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   1712篇
  免费   132篇
  国内免费   154篇
测绘学   105篇
大气科学   111篇
地球物理   433篇
地质学   689篇
海洋学   242篇
天文学   201篇
综合类   61篇
自然地理   156篇
  2024年   5篇
  2023年   19篇
  2022年   54篇
  2021年   70篇
  2020年   65篇
  2019年   65篇
  2018年   86篇
  2017年   64篇
  2016年   106篇
  2015年   70篇
  2014年   79篇
  2013年   78篇
  2012年   99篇
  2011年   109篇
  2010年   124篇
  2009年   80篇
  2008年   78篇
  2007年   60篇
  2006年   66篇
  2005年   34篇
  2004年   57篇
  2003年   35篇
  2002年   56篇
  2001年   28篇
  2000年   43篇
  1999年   28篇
  1998年   26篇
  1997年   19篇
  1996年   23篇
  1995年   21篇
  1994年   21篇
  1993年   13篇
  1992年   17篇
  1991年   11篇
  1990年   10篇
  1989年   10篇
  1988年   6篇
  1987年   22篇
  1986年   8篇
  1985年   9篇
  1984年   9篇
  1983年   10篇
  1982年   21篇
  1981年   14篇
  1980年   15篇
  1979年   5篇
  1978年   7篇
  1976年   11篇
  1974年   4篇
  1973年   6篇
排序方式: 共有1998条查询结果,搜索用时 15 毫秒
81.
动载作用下混凝土随应力变化的系列CT图像完整地记录了混凝土内部破损过程,在CT图像中能直接观察到细观裂纹时混凝土往往已发生宏观破坏,挖掘混凝土材料损伤的细观信息是混凝土细观力学分析的核心.本文基于混凝土单轴动态压缩CT试验,获得不同应力阶段的横断面CT图像,分别应用图像分割技术和灰度共生矩阵法提取孔隙率和4个特征值,研...  相似文献   
82.
Large organic falls to the benthic environment, such as dead wood or whale bones, harbour organisms relying on sulfide-oxidizing symbionts. Nothing is known however, concerning sulfide enrichment at the wood surface and its relation to wood colonization by sulfide-oxidizing symbiotic organisms.In this study we combined in situ hydrogen sulfide and pH measurements on sunken wood, with associated fauna microscopy analyses in a tropical mangrove swamp. This shallow environment is known to harbour thiotrophic symbioses and is also abundantly supplied with sunken wood. A significant sulfide enrichment at the wood surface was revealed. A 72 h sequence of measurements emphasized the wide fluctuation of sulfide levels (0.1–>100 μM) over time with both a tidal influence and rapid fluctuations. Protozoans observed on the wood surface were similar to Zoothamnium niveum and to vorticellids. Our SEM observations revealed their association with ectosymbiotic bacteria, which are likely to be sulfide-oxidizers. These results support the idea that sunken wood surfaces constitute an environment suitable for sulfide-oxidizing symbioses.  相似文献   
83.
84.
SⅡ型海洋重力仪凭借其动态测量精度高、系统可靠性强等优点已日益成为海洋调查中重力测量的重要仪器之一。从SⅡ型海洋重力仪在使用中出现的稳定平台倒台故障着手,分析了平台倒台故障的原因以及处理方法,并对仪器使用中的注意事项以及改进建议作了介绍。  相似文献   
85.
Indicators of the economic performance of fishing vessels are frequently computed in various countries. In this paper, we use different types of information to assess capital value and the economic performance of fishing vessels. On the one hand, field surveys provide technical and financial information on the main components of fishing capital. On the other hand, book values are given in bookkeeping databases. We use both sources of information on fishing capital are used for the same set of vessels, namely the commercial fishing fleet of the French region of Brittany. Based on these two sources, measures of economic performance can be produced for the short term using gross surplus; and for the long term including the cost of capital. The measures of performance obtained are presented and the differences between them are then discussed.  相似文献   
86.
