Ice draft and current measurements from the north-western Barents Sea, 1993–96     

Einar Povl Abrahamsen  Svein Østerhus  Tor Gammelsrød 《Polar research》2006,25(1):25-37

From 1993 to 1996, three oceanographic moorings were deployed in the north-western Barents Sea, each with a current meter and an upward-looking sonar for measuring ice drafts. These yielded three years of currents and two years of ice draft measurements. An interannual variability of almost I m was measured in the average ice draft. Causes for this variability are explored, particularly its possible connection to changes in atmospheric circulation patterns. We found that the flow of Northern Barents Atlantic-derived Water and the transport of ice from the Central Arctic into the Barents Sea appears to be controlled by winds between Nordaustlandet and Franz Josef Land, which in turn may be influenced by larger-scale variations such as the Arctic Oscillation/North Atlantic Oscillation.  相似文献   

Geomechanical model for fracture deformation under hydraulic, mechanical and thermal loads     

Chris McDermott  Olaf Kolditz 《Hydrogeology Journal》2006,14(4):485-498

Hydraulic flow and transport (heat and solute) within crystalline rocks is dominated by the fracture systems found within them. In situ stress conditions have a significant impact on the hydraulic, mechanical and thermal coupled processes, and quantification of these processes provides a key to understanding the often transient time-dependent behaviour of crystalline rocks. In this paper, a geomechanical model is presented which describes fracture closure as a function of effective stress and the changes in parameters such as storage, permeability, porosity and aperture. Allowing the fracture closure to be defined by the change in normal effective stress provides a link to the numerical consideration of parametrical changes due to rock stress alterations caused for example by changes in fracture fluid pressure, stress release, tectonic stress, thermal stress, orientation of the natural fracture in the pervasive stress system and local changes in a rock mass due to stress alteration. The model uses geometrical considerations based on a fractal distribution of apertures on the fracture surface, and applies well-established analytical elastic deformation solutions to calculate the deformation response to changes in effective stress. Analysis of the fractal generation method allows a standard normal distribution of fracture apertures to be predicted for all common fractal dimensions relating to a 2D surface. Changes in the fracture aperture are related to hydraulic functions such as permeability, storage and porosity of the fracture. The geomechanical model is experimentally validated against laboratory scale experimental data gained from the closure of a fractured sample recovered at a depth of 3,800 m from the KTB pilot borehole. Parameters for matching the experimental data were established externally, the only fitting parameters applied were the minimum and maximum contact area between the surfaces and the number of allowable contacts. The model provides an insight into the key processes determining the closure of a fracture, and can act as a material input function for numerical models linking the effects of changes in the stress field, hydraulic or thermal conditions, to the flow and transport parameters of a fractured system.

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Résumé L’écoulement et le transport (chaleur et soluté) dans les roches cristallines sont dominés par les systèmes de fracture. Les conditions de stress in-situ ont un impact significatif sur l’hydraulique, les processus couplés de mécanique et thermique et la quantification de ces processus apportent une clé pour comprendre le comportement transitoire des roches cristallines. Dans cet article un modèle géomécanique est présenté, modèle qui décrit la fermeture des fractures comme une fonction de la contrainte effective et des changements de paramètres tels le coefficient d’emmagasinement, la perméabilité, la porosité et l’ouverture. En s’accordant que la fermeture des fractures est définit par les changements de la contrainte effective normale, on apporte le lien avec la considération numérique des changements paramétriques dus aux altérations de la contrainte des roches, causés par exemple par des variations de la pression des fluides dans les fractures, du dégagement de la contrainte, des contraintes tectoniques et thermiques, des orientations des fractures naturelles dans le système de contraintes pénétrantes, et des changements locaux dans un massif de roches dus à l’altération des contraintes. Le modèle utilise des considérations géométriques basées sur une distribution fractale des ouvertures à la surface des fractures, et permet d’établir des solutions analytiques de la déformation élastique pour calculer la réponse de la déformation à la contrainte effective. L’analyse de la méthode par génération fractale permet de prédire une distribution normale standard de l’ouverture des fractures, pour toutes les dimensions fractales en relation avec les surfaces 2D. Les changements dans l’ouverture des fractures sont mis en relation avec les fonctions hydrauliques tels la perméabilité, l’emmagasinement et la porosité de la fracture. Le modèle géoméchanique est expérimentalement validé à l’échelle du laboratoire sur un échantillon fracturé récupéré à une profondeur de 3,800 mètres sur le puits du site pilote KTB. Les paramètres du calibrage des données expérimentales ont été établies extérieurement, les seuls paramètres utilisés étant les surfaces de contact minimum et maximum, et le nombre de contacts permis. Le modèle apporte une connaissance perspicace sur le processus clé déterminant la fermeture des fractures, et peut servir de fonction input dans les modèles numériques reliant les effets des variations de la contrainte du terrain, les conditions hydrauliques ou thermales, les paramètres de l’écoulement et du transport et les systèmes de fracture.

