全文获取类型
收费全文 | 1423篇 |
免费 | 210篇 |
国内免费 | 517篇 |
专业分类
测绘学 | 177篇 |
大气科学 | 95篇 |
地球物理 | 245篇 |
地质学 | 1002篇 |
海洋学 | 339篇 |
天文学 | 20篇 |
综合类 | 100篇 |
自然地理 | 172篇 |
出版年
2024年 | 3篇 |
2023年 | 27篇 |
2022年 | 45篇 |
2021年 | 54篇 |
2020年 | 61篇 |
2019年 | 80篇 |
2018年 | 61篇 |
2017年 | 61篇 |
2016年 | 71篇 |
2015年 | 76篇 |
2014年 | 100篇 |
2013年 | 98篇 |
2012年 | 101篇 |
2011年 | 97篇 |
2010年 | 90篇 |
2009年 | 89篇 |
2008年 | 93篇 |
2007年 | 112篇 |
2006年 | 98篇 |
2005年 | 84篇 |
2004年 | 82篇 |
2003年 | 72篇 |
2002年 | 86篇 |
2001年 | 47篇 |
2000年 | 40篇 |
1999年 | 39篇 |
1998年 | 58篇 |
1997年 | 31篇 |
1996年 | 33篇 |
1995年 | 24篇 |
1994年 | 27篇 |
1993年 | 23篇 |
1992年 | 20篇 |
1991年 | 11篇 |
1990年 | 12篇 |
1989年 | 10篇 |
1988年 | 4篇 |
1987年 | 5篇 |
1986年 | 4篇 |
1985年 | 4篇 |
1984年 | 3篇 |
1983年 | 5篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1971年 | 3篇 |
排序方式: 共有2150条查询结果,搜索用时 31 毫秒
111.
Wang Hongmei Ma Xiangru Liu Deng Yang Xiaofen Li Jihong 《Frontiers of Earth Science》2007,1(4):399-404
Understanding the dynamics of organic matter in modern marine water columns greatly favors the geobiological evaluation of
hydrocarbon source rocks. Biolipids could make great contribution to petroleum hydrocarbons due to their comparable chemical
components and the slightly refractory characteristics of biolipids during the microbial/thermal degradation. A variety of
environmental factors such as temperature, CO2 and salinity could affect the biochemical contents in microorganisms. As a result, microorganisms living in a changing environmental
condition might have a different contribution to the petroleum formation. Organic carbon flux is shown to bear a positive
correlation with the primary productivity only within a certain range of biomass volumes in a specific biohabitat. Furthermore,
organic matter is degraded much quickly in a water column with oxic conditions. Therefore, the anoxic condition, along with
the enhanced biological productivity, would be one of the significant factors in the formation of high-quality hydrocarbon
source rocks. The formation of biofilms and microbial mats favors the preservation of sedimentary organic matter by decreasing
the degradation rate of organic matter. Identification of biofilms and microbial mats in sedimentary rocks will thus greatly
help to understand the depositional processes of organic matter finally preserved in hydrocarbon source rocks.
__________
Translated from Earth Science—Journal of China University of Geosciences, 2007, 32(6): 748–754 [译自: 地球科学—中国地质大学学报 相似文献
112.
Sudhir Kumar Tewatia Pratima Rani Bose Asuri Sridharan Sunanda Rath 《Geotechnical and Geological Engineering》2007,25(2):239-255
It is shown that time compression curve obtained from one-dimensional consolidation curve in the laboratory may include six
phases. These are initial compression, first primary compression, transition from first primary compression to second primary
compression, second primary compression, and transition from second primary compression to creep and lastly creep. This paper
attempts to identify the quantitative beginnings and characteristics of these phases. A mathematical characteristic of all
the soils that follow primary consolidation as per Terzaghi’s one dimensional consolidation theory is derived. It is known
as the constant of primary consolidation. It is used to study the beginning of secondary consolidation and its effects on
primary consolidation. Another characteristic of soils for creep and total absence of primary compression is derived. Methods
are suggested for the determination of coefficients of Primary and Secondary consolidations and the compression index. 相似文献
113.
