very low-porosity (1–2 %) and low-permeability (10〈sup〉−18〈/sup〉 m〈sup〉2〈/sup〉 − 10〈sup〉−19〈/sup〉 m〈sup〉2〈/sup〉) crystalline rocks, which are usually the target of EGS, fluid-loss into the matrix and unsaturated flow impacts the formation breakdown pressure and the post-breakdown pressure trends. Therefore, underestimation of such parameters in numerical modeling can lead to significant underestimation of breakdown pressure. The simulation results also indicate the importance of implementing wellbore solvers for considering the effect of system compressibility and pressure drop due to friction in the injection line. The varying injection rate as a result of decompression at the instant of fracture initiation affects the fracture size, while the entry friction at the connection between the well and the initial notch may cause an increase in the measured breakdown pressure.
    Beschreibung: European Union's Horizon 2020 Research and Innovation programme
    Beschreibung: Commonwealth Scientific and Industrial Research Organisation http://dx.doi.org/10.13039/501100000943
    Beschreibung: RWTH Aachen (3131)
    Beschreibung: http://doi.org/10.5281/zenodo.3710746
    Schlagwort(e): ddc:550.78 ; Laboratory-experiments ; Hydraulic fracturing ; Simulation ; Leak-off ; Fracture toughness ; System compressibility ; Fracture radius ; Acoustic emission
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 21
    Publikationsdatum: 2023-11-15
    Beschreibung: This is a repond to the comments raised in Crisci´s et al. paper “Discussion on “Experimental Deformation of Opalinus Clay at Elevated Temperature and Pressure Conditions ¬ Mechanical Properties and the Influence of Rock Fabric” (2021). We are pleased to use the opportunity to clarify issues related to testing procedures and interpretation in more detail.
    Beschreibung: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Beschreibung: Helmholtz-Gemeinschaft http://dx.doi.org/10.13039/501100001656
    Beschreibung: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217)
    Schlagwort(e): ddc:550.78 ; Opalinus Clay ; Testing procedure ; Pore pressure generation ; Strain rate ; Drying-induced micro cracks
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 22
    Publikationsdatum: 2023-11-15
    Beschreibung: Silicate liquid immiscibility leading to formation of mixtures of distinct iron-rich and silica-rich liquids is common in basaltic and andesitic magmas at advanced stages of magma evolution. Experimental modeling of the immiscibility has been hampered by kinetic problems and attainment of chemical equilibrium between immiscible liquids in some experimental studies has been questioned. On the basis of symmetric regular solutions model and regression analysis of experimental data on compositions of immiscible liquid pairs, we show that liquid–liquid distribution of network-modifying elements K and Fe is linked to the distribution of network-forming oxides SiO2, Al2O3 and P2O5. We use the responding equation for testing chemical equilibrium in experiments not included in the regression analysis and compositions of natural immiscible melts found as glasses in volcanic rocks. Departures from equilibrium that the test revealed in crystal-rich multiphase experimental products and in natural volcanic rocks imply kinetic competition between liquid–liquid and crystal–liquid element partitioning. Immiscible liquid droplets in volcanic rocks appear to evolve along a metastable trend due to rapid crystallization. Immiscible liquids may be closer to chemical equilibrium in large intrusions where cooling rates are lower and crystals may be spatially separated from liquids.
