全文获取类型
收费全文 | 6134篇 |
免费 | 493篇 |
国内免费 | 616篇 |
专业分类
测绘学 | 269篇 |
大气科学 | 697篇 |
地球物理 | 369篇 |
地质学 | 1372篇 |
海洋学 | 237篇 |
天文学 | 20篇 |
综合类 | 363篇 |
自然地理 | 3916篇 |
出版年
2024年 | 11篇 |
2023年 | 58篇 |
2022年 | 237篇 |
2021年 | 247篇 |
2020年 | 246篇 |
2019年 | 285篇 |
2018年 | 202篇 |
2017年 | 212篇 |
2016年 | 267篇 |
2015年 | 230篇 |
2014年 | 315篇 |
2013年 | 359篇 |
2012年 | 334篇 |
2011年 | 389篇 |
2010年 | 248篇 |
2009年 | 302篇 |
2008年 | 316篇 |
2007年 | 332篇 |
2006年 | 364篇 |
2005年 | 304篇 |
2004年 | 280篇 |
2003年 | 253篇 |
2002年 | 253篇 |
2001年 | 190篇 |
2000年 | 203篇 |
1999年 | 178篇 |
1998年 | 141篇 |
1997年 | 114篇 |
1996年 | 69篇 |
1995年 | 60篇 |
1994年 | 52篇 |
1993年 | 50篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 21篇 |
1989年 | 12篇 |
1988年 | 12篇 |
1987年 | 11篇 |
1986年 | 11篇 |
1985年 | 6篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1980年 | 2篇 |
1977年 | 3篇 |
1954年 | 1篇 |
排序方式: 共有7243条查询结果,搜索用时 15 毫秒
1.
REGIONAL DEMOGRAPHIC STRUCTURE OF THE UNITED STATES 总被引:1,自引:0,他引:1
Richard L. Morrill 《The Professional geographer》1990,42(1):38-53
This paper analyzes the surprisingly great variation in demographic character across the states, utilizing data on fertility, mortality, age, sex, mobility, household character, abortion, race and ethnicity. A fairly simple regional pattern is revealed that is less related to levels of economic development than to long-standing historical cultural differences. 相似文献
2.
DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHEHEXIREGION,CHINA肖洪浪,高前兆,李福兴DEVELOPMENTSTRATEGIESOFWATERANDLANDRESOURCESINTHE... 相似文献
3.
4.
5.
N. H. S. OLIVER I. CARTWRIGHT V. J. WALL S. D. GOLDING 《Journal of Metamorphic Geology》1993,11(5):705-720
Abstract Large calcite veins and pods in the Proterozoic Corella Formation of the Mount Isa Inlier provide evidence for kilometre-scale fluid transport during amphibolite facies metamorphism. These 10- to 100-m-scale podiform veins and their surrounding alteration zones have similar oxygen and carbon isotopic ratios throughout the 200 × 10-km Mary Kathleen Fold Belt, despite the isotopic heterogeneity of the surrounding wallrocks. The fluids that formed the pods and veins were not in isotopic equilibrium with the immediately adjacent rocks. The pods have δ13Ccalcite values of –2 to –7% and δ18Ocalcite values of 10.5 to 12.5%. Away from the pods, metadolerite wallrocks have δ18Owhole-rock values of 3.5 to 7%. and unaltered banded calc-silicate and marble wallrocks have δ13Ccalcite of –1.6 to –0.6%, and δ18Ocalcite of 18 to 21%. In the alteration zones adjacent to the pods, the δ18O values of both metadolerite and calc-silicate rocks approach those of the pods. Large calcite pods hosted entirely in calc-silicates show little difference in isotopic composition from pods hosted entirely in metadolerite. Thus, 100- to 500-m-scale isotopic exchange with the surrounding metadolerites and calc-silicates does not explain the observation that the δ18O values of the pods are intermediate between these two rock types. Pods hosted in felsic metavolcanics and metasiltstones are also isotopically indistinguishable from those hosted in the dominant metadolerites and calc-silicates. These data suggest the veins are the product of infiltration of isotopically homogeneous fluids that were not derived from within the Corella Formation at the presently exposed crustal level, although some of the spread in the data may be due to a relatively small contribution from devolatilization reactions in the calc-silicates, or thermal fluctuations attending deformation and metamorphism. The overall L-shaped trend of the data on plots of δ13C vs. δ18O is most consistent with mixing of large volumes of externally derived fluids with small volumes of locally derived fluid produced by devolatilization of calc-silicate rocks. Localization of the vein systems in dilatant sites around metadolerite/calc-silicate boundaries indicates a strong structural control on fluid flow, and the stable isotope data suggest fluid migration must have occurred at scales greater than at least 1 km. The ultimate source for the external fluid is uncertain, but is probably fluid released from crystallizing melts derived from the lower crust or upper mantle. Intrusion of magmas below the exposed crustal level would also explain the high geothermal gradient calculated for the regional metamorphism. 相似文献
6.
本文简述了国际天球参考架的发展历史和现在射电参考架的现状—基准源选择的标准和参考架的稳定性。描述了地面上光学观测在依巴谷参考架的维持和加密的一系列工作。介绍由天体测量卫星GAIA和SIM给出的天球参考架可能逵到的精度。详述了在今后十年中地面天体测量的作用以及正在开展有关天球参考架的研究课题 ,同时也列出了我国正在和即将开展天体测量的几个研究课题 相似文献
7.
Aimed at promoting regional coalitions and expanding the approach to economic links, this paper puts forward some new concepts
such as link intensity and receiving coefficient, expounds the indexes of quantitative analysis of economic links and establishes
the quantitative-analysis model of economic links. With help of the model, this paper calculates the values of the link intensities
between Su-Xi-Chang (Suzhou-Wuxi-Changzhou) region and Shanghai, and analyzes the regional difference of the economic links. 相似文献
8.
DharmalingamVENUGOPAL 《山地科学学报》2004,1(1):74-80
The Nilgiri Mountains of south India is considered unique by anthropologists, geologists, climatologists, botanists as well as tourists. It has remained a subject of constant study and research over the last two centuries. Man-nature balance had continued undisturbed in the Nilgiris for thousands of years until the early 19th century when it became a British colony attracting, in due course, various developmental activities. Subsequently, the Nilgiris and its popular hill stations emerged as favourite places for the British population in India for rest and recuperation, game and for raising commercial plantations. In the process, the traditional indigenous crops were replaced by “English“ vegetables and the natural forests gave way to commercial plantations of coffee, tea and other exotic species of trees. 相似文献
9.
在对中国西部的区域划分进行界定并阐述中国西部区域特征的基础上,对中国西部的开发进行了历史回顾,并对中国西部开发的历史背景和现实基础、中国西部开发的优势和制约条件进行分析,最后提出了中国西部开发的发展战略和政策措施. 相似文献
10.