全文获取类型
收费全文 | 11929篇 |
免费 | 2078篇 |
国内免费 | 2109篇 |
专业分类
测绘学 | 969篇 |
大气科学 | 1987篇 |
地球物理 | 3571篇 |
地质学 | 5893篇 |
海洋学 | 1144篇 |
天文学 | 603篇 |
综合类 | 863篇 |
自然地理 | 1086篇 |
出版年
2024年 | 51篇 |
2023年 | 182篇 |
2022年 | 451篇 |
2021年 | 529篇 |
2020年 | 391篇 |
2019年 | 493篇 |
2018年 | 903篇 |
2017年 | 792篇 |
2016年 | 689篇 |
2015年 | 575篇 |
2014年 | 586篇 |
2013年 | 585篇 |
2012年 | 1130篇 |
2011年 | 866篇 |
2010年 | 568篇 |
2009年 | 540篇 |
2008年 | 469篇 |
2007年 | 429篇 |
2006年 | 430篇 |
2005年 | 1071篇 |
2004年 | 1045篇 |
2003年 | 790篇 |
2002年 | 331篇 |
2001年 | 254篇 |
2000年 | 221篇 |
1999年 | 245篇 |
1998年 | 195篇 |
1997年 | 203篇 |
1996年 | 178篇 |
1995年 | 161篇 |
1994年 | 147篇 |
1993年 | 105篇 |
1992年 | 84篇 |
1991年 | 89篇 |
1990年 | 65篇 |
1989年 | 59篇 |
1988年 | 52篇 |
1987年 | 28篇 |
1986年 | 23篇 |
1985年 | 16篇 |
1984年 | 11篇 |
1983年 | 11篇 |
1982年 | 7篇 |
1981年 | 7篇 |
1980年 | 8篇 |
1975年 | 4篇 |
1965年 | 3篇 |
1962年 | 3篇 |
1958年 | 4篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
Guadalupian (Middle Permian) Conodont Faunas at Shangsi Section, Northeast Sichuan Province 总被引:1,自引:0,他引:1
This study recovered the Guadalupian conodont faunas from Shangsi (上寺) Section in Northeast Sichuan (四川). Four genera and nine species were identified, and three conodont zones were recognized and established; they include Jinogondolella nankingensis Zone, J. aserrata Zone, and J. postserrata Zone. The Roadian and Wordian boundary is set in the interval 5 m from the top of Bed 86 by the first appearance of the conodont J. aserrata. The Wordian and Capitanian boundary is set in the interval 2.3 m from the top of the Bed 95 by the first appearance of J. postserrata. Most of the specimens demonstrated low color alteration index (CAI) as 1.5-3, indicating that most part of the Maokou (茅口) Formation may have the suitable thermal conditions for the formation of oil source rocks. 相似文献
82.
Calcimicrobialites across the Permian-Triassic boundary in Huayingshan (华蓥山) region were investigated using the fluorescence microscopic measurements to understand the occurrence of organic matter. The microbialites are composed of micrite matrix and coarse spar cement. Abundant rhombic or magnetic needle-like carbonate minerals were observed adrift within the cement. The fluorescence microscopic measurement indicates the micrite matrix in microbialites shows the most abundant organic matter, with the rhombic or magnetic needle-like carbonate minerals and coarse spar cement coming to the 2nd and the 3rd, respectively. Organic matter is mainly preserved in the space between the grains of the micrite minerals but almost evenly distributed in the rhombic or magnetic needle-like carbonate minerals. As one of the common diagenesis types, dolomitization is observed to occur in the microbialites in Huayingshan. However, the carbonate cement in microbialites still has high content of element Sr as shown by the microprobe analysis, reflecting that the dolomitization might have happened in a restricted environment. Observation under the fluorescence microscope shows that dolomitization just led to the redistribution of organic matter in the grain space of dolomite minerals, inferring that the diagenesis has a slight effect on the preservation, and thus on the content of organic matter in the microbialites. 相似文献
83.
植被波谱空间尺度效应及尺度转换方法初步研究 总被引:8,自引:1,他引:7
基于遥感图像的地物识别需要大量地物波谱数据的支持,而现有地物波谱库中收集到的同一地物的波谱,由于其测量尺度和方法的差异,波谱也存在很大差异.以冬小麦为例,首先介绍了材料波谱、端元波谱和像元波谱这3种不同尺度波谱的概念,并以实验数据分析了不同测量尺度下波谱的差别,以此说明波谱尺度转换的必要性.然后利用物理模型和统计模型建立不同测量尺度下的波谱转换关系.分别验证了SAILH模型和线性光谱混合模型在波谱转换中的精度.研究表明,在大尺度上采用统计模型,在小尺度上采用非线性的物理模型可以解释不同尺度观测植被波谱之间的差异. 相似文献
84.
85.
86.
87.
GPS observations of the ionospheric F2-layer behavior during the 20th November 2003 geomagnetic storm over South Korea 总被引:4,自引:1,他引:3
The ionospheric F2-layer peak density (NmF2) and its height (hmF2) are of great influence on the shape of the ionospheric
electron density profile Ne (h) and may be indicative of other physical processes within the ionosphere, especially those
due to geomagnetic storms. Such parameters are often estimated using models such as the semiempirical international reference
ionosphere (IRI) models or are measured using moderately priced to expensive instrumentation, such as ionosondes or incoherent
scatter radars. Global positioning system (GPS) observations have become a powerful tool for mapping high-resolution ionospheric
structures, which can be used to study the ionospheric response to geomagnetic storms. In this paper, we describe how 3-D
ionospheric electron density profiles were produced from data of the dense permanent Korean GPS network using the tomography
reconstruction technique. These profiles are verified by independent ionosonde data. The responses of GPS-derived parameters
at the ionospheric F2-layer to the 20th November 2003 geomagnetic storm over South Korea are investigated. A fairly large
increase in the electron density at the F2-layer peak (the NmF2) (positive storm) has been observed during this storm, which
is accompanied by a significant uplift in the height of the F2 layer peak (the hmF2). This is confirmed by independent ionosonde
observations. We suggest that the F2-layer peak height uplift and NmF2 increase are mainly associated with a strong eastward
electric field, and are not associated with the increase of the O/N2 ratio obtained from the GUVI instruments aboard the TIMED satellite. It is also inferred that the increase in NmF2 is not
caused by the changes in neutral composition, but is related to other nonchemical effects, such as dynamical changes of vertical
ion motions induced by winds and E × B drifts, tides and waves in the mesosphere/lower thermosphere region, which can be dynamically
coupled upward to generate ionospheric perturbations and oscillations. 相似文献
88.
89.
90.