    Beschreibung: DeutscheForschungsGemeinschaft http://dx.doi.org/10.13039/501100001659
    Beschreibung: Russian Science Foundation http://dx.doi.org/10.13039/501100006769
    Beschreibung: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217)
    Schlagwort(e): ddc:550.78 ; Experimental petrology ; Igneous rocks ; Silicate melts ; Liquid–liquid element distribution ; Symmetric regular solutions
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 23
    Publikationsdatum: 2023-11-15
    Beschreibung: The mechanical behavior of the sandy facies of Opalinus Clay (OPA) was investigated in 42 triaxial tests performed on dry samples at unconsolidated, undrained conditions at confining pressures (pc) of 50–100 MPa, temperatures (T) between 25 and 200 °C and strain rates (ε˙) of 1 × 10〈sup〉–3〈/sup〉–5 × 10〈sup〉–6〈/sup〉 s〈sup〉−1〈/sup〉. Using a Paterson-type deformation apparatus, samples oriented at 0°, 45° and 90° to bedding were deformed up to about 15% axial strain. Additionally, the influence of water content, drainage condition and pre-consolidation was investigated at fixed p〈sub〉c〈/sub〉–T conditions, using dry and re-saturated samples. Deformed samples display brittle to semi-brittle deformation behavior, characterized by cataclastic flow in quartz-rich sandy layers and granular flow in phyllosilicate-rich layers. Samples loaded parallel to bedding are less compliant compared to the other loading directions. With the exception of samples deformed 45° and 90° to bedding at p〈sub〉c〈/sub〉 = 100 MPa, strain is localized in discrete shear zones. Compressive strength (σ〈sub〉max〈/sub〉) increases with increasing p〈sub〉c〈/sub〉, resulting in an internal friction coefficient of ≈ 0.31 for samples deformed at 45° and 90° to bedding, and ≈ 0.44 for samples deformed parallel to bedding. In contrast, pre-consolidation, drainage condition, T and ε˙ do not significantly affect deformation behavior of dried samples. However, σ〈sub〉max〈/sub〉 and Young’s modulus (E) decrease substantially with increasing water saturation. Compared to the clay-rich shaly facies of OPA, sandy facies specimens display higher strength σmax and Young’s modulus E at similar deformation conditions. Strength and Young’s modulus of samples deformed 90° and 45° to bedding are close to the iso-stress Reuss bound, suggesting a strong influence of weak clay-rich layers on the deformation behavior.
    Beschreibung: Bundesministerium für Bildung und Forschung http://dx.doi.org/10.13039/501100002347
    Beschreibung: Helmholtz-Gemeinschaft http://dx.doi.org/10.13039/501100001656
    Beschreibung: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217)
    Schlagwort(e): ddc:550.78 ; Clay rock ; Sandy facies of Opalinus Clay ; Triaxial deformation experiments ; Microstructural deformation mechanisms ; Pressure-, temperature- and strain rate-dependent mechanical behaviour ; Anisotropy
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 24
    Publikationsdatum: 2023-11-16
    Beschreibung: This research work presents an experimental and numerical study of the coupled thermo-hydro-mechanical (THM) processes that occur during soil freezing. With focusing on the artificial ground freezing (AGF) technology, a new testing device is built, which considers a variety of AGF-related boundary conditions and different freezing directions. In the conducted experiments, a distinction is made between two thermal states: (1) The thermal transient state, which is associated with ice penetration, small deformations, and insignificant water suction. (2) The thermal (quasi-) steady state, which has a much longer duration and is associated with significant ice lens formation due to water suction. In the numerical modeling, a special focus is laid on the processes that occur during the thermal transient state. Besides, a demonstration of the micro-cryo-suction mechanism and its realization in the continuum model through a phenomenological retention-curve-like formulation is presented. This allows modeling the ice lens formation and the stiffness degradation observed in the experiments. Assuming a fully saturated soil as a biphasic porous material, a phase-change THM approach is applied in the numerical modeling. The governing equations are based on the continuum mechanical theory of porous media (TPM) extended by the phase-field modeling (PFM) approach. The model proceeds from a small-strain assumption, whereas the pore fluid can be found in liquid water or solid ice state with a unified kinematics treatment of both states. Comparisons with the experimental data demonstrate the ability and usefulness of the considered model in describing the freezing of saturated soils.
    Beschreibung: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Beschreibung: RWTH Aachen (3131)
    Schlagwort(e): ddc:550.78 ; Experimental study ; Frost heave ; Ice lens formation ; Phase-field modeling ; Soil freezing ; Thermo-hydro-mechanical coupling
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 25
    Publikationsdatum: 2023-09-14
    Beschreibung: Discrete randomly distributed fibers are commonly used to improve the engineering characteristics of the soil and thus soil properties such as shear strength, compressibility, density, and hydraulic conductivity. Most studies have so far focused on describing the behavior of soils containing randomly distributed fibers under dried or saturated conditions. However, the water table may seasonally fluctuate, thus generating unsaturated soil conditions. Therefore, a better understanding of the hydro-mechanical properties of unsaturated improved soils is of high necessity. In this research, the shear strength parameters of fine-grained soils were evaluated using the biaxial device available at Ruhr Universität Bochum. The applied device was modified to test unsaturated fine-grained soils with various degrees of saturation using axis translation and vapor equilibrium techniques. The experiments were conducted on fine soils containing 0, 0.5, and 1% fiber contents under a wide range of matric suctions. The ductile behavior was more noticeable in samples with lower suctions and higher straw contents. Furthermore, the shear strength of both unreinforced and reinforced fine-grained soils considerably increased by an increase in the suction. Finally, shear band inclination increased by the suction while decreasing by straw content.