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Resumen El flujo hidráulico y transporte (de calor y solutos) dentro de rocas cristalinas está dominado por los sistemas de fracturas que se encuentran en ellas. Las condiciones de esfuerzos in-situ tienen un impacto significativo en los procesos aparejados termales, mecánicos e hidráulicos y la cuantificación de estos procesos aporta una clave para entender el frecuente comportamiento transitorio dependiente de las rocas cristalinas. En este artículo se presenta un modelo geomecánico que describe el cierre de fracturas en función del esfuerzo efectivo y los cambios en parámetros tal como almacenamiento, permeabilidad, porosidad y apertura. El definir el cierre de fractura mediante el cambio en esfuerzo normal efectivo aporta un vínculo con la consideración numérica de cambios paramétricos ocasionados por alteraciones de esfuerzos en la roca causadas, por ejemplo, por cambios en presión de fluidos en fractura, liberación de esfuerzo, esfuerzo tectónico, esfuerzo termal, orientación de fracturas naturales en el sistema de esfuerzos penetrante, y cambios locales en una masa rocosa ocasionados por alteración de esfuerzos. El modelo utiliza consideraciones geométricas basadas en la distribución fractal de aperturas en la superficie de fractura y aplica soluciones analíticas bien establecidas de deformación elástica para calcular la respuesta de deformación a cambios en el esfuerzo efectivo. Los análisis del método de generación fractal permiten predecir una distribución normal standard para la distribución de aperturas de fracturas para todas las dimensiones fractales comunes que se relacionan con una superficie 2D. Los cambios en la apertura de fractura se relacionan con funciones hidráulicas tal como permeabilidad, almacenamiento y porosidad de la fractura. El modelo geomecánico se ha validado experimentalmente en contra de datos experimentales a escala de laboratorio obtenidos a partir del cierre de una muestra fracturada recuperada a una profundidad de 3,800 m en el pozo piloto KTB. Se establecieron externamente parámetros que se ajustan a los datos experimentales, con los parámetros de ajuste aplicados que fueron el área máxima y mínima de contacto entre las superficies y el número de contactos permisibles. El modelo arroja luz sobre los procesos clave que determinan el cierre de una fractura y puede actuar como un material de función de entrada para modelos numéricos que vinculan los efectos de cambios en el campo de esfuerzos, condiciones termales o hidráulicas, con los parámetros de flujo y transporte de un sistema fracturado.

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U–Pb ages of Neoarchean granitoids from the Black Hills, South Dakota, USA: implications for crustal evolution in the Archean Wyoming province     

James A. McCombs  Peter S. Dahl  Michael A. Hamilton 《Precambrian Research》2004,130(1-4):161-184

