Mullite in Libyan Desert Glass: Evidence for high-temperature/low-pressure formation          下载免费PDF全文

Ansgar Greshake  Richard Wirth  Jörg Fritz  Tomasz Jakubowski  Ute Böttger 《Meteoritics & planetary science》2018,53(3):467-481

Libyan Desert Glass (LDG) is a SiO₂-rich natural glass whose origin, formation mechanism, and target material are highly debated. We here report on the finding of a lens-shaped whitish inclusion within LDG. The object is dominantly composed of siliceous glass and separated from the surrounding LDG by numerous cristobalite grains. Within cristobalite, several regions rich in mullite often associated with ilmenite were detected. Mineral assemblage, chemical composition, and grain morphologies suggest that mullite was formed by thermal decomposition of kaolinitic clay at atmospheric pressure and T ≥ 1600 °C and also attested to high cooling rates under nonequilibrium conditions. Cristobalite contains concentric and irregular internal cracks and is intensely twinned, indicating that first crystallized β-cristobalite inverted to α-cristobalite during cooling of the SiO₂-rich melt. The accompanied volume reduction of 4% induced the high density of defects. The whitish inclusion also contains several partly molten rutile grains evidencing that at least locally the LDG melt was at T ≥ 1800 °C. Based on these observations, it is concluded that LDG was formed by high-temperature melting of kaolinitic clay-, rutile-, and ilmenite-bearing Cenozoic sandstone or sand very likely during an asteroid or comet impact onto Earth. While melting and ejection occurred at high pressures, the melt solidified quickly at atmospheric pressure.  相似文献   

Iron and Manganese in Groundwater: Using Kriging and GIS to Locate High Concentrations in Buncombe County,North Carolina          下载免费PDF全文

Crystal D. Johnson  Arpita Nandi  T. Andrew Joyner  Ingrid Luffman 《Ground water》2018,56(1):87-95

For health, economic, and aesthetic reasons, allowable concentrations (as suggested by the United States Environmental Protection Agency) of the secondary contaminants iron (Fe) and manganese (Mn) found present in drinking water are 0.3 and 0.05 mg/L, respectively. Water samples taken from private drinking wells in rural communities within Buncombe County, North Carolina contain concentrations of these metals that exceed secondary water quality criteria. This study predicted the spatial distribution of Fe and Mn in the county, and evaluated the effect of site environmental factors (bedrock geology, ground elevation, saprolite thickness, and drinking water well depth) in controlling the variability of Fe and Mn in groundwater. A statistically significant correlation between Fe and Mn concentrations, attributable to bedrock geology, was identified. Prediction models were created using ordinary kriging and cokriging interpolation techniques to estimate the presence of Fe and Mn in groundwater where direct measurements are not possible. This same procedure can be used to estimate the trend of other contaminants in the groundwater in different areas with similar hydrogeological settings.  相似文献   

How Different Analysis and Interpolation Methods Affect the Accuracy of Ice Surface Elevation Changes Inferred from Satellite Altimetry     

Strößenreuther  Undine  Horwath  Martin  Schröder  Ludwig 《Mathematical Geosciences》2020,52(4):499-525

Satellite altimetry has been widely used to determine surface elevation changes in polar ice sheets. The original height measurements are irregularly distributed in space and time. Gridded surface elevation changes are commonly derived by repeat altimetry analysis (RAA) and subsequent spatial interpolation of height change estimates. This article assesses how methodological choices related to those two steps affect the accuracy of surface elevation changes, and how well this accuracy is represented by formal uncertainties. In a simulation environment resembling CryoSat-2 measurements acquired over a region in northeast Greenland between December 2010 and January 2014, different local topography modeling approaches and different cell sizes for RAA, and four interpolation approaches are tested. Among the simulated cases, the choice of either favorable or unfavorable RAA affects the accuracy of results by about a factor of 6, and the different accuracy levels are propagated into the results of interpolation. For RAA, correcting local topography by an external digital elevation model (DEM) is best, if a very precise DEM is available, which is not always the case. Yet the best DEM-independent local topography correction (nine-parameter model within a 3,000 m diameter cell) is comparable to the use of a perfect DEM, which exactly represents the ice sheet topography, on the same cell size. Interpolation by heterogeneous measurement-error-filtered kriging is significantly more accurate (on the order of 50% error reduction) than interpolation methods, which do not account for heterogeneous errors.