    Beschreibung: Ruhr-Universität Bochum (1007)
    Schlagwort(e): ddc:550.78 ; Plane strain condition ; Shear strength ; Soil reinforcement ; Suction ; SWCC
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 26
    Publikationsdatum: 2023-12-19
    Beschreibung: The paper presents an experimental study on the effect of plastic fines content on the undrained behavior and liquefaction susceptibility of sand–fines mixtures under monotonic loading. The results of undrained monotonic triaxial compression tests conducted on mixtures of Hostun sand with varying amount (0–20%) and type (kaolin and calcigel bentonite) of plastic fines are presented. The specimens were prepared with different initial densities using the moist tamping method and consolidated at two different isotropic effective stresses. The results demonstrate that for both types of plastic fines, an increase in the fines content leads to a more contractive response and lower values of mobilized deviatoric stress. Despite similar relative density and fines content, the sand–kaolin mixtures showed a more contractive behavior than the sand–calcigel specimens. The steady-state lines (SSLs) in e–p´ space generally move downwards with increasing clay content. While the slopes of the SSLs for the clean Hostun sand and the mixtures with 10 and 20% kaolin are quite similar, the SSL lines for the specimens containing 10% or 20% calcigel run steeper or flatter, respectively. The inclination of the SSL in the q–p′ plane was found independent of clay type and content. The sand–kaolin mixtures were observed to be more susceptible to instability and flow liquefaction than the sand–calcigel mixtures.
    Schlagwort(e): ddc:550.78 ; Instability line ; Plastic fines content ; Sand–fines mixtures ; Steady-state line ; Undrained monotonic triaxial tests
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 27
    Publikationsdatum: 2023-06-19
    Beschreibung: The knowledge of tree species dependent turnover of soil organic matter (SOM) is limited, yet required to understand the carbon sequestration function of forest soil. We combined investigations of 13C and 15N and its relationship to elemental stoichiometry along soil depth gradients in 35-year old monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica) and red oak (Quercus rubra) growing on a uniform post-mining soil. We investigated the natural abundance of 13C and 15N and the carbon:nitrogen (C:N) and oxygen:carbon (O:C) stoichiometry of litterfall and fine roots as well as SOM in the forest floor and mineral soil. Tree species had a significant effect on SOM δ13C and δ15N reflecting significantly different signatures of litterfall and root inputs. Throughout the soil profile, δ13C and δ15N were significantly related to the C:N and O:C ratio which indicates that isotope enrichment with soil depth is linked to the turnover of organic matter (OM). Significantly higher turnover of OM in soils under deciduous tree species depended to 46% on the quality of litterfall and root inputs (N content, C:N, O:C ratio), and the initial isotopic signatures of litterfall. Hence, SOM composition and turnover also depends on additional—presumably microbial driven—factors. The enrichment of 15N with soil depth was generally linked to 13C. In soils under pine, however, with limited N and C availability, the enrichment of 15N was decoupled from 13C. This suggests that transformation pathways depend on litter quality of tree species.
    Beschreibung: Universität Trier (3163)
    Schlagwort(e): ddc:550.78 ; Stable isotopes ; Microbial turnover ; Litter ; Roots ; Common garden experiment ; Recultivated forest soil
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 28
    Publikationsdatum: 2023-06-19
    Beschreibung: Dissolved organic carbon (DOC) from Oa horizons has been proposed to be an important contributor for subsoil organic carbon stocks. We investigated the fate of DOC by directly injecting a DOC solution from 13C labelled litter into three soil depths at beech forest sites. Fate of injected DOC was quantified with deep drilling soil cores down to 2 m depth, 3 and 17 months after the injection. 27 ± 26% of the injected DOC was retained after 3 months and 17 ± 22% after 17 months. Retained DOC was to 70% found in the first 10 cm below the injection depth and on average higher in the topsoil than in the subsoil. After 17 months DOC in the topsoil was largely lost (− 19%) while DOC in the subsoil did not change much (− 4.4%). Data indicated a high stabilisation of injected DOC in the subsoils with no differences between the sites. Potential mineralisation as revealed by incubation experiments however, was not different between DOC injected in topsoil or subsoils underlining the importance of environmental factors in the subsoil for DOC stabilisation compared to topsoil. We conclude that stability of DOC in subsoil is primary driven by its spatial inaccessibility for microorganisms after matrix flow while site specific properties did not significantly affect stabilisation. Instead, a more fine-textured site promotes the vertical transport of DOC due to a higher abundance of preferential flow paths.
