全文获取类型
收费全文 | 2744篇 |
免费 | 396篇 |
国内免费 | 510篇 |
专业分类
测绘学 | 526篇 |
大气科学 | 246篇 |
地球物理 | 688篇 |
地质学 | 1221篇 |
海洋学 | 339篇 |
天文学 | 90篇 |
综合类 | 205篇 |
自然地理 | 335篇 |
出版年
2024年 | 9篇 |
2023年 | 17篇 |
2022年 | 63篇 |
2021年 | 93篇 |
2020年 | 81篇 |
2019年 | 111篇 |
2018年 | 81篇 |
2017年 | 121篇 |
2016年 | 117篇 |
2015年 | 152篇 |
2014年 | 156篇 |
2013年 | 196篇 |
2012年 | 185篇 |
2011年 | 148篇 |
2010年 | 149篇 |
2009年 | 172篇 |
2008年 | 168篇 |
2007年 | 194篇 |
2006年 | 172篇 |
2005年 | 143篇 |
2004年 | 151篇 |
2003年 | 142篇 |
2002年 | 94篇 |
2001年 | 94篇 |
2000年 | 93篇 |
1999年 | 96篇 |
1998年 | 70篇 |
1997年 | 61篇 |
1996年 | 59篇 |
1995年 | 49篇 |
1994年 | 42篇 |
1993年 | 36篇 |
1992年 | 36篇 |
1991年 | 25篇 |
1990年 | 18篇 |
1989年 | 9篇 |
1988年 | 12篇 |
1987年 | 10篇 |
1986年 | 2篇 |
1985年 | 6篇 |
1984年 | 4篇 |
1983年 | 1篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有3650条查询结果,搜索用时 31 毫秒
21.
The spectral methods and ice-induced fatigue analysis are discussed based on Miner's linear cumulative fatigue hypothesis and S-N curve data.According to the long-term data of full-scale tests on the platforms in the Bohai Sea,the ice force spectrum of conical structures and the fatigue environmental model are established.Moreover,the finite element model of JZ20-2MSW platform,an example of ice-induced fatigue analysis,is built with ANSYS software.The mode analysis and dynamic analysis in frequency domain under all kinds of ice fatigue work conditions are carried on,and the fatigue life of the structure is estimated in detail.The methods in this paper can be helpful in ice-induced fatigue analysis of ice-resistant platforms. 相似文献
22.
More and more researches show that neither the critical downward acceleration nor the critical slope of water waves is a universal constant. On the contrary, they vary with particular wave conditions. This fact moders the models either for the probability of wave breaking B or for the whitecap coverage W based on these criteria difficult to apply. In this paper and the one which follows we seek to develop models for the prediction of both B and W based on the kinematical criterion. First, several joint probabihstic distribution functions (PDFs) of wave characteristics are derived, based on which the breaking properties B and W are estimated. The estimation is made on the assumption that a wave breaks ff the horizontal velocity of water particles at its crest exceeds the local wave celerity, and whitecapping occurs in regions of fluid where water particles travel faster than the waves. The consequent B and W depend on wave spectral moments of orders 0 to 4.Then the JONSWAP spectrum is used to represent the fetch-limited sea waves in deep water, so as to relate the probahility of wave breaking and the whitecap coverage with wind parameters. To this end, the time-averaging technique proposed by Glazman (1986) is applied to the estimation of the spectral moments involved, and furthermore, the theoretical models are compared with available observations collected from published literature. From the comparison, the averaging time scale is determined. The final models show that the probability of wave breaking as well as the whitecap coverage depends on the dimensionless fetch. The agreement between these models and the database is reasonable. 相似文献
23.
