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排序方式: 共有405条查询结果,搜索用时 15 毫秒
51.
Georg Houben Ursula Noell Sara Vassolo Christoph Grissemann Mebus Geyh Susanne Stadler Eduardo J. Dose Sofia Vera 《Hydrogeology Journal》2014,22(8):1935-1952
The occurrence of a freshwater lens in the Paraguayan Chaco, 900 km away from the ocean, is reported. It is located underneath sandstone hills, surrounded by lowlands with predominantly saline groundwater. Its geometry was delineated using geoelectrical and electromagnetic investigations. The unusual height of the fresh groundwater level can be attributed to the presence of a confining layer at depth. The lens receives its recharge exclusively from rainfall during the hot and humid summer months. It predominantly contains water predating the atmospheric atomic bomb tests, some of it probably up to a thousand or more years old. The water balance shows that extraction currently does not exceed recharge in normal years. However, the available volume of groundwater leaves little room for a further increase of extraction in the future. Recharge is augmented by return flow from thousands of latrines and cess pits, and this has lead to widespread contamination of the groundwater by faecal bacteria. 相似文献
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53.
In this paper we review the possibilities for
magnetohydrodynamic processes to handle the angular momentum transport
in accretion disks. Traditionally the angular momentum transport has
been considered to be the result of turbulent viscosity in the disk,
although the Keplerian flow in accretion disks is linearly stable towards
hydrodynamic perturbations. It is on the other hand linearly unstable
to some magnetohydrodynamic (MHD) instabilities.
The most important instabilities are the Parker and Balbus-Hawley
instabilities that are related to the magnetic buoyancy and the shear
flow, respectively. We discuss these instabilities not only in the
traditional MHD framework, but also in the context of slender flux
tubes, that reduce the complexity of the problem while keeping most of
the stability properties of the complete problem. In the non-linear
regime the instabilities produce turbulence. Recent numerical
simulations describe the generation of magnetic fields by a dynamo in
the resulting turbulent flow. Eventually such a dynamo may generate a
global magnetic field in the disk. The relation of the MHD-turbulence
to observations of accretion disks is still obscure. It is commonly
believed that magnetic fields can be highly efficient in transporting
the angular momentum, but emission lines, short-time scale variability
and non-thermal radiation, which a stellar astronomer would take as
signs of magnetic variability, are more commonly observed during periods
of low accretion rates.
Received October 12, 1995 / Accepted November 16, 1995 相似文献
54.
Thomas Litt Sebastian Krastel Michael Sturm Rolf Kipfer Sefer Örcen Georg Heumann Sven Oliver Franz Umut B. Ülgen Frank Niessen 《Quaternary Science Reviews》2009,28(15-16):1555-1567
Lake Van is the fourth largest terminal lake in the world (volume 607 km3, area 3570 km2, maximum depth 460 m), extending for 130 km WSW–ENE on the Eastern Anatolian High Plateau, Turkey. The sedimentary record of Lake Van, partly laminated, has the potential to obtain a long and continuous continental sequence that covers several glacial–interglacial cycles (ca 500 kyr). Therefore, Lake Van is a key site within the International Continental Scientific Drilling Program (ICDP) for the investigation of the Quaternary climate evolution in the Near East (‘PALEOVAN’). As preparation for an ICDP drilling campaign, a site survey was carried out during the past years. We collected 50 seismic profiles with a total length of ~850 km to identify continuous undisturbed sedimentary sequences for potential ICDP locations. Based on the seismic results, we cored 10 different locations to water depths of up to 420 m. Multidisciplinary scientific work at positions of a proposed ICDP drill site included measurements of magnetic susceptibility, physical properties, stable isotopes, XRF scans, and pollen and spores. This core extends back to the Last Glacial Maximum (LGM), a more extended record than all the other Lake Van cores obtained to date. Both coring and seismic data do not show any indication that the deepest part of the lake (Tatvan Basin, Ahlat Ridge) was dry or almost dry during past times. These results show potential for obtaining a continuous undisturbed, long continental palaeoclimate record. In addition, this paper discusses the potential of ‘PALEOVAN’ to establish new results on the dynamics of lake level fluctuations, noble gas concentration in pore water of the lake sediment, history of volcanism and volcanic activities based on tephrostratigraphy, and paleoseismic and earthquake activities. 相似文献
55.
Landslides - The frictional resistance of rock and debris is supposed to induce stress anisotropy in the unsteady, non-uniform flow of gravitational mass flows, including debris flows. Though... 相似文献
56.
Xavier Bruna Catarino Egydio-Silva Marcos Sadowski Georg Robert de Assis Silva Bruno Takara Victor Junji 《Geotechnical and Geological Engineering》2022,40(3):1345-1361
Geotechnical and Geological Engineering - To optimize the prediction of structural geological conditions in the underground as of data collected at the surface, due to the usual great uncertainties... 相似文献
57.
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59.
Georg Marinos 《International Journal of Earth Sciences》1959,46(2):421-426
Zusammenfassung Aus den neueren geologischen Beobachtungen und Kartierungen in Griechenland folgt, daß die tektonische Aufgliederung Griechenlands nachRenz, zumindest in Ostgriechenland, nicht zu Recht besteht. An Stelle einer tektonischen Trennung haben wir kontinuierliche sedimentäre Übergänge. Gewiß gibt es tektonische Kontakte an kleineren und größeren Überschiebungen und einem verbreiteten Schuppenbau, aber diese Störungen beweisen keine tektonische Längsgliederung in Fazieszonen, für welcheRenz ein so reiches stratigraphisches Material erarbeitet hat. 相似文献
60.
A detailed analysis is presented of the hydraulic efficiency of plume management alternatives that combine a conventional pump-and-treat system with vertical, physical hydraulic barriers such as slurry walls or sheet piles. Various design settings are examined for their potential to reduce the pumping rate needed to obtain a complete capture of a given contaminated area. Using established modeling techniques for flow and transport, those barrier configurations (specified by location, shape, and length) that yield a maximum reduction of the pumping rate are identified assuming homogeneous aquifer conditions. Selected configurations are further analyzed concerning their hydraulic performance under heterogeneous aquifer conditions by means of a stochastic approach (Monte Carlo simulations) with aquifer transmissivity as a random space function. The results show that physical barriers are an appropriate means to decrease expected (mean) pumping rates, as well as the variance of the corresponding pumping rate distribution at any given degree of heterogeneity. The methodology presented can be transferred easily to other aquifer scenarios, provided some basic premises are fulfilled, and may serve as a basis for reducing the pumping rate in existing pump-and-treat systems. 相似文献