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   55300篇
  免费   5930篇
  国内免费   8546篇
测绘学   3863篇
大气科学   8563篇
地球物理   12119篇
地质学   26899篇
海洋学   4843篇
天文学   2966篇
综合类   5253篇
自然地理   5270篇
  2024年   174篇
  2023年   572篇
  2022年   1482篇
  2021年   1751篇
  2020年   1389篇
  2019年   1651篇
  2018年   6082篇
  2017年   5255篇
  2016年   4152篇
  2015年   1892篇
  2014年   2140篇
  2013年   1945篇
  2012年   2811篇
  2011年   4520篇
  2010年   3903篇
  2009年   4072篇
  2008年   3493篇
  2007年   3829篇
  2006年   1355篇
  2005年   1411篇
  2004年   1290篇
  2003年   1299篇
  2002年   1137篇
  2001年   979篇
  2000年   1124篇
  1999年   1488篇
  1998年   1204篇
  1997年   1124篇
  1996年   1037篇
  1995年   908篇
  1994年   787篇
  1993年   712篇
  1992年   563篇
  1991年   450篇
  1990年   330篇
  1989年   310篇
  1988年   270篇
  1987年   150篇
  1986年   143篇
  1985年   95篇
  1984年   99篇
  1983年   75篇
  1982年   65篇
  1981年   79篇
  1980年   60篇
  1979年   34篇
  1978年   11篇
  1977年   11篇
  1976年   12篇
  1958年   30篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
31.
法国是世界测绘强国之一。法国国家测绘局(IGN)是法国测绘的领导机构。本文介绍了IGN的概况,并对IGN大地测量、地图制图、对外服务、研究机构等情况作了较详细的描述。在此基础上对IGN的未来发展特点提出了几点看法。  相似文献   
32.
用逐步回归预测棉铃虫发生期和发生量   总被引:3,自引:0,他引:3  
收集了1973年至2000年的气象,农作物、棉铃虫虫害等196年因子,以运城、汾阳和临汾为山西省代表站,利用最优二分割法把发生量分为10级,5级和不分级3种情况;使用逐步回归计算了219个模型,从中选出27个棉铃虫二、三代发生期,发生量最优模型,在农气服务中应用,预报准确率为95%,效果良好。  相似文献   
33.
The early Tertiary Sanerlin uranium deposit is located near the southwestern margin of the Chaling-Yongxing pull-apart basin defined by the Chaling-Yongxing and Chenxian-Linwu sinistral strike-slip faults in southern China. The uranium ores are hosted in 15 breccia-vein bodies, which are separately located in the cores of three secondary anticlines of the Upper Permian Dangchong Formation. Individual breccia-vein bodies are composed of fragments of silicified shale and sandstone from the Dangchong Formation, and quartz veinlets as cements. These fragments, together with quartz veins, form a mosaic texture. Hydrothermal pitchblende is the only commercial uranium mineral, mainly occurring as disseminated grains within quartz veins or coating fragments. Other metallic minerals include molybdenite, pyrite, chalcopyrite, galena, sphalerite, and red microcrystalline hematite. Fluid inclusions in quartz veins have homogenization temperatures ranging from 150 to 280 °C, and calculated salinity values between 5.6 and 13.4 wt% NaCl equivalent. Stable isotope analyses show that the mineralizing fluid was characterized by '18O values of -2.2 to +2.6‰ and 'DH2O values of -134 to -110‰. These analytical data demonstrate that hydrothermal fluids were mainly derived from formation waters (brines) of the Chaling-Yongxing basin. Fluid overpressuring was caused by an abnormal geothermal gradient and impermeable shales in the deposit area. The geometry, texture, and structure of the breccia-vein system, along with the fluid pressure estimates, suggest that hydraulic fracturing generated the mineralized breccia-vein system. Pitchblende and associated minerals were deposited when gaseous phases were released abruptly from the ore fluids due to the hydraulic fracturing.  相似文献   
34.
The Skidaway River estuary is a tidally-dominated subtropical estuary in the southeastern USA surrounded by extensiveSpartina salt marshes. Weekly smapling at high and low tide began in 1986 for hydrography, nutrients, chlorophylla, particulate matter, and microbial and plankton biomass and composition; hydrographic and nutrient data during 1986–1996 are reported here. Salinity varied inversely with river discharge and exhibited variability at all time scales but with no long-term trend. Water temperature typically ranged over 25°C and was without apparent long-term frend. Seasonal cycles in concentrations of NO3, NH4, PO4, Si(OH)4, and DON were observed, with annual maxima generally occurring in late summer. Superimposed on seasonal cycles, all five nutrients exhibited steady increases in minimum, mean, and maximum concentrations; mean concentrations increased c. 50–150% during the decade. Nutrient concentrations were highly correlated with water temperature over the ten-year period, but weakly related to salinity and discharge. Nutrients were strongly correlated with one another, and the relative ratios among inorganic nutrients showed little long-term trend. Correlations among temperature and nutrient concentrations exhibited considerable inter-annual variability. Major spikes in organic and inorganic nutrient concentrations coincided with significant rainfall events; concentrations increased hyperbolically with rainfall. Although pristine compared to more heavily impacted waterways primarily outside the region, residential development and population density have been increasing rapidly during the past 15–20 years. Land use is apparently altering nutrient loading over the long-term (months-years), and superimposed on this are stochastic meteorological events that accelerate these changes over the short term (days-weeks).  相似文献   
35.
