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   76648篇
  免费   2694篇
  国内免费   3437篇
测绘学   2495篇
大气科学   6774篇
地球物理   14369篇
地质学   31716篇
海洋学   7221篇
天文学   14364篇
综合类   1204篇
自然地理   4636篇
  2023年   396篇
  2022年   906篇
  2021年   1306篇
  2020年   1211篇
  2019年   1282篇
  2018年   4556篇
  2017年   4170篇
  2016年   3418篇
  2015年   1460篇
  2014年   1963篇
  2013年   3039篇
  2012年   2996篇
  2011年   5008篇
  2010年   4565篇
  2009年   5084篇
  2008年   4227篇
  2007年   4838篇
  2006年   2373篇
  2005年   2134篇
  2004年   1955篇
  2003年   2053篇
  2002年   1900篇
  2001年   1522篇
  2000年   1347篇
  1999年   1170篇
  1998年   1093篇
  1997年   1131篇
  1996年   924篇
  1995年   909篇
  1994年   842篇
  1993年   747篇
  1992年   683篇
  1991年   603篇
  1990年   592篇
  1989年   605篇
  1988年   550篇
  1987年   616篇
  1986年   543篇
  1985年   649篇
  1984年   691篇
  1983年   624篇
  1982年   562篇
  1981年   613篇
  1980年   508篇
  1979年   484篇
  1978年   448篇
  1977年   449篇
  1976年   402篇
  1975年   397篇
  1973年   400篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
E. D. Golubeva 《Oceanology》2006,46(5):672-682
Based on the summarized geological and petrogeochemical information on the basaltic magmatism on the Pacific Ocean floor, the formation-geochemical rock types, their associations in the oceanic morpho-structures, and petrological provinces were distinguished. The features of the composition variations of the rocks presented mainly by magno-and ferrotholeiites in mid-oceanic ridges and by their petrological analogues at the plates (Fenner trend) were examined. The increase in the silica acidity and alkalinity of the rocks (Bowen trend) is moderately manifested in the eastern provinces of the ocean and is much more pronounced in the western areas (including the formation of complexes constituted by significant volumes of alkaline rocks).  相似文献   
952.
- Based on field investigations, this paper analyzes three types of harbour basinns and navigation channel excavated on seabed in Jiaozhou Bay, get a general rule of deposition for excavated trough, it found that pollution is one of crucial factors resulting in the deposition of the excavated trough in the east shore of Jiaozhou Bay. With these results, it predicted the annual deposition thickness for the excavated trough and disclosed the fact that it can't be deposited deadly during one storm. At the same time, with two-dimensional numerical model, it studied the effects of the excavated trough and the reclamation near shore on tidal cureent and said that the excavated trough can decrease the current velocity passing through the trough about 10- 15%, but only limited inside and near the trough and there are no effect on other regions; reclamation can cut off the pollution sources and no obvious effect on the currents of the Jiaozhou Bay. Connecting the deep trough and Cangkou tidal channel with a new  相似文献   
953.
利用1993年ENSO事件爆发(4月)前酝酿时期“热带大洋与全球大气-海洋耦合响应试验”强化观测阶段“向阳红五号”科学考察船155°E,2°S定点海洋气象和高空大气探测资料,分析了赤道西太平洋大气边界层特征。结果表明:赤道西太平洋上空辐合对流区边界层内也有等温或逆温层存在,大气边界层物理参数变化与大尺度海-气变异有密切关系,赤道太平洋沃克环流加强、东移,边界层上部逆温层出现概率明显增大,边界层内高湿层湿度减小。另外,边界层内实测风速、风向廓线随高度的变化基本上不服从Ekman规律,但由其平均,分量合成得到的平均风随高度的变化却基本符合Ekman规律。  相似文献   
954.
