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   3919篇
  免费   766篇
  国内免费   656篇
测绘学   334篇
大气科学   521篇
地球物理   1459篇
地质学   1425篇
海洋学   654篇
天文学   196篇
综合类   272篇
自然地理   480篇
  2024年   12篇
  2023年   50篇
  2022年   73篇
  2021年   102篇
  2020年   108篇
  2019年   150篇
  2018年   95篇
  2017年   150篇
  2016年   175篇
  2015年   153篇
  2014年   217篇
  2013年   345篇
  2012年   234篇
  2011年   209篇
  2010年   191篇
  2009年   259篇
  2008年   244篇
  2007年   271篇
  2006年   234篇
  2005年   221篇
  2004年   190篇
  2003年   160篇
  2002年   153篇
  2001年   134篇
  2000年   106篇
  1999年   124篇
  1998年   97篇
  1997年   90篇
  1996年   133篇
  1995年   175篇
  1994年   197篇
  1993年   56篇
  1992年   55篇
  1991年   49篇
  1990年   22篇
  1989年   30篇
  1988年   16篇
  1987年   19篇
  1986年   7篇
  1985年   10篇
  1984年   6篇
  1983年   7篇
  1982年   4篇
  1981年   2篇
  1980年   4篇
  1978年   1篇
  1954年   1篇
排序方式: 共有5341条查询结果,搜索用时 15 毫秒
41.
Recommendations for the certification of reference materials, as published by the International Organisation for Standardisation (ISO), are reviewed and proposals made as to how they can be adapted for the certification of new geological reference materials. Whilst acknowledging the important contribution made by the large number of existing matrix-matched geological reference materials, it is recommended that future characterisation programmes should follow the ISO guidelines for certification, not the least so that laboratories can readily use the resultant samples to establish the traceability of geoanalytical results.  相似文献   
42.
本文基于GRAPES全球模式的短期预报误差样本,利用赤道波动正规模态研究了热带风、压场平衡特征,并根据这些特征分析了线性平衡方程(LBE)在该区域应用时存在的问题。结果表明:(1)赤道波动能成功解释热带短期预报误差样本的大部分分量,对流层中层为60%~80%,对流层顶和平流层低层为80%以上。(2)在可解释的误差方差中,赤道罗斯贝波(ER)占比仅为30%~55%,其他赤道波动的作用不可忽视。(3)在ER模态基础上引入其他赤道波动会大幅削弱原有风、压场平衡约束,重力惯性波与Kelvin波的作用最为显著。此时,对流层中层位势高度h与u风、v风间的约束接近于零,而平流层低层h-u的平衡特征由Kelvin波主导。(4)LBE主要表达了ER模态下的风、压场平衡特征,与实际情形相比高估了热带风、压场的耦合程度,进一步的改进中需削弱这一虚假平衡,使得热带风、压场分析变得更加独立。  相似文献   
43.
煤岩动静力学参数关系试验研究   总被引:1,自引:0,他引:1  
煤岩的动、静力学参数关系的试验研究,对研究煤岩的位移和变形特征及工程支护设计具有重要意义。将取自新河煤矿3煤的煤样加工成Φ50 mm×100 mm的圆柱体标准试件;采用500 kHz频率的纵横波换能器分别进行纵横波速度测试;在MTS815.03电液伺服试验机上进行单轴压缩试验,得到静态弹性模量和泊松比。试验结果表明:煤岩动弹性模量与横波速度相关性强于与纵波速度的相关性,煤岩介质的纵波对孔隙和裂隙发育程度的敏感程度要比横波高;煤岩试件Ed/Es比值主要集中在1.4~1.7,室内煤岩试件动、静弹性模量和动静泊松比之间存在较好的线性相关关系。  相似文献   
44.
马斯京根法洪水演进反演计算方法的探讨   总被引:1,自引:0,他引:1  
从计算理论上系统分析和认识了马斯京根法洪水演进反演计算算法产生的坏条件,通过引入样条函数辅助进行修匀处理和对于x数值小的情况采用反演正算的算法,使得马斯京根法洪水演进反演计算可以进行多段连续演算并具有好的效果,在工程水文设计计算中具有实用价值。  相似文献   
45.
