全文获取类型
收费全文 | 1644篇 |
免费 | 187篇 |
国内免费 | 189篇 |
专业分类
测绘学 | 555篇 |
大气科学 | 160篇 |
地球物理 | 320篇 |
地质学 | 342篇 |
海洋学 | 127篇 |
天文学 | 141篇 |
综合类 | 161篇 |
自然地理 | 214篇 |
出版年
2023年 | 7篇 |
2022年 | 39篇 |
2021年 | 57篇 |
2020年 | 57篇 |
2019年 | 66篇 |
2018年 | 47篇 |
2017年 | 85篇 |
2016年 | 77篇 |
2015年 | 77篇 |
2014年 | 88篇 |
2013年 | 104篇 |
2012年 | 101篇 |
2011年 | 103篇 |
2010年 | 83篇 |
2009年 | 114篇 |
2008年 | 99篇 |
2007年 | 124篇 |
2006年 | 85篇 |
2005年 | 76篇 |
2004年 | 59篇 |
2003年 | 64篇 |
2002年 | 57篇 |
2001年 | 39篇 |
2000年 | 45篇 |
1999年 | 48篇 |
1998年 | 28篇 |
1997年 | 28篇 |
1996年 | 24篇 |
1995年 | 30篇 |
1994年 | 15篇 |
1993年 | 10篇 |
1992年 | 16篇 |
1991年 | 13篇 |
1990年 | 9篇 |
1989年 | 14篇 |
1988年 | 11篇 |
1987年 | 8篇 |
1986年 | 3篇 |
1985年 | 4篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1972年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有2020条查询结果,搜索用时 15 毫秒
1.
A method of structural damage identification using harmonic excitation force is presented. It considers the effects of both measurement and modelling errors in the baseline finite element model. Damage that accompanies changes in structural parameters can be estimated for a damaged structure from the change between measured vibration responses and ones calculated from the analytical model of the intact structure. In practice, modelling errors exist in the analytical model due to material and geometric uncertainties and a reduction in the degrees of freedom as well as measurement errors, making identification difficult. To surmount these problems, bootstrap hypothesis testing, which enables statistical judgment without information about these errors, was introduced. The method was validated by numerical simulation using a three‐dimensional frame structure and real vibration data for a three‐storey steel frame structure. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
2.
介绍了自动高精度测距系统。该系统能以5×10-7的测距精度测定多个观测点的形变,可作为跨断层动态连续观测的新仪器。 相似文献
3.
4.
Ralph A. M. J. Wijers 《Astrophysics and Space Science》1995,231(1-2):399-402
Since the discovery that dim gamma-ray bursts last longer on average than bright ones (time dilation) the cosmological origin of this effect has been contested by various researchers. I discuss the current status of this issue and conclude that current models for a non-cosmological time dilation only explain part of the observed phenomenon, and even then betray themselves by distinct signatures in the data. As those signatures have not been seen, the cosmological origin remains the favoured explanation of the time dilation. 相似文献
5.
AbstractThe ground vibrations during pile driving operation have a drastic potential to undermine the surrounding structures both in land and reclaimed land. Particularly, reclaimed land necessitates ample application of pile driving due to the weak land condition. To prevent the structural damage, attenuation of the ground vibrations to an allowable level through active isolation of circular open trench is the scope of this study. In this research, finite element simulations of continuous impact pile driving process from the ground surface was executed with particular attention to the pile-soil interaction, and thereby, the efficiency of open trench application in attenuation of the unsafe distance of different structures was surveyed using the vibration sensitivity degree. Regarding the crucial parameters of an open trench (depth, width, and location), it was concluded that a sufficient high depth can attenuate the unsafe distance up to 68%, the trench width variations are less effective, and an average pile-trench distance is the most efficient option. The excavation volume was also concluded as another crucial parameter in open trench design which takes all three parameters into account. The trench depth equal to the pile’s maximum critical depth of vibration was inferred for an optimum design. 相似文献
6.
本文研究模糊矩阵传递闭包的化简问题。通过定义 1类典型阵 ,给出传递闭包能够进一步化简的充要条件 ,为网络最大流分析提供了 1种新的模式。 相似文献
7.
Marco Ortiz 《Marine Ecology》2002,23(1):1-9
Abstract. The current article describes statistical power analysis as an efficient strategy for the estimation of the optimum sample size. The principle aim is constructively to criticise and enrich the results presented by Mouillot et al. (1999) , who estimate the optimum sample size for evaluating possible perturbations. The authors did not make any reference to statistical power analysis, even though their objective clearly went beyond a simple stock evaluation to assess management strategies in a particular marine ecosystem. Surprisingly, they proposed (a priori) an ANOVA design to test a hypothesis considering both space and temporal scales. However, the authors did not cover important topics related with power analysis and the precautionary principle, both used into environment impact assessment programmes for marine ecosystems. Based on their results and on statistical power analysis, it is demonstrated that the variability (dispersion statistics), a key factor they used to estimate the sample size, is less relevant than the magnitude of perturbation (effect size). Therefore, a greater effort must be devoted to estimate the effect size of a particular phenomenon rather than a desired variability. 相似文献
8.
本文考虑一维扩散方程的反问题,利用变分同化方法通过观测资料来确定方程中的未知初值,通过分析观测误差对于初值误差的影响,证明变分同化初值收敛于原问题的真实参数,并得到了参数的收敛精度。同时将得到的初值代入预报模式中,得到预报解,并分析了预报解的收敛性和预报误差。 相似文献
9.
10.
对煤炭资源勘查资源量与客观物质量之间差值的误差理论探讨,提出误差由真值误差、系统误差和离差3个基本部分组成,其中真值误差是不可消除的观测误差、系统误差是控制网度所决定的准确度标准、而离差是由地质构造及矿床形态的多解性产生的资源量波动。在此基础上,构筑了一个关于误差的数学模型,最后应用误差理论对现行勘查工作中常见的错误进行分析,并就如何避免错误及减少误差提出了见解。 相似文献