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  • Other Sources  (45)
  • 523  (27)
  • 550.724  (18)
  • 2020-2022  (45)
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  • 2020-2022  (45)
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  • 2021  (45)
  • 11
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    Central uplift collapse in acoustically fluidized granular targets: Insights from analog modeling (2021)
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    Publication Date: 2021-10-12
    Description: Depending on their sizes, impact craters have either simple or complex geometries. Peak-ring craters such as the Chicxulub impact structure possess a single interior ring of peaks and hills and a flat interior floor. The exact mechanisms leading to the formation of a morphological peak-ring are still a matter of debate. In this study, analog modeling was used to study the flow field of a collapsing central uplift. A 3-D-printed cast was used to bring the analog material in the shape of an overheightened central uplift that was based on numerical modeling. The cast was then quickly removed and the central peak collapsed, forming a flattened broad mound that spread out onto the annular moat of the crater cavity. A subwoofer was used to fluidize the granular target material. The kinematics of the collapse were analyzed with the aid of particle image velocimetry, revealing a downward and outward collapse of the central uplift. This mode of collapse is partly in agreement with numerical models, in particular for the initial and middle phases. The overthrusting of the collapsing central peak onto the inward moving crater floor predicted by numerical modeling was observed, though to a lesser degree. A peak-ring, however, could not be reproduced since the collapse came to a halt before the central peak was completely leveled. Nevertheless, the method provides qualitative insights into the kinematics of collapse phenomena. This experimental study provides independent support of the theory of acoustic fluidization, in addition to numerical simulations.
    Keywords: 550.724 ; 551.397 ; impact craters ; morphological peak-rings ; analog modeling
    Language: English
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  • 12
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    How Dynamic Boundary Conditions Induce Solute Trapping and Quasi-stagnant Zones in Laboratory Experiments Comprising Unsaturated Heterogeneous Porous Media (2021)
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    Publication Date: 2021-10-12
    Description: The vadose zone is subject to dynamic boundary conditions in the form of infiltration and evaporation. A better understanding of implications for flow and solute transport, arising from these dynamic boundary conditions in combination with heterogeneous structure, will help to improve the prediction of the fate of solutes. We present laboratory experiments and numerical simulations of heterogeneous porous media under unsaturated conditions where controlled, temporally varying precipitation and evaporation are applied to study the effect of dynamic boundary conditions on solute transport in the presence of material interfaces. Dye tracers Eosine Y and Brilliant Blue FCF are utilized to visualize solute transport and analyze redistribution processes in a flow cell. Water and solute fluxes in and out of the flow cell are quantified. While in dynamic experiments application of small infiltration rates (significantly below the saturated hydraulic conductivities of the materials) led to a reversal of transport paths between infiltration and succeeding evaporation, larger infiltration rates altered downward transport such that flow and transport paths differed from those observed during evaporation. Differences in transport paths ultimately led to a redistribution and trapping of solute in one material which manifested as pronounced tailing in breakthrough curves. Trapping was induced not by the formation of a stagnant zone as result of large parameter contrast but by an interplay of dynamic boundary conditions and material heterogeneity. This study thereby highlights the importance to consider dynamic boundary conditions in predictions of solute leaching.
    Keywords: 550.724 ; laboratory experiment ; unsaturated porous media ; conservative solute transport ; dynamic boundary conditions
    Language: English
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  • 13
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    Electrification of Experimental Volcanic Jets with Varying Water Content and Temperature (2021)
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    Publication Date: 2021-10-12
    Description: Volcanic lightning—a near ubiquitous feature of explosive volcanic eruptions—possesses great potential for the analysis of volcanic plume dynamics. To date, the lack of quantitative knowledge on the relationships between plume characteristics hinders efficient data analysis and application of the resulting parameterizations. We use a shock-tube apparatus for rapid decompression experiments to produce particle-laden jets. We have systematically and independently varied the water content (0–27 wt%) and the temperature (25–320 °C) of the particle-gas mixture. The addition of a few weight percent of water is sufficient to reduce the observed electrification by an order of magnitude. With increasing temperature, a larger number of smaller discharges are observed, with the overall amount of electrification staying similar. Changes in jet dynamics are proposed as the cause of the temperature-dependence, while multiple factors (including the higher conductivity of wet ash) can be seen responsible for the decreased electrification in wet experiments.