The strength of frozen soils has been one of the most extensively studied aspects in frozen soil mechanics. When carrying
out deep excavations using freezing methods, high stress states are often encountered. Therefore, the strength of frozen soils
under high confining pressures is of major concern. This paper first reviews the present failure criteria for frozen soils,
especially with regard to the effect of confining pressure on the strength. It is suggested that the strength consists of
two components, cohesion and friction, and can be expressed by the Mohr–Coulomb criterion and in the framework of the Drucker–Prager
criterion. However, these two components are both dependent on the stress state. Duncan’s equation for the friction angle
is extended to frozen soils. A frozen Lanzhou fine sand is taken as study subject. The frozen sand is compressed at a high
strain rate under three different temperatures and under a wide range of confining pressure. Experimental results have confirmed
the generally recognized principle that the strength increases with the confining pressure up to certain value. Thereafter,
it decreases with continued increase in confining pressure. Experimental data from our own tests and literature are fitted
to the new criterion, which shows its validity. 相似文献
114.
Characteristics of seasonal and spatial variations of primary production over the southeastern Bering Sea shelf 总被引:3,自引:1,他引:3
In situ primary production data collected during 1978–1981 period and 1997–2000 period were combined to improve understanding of seasonal and spatial distribution of primary production in the southeastern Bering Sea. Mean daily primary production rates showed an apparent seasonal cycle with high rates in May and low rates in summer over the entire shelf of the southeastern Bering Sea except for oceanic region due to lack of data. There was also an increasing trend of primary production rates in the fall over the inner shelf and the middle shelf. There was a decreasing trend of primary production rates between late April and mid-May over the inner shelf while there was an abrupt increase between late April and mid-May over the middle shelf and the outer shelf. In the shelf break region, there was an increasing pattern in late May. These suggest that there was a gradual progression of the development of the spring phytoplankton bloom from the inner shelf toward the shelf break region. There was also a latitudinal variability of primary production rate over the middle shelf, probably due to either spatial variations of the seasonal advance and retreat of sea ice or horizontal advection of saline water in the bottom layer. Annual rates of primary production across the southeastern Bering Sea shelf were 121, 150, 145, 110, and 84 g C m−2 yr−1 in the inner shelf, the middle shelf, the outer shelf, the shelf break, and oceanic region, respectively. High annual rates of primary production over the inner shelf can be attributed to continuous summer production based on regenerated nitrogen and/or a continuous supply of nitrogen at the inner front region, and to fall production. There were some possibilities of underestimation of annual primary production over the entire shelf due to lack of measurement in early spring and fall, which may be more apparent over the shelf break and oceanic region than the inner shelf, the middle, and the outer shelf. This study suggests that the response of primary production by climate change in the southeastern Bering Sea shelf can be misunderstood without proper temporal and seasonal measurement. 相似文献
115.
本文选取了商业化比较好的基于小波压缩原理的三个压缩软件 ,采用国际上通用的反映压缩影像质量还原精度的 PSNR(最佳峰值信噪比 )法 ,主要从压缩速度、压缩比率、压缩影像的还原质量等几方面对这些压缩软件的性能进行了比较 ,从而为今后更好地使用这些软件提供借鉴。 相似文献
116.
一种减少块效应的自适应后处理算法 总被引:3,自引:0,他引:3
针对在基于块变换的图像编码技术尤其在甚低速率视频编码技术中,块斑严重影响图像的主观质量问题,提出了一种复杂度低且效果比较明显的消除块效应的自适应后处理技术。模拟结果表明,该方法能够同时提高图像的客观峰值信噪比和主观视觉效果,具有较高的实用意义。 相似文献
117.
基于自适应四叉树分割的遥感图像压缩算法 总被引:4,自引:0,他引:4
遥感图像具有不同于一般图像的特点,针对遥感图像对压缩算法的特殊要求,提出了自适应四叉树分割算法实现图像的分块,以各像块的均值近似表示该像块的灰度值,从而以尺量少的数值获得对原始图像的最佳逼近,再对辐近误差进行自适应分割和基于像块方差的自适应量化,最后对各分量进行Huffman编码.实验表明,本算法能表现出良好的实用性能,比JPEG算法更适用于遥感图像的压缩. 相似文献
118.
从野外第一手资料入手,分析了本区的童子岩组第三段中亚段的沉积特征。在此基础上,归纳出本区主采煤层沉积环境,并研究了主采煤层的聚煤特征。 相似文献
119.
120.
数字滤波器的频谱窗和噪声压缩度 总被引:2,自引:0,他引:2
根据傅里叶理论导出了数字滤波器的频谱窗,根据随机过程理论导出了数字滤波的噪声压缩度。通过对二项式系数数字滤波器和多项式系数数字滤波器和频谱窗和噪声压缩度的分析,发现前者对噪声具有较强的压缩作用,同时能保持信号的凹凸性,是一种优于后者的数字滤波器。 相似文献