An analysis of published and original data on the meiobenthos abundance in the depth interval from 100 to 9807 m (in total, 665 records, 445 of them obtained for depths exceeding 1000 m) revealed general regularities in its distribution. The influence of the sampling and data processing methods on the quantitative estimates of the meiobenthos abundance is considered to demonstrate changes in the proportions of the main meiobenthic taxa at different depths and to characterize latitudinal changes in the meiobenthos abundance. The dependence of the abundance of free-living nematodes, the most abundant group of metazoan meiobenthos, on trophic conditions is analyzed. No significant differences in the meiobenthos abundance in the samples obtained by box-and multicorers are established. It is shown that the share of nematodes in metazoan meiobenthos communities increases with the depth. In temperate latitudes, a distinct maximum in the population density confined to depths exceeding 1 km is observed. The quantitative distribution of the meiobenthos at the depths gradient is controlled by the bottom macrotopography and trophic conditions.  相似文献   
955.
956.
Three different barge sizes were tested in uniform current in a tank of limited width. One of the barges was tested with and without bilge keels. Tests were conducted in both deep and shallow water. The barges were placed broadside to the current flow and the lateral forces on the barges were measured. The effect of the tank walls on these forces is clearly shown.  相似文献   
957.
模型沙干重度在动床模型试验中是影响起动流速比尺和冲淤时间比尺的重要参数,其随淤积历时、粒径变化、埋置深浅和加工方法的不同有着不同的变化规律。通过试验,结合已有的成果,对模型沙干重度受以上几种因素的影响进行了研究,并进一步分析了其变化规律在动床模型试验中的作用。  相似文献   
958.
硫酸皮肤素寡糖的分离与制备   总被引:2,自引:0,他引:2  
利用软骨素酶 ABC (Chondroitinase ABC,EC4 .2 .2 .4 )对硫酸皮肤素 (Dermatan Sul-fate,DS)进行控制降解。对得到的混合寡糖首先采用低压凝胶渗透色谱 (L PGPC)进行分级 ,然后对每一组分再利用强阴离子交换高压液相色谱 (SAX- HPL C)进行分离 ,最终制备出聚合度为 2 ,4 ,6 ,8,10 ,12的寡糖纯品。寡糖纯度采用 SAX- HPL C,毛细管电泳 (CE)以及聚丙烯酰胺凝胶电泳(PAGE)进行检验 ,结构采用电喷雾离子化质谱 (ESI- MS)以及核磁共振波谱 (NMR)技术确定  相似文献   
959.
960.
Aquatic surface microlayer contamination in chesapeake bay   总被引:1,自引:0,他引:1  
The aquatic surface microlayer (SMIC), 50 μm thick, serves as a concentration point for metal and organic contaminants that have low water solubility or are associated with floatable particles. Also, the eggs and larvae of many fish and shellfish species float on, or come in contact with, the water surface throughout their early development. The objectives of this study were (1) to determine the present degree of aquatic surface microlayer pollution at selected sites in Chesapeake Bay, and (2) to provide a preliminary evaluation of sources contributing to any observed contamination.Twelve stations located in urban bays, major rivers, and the north central bay were sampled three times, each at 5-day intervals during May 1986. Samples of 1.4–4.1 each were collected from the upper 30–60-μm water surface (surface microlayer, SMIC) using a Teflon-coated rotating drum microlayer sampler. One sample of subsurface water was collected in the central bay.At all stations, concentrations of metals, alkanes, and aromatic hydrocarbons in the SMIC were high compared with one bulk-water sample and with typical concentrations in water of Chesapeake Bay and elsewhere. SMIC contamination varied greatly among the three sampling times, but high mean contaminant levels (total polycyclic aromatic hydrocarbons, 1.9–6.2 μg 1−1; Pb, 4.9–24 μg 1−1; Cu, 4–16 μg 1−1; and Zn, 34–59 μg 1−1) were found at the upper Potomac and northern bay sites. Three separate areas were identified on the basis of relative concentrations of different aromatic hydrocarbons in SMIC samples - the northern bay, the Potomac River, and the cleaner southern and eastern portions of the sampling area.Suspected sources of surface contamination include gasoline and diesel fuel combustion, coal combustion, and petroleum product releases. Concentrations of metals and hydrocarbons, at approximately half the stations sampled, are sufficient to pose a threat to the reproductive stages of some fish and shellfish. Sampling and analysis of the surface microlayer provides a sensitive tool for source identification and monitoring of potentially harmful aquatic pollution.  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