    Keywords: 550.724 ; volcanic lightning ; atmospheric electricity ; Faraday cage ; volcanic jets ; temperature ; water content
    Language: English
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  • 14
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    Evaporation Study Based on Micromodel Experiments: Comparison of Theory and Experiment (2021)
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    Publication Date: 2021-10-11
    Description: Abstract Evaporation—a key process for water exchange between soil and atmosphere—is controlled by internal water fluxes and surface vapor fluxes. Recent studies demonstrated that the dynamics of the water flow in corners determine the time behavior of the evaporation rate. The internal water flux of the porous media is often described by capillary flow assuming complete wetting. Particularly, the crucial influence of partial wetting, that is, the nonlinear contact angle dependency of the capillary flow has been neglected so far. The focus of the paper is to demonstrate that SiO2-surfaces can exhibit contact angles of about 40°. This reduces the internal capillary flow by 1 order of magnitude compared to complete wetting. First, we derived the contact angle by inverse modeling. We conducted a series of evaporation experiments in a 2-D square lattice microstructure connected by lognormal distributed throats. We used an explicit analytical power series solution of the single square capillary model. A contact angle of 38° ± 1° was derived. Second, we directly measured the contact angle of the Si-SiO2 wafer using the Drop Shape Analyzer Krüss 100 and obtained an averaged contact angle of 42° ± 2°. The results support the single square capillary model as an appropriate model for the description of the evaporation process in an ideal square capillary.
    Keywords: 550.724 ; 551.5 ; evaporation ; experiments
    Language: English
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  • 15
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    Measuring vertical soil water content profiles by combining horizontal borehole and dispersive surface ground penetrating radar data (2021)
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    Publication Date: 2021-10-07
    Description: To investigate transient dynamics of soil water redistribution during infiltration, we conducted horizontal borehole and surface ground penetrating radar measurements during a 4-day infiltration experiment at the rhizontron facility in Selhausen, Germany. Zero-offset ground penetrating radar profiling in horizontal boreholes was used to obtain soil water content information at specific depths (0.2, 0.4, 0.6, 0.8 and 1.2 m). However, horizontal borehole ground penetrating radar measurements do not provide accurate soil water content estimates of the top soil (0–0.1 m depth) because of interference between direct and critically refracted waves. Therefore, surface ground penetrating radar data were additionally acquired to estimate soil water content of the top soil. Due to the generation of electromagnetic waveguides in the top soil caused by infiltration, a strong dispersion in the ground penetrating radar data was observed in 500 MHz surface ground penetrating radar data. A dispersion inversion was thus performed with these surface ground penetrating radar data to obtain soil water content information for the top 0.1 m of the soil. By combining the complementary borehole and surface ground penetrating radar data, vertical soil water content profiles were obtained, which were used to investigate vertical soil water redistribution. Reasonable consistency was found between the ground penetrating radar results and independent soil water content data derived from time domain reflectometry measurements. Because of the improved spatial representativeness of the ground penetrating radar measurements, the soil water content profiles obtained by ground penetrating radar better matched the known water storage changes during the infiltration experiment. It was concluded that the combined use of borehole and surface ground penetrating radar data convincingly revealed spatiotemporal soil water content variation during infiltration. In addition, this setup allowed a better quantification of water storage, which is a prerequisite for future applications, where, for example, the soil hydraulic properties will be estimated from ground penetrating radar data.
    Keywords: 550.724 ; Ground-penetrating radar ; Hydrogeophysics ; Data processing
    Language: English
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  • 16
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    Laboratory Study on Fluid-Induced Fault Slip Behavior: The Role of Fluid Pressurization Rate (2021)
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    Publication Date: 2021-10-06
    Description: Understanding the physical mechanisms governing fluid-induced fault slip is important for improved mitigation of seismic risks associated with large-scale fluid injection. We conducted fluid-induced fault slip experiments in the laboratory on critically stressed saw-cut sandstone samples with high permeability using different fluid pressurization rates. Our experimental results demonstrate that fault slip behavior is governed by fluid pressurization rate rather than injection pressure. Slow stick-slip episodes (peak slip velocity 〈 4 μm/s) are induced by fast fluid injection rate, whereas fault creep with slip velocity 〈 0.4 μm/s mainly occurs in response to slow fluid injection rate. Fluid-induced fault slip may remain mechanically stable for loading stiffness larger than fault stiffness. Independent of fault slip mode, we observed dynamic frictional weakening of the artificial fault at elevated pore pressure. Our observations highlight that varying fluid injection rates may assist in reducing potential seismic hazards of field-scale fluid injection projects.
