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   693篇
  免费   19篇
  国内免费   2篇
测绘学   12篇
大气科学   56篇
地球物理   166篇
地质学   271篇
海洋学   56篇
天文学   96篇
综合类   3篇
自然地理   54篇
  2021年   7篇
  2020年   7篇
  2018年   11篇
  2016年   12篇
  2014年   17篇
  2013年   25篇
  2012年   20篇
  2011年   20篇
  2010年   13篇
  2009年   29篇
  2008年   30篇
  2007年   24篇
  2006年   32篇
  2005年   24篇
  2004年   25篇
  2003年   15篇
  2002年   23篇
  2001年   19篇
  2000年   8篇
  1999年   14篇
  1998年   10篇
  1997年   6篇
  1996年   7篇
  1995年   7篇
  1994年   8篇
  1993年   7篇
  1992年   7篇
  1991年   10篇
  1987年   7篇
  1986年   5篇
  1985年   5篇
  1984年   8篇
  1983年   11篇
  1982年   9篇
  1981年   7篇
  1980年   8篇
  1977年   12篇
  1976年   7篇
  1975年   7篇
  1974年   8篇
  1972年   11篇
  1970年   7篇
  1968年   6篇
  1964年   5篇
  1959年   5篇
  1955年   7篇
  1954年   9篇
  1952年   6篇
  1950年   8篇
  1948年   8篇
排序方式: 共有714条查询结果,搜索用时 15 毫秒
701.
Conceptual models are applied during the various stages of the scientific method and are part of the numerous techniques employed in geologic field, laboratory and interpretative work. Uses of models are manifold, e.g., to show complex relationships, to help in memorizing complicated phenomena and in formulizing meaningful research programmes, to improve critical appraisal of hypotheses and theories, to compel investigators to consider all parameters and all possible alternative interpretations, to visualize phenomena in teaching, to prepare flow charts in computer geology, and to predict concealed and unexplored stratigraphic sections.Nine conceptual models are offered here which cover: (1) hierarchy of factors determining the origin of sediments; (2) depositional sedimentary environments; (3) limestone environments; (4) reef genesis; (5) soil genesis; (6) chemical sedimentary ores; (7) placer sedimentary ores, exemplified by: (8) the Witwatersrand uranium—gold deposits; and (9) an idealized stratigraphic-environmental model.  相似文献   
702.
703.
Zusammenfassung Zuerst wird eine gedrängte Übersicht über die Stratigraphie der basko-kantabrischen Kreidegeosynklinale und die Tektonik ihres baskischen, strukturell noch zu den Pyrenäen gehörigen Anteils gegeben. Dann werden gewisse, den klastischen Kreide-Schichtfolgen im allgemeinen fremde, sauber klassierte Grobpsammite und Geröllfacien als Spuren lokaler Ausgleichsströmungen an Untergrundsunebenheiten gedeutet. Diese ordnen sich, zusammen mit Mächtigkeitsminima insbesondere des Albien, und mit Diskordanzen in Albien und Cenoman auf Nordnordwest- bis Nord-streichenden Zonen an. Parallel dazu verlaufen Bruchstrukturen innerhalb der Kreide, die den Gang der posteozänen Pyrenäenfaltung örtlich beeinflußten und von dieser, so beherrschend und tiefgreifend sie hier im übrigen scheinen mag, nur unvollkommen überprägt wurden. Auf eine ältere Strukturierung des voralpidischen Untergrundes wird geschlossen. Tatsächlich ergab ein zu Vergleichszwecken angestelltes Literaturstudium in dem ostwärts anschließenden Gebiet der Westpyrenäen das Vorhandensein variscischer Achsen und Brüche gleicher Art und Orientierung.
Stratigraphy and tectonics of the Basco-Cantabrian cretaceous geosyncline (Northern Spain) are briefly resumed. Lithofacial and structural peculiarities in it are described. They suggest meridional faulting of the basement, similarly as described by others in the older rocks exposed in the Western Pyrenees.

