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   60535篇
  免费   944篇
  国内免费   439篇
测绘学   1219篇
大气科学   4225篇
地球物理   12074篇
地质学   21683篇
海洋学   5299篇
天文学   13489篇
综合类   132篇
自然地理   3797篇
  2021年   552篇
  2020年   652篇
  2019年   748篇
  2018年   1414篇
  2017年   1378篇
  2016年   1555篇
  2015年   960篇
  2014年   1531篇
  2013年   2877篇
  2012年   1765篇
  2011年   2434篇
  2010年   2139篇
  2009年   2760篇
  2008年   2485篇
  2007年   2550篇
  2006年   2348篇
  2005年   1739篇
  2004年   1784篇
  2003年   1769篇
  2002年   1614篇
  2001年   1432篇
  2000年   1326篇
  1999年   1206篇
  1998年   1217篇
  1997年   1208篇
  1996年   966篇
  1995年   923篇
  1994年   818篇
  1993年   747篇
  1992年   678篇
  1991年   661篇
  1990年   705篇
  1989年   635篇
  1988年   576篇
  1987年   668篇
  1986年   645篇
  1985年   815篇
  1984年   885篇
  1983年   859篇
  1982年   782篇
  1981年   717篇
  1980年   688篇
  1979年   646篇
  1978年   649篇
  1977年   609篇
  1976年   576篇
  1975年   587篇
  1974年   546篇
  1973年   572篇
  1972年   375篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
241.
Comprehensive distribution patterns of physical and chemical characteristics have been obtained from a series of cruises in Liverpool Bay. The marked feature of these distributions was their temporal variability, suggesting that the surface residual circulation is also temporally variable. The influence of wind stress upon the circulation pattern and hence water quality of this sea area is illustrated.  相似文献   
242.
Summary Asa step in the development of a fully coupled regional model of the atmosphere-ice-ocean system, atmospheric and sea ice models have been adapted to a western Arctic domain centered on the Bering Strait. Lateral boundary conditions derived from operational analyses drive the models through simulations on grids having horizontal resolutions of 21 km and 7 km. Sensitivities to the presence of sea ice are large after only 48 hours, by which time the surface temperatures in the Bering and Chukchi Seas are 10–15°C higher without sea ice than with sea ice. The temperatures, in turn, modify the fields of sea level pressure, surface wind and precipitation. By influencing the surface wind stress through the static static stability, the surface state feeds back to the surface momentum exchange, ice/ocean transport, and the rate of formation of new ice. The results also show a resolution-dependence of the surface winds, precipitation rates and new ice formation rates, particularly in areas in which the coastal configuration and topography are spatially complex. The experiments will be augmented by the implementation of an ocean model on the same grids.With 12 Figures  相似文献   
243.
A thick (200 m) rock salt mass covering Late Archean granitoids was exposed for the first time in the Early Proterozoic volcanogenic-sedimentary formations in the Onega trough of the east Baltic Shield by the Onega parametric well. The mineral composition of salts, their geochemical features, and the isotopic composition of carbonate carbon and oxygen have been studied. After fluid inclusions present in salts, their metamorphism temperature and isotopic composition of helium and argon were determined. The obtained results give evidence of the fact that rock salts and magnesites associated with them were formed in an evaporate basin with participation of deep crustal processes. The age of the underlying granitoids (2.716 ± 9 Ma) is determined using the Pb—Pb method.  相似文献   
244.
Bjune, A. E., Birks, H. J. B., Peglar, S. M. & Odland, A. 2010: Developing a modern pollen–climate calibration data set for Norway. Boreas, Vol. 39, pp. 674–688. 10.1111/j.1502‐3885.2010.00158.x. ISSN 0300‐9483. Modern pollen–climate data sets consisting of modern pollen assemblages and modern climate data (mean July temperature and mean annual precipitation) have been developed for Norway based on 191 lakes and 321 lakes. The original 191‐lake data set was designed to optimize the distribution of the lakes sampled along the mean July temperature gradient, thereby fulfilling one of the most critical assumptions of weighted‐averaging regression and calibration and its relative, weighted‐averaging partial least‐squares regression. A further 130 surface samples of comparable taphonomy, taxonomic detail and analyst became available as a result of other projects. These 130 samples, all from new lakes, were added to the 191‐lake data set to create the 321‐lake data set. The collection and construction of these data sets are outlined. Numerical analyses involving generalized linear modelling, constrained ordination techniques, weighted‐averaging partial least‐squares regression, and two different cross‐validation procedures are used to asses the effects of increasing the size of the calibration data set from 191 to 321 lakes. The two data sets are used to reconstruct mean July temperature and mean annual precipitation for a Holocene site in northwest Norway and a Lateglacial site in west‐central Norway. Overall, little is to be gained by increasing the modern data set beyond about 200 lakes in terms of modern model performance statistics, but the down‐core reconstructions show less between‐sample variability and are thus potentially more plausible and realistic when based on the 321‐lake data set.  相似文献   
245.
