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   6469篇
  免费   284篇
  国内免费   80篇
测绘学   236篇
大气科学   516篇
地球物理   1533篇
地质学   2166篇
海洋学   529篇
天文学   1177篇
综合类   29篇
自然地理   647篇
  2022年   30篇
  2021年   74篇
  2020年   90篇
  2019年   123篇
  2018年   179篇
  2017年   163篇
  2016年   233篇
  2015年   174篇
  2014年   200篇
  2013年   411篇
  2012年   258篇
  2011年   344篇
  2010年   291篇
  2009年   382篇
  2008年   339篇
  2007年   293篇
  2006年   269篇
  2005年   260篇
  2004年   261篇
  2003年   205篇
  2002年   215篇
  2001年   116篇
  2000年   145篇
  1999年   103篇
  1998年   116篇
  1997年   82篇
  1996年   85篇
  1995年   83篇
  1994年   90篇
  1993年   73篇
  1992年   88篇
  1991年   67篇
  1990年   57篇
  1989年   50篇
  1988年   57篇
  1987年   52篇
  1986年   56篇
  1985年   67篇
  1984年   64篇
  1983年   76篇
  1982年   56篇
  1981年   65篇
  1980年   55篇
  1979年   65篇
  1978年   51篇
  1977年   34篇
  1976年   28篇
  1975年   26篇
  1974年   26篇
  1973年   30篇
排序方式: 共有6833条查询结果,搜索用时 15 毫秒
61.
62.
63.
The analysis of a pneumatic-type wave-energy conversion buoy is developed assuming independence of the buoy heaving motion and the motion of the water column within the center pipe. Results of the analysis are then compared with experimental data in a study of the relative air velocity within the turbine passage. The results compare very well. The effect of the variation of the center pipe length is found to be significant for periods about the surge chamber resonance but is negligible in the neighborhood of the heaving resonance period. Further, the theory is applied to a prototype buoy study of the U.S. Coast Guard, and a dimensionless design curve is developed from the results of the prototype analysis.  相似文献   
64.
The purpose of this article is to draw attention to a change in the trend of Soviet naval developments. This change stems from a combination of: (1) a sharp increase in the allocation of resources to naval shipbuilding; (2) a marked rise in the navy's political influence; and (3) a new approach to the role of seapower in Soviet policy. Professor MccGwire, Senior Fellow at the Brookings Institution, USA, believes that both the nature and the significance of the change are being obscured by the fact that for the past decade naval leaders in the West have been talking of ‘a Soviet naval build-up’, and continue to use the same language to describe the very different present situation. There is no general awareness, even within naval circles, that we are now facing a significant change in established trends. This article draws on a longer and more general article.1  相似文献   
65.
66.
67.
Studies in marine microbiology relevant to the cultivation of lobster in Fatty Basin were made. Biomass of bacteria and allied microorganisms in whole seawater column of the basin was very small (2×104 gC), but a large biomass was found in the sediments (3×105 gC). The decomposition of chitin occurred chiefly in the sediments. The rate of decomposition (500 g/day) was approximately half of the rate of production. However, the remaining production was considered not to be involved in the chitin cycle of the basin. This hypothesis was supported by the results of the analysis of the budget of organic matter in the area. Shell disease of lobster caused by chitinoclastic bacteria was detected, although it was not serious. A destructive yeast parasite of crustaceans,Metschnikowia, was collected only from a crab in the basin. This report is dedicated to Professor KenSugawara for his 70 years old anniversary.  相似文献   
68.
69.
Two single-channel seismic (SCS) data sets collected in 2000 and 2005 were used for a four-dimensional (4D) time-lapse analysis of an active cold vent (Bullseye Vent). The data set acquired in 2000 serves as a reference in the applied processing sequence. The 4D processing sequence utilizes time- and phase-matching, gain adjustments and shaping filters to transform the 2005 data set so that it is most comparable to the conditions under which the 2000 data were acquired. The cold vent is characterized by seismic blanking, which is a result of the presence of gas hydrate in the subsurface either within coarser-grained turbidite sands or in fractures, as well as free gas trapped in these fracture systems. The area of blanking was defined using the seismic attributes instantaneous amplitude and similarity. Several areas were identified where blanking was reduced in 2005 relative to 2000. But most of the centre of Bullseye Vent and the area around it were seen to be characterized by intensified blanking in 2005. Tracing these areas of intensified blanking through the three-dimensional (3D) seismic volume defined several apparent new flow pathways that were not seen in the 2000 data, which are interpreted as newly generated fractures/faults for upward fluid migration. Intensified blanking is interpreted as a result of new formation of gas hydrate in the subsurface along new fracture pathways. Areas with reduced blanking may be zones where formerly plugged fractures that had trapped some free gas may have been opened and free gas was liberated.  相似文献   
70.
Bridge scour modeling requires storm surge hydrographs as open ocean boundary conditions for coastal waters surrounding tidal inlets. These open coast storm surge hydrographs are used to accurately determine both horizontal and vertical circulation patterns, and thus scour, within the inlet and bay for an extreme event. At present, very little information is available on the effect that tidal inlets have on these open coast storm surge hydrographs. Furthermore, current modeling practice enforces a single design hydrograph along the open coast boundary for bridge scour models. This study expands on these concepts and provides a more fundamental understanding on both of these modeling areas.  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