Résumé Stratigraphie et tectonique du géosynclinal crétacé basco-cantabrique sont résumées brèvement. Des surélévements au fond de la mer aptienne-cénomanienne, conclus de certains faciès grossiers, d'épaisseurs-minimum et de discordances, s'élongeaient parallèles à des failles et des axes pré-pyrénéennes, disharmoniques, en direction Nord-nord-ouest ou Nord. Celles-ci ne sont retouchées qu'imparfaitement par le plissement post-éocène, entraînant et catégorique qu'il bien y paraisse. A l'Est de Tolosa, jusqu'à Pau, le sous-sol élevé du géosynclinal crétacé montre des axes et des failles méridionales, hercyniennes, tout-à-fait pareilles à celles qu'il me fallut admettre au-dessous du Crétacé basque pour y expliquer les particularités mentionnées ci-dessus.

, , - . .


Erweitertes Manuskript eines Vortrages vor der Hauptversammlung der Geologischen Vereinigung in Wetzlar am 13. März 1961.  相似文献   
704.
Summary From station's observations much information on top-frequencyfEs, less on blanketing frequencyfbEs is obtained. The results must be interpretated in statistical terms. Time- and distance-correlation functions are established. Daily, seasonal and geographic regularities are discussed. A sharp maximum exists at the magnetic equator but only for the top-frequency. No well defined influence of the solar cycle has been found, only a very weak one of lunar tides.Apart the routine evaluation observations of the variation of the reflection coefficient with frequency have been made. In temperate latitudes in about 1/3 of all cases there is no partial reflection; in the other cases local variations of electron concentration up to 1:2 are found, higher values are rare. At low latitudes the variation may be more important. This is found by a qualitative classification of ionograms of different stations. Such classification has been made for transparency, scatter, angle of incidence and layer developpement. Diffuse echoes exist often near the magnetic equator.Es is always vertically sounded at temperate latitudes. In most cases the ionization originates as a thin layer of constant altitude. Transitory phenomena coming downwards are responsible for «E2s» in daytine.It appears that a cumulo-cirrus-cloud layer is a good model forEs-ionization. An atmospheric ionization process using stored energy from dissociation or ion existence seems to be most probable.  相似文献   
705.
Zusammenfassung Es werden Beziehungen abgeleitet zwischen dem vertikalen Gradienten des modifizierten BrechungsindexM (für Centimeter- bis Meterwellen) in der maritimen Grenzschicht der Atmosphäre und der Temperatur- und Wasserdampfdifferenz Luft-Wasser. Diese Beziehungen ergeben sich aus unseren Kenntnissen von dem Einflusse der Temperaturschichtung und der Windgeschwindigkeit auf die vertikale Wasserdampfschichtung über See und ermöglichen die Absehätzung der Refraktionsschichtung (Duct-Dicke,mittlerer Gradient u.a.) für elektromagnetische Wellen (Cm- bis M-Band) in den unteren Dekametern über dem Meer. Es werden Tabellen und Diagramme für eine praktische Anwendung des Verfahrens angegeben.Als Beispiel werden Häufigkeitsverteilungen der Duct-Dicke und des mittleren Brechungsindexgradienten für Sommer und Winter gezeigt, die sich aus vierjährigen Beobachtungen auf einem nordatlantischen Wetterschiff ergaben.
On the gradient of the refractive index of electro-magnetic waves (centimetre- to metre-waves) in the maritime boundary layer of the atmosphere
Summary Correlations between the vertical gradient of the modified refractive indexM (of centimetre- to metre-waves) in the maritime boundary layer of the atmosphere and the difference air-water in temperature and water vapour are stated. The correlations are known from the influences exercised by the temperature stratification and the wind velocity on the vertical stratification of water vapour above the sea surface; they allow to estimate the refractive stratification (thickness of duct, medium gradient etc.) of the electro-magnetic waves (centimetre- to metre-waves) in the lower decametres above the ocean surface. Tables and diagrams are stated in the paper for a practical application of the method under discussion.As an example, the author included in this paper data of frequency distribution of the duct thickness and of the medium gradient of the refractive index for summer and winter which are derived from four years' observations on board of a weather ship in the North Atlantic.

Sur le gradient de l'indice de réfraction des ondes électromagnétiques (de 0,01 m á 1m) présentes dans la couche limite maritime de l'atmosphère
Résumé On révèle des corrélations entre le gradient vertical de l'indice modifié de réfractionM (par rapport aux ondes de 0,01 à 1 m) et la différence: air eau de la température et du vapeur d'eau. Ces corrélations, nous le savons, résultent de l'effect que la stratification de la température a sur la stratification verticale du vapeur d'eau au-dessus de la surface de la mer. La connaissance de ces corrélations nous permet d'estimer la stratification de réfraction (l'épaisseur du duct, gradient moyen etc.) des ondes électromagnétiques présentes dans les bas décamètres voisins de la surface de l'océan. Des tableaux et des diagrammes permettant d'appliquer au travail pratique la méthode discutée ici, sont indiqués.Pour offrir un exemple, l'auteur décrit dans cet article la distribution en fréquence de l'épaisseur du duct et du gradient moyen de l'indice de réfraction pour l'hiver et pour l'été en se basant sur les observations faites à bord d'un bateau météorologique en Océan Atlantique Nord pendant une période de quatre ans.