分形在地貌学中应用的几个问题的分析   总被引:5,自引:0,他引:5  
李锰  朱令人  龙海英 《地震研究》2002,25(2):155-162
就实际地貌分形研究工作中经常遇到的截面约定、自仿射分形局域维与渡越长度、分维的比较以及过渡分形等几个问题进行了分析和探讨,并提出了在应用过程中的注意事项和解决方案。  相似文献   
36.
李桂华  何家斌 《地震研究》2002,25(2):163-165
采用一致的地震震相和台网布局,应用昆明数字地震台网资料,对2002年月1月4日-1月7日的漾濞地震及序列作进一步核定,得出了更精确的地震震源位置,并作出三维空间图。  相似文献   
37.
Field observations on ground motions from recent earthquakes imply that current knowledge is limited with regard to relating vertical and horizontal motions at liquefiable sites. This paper describes a study with the purpose of clarifying this emerging issue to some extent. A series of numerical analyses is carried out on a liquefiable soil deposit with a verified, fully coupled, nonlinear procedure. It is shown that the transformation of vertical motions in the deposit differs considerably from the transformation of horizontal motions. Both the amplitude and frequency content of the horizontal motions are strongly dependent on the shaking level or the associated nonlinear soil behavior. The transfer function for vertical motions is however likely to be independent of the intensity of input motions; no reduction in the amplitude occurs even in the case of strong shaking. The results are shown to be in consistence with the laboratory observations on shaking table tests and recent field observations that less nonlinearity exists for vertical motions. It is also shown that the possibility exists for using information on spectral ratios between the horizontal and vertical surface motions to quickly identify in situ soil behavior and liquefaction that are not readily covered by conventional field or laboratory experimentation procedures.  相似文献   
38.
In east Asia, acidic gases derived from fossil fuel combustion have increased in the past decades. On the other hand, the Asian dust, also called Kosa (yellow sand) is transported following windstorms from arid lands in the Asian continent. Many researchers have been interested in the reaction between acidic aerosols and Kosa aerosols as well as the long-range transport of these emissions. To investigate the characteristics of chemical components in precipitation on a long-term basis over Japan, precipitation was sequentially collected from April 1984 to March 1997 at Kanazawa located near the coast of the Sea of Japan. Precipitation samples were collected at 1 mm intervals for the first 5 mm rainfall and all volume of rainwater after 6 mm for all precipitation events with an automatic wet only precipitation collector. According to the analyses of precipitation including Kosa aerosols during Kosa periods, the reaction in the air between Kosa and acidic components during the long-range transport was discussed.  相似文献   
39.
长江巨洪前期物理因子的配置   总被引:4,自引:4,他引:4  
冯利华 《地理科学》2002,22(4):504-507
提出了长江巨洪前期物理因子配置的概念,在每一次巨洪发生之前,影响巨洪的主要物理因子之间一般都会表现出相似的特征,即出现一些固有的配置,这些配置正是长江发生巨洪的强信号,对长江巨洪的预测研究具有重要的指示作用。  相似文献   
40.
区域可持续发展评价:进展与展望   总被引:55,自引:4,他引:55  
可持续发展的评价是目前可持续发展研究的热点和前沿。当前评价可持续发展的单项指标和指标体系在指标与可持续性的关系、指标权重的选择、指标的定量化、可持续性的总体判别方法、资本替代性与替代速度、指标阈值的确定等方面具有不同的局限性 ,这也是可持续发展评价的主要难点。可持续发展的评价必然要落实到不同尺度的空间地理单元上 ,区域发展是评价的对象 ,因而可持续发展的评价必然具有显著的区域性 ,不同区域的评价指标或指标体系必然会有所不同。社会、经济与人口的发展是人类社会发展的主要目标 ,而发展的可持续性依赖于自然环境的质量 ,包括自然资源的再生或替代、生命支持系统与生物多样性的维持或改善。据此 ,论文构建了区域可持续发展评价的理论框架和发展——可持续性二维评价坐标体系 ,并提出了“自上而下”和“自下而上”的指标遴选方法。  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