首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   344篇
  免费   16篇
  国内免费   2篇
测绘学   17篇
大气科学   37篇
地球物理   72篇
地质学   147篇
海洋学   15篇
天文学   45篇
综合类   3篇
自然地理   26篇
  2022年   3篇
  2019年   3篇
  2018年   3篇
  2017年   8篇
  2016年   7篇
  2015年   6篇
  2014年   12篇
  2013年   16篇
  2012年   12篇
  2011年   15篇
  2010年   21篇
  2009年   14篇
  2008年   11篇
  2007年   9篇
  2006年   13篇
  2005年   7篇
  2004年   13篇
  2003年   13篇
  2002年   8篇
  2001年   9篇
  2000年   3篇
  1999年   8篇
  1998年   3篇
  1997年   4篇
  1996年   5篇
  1994年   6篇
  1993年   8篇
  1992年   5篇
  1991年   4篇
  1990年   3篇
  1987年   5篇
  1986年   4篇
  1985年   7篇
  1984年   14篇
  1983年   8篇
  1982年   8篇
  1981年   6篇
  1980年   3篇
  1979年   3篇
  1978年   4篇
  1977年   3篇
  1976年   2篇
  1975年   7篇
  1974年   3篇
  1972年   2篇
  1968年   5篇
  1967年   2篇
  1966年   3篇
  1964年   2篇
  1937年   2篇
排序方式: 共有362条查询结果,搜索用时 93 毫秒
261.
Zusammenfassung Es werden die typischen Faseraggregate von kohliger Substanz beschrieben, die oft an den Liegendkontakten wirtschaftlich wichtiger Gold-Uran-Konglomerate des südafrikanischen Witwatersrand-Systems in dünnen Lagen auftreten. Der Mineralbestand dieser Aggregate wird erzmikroskopisch untersucht, wobei Gold und Uranpecherz besonders berücksichtigt werden. Die Faserung wird als Wachstum senkrecht zur Wandung schichtparalleler Ablösungsklüfte erklärt (parallelstengelige Kluftfüllung im Sinne vonMügge, 1928).An primären detritischen Mineralien finden sich in diesen Aggregaten Pyrit, Kobaltglanz, Chromit, Zirkon und Iridosmium; umgelagerte authigene Bestandteile der Schwerefraktion sind im wesentlichen Gold, Linneit, Kupferkies, Bleiglanz. Zinkblende und Spuren von Magnetkies. Der primär ebenfalls detritische Uranpecherzanteil liegt ausschließlich in stark resorbierter Form vor, d. h. die UO2-Partikel lösen sich in den verfestigten Kohlenwasserstoffen zu einzelnen Bruchstücken auf, die dann allmählich adsorptiv verdaut werden.Auffallend sind die charakteristischen Fließgefüge der kohligen Substanz um die allogenen Bestandteile der Paragenese, insbesondere um den mengenmäßig vorherrschenden Pyrit. Es kommt zu typischen Augenbildungen, wobei sich häufig Gold, Bleiglanz oder sekundärer Quarz in den Druckschattenräumen an beiden Enden der Streckungshöfe absetzen. Ähnliche Fließgefüge lassen sich auch um einige frühe Neubildungen wie Zinkblende beobachten.Es ergibt sich somit aus den Untersuchungen, daß die kohlige Substanz in ihrer heutigen Form jünger ist als die primären detritischen Mineralien, daß aber andererseits die pseudohydrothermale Umlagerung der Hauptmasse des Goldes (das ursprünglich ebenfalls in detritischer Form vorgelegen haben muß) teilweise noch später erfolgt ist. Dasselbe gilt für einige genetisch junge Sulfide wie Linneit, z. T. auch für Kupferkies.