Herrn o. ö. Professor Dr. Paul Raethjen, Direktor des Geophysikalischen Instituts der Universität Hamburg, zum 60. Geburtstag gewidmet.  相似文献   
706.
707.
The heat waves of 2003 in Western Europe and 2010 in Russia, commonly labelled as rare climatic anomalies outside of previous experience, are often taken as harbingers of more frequent extremes in the global warming-influenced future. However, a recent reconstruction of spring–summer temperatures for WE resulted in the likelihood of significantly higher temperatures in 1540. In order to check the plausibility of this result we investigated the severity of the 1540 drought by putting forward the argument of the known soil desiccation-temperature feedback. Based on more than 300 first-hand documentary weather report sources originating from an area of 2 to 3 million km2, we show that Europe was affected by an unprecedented 11-month-long Megadrought. The estimated number of precipitation days and precipitation amount for Central and Western Europe in 1540 is significantly lower than the 100-year minima of the instrumental measurement period for spring, summer and autumn. This result is supported by independent documentary evidence about extremely low river flows and Europe-wide wild-, forest- and settlement fires. We found that an event of this severity cannot be simulated by state-of-the-art climate models.  相似文献   
708.
709.
The recent vertical displacement field of the Upper Rhine Graben (URG) located in the tri-national region between Germany, France and Switzerland is investigated using repeatedly measured leveling data. We estimate vertical displacement rates at leveling benchmarks by applying a kinematic network adjustment on more than 40,000 height differences measured by German, French and Swiss surveying agencies. Focusing on an optimal solution for the adjusted rates in the URG area also historical data (measured before 1900) are used, significantly increasing the time span of available measurements and the number of transnational connections between the three countries. To account for inhomogeneities apparent in the database, we apply an iterative variance component estimation within the adjustment procedure, particularly revealing more realistic information on the accuracy of the estimated rates. A special focus within our analysis is put on the statistical testing of gross errors in the observations and model-related errors at benchmarks with non-linear movement. As some of the estimated vertical rates behave significantly different compared to the vertical rates of adjacent benchmarks, a filtering of outliers is applied after the adjustment procedure. The resulting map of linear height changes in an area of 280 km in N–S and 230 km in E–W direction provides detailed insight into the recent vertical displacements of the URG and neighboring regions. In the German part of the study area, it was possible for the first time to consistently constrain an average subsidence rate of 0.5 mm/a ( \(\pm \) 0.2 mm/a) of the Graben interior w.r.t. the Black Forest. In addition to the tectonic displacements, some man-induced surface movements, e.g., caused by oil and groundwater extraction, are observed and discussed.  相似文献   
710.
A hydrogeological study was conducted in northwestern New Brunswick, Canada, to improve the predictability of fracture-dominated groundwater flow within folded bedrock composed of fine-grained turbidites. Borehole televiewer logging and outcrop mapping, integrated with hydraulic packer tests revealed enhanced hydraulic conductivity associated with northeasterly striking bedding-plane fractures formed during folding and flexural slip. These fractures impart azimuthal anisotropy to the aquifer because of moderately dipping fold limbs. High-angle fractures form a well-developed non-stratabound network, comprising two open fracture sets striking NNE parallel to the current direction of principal stress, and WNW parallel to the direction of principal stress that dominated during the Acadian orogeny. The subset of fractures showing significant oxidation, deemed most important to the groundwater flow system, is dominated by bedding-plane and high-angle fractures striking near-parallel to the maximum principal stress direction, resulting in extensional opening and enhanced hydraulic conductivities. An equivalent porous media model, incorporating anisotropy and varying hydraulic conductivity with depth, indicates that horizontal flow dominates the aquifer with relatively minor exchange between different model layers. These findings have implications for understanding flow directions in the Black Brook Watershed and elsewhere in the Matapédia Basin where fractures formed under similar stress conditions.  相似文献   
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