Zusammenfassung Bei der Durchführung quantitativer Gemenge-Analysen mit Hilfe der Röntgendiffraktionsmethode kann man den Massenschwächungskoeffizienten (*) verwenden, den man bei der Röntgenfluoreszenzanalyse Röhrenhauptlinienverfahren vonSchroll-Stepon erhalten hat. Er wird von der Wellenlänge der Röntgenfluoreszenzstreustrahlung auf die Wellenlänge der Diffraktionsröhre transformiert. Ein Intensitäts-Konzentrationsdiagramm, in dem die Intensität der gewählten Diffraktionslinie durch das Produkt von Intensität und dem jeweiligen Wert von * ersetzt wird, ergibt eine allgemeingültige Eichgerade. Als Beispiel werden Messungen an Zweistoffsystemen wie Magnesit-Quarz, Albit-Quarz, Calcit-Quarz, Stilpnomelan-Quarz und Siderit-Quarz angeführt.Die vorgeschlagene Methode ist für kombinierte Diffraktions- und Röntgenfluoreszenzanalysen sehr zweckmäßig.
Summary Applicating the quantitative X-ray-diffraction-analysis für mixtures of minerals the use of the mass absorption coefficient (*) is possible, which is received by the X-ray-fluorescence-analysis of the same sample following the method proposed by the authors. The mass absorption coefficients must be transformed to the new wave length of the X-ray-diffraction tube. In the intensity-concentration-diagram the intensity is substituted by the product of intensity and mass absorption coefficient. An uniform standard line is received, as the measurement of quartz shows in the systems of two-substances, such as magnesite-quartz, albite-quartz, calcite-quartz, stilpnomelan-quartz and siderite-quartz.The proposed method is very useful for the combined X-ray-diffraction and X-ray fluorescence analysis.


Mit 6 Textabbildungen  相似文献   
246.
247.
Zusammenfassung Nach einer knappen Einführung in die Magnitudenberechnung wird über die Ableitung der Stationsgleichungen für Wien (12) und Graz (13) berichtet. Den Berechnungen lagen 158, bzw. 121 Erdbeben normaler Herdtiefe und von der GrößeM7,0 zugrunde. Für beide Stationskonstanten wurde der gleiche WertC=+0,15 gefunden. Die Tabelle 2 enthält 94 von beiden Stationen registrierte Erdbeben mit Angabe der UnterschiededM (=D) von Wien und Graz gegenüber denM-Werten von Pasadena.Verschiedene Schwierigkeiten, die bei der Aufstellung der maximalen Horizontalamplitude auftauchen können, werden besprochen und an Hand eines Beispieles (Tabelle 3) dargelegt. Ferner wird angedeutet, daß das letzte GliedD der Magnitudengleichung definitionsmäßig überlastet ist und daß eine Aufteilung nach den wirklich individuellen Eigenschaften und den Gruppeneigenschaften des Einzelbebens notwendig wäre. Alle Erdbeben aus einer bestimmten Region tragen gemeinsame Merkmale, jedes einzelne hat dazu einen eigenen Habitus.
Summary After a short introduction into the computation of magnitude we show the derivation of the respective equations for Vienna (12) and Graz (13). The basis of the computations were 158, 121 respectively, teleseisms of normal depth of focus and of the magnitudeM7.0. The same value was found for both station constants, namelyC=+0.15. Table 2 contains 94 earthquakes recorded at both stations, showing the differencesdM (=D) of Vienna and Graz, in comparison with theM-values of Pasadena.Various difficulties that can arise by establishing the total horizontal amplitude, are considered and discussed by means of an example (Table 3). Further we find that the last termD of the equation of magnitude is overcharged according to the definition, and that a separation according to the really individual qualities and to the group qualities of each earthquake would be necessary. All earthquakes from a certain area have specified qualities, and in addition each one shows a special behaviour of its own.

Résumé Après une introduction concise du calcul de magnitude on est informé sur les équations qui en dérivent et qui sont valables pour les stations de Vienne (12) et de Graz (13). Resp. 158 et 121 séismes d'une profondeur de foyer normale et de magnitudeM7.0 forment la base de ces calculs. Une valeur identiqueC=+0,15 a été trouvée comme constante des deux stations. Le tableau 2 contient 94 séismes, enregistrés par les deux stations, avec indication des différencesdM (=D) de Vienne et de Graz vis-à-vis des valeursM de Pasadena.Diverses difficultés qui peuvent surgir en établissant l'amplitude horizontale maximum, sont discutées et démontrées par un exemple (tableau 3). En outre on trouve que le dernier coefficientD de l'équation est «surchargé» selon la définition. Une distribution d'après les véritables qualités individuelles et d'après les qualités de groupe du séisme particulier serait nécessaire. Tous les séismes provenant d'une certaine région ont des propriétés communes, de plus chacun a son caractère particulier.
  相似文献   
248.
The mechanism of concentration on the shaking table is discussed and a fresh attempt is made to obtain the theoretical model describing the velocity of a fluid in depth and time when in contact with a symmetrically reciprocating surface.The significance of the many design and operational variables and their interrelationships are examined. The various areas where the shaking table plays its role are then considered.The relevant features of micropanner, superpanner and the subsequent macropanner and pulsepanner are also treated since they all, like the shaking table, have the differential motion in common.  相似文献   
249.
Wuyep  E. O.  Oluyemi  G. F.  Yates  K.  Akisanya  A. R. 《Natural Resources Research》2020,29(2):1239-1258
Natural Resources Research - Sand failure and production occurs when the formation stress exceeds the strength of the formation, which is derived majorly from the natural material that cements the...  相似文献   
250.
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