    Keywords: 550.724 ; fault slip ; fluid injection ; induced seismicity ; fluid pressurization rate ; stick-slip ; fault creep
    Language: English
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  • 17
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    Controls on the lateral channel-migration rate of braided channel systems in coarse non-cohesive sediment (2021)
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    Publication Date: 2021-10-06
    Description: Lateral movements of alluvial river channels control the extent and reworking rates of alluvial fans, floodplains, deltas, and alluvial sections of bedrock rivers. These lateral movements can occur by gradual channel migration or by sudden changes in channel position (avulsions). Whereas models exist for rates of river avulsion, we lack a detailed understanding of the rates of lateral channel migration on the scale of a channel belt. In a two-step process, we develop here an expression for the lateral migration rate of braided channel systems in coarse, non-cohesive sediment. On the basis of photographic and topographic data from laboratory experiments of braided channels performed under constant external boundary conditions, we first explore the impact of autogenic variations of the channel-system geometry (i.e. channel-bank heights, water depths, channel-system width, and channel slope) on channel-migration rates. In agreement with theoretical expectations, we find that, under such constant boundary conditions, the laterally reworked volume of sediment is constant and lateral channel-migration rates scale inversely with the channel-bank height. Furthermore, when channel-bank heights are accounted for, lateral migration rates are independent of the remaining channel geometry parameters. These constraints allow us, in a second step, to derive two alternative expressions for lateral channel-migration rates under different boundary conditions using dimensional analysis. Fits of a compilation of laboratory experiments to these expressions suggest that, for a given channel bank-height, migration rates are strongly sensitive to water discharges and more weakly sensitive to sediment discharges. In addition, external perturbations, such as changes in sediment and water discharges or base level fall, can indirectly affect lateral channel-migration rates by modulating channel-bank heights. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd. © 2019 The Author. Earth Surface Processes and Landforms published by John Wiley & Sons, Ltd.
    Keywords: 550.724 ; 551.35 ; braided alluvial rivers ; physical experiments ; channel migration
    Language: English
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  • 18
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    Velocity Field Estimation on Density-Driven Solute Transport With a Convolutional Neural Network (2021)
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    Publication Date: 2021-10-06
    Description: Recent advances in machine learning open new opportunities to gain deeper insight into hydrological systems, where some relevant system quantities remain difficult to measure. We use deep learning methods trained on numerical simulations of the physical processes to explore the possibilities of closing the information gap of missing system quantities. As an illustrative example we study the estimation of velocity fields in numerical and laboratory experiments of density-driven solute transport. Using high-resolution observations of the solute concentration distribution, we demonstrate the capability of the method to structurally incorporate the representation of the physical processes. Velocity field estimation for synthetic data for both variable and uniform concentration boundary conditions showed equal results. This capability is remarkable because only the latter was employed for training the network. Applying the method to measured concentration distributions of density-driven solute transport in a Hele-Shaw cell makes the velocity field assessable in the experiment. This assessability of the velocity field even holds for regions with negligible solute concentration between the density fingers, where the velocity field is otherwise inaccessible.
