全文获取类型
收费全文 | 10639篇 |
免费 | 1969篇 |
国内免费 | 2864篇 |
专业分类
测绘学 | 960篇 |
大气科学 | 2266篇 |
地球物理 | 2492篇 |
地质学 | 5531篇 |
海洋学 | 1410篇 |
天文学 | 346篇 |
综合类 | 979篇 |
自然地理 | 1488篇 |
出版年
2024年 | 35篇 |
2023年 | 141篇 |
2022年 | 413篇 |
2021年 | 537篇 |
2020年 | 362篇 |
2019年 | 520篇 |
2018年 | 589篇 |
2017年 | 518篇 |
2016年 | 611篇 |
2015年 | 543篇 |
2014年 | 614篇 |
2013年 | 631篇 |
2012年 | 695篇 |
2011年 | 744篇 |
2010年 | 715篇 |
2009年 | 661篇 |
2008年 | 570篇 |
2007年 | 651篇 |
2006年 | 548篇 |
2005年 | 522篇 |
2004年 | 436篇 |
2003年 | 363篇 |
2002年 | 528篇 |
2001年 | 443篇 |
2000年 | 420篇 |
1999年 | 449篇 |
1998年 | 281篇 |
1997年 | 294篇 |
1996年 | 246篇 |
1995年 | 260篇 |
1994年 | 213篇 |
1993年 | 211篇 |
1992年 | 149篇 |
1991年 | 119篇 |
1990年 | 91篇 |
1989年 | 71篇 |
1988年 | 66篇 |
1987年 | 55篇 |
1986年 | 32篇 |
1985年 | 22篇 |
1984年 | 18篇 |
1983年 | 13篇 |
1982年 | 20篇 |
1981年 | 9篇 |
1980年 | 10篇 |
1979年 | 5篇 |
1972年 | 2篇 |
1971年 | 3篇 |
1958年 | 15篇 |
1947年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 859 毫秒
711.
Qingchun Wen Xiuzhen Li Hongshi He Yuanman Hu Xin Chen Yu Chang Wei Wang Rencang Bu 《中国地理科学(英文版)》2014,24(1):50-59
Traditionally,patch-based analysis at the landscape scale fragmentation has been used in the study of landscape ecology while the study of boundary and node has not been considered as much detail until recently.This study investigated the possibility of applying boundary-and node-based methods in landscape pattern analysis to the upper reaches of the Minjiang River in the southwestern China.Boundary-based and node-based landscape indices were selected to be used in analyzing changes in landscape patterns,and the results were compared with analysis using traditional pattern indices.We compared the responses of patch-area-based,boundary-length-based and node-number-based indices,and concluded that boundary-based and node-based indices are more sensitive to disturbance than patch-based indices with various patterns,and node-based indices are even more sensitive than boundary-based ones.Thus,the results suggest that boundary-based and node-based pattern analysis methods provide helpful supplementary information to traditional patch-based pattern analysis methods.The results about pattern dynamics of landscapes in the upper reaches of the Minjiang River based on boundaries and nodes showed that with human disturbance,the dominance of forest landscape was weakened by other landscape types; thus the landscape pattern of the study area became more homogeneous and the boundary network became more complex.These changes further augmented disturbance interfaces in the landscape and increased the possibilities of further landscape fragmentation. 相似文献
713.
In this paper, we assessed the ecological and biodiversity status in the Bohai Sea through a quantitative survey on mac-rofaunal community at 25 stations in Laizhou Bay and adjacent waters in the autumn of 2006.We tested the robustness and effectiveness of taxonomic distinctness as an ecological indictor by analyzing its correlation with species richness and natural environmental variables and by analyzing other ecological indicators (Shannon-Wiener H' and W statistics from Abundance Biomass Comparison curve).Results so obtained indicated that the benthic environment of the study waters in general is not under major impact of anthropogenic disturbance, but some stations in Laizhou Bay and along the coast of the Shandong Peninsula and even in the central Bohai Sea might be moderately disturbed and showed signs of ecological degradation.The taxonomic distinctness measures △+ and Λ+ were independent of sampling effort and natural environment factors and were compliant to other ecological indicators.Further application of the taxonomic distinctness indicator to assess marine biodiversity and ecosystem health on a larger regional scale with historical data seems promising. 相似文献
714.