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A multi‐storage groundwater concept for the SWAT model to emphasize nonlinear groundwater dynamics in lowland catchments          下载免费PDF全文

Matthias Pfannerstill  Björn Guse  Nicola Fohrer 《水文研究》2014,28(22):5599-5612

Hydrological models are useful tools to analyze present and future conditions of water quantity and quality. The integrated modelling of water and nutrients needs an adequate representation of the different discharge components. In common with many lowlands, groundwater contribution to the discharge in the North German lowlands is a key factor for a reasonable representation of the water balance, especially in low flow periods. Several studies revealed that the widely used Soil and Water Assessment Tool (SWAT) model performs poorly for low flow periods. This paper deals with the extension of the groundwater module of the SWAT model to enhance low flow representation. The current two‐storage concept of SWAT was further developed to a three‐storage concept. This was realized due to modification of the groundwater module by splitting the active groundwater storage into a fast and a slow contributing aquifer. The results of this study show that the groundwater module with three storages leads to a good prediction of the overall discharge especially for the recession limbs and the low flow periods. The improved performance is reflected in the signature measures for the mid‐segment (percent bias ?2.4% vs ?15.9%) and the low segment (percent bias 14.8% vs 46.8%) of the flow duration curve. The three‐storage groundwater module is more process oriented than the original version due to the introduction of a fast and a slow groundwater flow component. The three‐storage version includes a modular approach, because groundwater storages can be activated or deactivated independently for subbasin and hydrological response unit level. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Coupled groundwater flow and heat transport simulation for estimating transient aquifer–stream exchange at the lowland River Spree (Germany)          下载免费PDF全文

Gunnar Nützmann  Christian Levers  Jörg Lewandowski 《水文研究》2014,28(13):4078-4090

Subsurface flow and heat transport near Freienbrink, NE Germany, was simulated in order to study groundwater–surface water exchange between a floodplains aquifer and a section of the lowland River Spree and an adjacent oxbow. Groundwater exfiltration was the dominant process, and only fast surface water level rises resulted in temporary infiltration into the aquifer. The main groundwater flow paths are identified based on a 3D groundwater flow model. To estimate mass fluxes across the aquifer–surface water interfaces, a 2D flow and heat transport modelling approach along a transect of 12 piezometers was performed. Results of steady‐state and transient water level simulations show an overall high accuracy with a Spearman coefficient ρ = 0.9996 and root mean square error (RMSE) = 0.008 m. Based on small groundwater flow velocities of about 10^?7 to 10^?6 ms^?1, mean groundwater exfiltration rates of 233 l m^?2 d^?1 are calculated. Short periods of surface water infiltration into the aquifer do not exceed 10 days, and the infiltration rates are in the same range. The heat transport was modelled with slightly less accuracy (ρ = 0.8359 and RMSE = 0.34 °C). In contrast to the predominant groundwater exfiltration, surface water temperatures determine the calculated temperatures in the upper aquifer below both surface water bodies down to 10 m during the whole simulation period. These findings emphasize prevailing of heat conduction over advection in the upper aquifer zones, which seems to be typical for lowland streams with sandy aquifer materials and low hydraulic gradients. Moreover, this study shows the potential of coupled numerical flow and heat transport modelling to understand groundwater–surface water exchange processes in detail. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Temporal variation of suspended sediment transport in the Koga catchment,North Western Ethiopia and environmental implications          下载免费PDF全文

Eleni Yeshaneh  Alexander Eder  Günter Blöschl 《水文研究》2014,28(24):5972-5984

Event sediment transport and yield were studied for 45 events in the upstream part of the 260 km² agricultural Koga catchment that drains to an irrigation reservoir. Discharge and turbidity data were collected over a period of more than a year, accompanied by grab sampling. Turbidity was very well correlated with the sediment concentrations from the samples (r = 0.99), which allowed us to estimate the temporal patterns of sediment concentrations within events. The hysteresis patterns between discharge and sediment concentrations were analysed to provide insight into the different sediment sources. Anticlockwise patterns are the dominant hysteresis patterns in the area, suggesting smaller contributions of suspended sediment from the river channels than from the hillslopes and agricultural areas. Complicated types of hysteresis patterns were mostly observed for long events with multiple peaks. For a given discharge, sediment yields in August and September, when the catchment was almost completely covered with vegetation, were much smaller than during the rest of the rainy season. The hysteresis patterns and timing suggest that the sediment availability from the agricultural areas and hillslopes affects sediment yields more strongly than does peak discharge. Two distinct types of sediment rating curves were observed for the season when the agricultural land was covered with vegetation and when it was not, indicating the dominating contribution of land use/cover to sediment yields in the catchment. The rate of suspended sediment transport in the area was estimated as 25.6 t year^?1 ha^?1. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Analysis of uncertainties in the hydrological response of a model‐based climate change impact assessment in a subcatchment of the Spree River,Germany          下载免费PDF全文