Acoustic plane-wave scattering from a rough surface overlying a fluid half-space with a sound-speed distribution subject to a small random variation is considered. Under the assumption that the surface roughness and medium randomness are statistically independent, the scattered field may be derived by first solving for the mean field in the medium, and then incorporating with boundary-perturbation method to obtain the total mean field and the power spectral density of the scattered field. The employed algorithm is compatible to the analysis available in the existing literature so that the formulations are conveniently integrated. The results for the power spectral density have shown that the effects of medium inhomogeneities on the rough surface scattering are limited in a spectral regime where the scattered components have shallow grazing angles. The distribution of the power spectral density over the space is primarily governed by the power spectrum and correlation lengths of the rough surface. 相似文献
24.
空间扰动引力的谱分析 总被引:1,自引:0,他引:1
基于重力场的频谱理论,给出了扰动引力在全球平均意义下的功率谱表达式,揭示了扰动引力的传播特性。通过数值试验,给出了扰动引力随高度变化规律,并分析了不同高度上,扰动引力恢复重力场的最高阶数及相应分辨率。从理论上分析了航空重力测量探测重力场中高频信息的能力,对制定飞行方案有一定参考价值。 相似文献
25.
26.
Fluctuations of peak energy and peak frequency o local wavelet energy spectrum for wind wavesTXFluctuationsofpeakenergyandpeak... 相似文献
27.
姜福德 《中国海洋大学学报(自然科学版)》1987,(3)
本文研究一类二阶非线性微分方程 X+q(t)φ(X,X)+p(t)g(X)h(X)=0的解的振动性。文[1]研究了方程 X+q(t)φ(X,X)+p(t)g (X)X~α=0的解的振动性,而文[2]研究了方程 X+p(t)g(X)h(X)=0的解的振动性。本文所得的结果包含了文[1]的全部结果及文[2]的部份结果。 相似文献
28.
研究外荷载为长期非平稳随机过程。考虑长期荷载的特性 ,采用 1个概率谱密度函数来反映长期非平稳随机荷载及其特征 ;概率谱密度函数是基于大量的一般谱密度函数的统计特性获得。以延长结构的抗疲劳使用寿命为目标函数 ,提出了调谐质量减振阻尼器的优化设计方法 ,这在实际工程中有着极为广阔的应用前景。本文旨在从理论上发展长期非平稳随机荷载作用下调谐质量减振阻尼器的优化设计方法 ;文中采用长期波浪实测数据 ,给出了 1个数值算例说明整个设计过程。 相似文献
29.
Long-term variability in the intermediate layer of the eastern Japan Basin has been investigated to understand the variability
of water mass formation in the East Sea. The simultaneous decrease of temperature at shallower depths and oxygen increasing
at deeper depths in the intermediate layer took place in the late 1960’s and the mid-1980’s. Records of winter sea surface
temperatures and air temperatures showed that there were cold winters that persisted for several years during those periods.
Therefore, it was assumed that a large amount of newly-formed water was supplied to the intermediate layer during those cold
winters. Close analysis suggests that the formation of the Upper Portion of Proper Water occurred in the late 1960’s and the
Central Water in the mid-1980’s. 相似文献
30.
厦门湾有色溶解有机物的光吸收特性研究 总被引:3,自引:0,他引:3
研究了厦门湾九龙江河口区、西海域、同安湾及东侧水道海水中有色溶解有机物(CDOM)的光吸收特性,分析了CDOM的河口行为,并讨论了CDOM光吸收特性与其荧光性质之间的关系。结果表明,厦门湾表层海水CDOM光吸收系数a(355)的水平分布表现为河口区最高、东侧水道最低、西海域和同安湾介于两者之间,底层水a(355)的分布与表层基本相似,表明陆源河流输入是厦门湾CDOM的主要来源;a(355)的垂直分布为表层高于底层,主要受水文和生物因素控制。厦门湾表层水CDOM光谱斜率S的平均值介于0.014—0.018nm-1,但河口低盐度区S值较小,反映陆源腐殖质的影响。a(355)在河口混合中呈保守行为,表明CDOM具有良好的保守性质。CDOM的吸收系数a(355)与其荧光强度之间表现为较好的相关关系,指示可以用灵敏度更高的荧光方法来研究CDOM的分布和行为。 相似文献