Zircons in basement rocks from the eastern Wyoming province (Black Hills, South Dakota, USA) have been analyzed by ion microprobe (SHRIMP) in order to determine precise ages of Archean tectonomagmatic events. In the northern Black Hills (NBH) near Nemo, Phanerozoic and Proterozoic (meta)sedimentary rocks are nonconformably underlain by Archean biotite–feldspar gneiss (BFG) and Little Elk gneissic granite (LEG), both of which intrude older schists. The Archean granitoid gneisses exhibit a pervasive NW–SE-trending fabric, whereas an earlier NE–SW-trending fabric occurs sporadically only in the BFG, which is intruded by the somewhat younger LEG. Zircon crystals obtained from the LEG and BFG exhibit double terminations, oscillatory zoning, and Th/U ratios of 0.6±0.3—thereby confirming a magmatic origin for both lithologies. In situ analysis of the most U–Pb concordant domains yields equivalent ²⁰⁷Pb/²⁰⁶Pb ages (upper intercept, U–Pb concordia) of 2559±6 and 2563±6 Ma (both ±2σ) for the LEG and BFG, respectively, which constrains a late Neoarchean age for sequential pulses of magmatism in the NBH. Unzoned (in BSE) patches of 2560 Ma zircon commonly truncate coeval zonation in the same crystals with no change in Th/U ratio, suggesting that deuteric, fluid-assisted recrystallization accompanied post-magmatic cooling. A xenocrystic core of magmatic zircon observed in one LEG zircon yields a concordant age of 2894±6 Ma (±2σ). This xenocryst represents the oldest crustal material reported thus far in the Black Hills. Whether this older zircon originated as unmelted residue of 2900 Ma crust that potentially underlies the Black Hills or as detritus derived from 2900 Ma crustal sources in the Wyoming province cannot be discerned. In the southern Black Hills (SBH), the peraluminous granite at Bear Mountain (BMG) of previously unknown age intrudes biotite–plagioclase schist. Zircon crystals from the BMG are highly metamict and altered, but locally preserve small domains suitable for in situ analysis. A U–Pb concordia upper intercept age of 2596±11 Ma (±2σ) obtained for zircon confirms both the late Neoarchean magmatic age of the BMG and a minimum age for the schist it intrudes. Taken together, these data indicate that the Neoarchean basement granitoids were emplaced at 2590–2600 Ma (SBH) and 2560 Ma (NBH), most likely in response to subduction associated with plate convergence (final assembly of supercontinent Kenorland?). In contrast, thin rims present on some LEG–BFG zircons exhibit strong U–Pb discordance, high common Pb, and low Th/U ratios—suggesting growth or modification under hydrothermal conditions, as previously suggested for similar zircons from SE Wyoming. The LEG–BFG zircon rims yield a nominal upper intercept date of 1940–2180 Ma, which may represent a composite of multiple rifting events known to have affected the Nemo area between 2480 and 1960 Ma. Together, these observations confirm the existence of a Paleoproterozoic rift margin along the easternmost Wyoming craton. Moreover, the 2480–1960 Ma time frame inferred for rifting in the Black Hills (Nemo area) corresponds closely to a 2450–2100 Ma time frame previously inferred for the fragmentation of supercontinent Kenorland.  相似文献   

Simulation of site-scale water fluxes in desert and natural oasis ecosystems of the arid region in Northwest China     

Mingjuan Xie  Geping Luo  Olaf Hellwich  Amaury Frankl  Wenqiang Zhang  Chunbo Chen  Chen Zhang  Philippe De Maeyer 《水文研究》2021,35(12):e14444

The study of water fluxes is important to better understand hydrological cycles in arid regions. Data-driven machine learning models have been recently applied to water flux simulation. Previous studies have built site-scale simulation models of water fluxes for individual sites separately, requiring a large amount of data from each site and significant computation time. For arid areas, there is no consensus as to the optimal model and variable selection method to simulate water fluxes. Using data from seven flux observation sites in the arid region of Northwest China, this study compared the performance of random forest (RF), support vector machine (SVM), back propagation neural network (BPNN), and multiple linear regression (MLR) models in simulating water fluxes. Additionally, the study investigated inter-annual and seasonal variation in water fluxes and the dominant drivers of this variation at different sites. A universal simulation model for water flux was built using the RF approach and key variables as determined by MLR, incorporating data from all sites. Model performance of the SVM algorithm (R² = 0.25–0.90) was slightly worse than that of the RF algorithm (R² = 0.41–0.91); the BPNN algorithm performed poorly in most cases (R² = 0.15–0.88). Similarly, the MLR results were limited and unreliable (R² = 0.00–0.66). Using the universal RF model, annual water fluxes were found to be much higher than the precipitation received at each site, and natural oases showed higher fluxes than desert ecosystems. Water fluxes were highest during the growing season (May–September) and lowest during the non-growing season (October–April). Furthermore, the dominant drivers of water flux variation were various among different sites, but the normalized difference vegetation index (NDVI), soil moisture and soil temperature were important at most sites. This study provides useful insights for simulating water fluxes in desert and oasis ecosystems, understanding patterns of variation and the underlying mechanisms. Besides, these results can make a contribution as the decision-making basis to the water management in desert and oasis ecosystems.  相似文献   

Analysis of the THM behaviour in a clay-based engineered barrier system (EBS): modelling of the HE-E experiment (Mont Terri URL)     

Xuerui Wang  Hua Shao  Jürgen Hesser  Olaf Kolditz 《Environmental Earth Sciences》2016,75(20):1350