The footwall of the principal uraniferous congiomerates occurring within the Witwatersrand System (South Africa) is often characterized by a fibrous carbon or thucholite seam, ranging in thickness from parts of a mm to several cm (Figs. 1 and 2). The carbonaceous material has formed by radiolytic polymerization of originally mobile (preferably gaseous) hydrocarbons percolating through the strata, the prerequisite for its formation thus being the presence of detrital uraninite within the heavy fraction of the individual reefs. The conspicuous columnar fabric of the aggregates is explained as being due to growth at right angles to the bedding planes comparable with several other fibrous minerals (Mügge, 1928).The association between heavy minerals and carbonaceous matter must be particularly intimate, since the latter has accumulated in the lowermost footwall portions where the bulk of the heavy suite of the conglomerates is concentrated. Primary detrital constituents within the fibrous aggregates are pyrite, cobaltite, chromite, zircon, and iridosmine (Figs. 4 and 5); authigenic recrystallized heavy minerals are gold, linnaeite, chalcopyrite, galena, sphalerite, as well as traces of pyrrhotite (Figs. 3 and 6). The detrital uraninite fraction is to a large extent replaced by carbonaceous matter, i. e. the UO2-particles are in the state of floating apart in, and being gradually digested by, the solid hydrocarbons (Fig. 8). Very often the carbonaceous matter displays a prominent flow lamination around the allogenic constituents, especially around the ubiquitous pyrite (Fig. 4). Rise is given thereby to a typical augen structure; gold, galena or secondary quartz are commonly deposited in the pressure-shadows on either side of the inclusions. Similar fabrics may be observed around some early-formed authigenic minerals like sphalerite (Fig. 6); the latter mostly appears to be dragged apart (boudinagedrd) and carbon usually comes to occupy the interstices between the individual fragments. Elongated fragments often tend to arrange themselves parallel to the flow laminae.It is evident from the investigation that carbonaceous matter is distinctly younger than the primary detrital minerals. On the other hand, it is likewise obvious that the pseudohydrothermal recrystallization of the main mass of the gold (which originally also formed part of the heavy suite) must have taken place at a still later stage. The same applies to several genetically young Sulfides as linnaeite, in part also to chalcopyrite.


Erweiterte Fassung eines Vortrages, den der Verfasser am 5. 9. 65 auf der 43. Jahrestagung der Deutschen Mineralogischen Gesellschaft in Hannover gehalten hat.  相似文献   
262.
263.
264.
Knowledge of the environmental controls of carbon dioxide fluxes is essential for understanding the dynamics of carbon exchange between ecosystems and atmosphere. In this study we investigated soil respiration and moss photosynthesis as well as their contribution to the net carbon dioxide flux of two different wet tundra systems. During two summers, in situ carbon dioxide fluxes were measured in a tussock tundra and in a low-centre polygonal tundra on Taimyr Peninsula, central Siberia. Measurements were carried out by means of a multichannel gas exchange system. Results show pronounced differences in soil respiration rates as related to microscale topography, mainly due to differences of soil water table and soil temperatures. Modelling of soil respiration for individual microsites revealed differences of process performance with respect to both factors. The wet microsites showed the highest potential regarding an increase of soil respiration rates in warmer and drier climate change scenarios. Another important process compensating the CO2 release from the soil was the photosynthesis of the moss layer, assimilating as much as 51% to 98% of the daily amount of carbon dioxide released from wet tundra soils. This result demonstrates the importance of mosses in the context of tundra ecosystem processes. The magnitude of net system fluxes of the whole system at the depression of the polygonal tundra was strongly influenced by changes in soil water table. Consequently, any changes of the hydrology, as anticipated in the context of global change, would effectively alter the carbon balance of wet tundra systems.  相似文献   
265.
At the geothermal test site near Groß Schönebeck (NE German Basin), a new 3D seismic reflection survey was conducted to study geothermal target layers at around 4 km depth and 150°C. We present a workflow for seismic facies classification and modelling which is applied to a prospective sandstone horizon within the Rotliegend formation. Signal attributes are calculated along the horizon using the continuous Morlet wavelet transform. We use a short mother wavelet to allow for the temporal resolution of the relatively short reflection signals to be analysed. Time-frequency domain data patterns form the input of a neural network clustering using self-organizing maps. Neural model patterns are adopted during iterative learning to simulate the information inherent in the input data. After training we determine a gradient function across the self-organizing maps and apply an image processing technique called watershed segmentation. The result is a pattern clustering based on similarities in wavelet transform characteristics. Three different types of wavelet transform patterns were found for the sandstone horizon. We apply seismic waveform modelling to improve the understanding of the classification results. The modelling tests indicate that thickness variations have a much stronger influence on the wavelet transform response of the sandstone horizon compared with reasonable variations of seismic attenuation. In our interpretation, the assumed thickness variations could be a result of variable paleo-topography during deposition of predominantly fluvial sediments. A distinct seismic facies distribution is interpreted as a system of thicker paleo-channels deposited within a deepened landscape. The results provide constraints for the ongoing development of the geothermal test site.  相似文献   
266.