Management of the coastal zone often focuses on “islands” of high value ecosystems, in terms of economic value or conservation. However, integrated management requires landscape-level analysis of all ecosystem values. The biodiversity portfolio analysis (BPA) method is derived from the logic used in share (equity) portfolio management in terms of balancing within a portfolio the returns with the risks. Optimising the returns from a share portfolio, or a suite of ecosystems in a landscape, is dependent on the relationship between the units in terms of risk and return. Three case studies are then presented to test the applicability of the BPA method at the international (North West Europe), regional (Durham Heritage Coast, UK) and local (part of South Uist, Outer Hebrides, UK) spatial scale. The Biodiversity Portfolio Analysis for NW Europe showed that risk and return were highly correlated in the studied Member States. The ranking of risk and return, with the highest first, was Ireland > UK > France=Netherlands > Belgium. For these Member States the risks to ecosystem service provision were positively correlated with GNI (r=0.97, P<0.01); suggesting that the higher the economic importance of coastal and marine resources in a Member State the more at risk the resources are. The regional and local case studies were more focussed on providing information on which to base Integrated Coastal Zone Management (ICZM) decision making; both case studies used stakeholder participation to determine risks and returns. The conclusions from these two case studies show how the BPA method can be useful in terms of setting ICZM priorities and in addressing local coastal issues. The BPA involves making a number of assumptions, however, it does provide coastal managers with a potential tool to strategically plan due to increased awareness of the interaction between the ecosystems in the portfolio.There is a need for such techniques, which involve stakeholder participation and which create objective outcomes, to support the implementation of ICZM.  相似文献   
87.
For any specific wind speed, waves grow in period, height and length as a function of the wind duration and fetch until maximum values are reached, at which point the waves are considered to be fully developed. Although equations and nomograms exist to predict the parameters of developing waves for shorter fetch or duration conditions at different wind speeds, these either do not incorporate important variables such as the air and water temperature, or do not consider the combined effect of fetch and duration. Here, the wind conditions required for a fully developed sea are calculated from maximum wave heights as determined from the wind speed, together with a published growth law based on the friction velocity. This allows the parameters of developing waves to be estimated for any combination of wind velocity, fetch and duration, while also taking account of atmospheric conditions and water properties.  相似文献   
88.
The method of Wang [Wang, Y.-H., 2007. Formula for predicting bedload transport rate in oscillatory sheet flow. Coastal Engineering 54, 594–601] to predict the wave friction factor is discussed. At the threshold of sediment entrainment the proposed equation produces a wide scatter of data points when plotted against an equation based on the Shields parameter. It is shown that a better correlation coefficient can be obtained by calculating the critical wave friction factor directly from the wave period, as well as the sediment and water properties.  相似文献   
89.
The geophysical model function (GMF) describes the relationship between backscattering and sea surface wind, so that wind vec- tors can be retrieved from backscattering measurement. The GMF plays an important role in ocean wind vector retrievals, its performance will directly influence the accuracy of the retrieved wind vector. Neural network (NN) approach is used to develop a unified GMF for C-band and Ku-band (NN-GMF). Empirical GMF CMOIM and QSCAT-1 are used to generate the simulated training data-set, and Gaussian noise at a signal noise ratio of 30 dB is added to the data-set to simulate the noise in the backscat- tering measurement. The NN-GMF employs radio frequency as an additional parameter, so it can be applied for both C-band and Ku-band. Analyses show that the %predicted by the NN-GMF is comparable with the σpredicted by CMOIM and QSCAT-1. Also the wind vectors retrieved from the NN-GMF and empirical GMF CMOIM and QSCAT-1 are comparable, indicating that the NN-GMF is as effective as the empirical GMF, and has the advantages of the universal form.  相似文献   
90.
2009年2月(冬季)和8月(夏季)在南海北部海域(nSCS)采用流式细胞术对聚球藻、原绿球藻、超微型光合真核生物3类超微型光合浮游生物和异养浮游细菌的丰度和碳生物量的时空分布特征进行了研究,并分析了其与环境因子之间的关系。结果表明,夏季聚球藻和原绿球藻的平均丰度高于冬季,超微型光合真核生物和异养浮游细菌的丰度反之,为冬季高于夏季。聚球藻、超微型光合真核生物和异养浮游细菌在富营养的近岸陆架海域丰度较高,而原绿球藻高丰度则出现在陆坡开阔海域。在垂直分布上,聚球藻主要分布在跃层以上,跃层以下丰度迅速降低;原绿球藻高丰度主要出现在真光层底部;超微型光合真核生物在水层中的高值同样出现在真光层底部,且与Pico级份叶绿素a浓度分布一致;异养浮游细菌在水体中的分布与聚球藻类似。这些分布格局的差异,取决于环境条件的变化和4类超微型浮游生物生态生理适应性的差异。在超微型光合浮游生物群落中,各类群碳生物量的贡献因季节和海域类型的不同而发生变化:聚球藻在夏季近岸陆架区占超微型光合浮游生物总碳生物量的41%,原绿球藻在陆坡开阔海成为主要贡献者(50%),超微型光合真核生物碳生物量以冬季为高(在近岸陆架区占比68%)。冬、夏季异养浮游细菌碳生物量均高于超微型光合浮游生物碳生物量。  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