全文获取类型
收费全文 | 34294篇 |
免费 | 5449篇 |
国内免费 | 6719篇 |
专业分类
测绘学 | 5378篇 |
大气科学 | 4842篇 |
地球物理 | 9785篇 |
地质学 | 13708篇 |
海洋学 | 4282篇 |
天文学 | 1478篇 |
综合类 | 2351篇 |
自然地理 | 4638篇 |
出版年
2024年 | 128篇 |
2023年 | 387篇 |
2022年 | 1023篇 |
2021年 | 1287篇 |
2020年 | 1481篇 |
2019年 | 1766篇 |
2018年 | 1250篇 |
2017年 | 1677篇 |
2016年 | 1624篇 |
2015年 | 1748篇 |
2014年 | 2207篇 |
2013年 | 2500篇 |
2012年 | 2207篇 |
2011年 | 2348篇 |
2010年 | 1855篇 |
2009年 | 2304篇 |
2008年 | 2340篇 |
2007年 | 2370篇 |
2006年 | 2322篇 |
2005年 | 1903篇 |
2004年 | 1740篇 |
2003年 | 1424篇 |
2002年 | 1186篇 |
2001年 | 975篇 |
2000年 | 876篇 |
1999年 | 816篇 |
1998年 | 797篇 |
1997年 | 686篇 |
1996年 | 566篇 |
1995年 | 488篇 |
1994年 | 412篇 |
1993年 | 395篇 |
1992年 | 286篇 |
1991年 | 213篇 |
1990年 | 165篇 |
1989年 | 146篇 |
1988年 | 122篇 |
1987年 | 69篇 |
1986年 | 61篇 |
1985年 | 58篇 |
1984年 | 33篇 |
1983年 | 15篇 |
1982年 | 26篇 |
1981年 | 19篇 |
1980年 | 23篇 |
1979年 | 17篇 |
1978年 | 15篇 |
1977年 | 20篇 |
1976年 | 25篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
61.
62.
63.
Jacques Laskar 《Celestial Mechanics and Dynamical Astronomy》1993,56(1-2):191-196
Frequency analysis is a new method for analyzing the stability of orbits in a conservative dynamical system. It was first devised in order to study the stability of the solar system (Laskar, Icarus, 88, 1990). It is a powerful method for analyzing weakly chaotic motion in hamiltonian systems or symplectic maps. For regular motions, it yields an analytical representation of the solutions. In cases of 2 degrees of freedom system with monotonous torsion, precise numerical criterions for the destruction of KAM tori can be found. For a 4D symplectic map, plotting the frequency map in the frequency plane provides a clear representation of the global dynamics and describes the actual Arnold web of the system. 相似文献
64.
65.
66.
案例推理的地学应用背景和方法 总被引:3,自引:1,他引:2
案例推理 (CBR)研究 ,是人类认知的一种合理性模型 ,接近于人类认识、解决问题最原始的思维方式 ,具有在无法获取机理模型、确定规则或统计模型时 ,采用简单的历史相似性推理实现问题的定量求解和预测 ,成为人工智能领域近年来的研究热点。 相似文献
67.
张晓晖 《成都信息工程学院学报》2002,17(3):166-169
通过对影响能见度的因素的分析,提出了一种能见度预报方法-综合分析法,极大地提高了能见度的预报准确率。 相似文献
68.
Dan Gill 《Mathematical Geology》1993,25(4):471-482
The ability of association analysis to discriminate sedimentary facies was tested on Purdy's modal analyses of modern sediments of the Great Bahama Bank. Purdy's data set has served in the past as a standard reference for evaluating various multivariate classification algorithms. In order to adapt Purdy's data to association analysis, the percent abundance of the 12 constituents was converted to binary form by dichotomizing each variable on its mean value. The results obtained by association analysis are virtually identical to those obtained by Purdy and other authors. The same four main sedimentary facies were discriminated; 86% of the samples were identically classified (97% when misclassified borderline cases are counted as matches); the total partition variance of the classification is only negligibly greater (4%); and the grouping of the variables yielded the same four groups. The rank order of the three division-attributes responsible for the sample classification is fines, oolites, and corals. Association analysis has been employed by other authors to differentiate meaningful facies groups in studies of ancient reef carbonates, modern reef sediments, and heavy minerals in stream sediments. In all these studies, the results were found to be compatible with those obtained by using the continuous quantitative measurements, indicating that qualitative binary data may often be sufficient for the purpose of facies discrimination in many branches of geology and that association analysis is an effective method for this purpose. 相似文献
69.
寇新建 《大地测量与地球动力学》1994,(3)
由于观测条件的变化,观测值方差将产生随机波动。本文根据Bayes估计理论,推导了线性模型待估参数的分布函数及观测值方差估计公式,并将其用于位移检验。计算结果表明,在同样的置信度下,本方法更符合实际。文中还对参数估值的统计性质进行了讨论。 相似文献
70.
Dr M. J. Wooller D. L. Swain K. J. Ficken A. D. Q. Agnew F. A. Street-Perrott G. Eglinton 《第四纪科学杂志》2003,18(1):3-15
Woody, subalpine shrubs and grasses currently surround Lake Rutundu, Mount Kenya. Multiple proxies, including carbon isotopes, pollen and grass cuticles, from a 755‐cm‐long core were used to reconstruct the vegetation over the past 38 300 calendar years. Stable carbon‐isotope ratios of total organic carbon and terrestrial biomarkers from the lake sediments imply that the proportion of terrestrial plants using the C4 photosynthetic pathway was greater during the Late Pleistocene than in the Holocene. Pollen data show that grasses were a major constituent of the vegetation throughout the Late Pleistocene and Holocene. The proportion of grass pollen relative to the pollen from other plants was greatest at the last glacial maximum (LGM). Grass cuticles confirm evidence that C4 grass taxa were present at the LGM and that the majority followed the cold‐tolerant NADP‐MEC4 subpathway. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献