    Beschreibung: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659
    Schlagwort(e): ddc:550.78 ; Forest subsoils ; Cascade model ; Incubation experiment ; 13C ; Field experiment
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 29
    Publikationsdatum: 2023-06-12
    Beschreibung: A multiphase-field approach for elasto-plastic and anisotropic brittle crack propagation in geological systems consisting of different regions of brittle and ductile materials is presented and employed to computationally study crack propagation. Plastic deformation in elasto-plastic materials such as frictional, granular or porous materials is modelled with the pressure-sensitive Drucker-Prager plasticity model. This plasticity model is combined with a multiphase-field model fulfilling the mechanical jump conditions in diffuse solid-solid interfaces. The validity of the plasticity model with phase-inherent stress and strain fields is shown, in comparison with sharp interface finite element solutions. The proposed model is capable of simulating crack formation in heterogeneous multiphase systems comprising both purely elastic and inelastic phases. We investigate the influence of different material parameters on the crack propagation with tensile tests in single- and two-phase materials. To show the applicability of the model, crack propagation in a multiphase domain with brittle and elasto-plastic components is performed.
    Schlagwort(e): ddc:550.78 ; Multiphase-field ; Drucker-Prager plasticity ; Brittle fracture ; Elasto-plastic fracture
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 30
    Publikationsdatum: 2023-06-23
    Beschreibung: Many explosive volcanic eruptions produce underexpanded starting gas-particle jets. The dynamics of the accompanying pyroclast ejection can be affected by several parameters, including magma texture, gas overpressure, erupted volume and geometry. With respect to the latter, volcanic craters and vents are often highly asymmetrical. Here, we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent. The vent geometries chosen for this study are based on field observations. The novel element of the vent geometry investigated herein is an inclined exit plane (5, 15, 30° slant angle) in combination with cylindrical and diverging inner geometries. In a vertical setup, these modifications yield both laterally variable spreading angles as well as a diversion of the jets, where inner geometry (cylindrical/diverging) controls the direction of the inclination. Both the spreading angle and the inclination of the jet are highly sensitive to reservoir (conduit) pressure and slant angle. Increasing starting reservoir pressure and slant angle yield (1) a maximum spreading angle (up to 62°) and (2) a maximum jet inclination for cylindrical vents (up to 13°). Our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents.
    Beschreibung: European Research Council http://dx.doi.org/10.13039/501100000781
    Schlagwort(e): ddc:550.78 ; Explosive eruptions ; Crater asymmetry ; Vent asymmetry ; Gas jets ; Inclined jets ; Tilted eruptions
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 31
    Publikationsdatum: 2023-06-23
    Beschreibung: The ascent of hydrous magma prior to volcanic eruptions is largely driven by the formation of H2O vesicles and their subsequent growth upon further decompression. Porosity controls buoyancy as well as vesicle coalescence and percolation, and is important when identifying the differences between equilibrium or disequilibrium degassing from textural analysis of eruptive products. Decompression experiments are routinely used to simulate magma ascent. Samples exposed to high temperature (T) and pressure (P) are decompressed and rapidly cooled to ambient T for analysis. During cooling, fluid vesicles may shrink due to decrease of the molar volume of H2O and by resorption of H2O back into the melt driven by solubility increase with decreasing T at P 〈 300 MPa. Here, we quantify the extent to which vesicles shrink during cooling, using a series of decompression experiments with hydrous phonolitic melt (5.3–3.3 wt% H2O, T between 1323 and 1373 K, decompressed from 200 to 110–20 MPa). Most samples degassed at near-equilibrium conditions during decompression. However, the porosities of quenched samples are significantly lower than expected equilibrium porosities prior to cooling. At a cooling rate of 44 K·s−1, the fictive temperature Tf, where vesicle shrinkage stops, is up to 200 K above the glass transition temperature (Tg), Furthermore, decreasing cooling rate enhances vesicles shrinkage. We assess the implications of these findings on previous experimental degassing studies using phonolitic melt, and highlight the importance of correctly interpreting experimental porosity data, before any comparison to natural volcanic ejecta can be attempted.
