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   6449篇
  免费   1292篇
  国内免费   1643篇
测绘学   538篇
大气科学   1291篇
地球物理   1690篇
地质学   3206篇
海洋学   783篇
天文学   356篇
综合类   717篇
自然地理   803篇
  2024年   24篇
  2023年   95篇
  2022年   353篇
  2021年   384篇
  2020年   319篇
  2019年   357篇
  2018年   358篇
  2017年   376篇
  2016年   398篇
  2015年   346篇
  2014年   417篇
  2013年   404篇
  2012年   449篇
  2011年   406篇
  2010年   388篇
  2009年   364篇
  2008年   332篇
  2007年   332篇
  2006年   264篇
  2005年   243篇
  2004年   199篇
  2003年   182篇
  2002年   190篇
  2001年   207篇
  2000年   226篇
  1999年   228篇
  1998年   234篇
  1997年   217篇
  1996年   172篇
  1995年   162篇
  1994年   163篇
  1993年   121篇
  1992年   113篇
  1991年   79篇
  1990年   56篇
  1989年   54篇
  1988年   39篇
  1987年   27篇
  1986年   23篇
  1985年   10篇
  1984年   18篇
  1983年   11篇
  1982年   8篇
  1981年   6篇
  1980年   5篇
  1979年   4篇
  1976年   2篇
  1975年   2篇
  1958年   10篇
  1957年   3篇
排序方式: 共有9384条查询结果,搜索用时 31 毫秒
11.
本文以成矿地质模型为依据,对观驾山—黑山岛一带的化探、物探、遥感及地质等信息变量进行了详细的分析,选取了10个信息变量24个标志状态;在此基础上,运用信息量法对研究区内458个单元的信息量值进行计算,并由此确定了研究范围内的成矿远景区。事实表明,研究区信息量成矿预测是非常有效的  相似文献   
12.
13.
14.
Microwave Type III bursts with positive frequency drifting rate were found by Stahli and Benz (1987) for first time. Type III events are especially interesting because they are well-known to be signatures of electron beams in coronal plasma, and they are effective means for diagnosting of source plasma. A microwave burst consisting of some microwave type III burst groups was registered at Beijing Astronomical Observatory with the 2545–2645–2840 MHz synchronous observing system. The distributions of frequency drifting rate, half power duration, and intensity for each impulse in the groups have been statistically analysed. From this analysis, some important parameters for the dynamic process in the flare are deduced and discussed.  相似文献   
15.
从桐柏─大别山成矿带区域地质、地球物理、地球化学特征的研究入手,分析与金成矿有关的地层、构造、岩浆岩等控矿因素的地球物理场及地球化学特征;建立区域金矿找矿模型。  相似文献   
16.
洛坝铅锌矿床位于黄诸关韧脆性变形带所波及的范围内,断裂、褶皱、劈理、节理等形态构造类型均很发育,经对小型构造的统计及应力场分析证明,矿区变形主要是在海西—印支期长期存在的南北向古应力场持续作用下形成的。  相似文献   
17.
本文在已有数据处理方法的基础上,利用近代数值逼近理论,给出了从时空域角度描述地壳垂直运动过程的一种具体的函数解析形式。最后给出了一个实际算例。  相似文献   
18.

Long-term measurement of carbon metabolism of old-growth forests is critical to predict their behaviors and to reduce the uncertainties of carbon accounting under changing climate. Eddy covariance technology was applied to investigate the long-term carbon exchange over a 200 year-old Chinese broad-leaved Korean pine mixed forest in the Changbai Mountains (128°28′E and 42°24′N, Jilin Province, P. R. China) since August 2002. On the data obtained with open-path eddy covariance system and CO2 profile measurement system from Jan. 2003 to Dec. 2004, this paper reports (i) annual and seasonal variation of F NEE, F GPP and R E; (ii) regulation of environmental factors on phase and amplitude of ecosystem CO2 uptake and release Corrections due to storage and friction velocity were applied to the eddy carbon flux.

LAI and soil temperature determined the seasonal and annual dynamics of FGPP and RE separately. VPD and air temperature regulated ecosystem photosynthesis at finer scales in growing seasons. Water condition at the root zone exerted a significant influence on ecosystem maintenance carbon metabolism of this forest in winter.

The forest was a net sink of atmospheric CO2 and sequestered −449 g C·m−2 during the study period; −278 and −171 gC·m−2 for 2003 and 2004 respectively. F GPP and F RE over 2003 and 2004 were −1332, −1294 g C·m−2. and 1054, 1124 g C·m−2 respectively. This study shows that old-growth forest can be a strong net carbon sink of atmospheric CO2.

There was significant seasonal and annual variation in carbon metabolism. In winter, there was weak photosynthesis while the ecosystem emitted CO2. Carbon exchanges were active in spring and fall but contributed little to carbon sequestration on an annual scale. The summer is the most significant season as far as ecosystem carbon balance is concerned. The 90 days of summer contributed 66.9, 68.9% of F GPP, and 60.4, 62.1% of R E of the entire year.

  相似文献   
19.
Analysis of Zn, Cu, Pb, Co, Cr, Li, Ni, K, Al, Fe extracted by 1 mol/L HCl or 0.5 mol/LHCl/H_2O_2, showed concentrations of Zn, Cu, Pb, Co, Cr, Fe, Ni were significantly correlated with Li, Al,K, and clay. Two methods are used to indicate the background value of the non-residual phase of elementsin sediments, and are the same as the methods used to indicate the background value of totalconcentrations in sediments. The first method uses correlograms and regression equations,the second usesthe mean element concentrations normalized with grain size. Li, Al, K can be used as reference elements to determine the background value of Zn, Cu, Pb, Co,Cr, Ni, Fe, while the clay concentration's correlation with some extractable concentrations can be used tocalculate the background value of the non-residual phase of elements as a percentage of clay concentrationin the sediments. Based on this study, the concept of using the background value of the non-residualphase of elements to compare the pollution level in differ  相似文献   
20.
应用地壳波浪与镶嵌构造学说对富氏谱分析法提取地壳垂直形变信息的科学性做了地质学意义上的阐释 ,并提出了根据多期形变资料提取特定波段上构造策应力的数学模型  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