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SEAWATCH system: An operational marine surveillance and information system     

Svein Tryggestad  Sivert Moen Jr 《Marine pollution bulletin》1994,29(6-12):577-579

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Ulvöspinel in native iron-bearing assemblages and the origin of these assemblages in basalts from Ovifak,Greenland, and Bühl,Federal Republic of Germany     

Olaf Medenbach  Ahmed ElGoresy 《Contributions to Mineralogy and Petrology》1982,80(4):358-366

Euhedral crystals of ulvöspinel are found in many of the native-iron-bearing xenoliths from the basalt of Bühl near Kassel (West Germany) and Ovifak on Disko Island (West Greenland). The typical assemblage of these xenoliths at both localities is: native Fe, troilite, cohenite, ulvöspinel, ilmenite, olivine, and plagioclase, as well as silicate glass containing droplets of former Fe and troilite melt. The ulvöspinel subsolidus textures and intergrowths also indicate identical cooling histories for the xenoliths in both cases. Ulvöspinel crystallized after the formation of iron, but still above the Fe-FeS eutectic at 988° C. A subsequent strong drop in oxygen fugacity revealed partial breakdown of ulvöspinel according to the reaction .Microprobe analyses of a Bühl xenolith indicate that ulvöspinel contains up to 4.7 w.t.% MnO, while olivine compositions correspond to Fa_64–74Fo_12–24Te_12–15. The entire xenolith contains 1.9 w.t.% MnO. This fact, together with the geological evidence and the occurrence of corroded quartz relicts within some of the xenoliths provides clear evidence for reduction under near-surface conditions in a blast-furnace-like process. The reducing agent was coal from the Tertiary seams cut by the erupting basalt, while the xenolith source material most probably was spherosiderite, which is very common in the coals and would explain the high MnO content. Consequently, the presence of cohenite is not necessarily an indicator of high pressure.The analogies between the Bühl and Ovifak localities and their xenoliths strongly suggest a similar formation through near surface reduction and not derivation from the mantle.  相似文献   

Direct measurements of volume transports through Fram Strait   总被引：9，自引：1，他引：9  

Eberhard Fahrbach  Jens Meincke  Svein Østerhus  Gerd Rohardt  Ursula Schauer  Vigdis Tverberg  Jennifer Verduin 《Polar research》2001,20(2):217-224

Heat and freshwater transports through Fram Strait are understood to have a significant influence on the hydrographic conditions in the Arctic Ocean and on water mass modifications in the Nordic seas. To determine these transports and their variability reliable estimates of the volume transport through the strait are required. Current meter moorings were deployed in Fram Strait from September 1997 to September 1999 in the framework of the EU MAST III Variability of Exchanges in the Northern Seas programme. The monthly mean velocity fields reveal marked velocity variations over seasonal and annual time scales, and the spatial structure of the northward flowing West Spitsbergen Current and the southward East Greenland Current with a maximum in spring and a minimum in summer. The volume transport obtained by averaging the monthly means over two years amounts to 9.5 ± 1.4 Sv to the north and 11.1 ± 1.7 Sv to the south (1 Sv = 10⁶ m³s⁻¹). The West Spitsbergen Current has a strong barotropic and a weaker baroclinic component; in the East Greenland Current barotropic and baroclinic components are of similar magnitude. The net transport through the strait is 4.2 ± 2.3 Sv to the south. The obtained northward and southward transports are significantly larger than earlier estimates in the literature; however, within its range of uncertainty the balance obtained from a two year average is consistent with earlier estimates.  相似文献   

Crustal Stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms     

C.D. Lindholm  H. Bungum  R.K. Bratli  B.S. Aadnøy  N. Dahl  B. Tørudbakken  K. Atakan 《地学学报》1995,7(1):51-59

The regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of directions of maximum horizontal compression (_H) as extracted from borehole breakouts and from earthquake focal mechanism solutions.
The results indicate that the regional stress field is dominated by NW-SE compression, with good consistency between shallow borehole breakouts (2–5 km depth) and deeper earthquakes (10–25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations.
The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE-SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.  相似文献   

Authigenic sericite record of a fossil geothermal system: the Offenburg trough,central Black Forest,Germany     

Olaf?Brockamp  Norbert?Clauer  Michael?ZutherEmail author 《International Journal of Earth Sciences》2003,92(6):843-851

A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K–Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of ¹⁸O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150–210 °C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.  相似文献