全文获取类型
收费全文 | 67440篇 |
免费 | 12120篇 |
国内免费 | 16946篇 |
专业分类
测绘学 | 5083篇 |
大气科学 | 13159篇 |
地球物理 | 11894篇 |
地质学 | 36302篇 |
海洋学 | 9228篇 |
天文学 | 3673篇 |
综合类 | 4727篇 |
自然地理 | 12440篇 |
出版年
2024年 | 539篇 |
2023年 | 1368篇 |
2022年 | 2563篇 |
2021年 | 3010篇 |
2020年 | 2784篇 |
2019年 | 3305篇 |
2018年 | 2587篇 |
2017年 | 2979篇 |
2016年 | 3039篇 |
2015年 | 3308篇 |
2014年 | 4175篇 |
2013年 | 4104篇 |
2012年 | 4361篇 |
2011年 | 4501篇 |
2010年 | 3828篇 |
2009年 | 4580篇 |
2008年 | 4415篇 |
2007年 | 4819篇 |
2006年 | 4690篇 |
2005年 | 4228篇 |
2004年 | 3683篇 |
2003年 | 3444篇 |
2002年 | 2909篇 |
2001年 | 2625篇 |
2000年 | 2451篇 |
1999年 | 2116篇 |
1998年 | 1807篇 |
1997年 | 1455篇 |
1996年 | 1265篇 |
1995年 | 1089篇 |
1994年 | 985篇 |
1993年 | 829篇 |
1992年 | 648篇 |
1991年 | 532篇 |
1990年 | 362篇 |
1989年 | 345篇 |
1988年 | 242篇 |
1987年 | 134篇 |
1986年 | 79篇 |
1985年 | 69篇 |
1984年 | 37篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 29篇 |
1980年 | 24篇 |
1979年 | 21篇 |
1978年 | 27篇 |
1977年 | 21篇 |
1976年 | 7篇 |
1954年 | 10篇 |
排序方式: 共有10000条查询结果,搜索用时 11 毫秒
11.
利用图像分形编码中定义域块和最小均方误差一一对应的特点,提出了1种基于分形编码的多姿态、表情的人脸图像检索方法。该方法将待检索图像分割为相同大小的值域块,然后将每一值域块按给定的定义域块进行分形编码,得到最小均方误差,计算该最小均方误差与图像库中最小均方误差的欧氏距离。将待检索图像所有值域块的欧氏距离求平均,此平均欧氏距离较小的几幅图像即为检索出的图像。实验证明该方法能够准确地检索出图像库中存储的同一人的不同姿态、表情的图像。 相似文献
12.
The properties of salinity in the South China Sea (SCS), a significant marginal sea connecting the Pacific andIndian Oceans, are greatly influenced by the transport of fresh water flux between the two oceans. However, the long-termchanges in the intermediate water in the SCS have not been thoroughly studied due to limited data, particularly in relationto its thermodynamic variations. This study utilized reanalysis data products to identify a 60-year trend of freshening in theintermediate waters of the northern South China Sea (NSCS), accompanied by an expansion of low-salinity water. Thestudy also constructed salinity budget terms, including advection and entrainment processes, and conducted an analysis ofthe salinity budget to understand the impacts of external and internal dynamic processes on the freshening trend of theintermediate water in the NSCS. The analysis revealed that the freshening in the northwest Pacific Ocean and theintensification of intrusion through the Luzon Strait at intermediate levels are the primary drivers of the salinity changes inthe NSCS. Additionally, a weakened trend in the intensity of vertical entrainment also contributes to the freshening in theNSCS. This study offers new insights into the understanding of regional deep sea changes in response to variations in boththermodynamics and oceanic dynamic processes. 相似文献
13.
14.
The reservoir quality of Jurassic and Triassic fluvial and lacustrine-deltaic sandstones of the Yanchang Oil Field in the Ordos Basin is strongly influenced by the burial history and facies-related diagenetic events. The fluvial sandstones have a higher average porosity (14.8%) and a higher permeability (12.7×10?3 ?m2) than those of the deltaic sandstones (9.8% and 5.8 ×10?3 ?m2, respectively). The burial compaction, which resulted in 15% and 20% porosity loss for Jurassic and Triassic sandstones, respectively, is the main factor causing the loss of porosity both for the Jurassic and Triassic sandstones. Among the cements, carbonate is the main one that reduced the reservoir quality of the sandstones. The organic acidic fluid derived from organic matter in the source rocks, the inorganic fluid from rock-water reaction during the late diagenesis, and meteoric waters during the epidiagenesis resulted in the formation of dissolution porosity, which is the main reason for the enhancement of reservoir-quality. 相似文献
15.
王周琼 《中国科学D辑(英文版)》2002,45(Z1)
Only by providing the good conditions for the growth of plants can a favorable ecologicalenvironment on which human beings rely for existence be created. The upside-down-T dou-ble-layer water-conserving afforestation way is developed according to the situation of the short-age of water resources, low soil fertility and vast land in the arid areas. The characteristics of theafforestation way are to change the microenvironment in the root area of the plants, provide thefavorable conditions for the growth of plants, and reduce the necessary conditions for the growth ofplants in large areas in the arid regions. Meanwhile, the size of its water-conserving layer can bechanged according to the size of the planted trees. The different ways of the bottom wa-ter-conserving layer can be used according to the requirements. The afforestation way is suitablefor planting trees on a small scale and also for afforesting on a large scale under the adverse cir-cumstances in the arid areas, and has been effectively used in the afforestation in the hinterland ofTaklamakan Desert and the southern marginal zone of Gurbantonggut Desert. The prospects ofthe afforestation way are broad in afforestation and desertification control in the desert regions. 相似文献
16.
17.
该文用几何光学与辐射传输混合模型研究不连续植被冠层的几何光学反射模型的四分量(承照树冠、承照地面、阴影树冠、阴影地面)的参数化。用一个修正的均匀介质层路径散射(反射与传输)参数的解析算法估计路径散射参数(反射与传输),其中也考虑了冠层间隙的影响。光谱分量特征是不连续植被冠层的传输与反射,背景反照率,以直射光通量与天空漫射光通量比例的函数。光谱分量特征的模型与在美国缅因州Holand采集的针叶林数据吻合。基于LiStrahler几何光学相互遮蔽模型,用参数化的光谱分量特征对老松林和老云杉林的方向反射进行估计,其结果与在不同太阳与观测方向上的PARABOLA测量值匹配得很好。 相似文献
18.
POLDER(地球反射极化和方向)仪器在BOREAS实验中曾搭载NASA的一架C130飞机飞行。在BOREAS的南部地区,POLDER获取了各种实验场地上的BRDF测量值。森林覆盖地区的大的热点特征,以及十个地区上的镜面反射分量得到了描述。该文通过POLDER的测量数据,对向常规的光谱特征中加入通过遥感获得的方向性特征后,对各种森林覆盖的分类和区分能力的提高给出了定量化的解。当将方向信息加入常规光谱信息后,采用无监督分类时,类间耦合矩阵的各项显著减小(减小倍数为2-5倍)。这一事实证明了用遥感方向特征可以增强对BOREAL森林覆盖的区分能力。 相似文献
19.
20.
The characteristics of atmospheric heat source associated with the summer monsoon onset in the South China Sea (SCS) are studied using ECMWF reanalysis data from 1979 to 1993. A criterion of the SCS summer monsoon onset is defined by the atmospheric hea… 相似文献