Anne Gädeke  Herwig Hölzel  Hagen Koch  Ina Pohle  Uwe Grünewald 《水文研究》2014,28(12):3978-3998

Climate change impact assessments form the basis for the development of suitable climate change adaptation strategies. For this purpose, ensembles consisting of stepwise coupled models are generally used [emission scenario → global circulation model → downscaling approach (DA) → bias correction → impact model (hydrological model)], in which every item is affected by considerable uncertainty. The aim of the current study is (1) to analyse the uncertainty related to the choice of the DA as well as the hydrological model and its parameterization and (2) to evaluate the vulnerability of the studied catchment, a subcatchment of the highly anthropogenically impacted Spree River catchment, to hydrological change. Four different DAs are used to drive four different model configurations of two conceptually different hydrological models (Water Balance Simulation Model developed at ETH Zürich and HBV‐light). In total, 452 simulations are carried out. The results show that all simulations compute an increase in air temperature and potential evapotranspiration. For precipitation, runoff and actual evapotranspiration, opposing trends are computed depending on the DA used to drive the hydrological models. Overall, the largest source of uncertainty can be attributed to the choice of the DA, especially regarding whether it is statistical or dynamical. The choice of the hydrological model and its parameterization is of less importance when long‐term mean annual changes are compared. The large bandwidth at the end of the modelling chain may exacerbate the formulation of suitable climate change adaption strategies on the regional scale. Copyright © 2013 John Wiley & Sons, Ltd.  相似文献   

Isotropic and anisotropic pointing models     

András Pál  Krisztián Vida  László Mészáros  György Mező 《Experimental Astronomy》2016,41(1-2):1-15

This paper describes an alternative approach for generating pointing models for telescopes equipped with serial kinematics, esp. equatorial or alt-az mounts. Our model construction does not exploit any assumption for the underlying physical constraints of the mount, however, one can assign various effects to the respective components of the equations. In order to recover the pointing model parameters, classical linear least squares fitting procedures can be applied. This parameterization also lacks any kind of parametric singularity. We demonstrate the efficiency of this type of model on real measurements with meter-class telescopes where the results provide a root mean square accuracy of 1.5?2 arcseconds.  相似文献   

The design and flight performance of the PoGOLite Pathfinder balloon-borne hard X-ray polarimeter     

M. Chauvin  H.-G. Florén  M. Jackson  T. Kamae  T. Kawano  M. Kiss  M. Kole  V. Mikhalev  E. Moretti  G. Olofsson  S. Rydström  H. Takahashi  J. Lind  J.-E. Strömberg  O. Welin  A. Iyudin  D. Shifrin  M. Pearce 《Experimental Astronomy》2016,41(1-2):17-41

In the 50 years since the advent of X-ray astronomy there have been many scientific advances due to the development of new experimental techniques for detecting and characterising X-rays. Observations of X-ray polarisation have, however, not undergone a similar development. This is a shortcoming since a plethora of open questions related to the nature of X-ray sources could be resolved through measurements of the linear polarisation of emitted X-rays. The PoGOLite Pathfinder is a balloon-borne hard X-ray polarimeter operating in the 25-240 keV energy band from a stabilised observation platform. Polarisation is determined using coincident energy deposits in a segmented array of plastic scintillators surrounded by a BGO anticoincidence system and a polyethylene neutron shield. The PoGOLite Pathfinder was launched from the SSC Esrange Space Centre in July 2013. A near-circumpolar flight was achieved with a duration of approximately two weeks. The flight performance of the Pathfinder design is discussed for the three Crab observations conducted. The signal-to-background ratio for the observations is shown to be 0.25 ±0.03 and the Minimum Detectable Polarisation (99 % C.L.) is (28.4 ±2.2) %. A strategy for the continuation of the PoGOLite programme is outlined based on experience gained during the 2013 maiden flight.  相似文献   

Automatic detection of earthquake-induced ground failure effects through Faster R-CNN deep learning-based object detection using satellite images     

Hacıefendioğlu  Kemal  Başağa  Hasan Basri  Demir  Gökhan 《Natural Hazards》2021,105(1):383-403

Natural Hazards - The seismically induced ground failure is defined as any earthquake-generated process that leads to deformations within a soil medium, which in turn results in permanent...  相似文献