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   13795篇
  免费   3079篇
  国内免费   978篇
测绘学   520篇
大气科学   261篇
地球物理   11532篇
地质学   2530篇
海洋学   372篇
天文学   1003篇
综合类   1058篇
自然地理   576篇
  2024年   19篇
  2023年   118篇
  2022年   308篇
  2021年   480篇
  2020年   448篇
  2019年   494篇
  2018年   482篇
  2017年   497篇
  2016年   399篇
  2015年   598篇
  2014年   716篇
  2013年   724篇
  2012年   690篇
  2011年   761篇
  2010年   705篇
  2009年   1023篇
  2008年   768篇
  2007年   902篇
  2006年   845篇
  2005年   807篇
  2004年   758篇
  2003年   721篇
  2002年   569篇
  2001年   543篇
  2000年   477篇
  1999年   421篇
  1998年   426篇
  1997年   347篇
  1996年   381篇
  1995年   310篇
  1994年   293篇
  1993年   237篇
  1992年   188篇
  1991年   116篇
  1990年   85篇
  1989年   59篇
  1988年   50篇
  1987年   22篇
  1986年   18篇
  1985年   10篇
  1984年   6篇
  1983年   3篇
  1982年   1篇
  1981年   1篇
  1980年   1篇
  1979年   15篇
  1978年   1篇
  1977年   2篇
  1954年   7篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
131.
132.
133.
134.
135.
136.
Preparedness is a key dependent variable in many studies examining people’s response to disasters such as earthquakes. A feature of many studies on this issue, however, is the lack of attention given to psychometric issues when constructing measures of preparedness. With regard to earthquake preparation, for example, many studies could be greatly improved by the use of a valid and reliable measure of preparedness. This research developed such a measure that assessed both low-level preparedness, such as having an emergency kit, and high-level preparedness, such as altering home structures to mitigate damage. Studies of Wellington (New Zealand) residents using two samples totalling n=652 showed that 23 items measuring these different aspects of earthquake preparation could be combined into a reliable, valid, unifactorial scale. This brief scale should have utility in multivariate studies of earthquake preparation, either as a dependent variable, where preparation is the outcome variable of primary interest, or as one of several independent variables, where preparation and other measures predict another outcome variable.  相似文献   
137.
PDC钻头在寺河煤矿瓦斯预抽放钻井中的应用   总被引:2,自引:1,他引:2  
在寺河矿区一期瓦斯地面预抽放钻井施工中,使用传统的镶齿三牙轮钻头钻进,钻进速度低,仅为2.0m/h,且井斜不易控制、钻头寿命短、成本较高。通过对寺河煤矿地层特征的分析,在二期工程施工中,9#煤以上地层优选六翼内凹式PDC钻头钻进,采取一定技术措施并合理调整钻压、转速及泵量等钻进参数,钻速可达7.0m/h,井斜也得到有效控制,降低了钻井成本。  相似文献   
138.
River Vrishabhavathy, a tributary of Cauvery River was studied for 12 physico-chemical parameters at four sites over a distance of 50 km for a period of 2 years (1999–2001) at monthly intervals. Water was faintly alkaline, with pH showing negative correlation with temperature. The dissolved oxygen content increased downstream with negative correlation to biological oxygen demand (BOD), chemical oxygen demand (COD) and turbidity. Bicarbonate alkalinity was very low compared with carbonate alkalinity. Carbonate alkalinity, total hardness, total dissolved solids, total suspended solids, electrical conductivity, BOD and COD decreased downstream, with an upward trend in the middle reaches due to the introduction of raw sewage. The seasonal and yearly trends are also discussed. The river is a sewer collector undergoing self-purification.  相似文献   
139.
A comparative tectonic quiescence and lack of earthquakes make the stable centres of continents attractive for siting long-term radioactive waste storage facilities. The low rates of deformation in such regions, however, make it difficult to characterize their long-term seismotectonic behaviour, leading to uncertain estimates for the very low probability hazard estimates required by society. In an attempt to overcome the deficiency of both contemporary seismicity and paleoseismic data in central Canada, we have used earthquake histories from regions with similar seismotectonic characteristics from around the world. Substituting space for time, we estimate a long-term rate per 106 km2 of 0.004 magnitude ≥6 earthquakes per annum, of which 33–100% might rupture to the surface.  相似文献   
140.
A morphometric analysis was done to determine the drainage characteristics of Lules River basin using land-sat imageries and topographical maps. This catchment was divided into seven sub-basins for the analysis: Liquimayo, Hoyada, Ciénaga, De Las Tablas, Siambón, Potrerillo and San Javier. Yungas ecoregion covers almost all the watershed. The drainage patterns of the sub-basins are dendritic and parallel. The basin includes seventh order stream and lower streams order mostly dominate the basin. The development of stream segments is affected by slope and local relief. The mean bifurcation ratio indicates that the drainage pattern is not much influenced by geological structures. The shape parameters also reveal the elongation of the basin and sub-basins.  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