    Beschreibung: German Science Foundation
    Schlagwort(e): ddc:550.78 ; Decompression experiments ; Vesiculation ; Vesicle shrinkage ; Quench effect ; H2O resorption ; Fictive temperature
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 32
    Publikationsdatum: 2023-06-23
    Beschreibung: Supersaturation of H2O during magma ascent leads to degassing of melt by formation and growth of vesicles that may power explosive volcanic eruptions. Here, we present experiments to study the effect of initially dissolved H2O concentration (cH2Oini) on vesicle formation, growth, and coalescence in phonolitic melt. Vesuvius phonolitic melts with cH2Oini ranging between 3.3 and 6.3 wt% were decompressed at rates of 1.7 and 0.17 MPa·s−1 and at temperatures ≥ 1323 K. Decompression started from 270 and 200 MPa to final pressures of 150–20 MPa, where samples were quenched isobarically. Optical microscopy and Raman spectroscopic measurements confirm that the glasses obtained were free of microcrystals and Fe-oxide nanolites, implying that the experiments were superliquidus and phase separation of the hydrous melt was homogeneous. A minimum number of the initially formed vesicles, defined by the number density normalized to vesicle-free glass volume (VND), is observed at ~ 5 wt% cH2Oini with a logVND of ~ 5 (in mm−3). The logVND increases strongly towards lower and higher cH2Oini by one order of magnitude. Furthermore, an important transition in evolution of vesiculation occurs at ~ 5.6 wt% cH2Oini. At lower cH2Oini, the initial VND is preserved during further decompression up to melt porosities of 30–50%. At higher cH2Oini, the initial vesicle population is erased at low melt porosities of 15–21% during further decompression. This observation is attributed to vesicle coalescence favored by low melt viscosity. In conclusion, cH2Oini determines the VND of initial phase separation and the evolution of vesiculation during decompression that controls the style of volcanic eruptions.
    Beschreibung: German Science Foundation
    Schlagwort(e): ddc:550.78 ; Vesicle number density ; Phonolitic melt ; Spinodal decomposition ; Melt degassing ; H2O concentration ; Fe-oxide nanolites
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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  • 33
    Publikationsdatum: 2023-06-17
    Beschreibung: Knowledge of pressure-dependent static and dynamic moduli of porous reservoir rocks is of key importance for evaluating geological setting of a reservoir in geo-energy applications. We examined experimentally the evolution of static and dynamic bulk moduli for porous Bentheim sandstone with increasing confining pressure up to about 190 MPa under dry and water-saturated conditions. The static bulk moduli (Ks) were estimated from stress–volumetric strain curves while dynamic bulk moduli (Kd) were derived from the changes in ultrasonic P- and S- wave velocities (~ 1 MHz) along different traces, which were monitored simultaneously during the entire deformation. In conjunction with published data of other porous sandstones (Berea, Navajo and Weber sandstones), our results reveal that the ratio between dynamic and static bulk moduli (Kd/Ks) reduces rapidly from about 1.5 − 2.0 at ambient pressure to about 1.1 at high pressure under dry conditions and from about 2.0 − 4.0 to about 1.5 under water-saturated conditions, respectively. We interpret such a pressure-dependent reduction by closure of narrow (compliant) cracks, highlighting that Kd/Ks is positively correlated with the amount of narrow cracks. Above the crack closure pressure, where equant (stiff) pores dominate the void space, Kd/Ks is almost constant. The enhanced difference between dynamic and static bulk moduli under water saturation compared to dry conditions is possibly caused by high pore pressure that is locally maintained if measured using high-frequency ultrasonic wave velocities. In our experiments, the pressure dependence of dynamic bulk modulus of water-saturated Bentheim sandstone at effective pressures above 5 MPa can be roughly predicted by both the effective medium theory (Mori–Tanaka scheme) and the squirt-flow model. Static bulk moduli are found to be more sensitive to narrow cracks than dynamic bulk moduli for porous sandstones under dry and water-saturated conditions.
    Beschreibung: Helmholtz-Zentrum Potsdam Deutsches GeoForschungsZentrum - GFZ (4217)
    Schlagwort(e): ddc:550.78 ; Porous sandstone ; Static bulk modulus ; Dynamic bulk modulus ; Narrow (compliant) cracks ; Equant (stiff) pores
    Sprache: Englisch
    Materialart: doc-type:article
    Standort Signatur Erwartet Verfügbarkeit
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