Trade-wind cumuli constitute the cloud type with the highest frequency of occurrence on Earth, and it has been shown that their sensitivity to changing environmental conditions will critically influence the magnitude and pace of future global warming. Research over the last decade has pointed out the importance of the interplay between clouds, convection and circulation in controling this sensitivity. Numerical models represent this interplay in diverse ways, which translates into different responses of trade-cumuli to climate perturbations. Climate models predict that the area covered by shallow cumuli at cloud base is very sensitive to changes in environmental conditions, while process models suggest the opposite. To understand and resolve this contradiction, we propose to organize a field campaign aimed at quantifying the physical properties of trade-cumuli (e.g., cloud fraction and water content) as a function of the large-scale environment. Beyond a better understanding of clouds-circulation coupling processes, the campaign will provide a reference data set that may be used as a benchmark for advancing the modelling and the satellite remote sensing of clouds and circulation. It will also be an opportunity for complementary investigations such as evaluating model convective parameterizations or studying the role of ocean mesoscale eddies in air–sea interactions and convective organization.  相似文献   
267.
Two stations monitoring concentrations of carbon dioxide and radon in soil gas (Oldřišská and Novy Kostel) and one station monitoring flow of carbon dioxide at a mofette (Soos) have been operated in the area of the West Bohemian earthquake swarms. We present preliminary results obtained on the base of four-year observations. We found that data are not influenced considerably by barometric pressure. Although the CO2 concentration varies greatly, the long-term trends at stations Oldřišská and Novy Kostel are similar, which indicates that the CO2 flow is controlled by common geogenic processes. Also temporal trends of CO2 and Rn concentrations in soil gas at individual stations are analogous. We found diurnal variations of both CO2 concentration in soil gas and the CO2 flow at mofettes due to the earth tides. A response to tides of semi-diurnal period is insignificant in CO2 concentration and only weak in the CO2 flow. We also examined possible pre-seismic, co-seismic and post-seismic effects of the intensive 2008 earthquake swarm on the CO2 concentration at Oldřišská and Novy Kostel, and on the CO2 flow at Soos. However, all potential indications were insignificant and there has not been proven any influence of the swarm on the CO2 concentration as well as on the CO2 flow. Nevertheless, a gradual decrease of amplitudes of diurnal variations before the swarm and the lowest amplitudes during the swarm is a noteworthy phenomenon, which might indicate the strain changes of the rock associated with earthquake swarm.  相似文献   
268.
Anomalous fading of the infrared stimulated luminescence (IRSL) signal from the polymineral fine-grain and K-feldspar fractions of aeolian sediments from Hungary has been studied. The samples in this study have previously been dated using the multiple aliquot additive dose (MAAD) protocol to measure the IRSL signal. The IRSL measurements using MAAD were conducted ~4 weeks after the irradiation, making it difficult to assess to what extent these age estimates were affected by anomalous fading. In this study, equivalent doses were obtained using the single aliquot regenerative dose (SAR) protocol. The fading rate for each sample was calculated using the different IRSL components and different parts of the decay curve. For each sample, the middle part of the decay curve always showed a lower fading rate than the initial part of the decay curve. The difference between the fading rates for different parts of the decay curve was greater for the K-feldspars than for the polymineral fine grains. Fading corrected ages were calculated by integrating both the initial and the middle part of the decay curve. These ages were compared with optically stimulated luminescence (OSL) ages from quartz, infrared radiofluorescence (IR-RF) ages obtained from K-feldspars and also with independent ages, provided by radiocarbon dating of shells and charcoal, and uranium-series dating of travertine.  相似文献   
269.
New magnetotelluric data from the Münchberg Gneiss complex in Southern Germany reveal a zone of extremely high electrical conductivity. 1-D modelling of the data is justified in the period range 0.01 to 10  s. At least three layers are required to explain the steepness of the apparent resistivity curves, and the best-fitting models comprise four layers with successively higher conductivities. The layers of highest conductivity at depths between 2.2 and 3.6  km correlate with pronounced bands of high seismic reflectivity (profile DEKORP 85-4N). The Münchberg complex is today widely recognized as a tectonic klippe, consisting of rocks whose metamorphic and stratigraphic order is inverted rather than overturned. The material was transported into its present position by predominantly horizontal tectonic forces along shear zones. We interpret the high conductivity and high reflectivity as remnants of this transport process.  相似文献   
270.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号