?????????????????????????????????????GPS????????????????????????????????????????4?????????????????????????????????????????????????????????????????????????壻??????????????????????????????????????????壻?齭???????????????????????????????????塣???????????????????????ζ?????????????????????????????????????????????κ??齭??????????????????????????????????????????????? 相似文献
715.
熊耳群为玄武粗安岩-英安流纹岩组合,大红口组为粗面岩组合,属B类的过渡型拉斑玄武岩浆系列,具以太华群为岩浆房的壳幔混染型成因;秦岭群和宽坪群为变拉斑玄武岩建造,属A类拉斑玄武岩浆系列,具幔源型成因;二郎坪群和丹凤群属细碧岩-石英角斑岩建造,C类石英角斑岩浆系列与A类拉斑玄武岩浆系列共存,具壳幔双层混合型成因。 相似文献
716.
内蒙迪彦庙地区早二叠世寿山沟组地层发育良好,岩性相对稳定,变质、变形程度较低,是研究区范围内早二叠世地层层序研究的理想地区。该课题逐层实测了寿山沟组剖面,从剖面的岩性地层入手,着重分析了迪彦庙地区早二叠世的岩性组合特征、基本层序、沉积旋回特征;同时根据古生物化石类型和组合,推断该区域形成的地质时代和沉积环境。研究表明:寿山沟组剖面厚度为9598.98m,根据岩性和粒度的大小,可分3个岩段:一段为滨浅海浅海陆棚沉积;二段、三段为一套浊流沉积岩系,发育特征性浊流沉积构造,为典型的复理石建造。 相似文献
717.
An air-gun source is the most commonly used excitation method in offshore seismic exploration.The excitation characteristics of an air-gun source affect seismic... 相似文献
718.
Banana is one of the main economic agrotypes in Zhangzhou, Fujian Province. The multi-temporal ENVISAT ASAR data with different
polarization are used to classify the banana fields in this paper. Principal component analysis (PCA) was applied for six
pairs of ASAR dual-polarization data. For its large leaves, banana has high backscatter. So the value of banana fields is
high and shows very bright in the 1st component, which makes it much easier for banana fields extraction. Dual-polarization
data provide more information, and the VV and VH backscatter of banana show different characters with other land covers. Based
on the analysis of the radar signature of banana fields and other land covers and the 1st component, banana fields are classified
using object-oriented classifier. Compared to the field survey data and ASTER data, the accuracy of banana fields in the study
area is 83.5%. It shows that the principal component analysis provides the useful information in SAR images analysis and makes
the extraction of banana fields easier.
Supported by the Program for New Century Excellent Talents in University (NCET-05-0573), Fujian Science and Technology Project
(No.2006I0018), the Science Project of the Education Department of Fujian Province(No. 2006F5022). 相似文献
719.
720.
NUFFT-Based Iterative Reconstruction Algorithm for Synthetic Aperture Imaging Radiometers 总被引:2,自引:0,他引:2
Synthetic aperture imaging radiometers (SAIRs) are powerful sensors for high-resolution observation of the Earth at low microwave frequencies. However, the future application of this new technology is limited by its large number of element antennas and receiving channels. In order to further reduce the complexity of the whole system, the circular and rotating array configurations have been proposed. Their disadvantages lie primarily in more complicated image reconstruction, because the conventional fast Fourier transform (FFT) method could not be used directly for image reconstruction. This letter introduces an iterative method for the reconstruction of radiometric brightness temperature maps from nonuniform visibility function samples. This method combines the conjugate gradient algorithm with min-max nonuniform FFT approach, which has been shown to provide considerably improved image quality relative to the traditional gridding method. Simulations for SAIRs were performed and showed better image quality in comparison to the traditional frequency interpolation method. 相似文献