全文获取类型
收费全文 | 3265篇 |
免费 | 807篇 |
国内免费 | 1271篇 |
专业分类
测绘学 | 371篇 |
大气科学 | 887篇 |
地球物理 | 707篇 |
地质学 | 1911篇 |
海洋学 | 553篇 |
天文学 | 182篇 |
综合类 | 318篇 |
自然地理 | 414篇 |
出版年
2024年 | 41篇 |
2023年 | 137篇 |
2022年 | 250篇 |
2021年 | 325篇 |
2020年 | 254篇 |
2019年 | 281篇 |
2018年 | 277篇 |
2017年 | 215篇 |
2016年 | 241篇 |
2015年 | 200篇 |
2014年 | 196篇 |
2013年 | 207篇 |
2012年 | 221篇 |
2011年 | 207篇 |
2010年 | 226篇 |
2009年 | 176篇 |
2008年 | 176篇 |
2007年 | 136篇 |
2006年 | 165篇 |
2005年 | 118篇 |
2004年 | 91篇 |
2003年 | 70篇 |
2002年 | 94篇 |
2001年 | 105篇 |
2000年 | 93篇 |
1999年 | 120篇 |
1998年 | 86篇 |
1997年 | 92篇 |
1996年 | 112篇 |
1995年 | 87篇 |
1994年 | 78篇 |
1993年 | 52篇 |
1992年 | 43篇 |
1991年 | 34篇 |
1990年 | 22篇 |
1989年 | 29篇 |
1988年 | 22篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 3篇 |
1984年 | 7篇 |
1983年 | 5篇 |
1982年 | 9篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 2篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1958年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有5343条查询结果,搜索用时 31 毫秒
41.
支持向量机(SVM)是近年来发展起来的机器学习的新方法,它较好地解决小样本、非线性、高维数、局部极小点等实际问题.文中研究支持向量机的拓展算法--最小二乘支持向量机(LSSVM),并将其应用于确定大面积复杂似大地水准面.通过工程实例并与神经网络模型和二次曲面多项式拟合模型相比较,验证确定区域似大地水准面的LSSVM方法的有效性. 相似文献
42.
43.
Automated Target Detection and Discrimination Using Constrained Kurtosis Maximization 总被引:1,自引:0,他引:1
Exploiting hyperspectral imagery without prior information is a challenge. Under this circumstance, unsupervised target detection becomes an anomaly detection problem. We propose an effective algorithm for target detection and discrimination based on the normalized fourth central moment named kurtosis, which can measure the flatness of a distribution. Small targets in hyperspectral imagery contribute to the tail of a distribution, thus making it heavier. The Gaussian distribution is completely determined by the first two order statistics and has zero kurtosis. Consequently, kurtosis measures the deviation of a distribution from the background and is suitable for anomaly/target detection. When imposing appropriate inequality constraints on the kurtosis to be maximized, the resulting constrained kurtosis maximization (CKM) algorithm will be able to quickly detect small targets with several projections. Compared to the widely used unconstrained kurtosis maximization algorithm, i.e., fast independent component analysis, the CKM algorithm may detect small targets with fewer projections and yield a slightly higher detection rate. 相似文献
44.
如何加速成图周期,快速、高效地生产各种地理信息产品是摄影测量应用研究的主要内容.本文介绍了非量测数码航摄影像在全数字摄影测量工作站上进行地理信息产品生产的技术方法和工作流程,研究了关键技术环节. 相似文献
45.
There has been increasing concern about the lack of involvement by social scientists and humanists in a global change program,
although many social scientists are already directly involved in various aspects of research on environmental change, and
their research interests are clearly central to a global change research agenda. Based on a historical review, the role of
social science disciplines as well as social science institutes in an emerging multidecadal global change program is discussed.
Both “plan of action” and “plan of inaction” are suggested to avoid potential pitfalls due to the rush development of a social
science program into the existing global climate change problem. 相似文献
46.
47.
Qian Y Engel MH Macko SA Carpenter S Deming JW 《Geochimica et cosmochimica acta》1993,57(14):3281-3293
Dipeptide hydrolysis and amino acid decomposition appear to follow a first-order rate law. The hydrolysis rate increases exponentially with increasing temperature in aqueous solution at both 265 atm and water steam pressures over the temperature range of 100 to 220 degrees C. Dipeptide hydrolysis has a lower apparent activation energy at 265 atm (44.1 KJ/mol) than at water steam pressure (98.9 KJ/mol). At lower temperatures (<200-220 degrees C), the rate of peptide bond hydrolysis is faster at 265 atm than at water steam pressure. At higher temperatures (>200-220 degrees C), however, peptide bond hydrolysis is slower at 265 atm than at water steam pressure. In aqueous solution, amino acid decomposition rates also increase exponentially with increasing temperature. Amino acid decomposition rates are much higher at 265 atm than at water steam pressure over the entire temperature range investigated. 相似文献
48.
49.
Porous fence is a kind of artificial windbreak that has many practical applications. The threshold wind velocities at different distances downwind from porous fences were measured and the corresponding characteristics of particle movement observed to assess their shelter effect. It is found that the fence’s porosity is the key factor that determines the resulting shelter effect. The area near a fence can be typically classified into five regions, each with a different mode of particle movement. Dense fences, and especially solid fences, favor the accumulation of sand upwind of the fences. Fences with porosities of 0.3–0.4 produce the maximum threshold wind velocity; those with porosities of 0.3–0.6 (depending on the fence height) provide the maximum effective shelter distance. It is confirmed that the fence porosities of 0.3–0.4 that have been proposed for practical application in previous research are the most effective for abating wind erosion. 相似文献
50.
In the process of steam stimulation exploitation of viscous crude oil, the injected water, at high temperature and under high pressure, reacts intensively with the host rock. This kind of water–rock interaction in Liaohe Shuguang Oil Field was studied on the basis of analysis of water composition changes, laboratory experiments, mineral saturation indices analysis, and mass balance calculation. Compared with the injected water, the changes of the composition of discharged water are mainly the distinct decrease of pH, Na+, SiO2 and Cl−, as well as the increase of K+, Ca2+, Mg2+, SO
4
2−
and HCO
3
−
. Laboratory experiments under field conditions showed: the dissolution sequence of minerals quantitatively is quartz>potassium feldspar>albite, and the main change of clay minerals is the conversion of kaolinite to analcime. Mass balance calculation indicated during the process of steam stimulation, large quantities of analcime are precipitated with the dissolution of large amounts of quartz, kaolinite, potassium feldspar, and CO2. These results correlated very well with the experimental results. The calculated results of Liaohe Shuguang Oil Field showed that during the steam stimulation for viscous crude oil, the amounts of minerals dissolved (precipitated) are huge. To control the clogging of pore spaces of oil reservoirs, increased study of water–rock interaction is needed. 相似文献