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   65942篇
  免费   897篇
  国内免费   1198篇
测绘学   2252篇
大气科学   4809篇
地球物理   12764篇
地质学   26508篇
海洋学   4876篇
天文学   10842篇
综合类   2250篇
自然地理   3736篇
  2022年   292篇
  2021年   490篇
  2020年   558篇
  2019年   628篇
  2018年   5696篇
  2017年   5011篇
  2016年   3721篇
  2015年   891篇
  2014年   1221篇
  2013年   2053篇
  2012年   2298篇
  2011年   4436篇
  2010年   3536篇
  2009年   4199篇
  2008年   3617篇
  2007年   4089篇
  2006年   1781篇
  2005年   1430篇
  2004年   1718篇
  2003年   1694篇
  2002年   1360篇
  2001年   1008篇
  2000年   935篇
  1999年   804篇
  1998年   813篇
  1997年   770篇
  1996年   617篇
  1995年   601篇
  1994年   535篇
  1993年   481篇
  1992年   444篇
  1991年   444篇
  1990年   464篇
  1989年   402篇
  1988年   384篇
  1987年   418篇
  1986年   431篇
  1985年   523篇
  1984年   561篇
  1983年   558篇
  1982年   508篇
  1981年   490篇
  1980年   454篇
  1979年   415篇
  1978年   396篇
  1977年   402篇
  1976年   365篇
  1975年   370篇
  1974年   356篇
  1973年   384篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
52.
53.
Karst database development in Minnesota: design and data assembly   总被引:1,自引:0,他引:1  
The Karst Feature Database (KFD) of Minnesota is a relational GIS-based Database Management System (DBMS). Previous karst feature datasets used inconsistent attributes to describe karst features in different areas of Minnesota. Existing metadata were modified and standardized to represent a comprehensive metadata for all the karst features in Minnesota. Microsoft Access 2000 and ArcView 3.2 were used to develop this working database. Existing county and sub-county karst feature datasets have been assembled into the KFD, which is capable of visualizing and analyzing the entire data set. By November 17 2002, 11,682 karst features were stored in the KFD of Minnesota. Data tables are stored in a Microsoft Access 2000 DBMS and linked to corresponding ArcView applications. The current KFD of Minnesota has been moved from a Windows NT server to a Windows 2000 Citrix server accessible to researchers and planners through networked interfaces.  相似文献   
54.
55.
A recent article by Beer et al. (1974) in Marine Geology describes an in-situ suspended sediment sampling system which utilizes a pump/motor unit, filter holders, and a lead-acid battery power source. Two aspects of their article may be misleading: the use of certain filter material as X-ray diffraction mounts, and the technique by which an oil-filled battery case may be operated.  相似文献   
56.
The sodian stellerite (a zeolite) occurs as lustrous white or slightly pink, easily cleavable lamellae, filling cavities and fractures in an acid lava (andesite or rhyolite) in the cliff below S. Efisio Tower, near Capo Pula, Cagliari, Sardegna, Italy. The principal forms present are {010}, {011}, {111}. Single crystal photographs show an orthorhombic symmetry and Ammm (or Ammm) space group. An indexed X-ray diffraction powder pattern is provided. Unit cell dimensions, chemical formula, density, optical properties and thermal behaviour are given.  相似文献   
57.
Investigations in a newly constructed subsea road tunnel in the Iddefjord granite at Hvaler and test-pumping of boreholes on land indicate that a topographic or geophysical anomaly is no guarantee of a substantially transmissive fracture zone. Many prominent fracture zones appear to have depressed transmissivity due to secondary swelling-clay mineral infillings. No current geophysical technique can adequately distinguish these zones from transmissive ones. Given that siting of boreholes on the basis of geology alone can be unreliable, hydrogeologists should concentrate on quantifying the chances of a successful boring, and optimizing those chances by sensible location, favourable borehole orientation and use of artificial enhancement techniques.  相似文献   
58.
The gradient (ordination) analysis was used to find the tendencies and the rate of changes in discrete parameters of the cycle of forest phytomasses accompanying a growing thermoarid climatic trend. The prognostic estimates of biotic regulation of the carbon cycle by forest biogeocenoses according to the HadCM3 (version A2) climatic scenarios, describing the most powerful coming global warming, are presented.  相似文献   
59.
The physical-synoptic principles of the automated method of summer precipitation forecasting introduced into operational prognostic practice in 2006 by the decree of the Central Methodical Commission of the Federal Service for Hydrometeorology and Environmental Monitoring (Roshydromet) are presented. Beside that, improved modifications of this method, which are automatically updated, are shown. Validation of those updates based on observational data collected in 2004 has shown improvement of all major criteria of forecasts efficiency. The performed improvement is based on the more detailed quantitative realization of mechanism of various showers formation (including heavy rainfalls).  相似文献   
60.
The kinematic α-effect dynamo problem is investigated in the case of an exterior perfect conductor. It is shown that certain approximate symmetries discovered in the numerical analysis of ROBERTS (1972) are exact for this case. As an illustration, an exact solution is given in a cylindrical geometry, where the equations can be written in terms of one variable. The implications for the earth's dynamo are discussed.  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