A Modified Form of Mild-Slope Equation with Weakly Nonlinear Effect   总被引:6,自引:0,他引:6  
Nonlinear effect is of importance to waves propagating from deep water to shallow water.Thenon-linearity of waves is widely discussed due to its high precision in application.But there are still someproblems in dealing with the nonlinear waves in practice.In this paper,a modified form of mild-slope equa-tion with weakly nonlinear effect is derived by use of the nonlinear dispersion relation and the steady mild-slope equation containing energy dissipation.The modified form of mild-slope equation is convenient to solvenonlinear effect of waves.The model is tested against the laboratory measurement for the case of a submergedelliptical shoal on a slope beach given by Berkhoff et al,The present numerical results are also comparedwith those obtained through linear wave theory.Better agreement is obtained as the modified mild-slope e-quation is employed.And the modified mild-slope equation can reasonably simulate the weakly nonlinear ef-fect of wave propagation from deep water to coast.  相似文献   
46.
The management of marine ecosystems requires adequate knowledge of both environmental and human dimensions, as well as their interrelationships. In this study, the aquaculture and fisheries activities are analyzed in one of most important fishing regions in Europe, Galicia (NW Spain). In particular, the intensity and characteristics of the fishing dependency are evaluated in terms of income and employment. Thus, nine marine economic activities for nine Maritime Zones have been defined, considering the social and economic relevance of the provided ecosystem services to these communities. This paper highlights the entire Galician coast as fishing-dependent, independently of the urbanization level. Furthermore, the contribution of different fishery segments to the income and employment of these coastal communities is reviewed, including fluctuations on whether the activity is small or large-scale. Finally, this study establishes a strong relationship among the marine activities, which generate most employment, and the regulatory framework of the Regional Government. Therefore, the conclusions are relevant to design and implement policies that affect Galicia׳s Region and all its related marine ecosystems.  相似文献   
47.
长江口水域波浪数值计算   总被引:2,自引:0,他引:2  
利用高阶非线性抛物型缓坡方程对长江口水域的波浪传播变形作了推算,依据测站的资料分析数值模式中的底摩阻因子和风能输入因子对波浪传播的影响,进而确定其参数。针对不同导堤结构型式,分析了潜堤的波浪传递系数,最后对长江口二期整治工程完成后水域的波浪场作了推算。  相似文献   
48.
航道对多方向波传播影响   总被引:2,自引:0,他引:2  
应用Boussinesq方程对不同入射角、不同方向集中度的波浪与航道的相互作用进行模拟,得到了航道的折射影响规律以及不同入射角、不同方向集中度的波浪对航道作用的差别.结果对试验研究及工程实践有指导意义.  相似文献   
49.
Two mathematical models for combined refraction-diffraction of regular and irregular waves on non-uniform current in water of slowly varying topography are presented in this paper. Model I is derived by wave theory and variational principle separately. It has two kinds of expressions including the dissipation term. Model n is based on the energy conservation equation with energy flux through the wave crest lines in orthogonal curvilinear coordinates and the wave kinematic conservation equation. The analysis and comparison and special cases of these two models are also given.  相似文献   
50.
Monopiles are considered to be as a kind of viable foundation types for offshore wind turbines. The effect of negative skin friction on pile foundation is always an important problem. There are very important theoretical and practical significance to study the distribution law of negative skin friction and the calculation method. Based on the special stratum, the stress and strain of the monopile and soil are simplified, and the improved Kezdi’s double-broken-line model is adopted. The analytical solution of negative skin friction of monopile is deduced according to the degree of skin friction. An engineering case was analyzed by the method, and the calculated results agree well with the measured data. The calculation method proposed can accurately describe the range of the monopile skin frictional distribution and the position of the neutral point, and it is simple and convenient to calculate, that is also a feasible method for calculating the negative skin friction of monopile of offshore wind turbines in practical engineering.  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