    Keywords: 550.724 ; Hele-Shaw cell experiment ; density-driven active solute transport ; convolutional neural network ; velocity field estimation
    Language: English
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  • 19
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    Formation of Small Craters in the Lunar Regolith: How Do They Influence the Preservation of Ancient Melt at the Surface? (2021)
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    Publication Date: 2021-09-29
    Description: The impact melt that records the formation time of basins is essential for the understanding of the lunar bombardment history. To better understand melt distribution on the Moon, this study investigates mixing of melt by small impacts using a Monte Carlo numerical model. The obtained mixing behavior is then integrated into a larger scale model developed in previous work. While large impacts produce most of the melt volume in both the regolith and megaregolith, we find that the dominant source of melt near the surface is small impacts. Material in the top meter is affected mainly by impacts that form craters 〈5 km in diameter. In the uppermost 10 cm, melt with age 〈0.5 Ga is abundant; while as depth increases older melt is increasingly present. This may indicate that the excess of impact melt 〈0.5 Ga in lunar samples from the near surface is caused by the cumulative mixing of small impacts. A comparison of the age distribution of melt derived from craters of different sizes with that of impact glass constrains the size of spherule‐forming impacts. Our model is consistent with observations if most impact glass spherules from the near surface are produced by 〈100 m craters and 〉100 m craters do not contribute abundant spherules. The distribution of the datable melt with depth is also analyzed, which is essential for future sampling missions. Excavated materials of young and large craters (〉100 m on highlands; 〉10 km on maria) appear to be the most fruitful targets.
    Description: Plain Language Summary: Hypervelocity impact events on the Moon generate great energy that melts materials in the near‐surface. The generated melt products record the age of impact craters. The abundance of impact melts of different ages is therefore essential for our understanding of the lunar bombardment history. Most of the returned samples are derived from the near‐surface, where the material composition has been significantly affected by the frequent gardening of small impacts. Improving our understanding of how impact processes change the material composition is helpful for sample interpretations. Here, we build a numerical model to investigate this issue. The simulation results show that craters 〈100 m likely lead to the excess of datable impact melt 〈0.5 Ga that has been found in returned samples. In addition, we delineate the distribution of datable melt in various depths. It provides insight into future lunar missions aiming to collect melt that can be easier to date. We suggest that ejecta blankets of young and large craters (〉100 m on highlands; 〉10 km on maria) would be the optimal targets.
    Description: Key Points: A numerical model is developed to investigate the effect of small impact gardening on ancient melt in the lunar near‐surface. Gardening of craters 〈100 m in diameter likely lead to the excess of datable impact melt 〈0.5 Ga in lunar regolith samples. Distribution of radiometrically datable melt in the regolith and the megaregolith is analyzed.
    Description: Deutche Forschungsgemeinshaft (DFG, German Research Foundation)
    Keywords: 523 ; 551.701 ; lunar regolith ; impact melts ; dating techniques ; datable samples ; simulation
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  • 20
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    Influence of Mercury's Exosphere on the Structure of the Magnetosphere (2021)
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    Publication Date: 2021-09-24
    Description: Mercury is embedded in a tenuous and highly anisotropic sodium exosphere, generated mainly by plasma-surface interactions. The absolute values of the sodium ion density are still under debate. Observations by MESSENGER's Fast Imaging Plasma Spectrometer (FIPS) instrument suggest the density of exospheric ions to be several orders of magnitude lower than the upstream solar wind density, indicating that the sodium exosphere has no substantial influence on the magnetospheric current systems. However, MESSENGER magnetic field observations of field line resonances revealed sodium ion densities comparable to the upstream solar wind density. To investigate how a dense exosphere would affect the current systems within Mercury's magnetosphere, we apply an established hybrid (kinetic ions, fluid electrons) model and conduct multiple model runs with gradually increasing exospheric density, ranging from no sodium ions at all to comet-like configurations. We demonstrate how a sufficiently dense exosphere leads to self-shielding of the sodium ion population from the ambient electric field and a significant inflation and symmetrization of Mercury's magnetosphere, which is decreasingly affected by the dipole offset. Once the sodium ion density is sufficiently high, Region 2 field-aligned currents emerge close to the planet. The modeled Region 2 currents are located below the orbit of MESSENGER, thereby providing a possible explanation for the absence of these currents in observations. The sodium exosphere also closes a significant fraction of the Region 1 currents through Pedersen and Hall currents before the “guiding” magnetic field lines even reach the planetary surface. The modeled sodium ion and solar wind densities agree well with observations.
    Keywords: 523 ; exosphere ; magnetosphere ; field-aligned currents ; hybrid-model ; Mercury ; sodium
    Language: English
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