全文获取类型
收费全文 | 40529篇 |
免费 | 19349篇 |
国内免费 | 44745篇 |
专业分类
测绘学 | 4286篇 |
大气科学 | 21946篇 |
地球物理 | 13463篇 |
地质学 | 38402篇 |
海洋学 | 17240篇 |
天文学 | 912篇 |
综合类 | 4859篇 |
自然地理 | 3515篇 |
出版年
2024年 | 300篇 |
2023年 | 678篇 |
2022年 | 1173篇 |
2021年 | 1553篇 |
2020年 | 2480篇 |
2019年 | 5800篇 |
2018年 | 5922篇 |
2017年 | 5644篇 |
2016年 | 5682篇 |
2015年 | 5207篇 |
2014年 | 4629篇 |
2013年 | 5305篇 |
2012年 | 5136篇 |
2011年 | 4911篇 |
2010年 | 4869篇 |
2009年 | 4110篇 |
2008年 | 3473篇 |
2007年 | 3436篇 |
2006年 | 3031篇 |
2005年 | 2815篇 |
2004年 | 3033篇 |
2003年 | 2699篇 |
2002年 | 2462篇 |
2001年 | 2209篇 |
2000年 | 1759篇 |
1999年 | 1702篇 |
1998年 | 1726篇 |
1997年 | 1789篇 |
1996年 | 1383篇 |
1995年 | 1319篇 |
1994年 | 1190篇 |
1993年 | 1170篇 |
1992年 | 991篇 |
1991年 | 734篇 |
1990年 | 713篇 |
1989年 | 591篇 |
1988年 | 490篇 |
1987年 | 391篇 |
1986年 | 311篇 |
1985年 | 248篇 |
1984年 | 276篇 |
1983年 | 165篇 |
1982年 | 202篇 |
1981年 | 145篇 |
1980年 | 111篇 |
1979年 | 110篇 |
1978年 | 50篇 |
1977年 | 48篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
本文讨论了河西走廊东部地区早二叠世地层研究的有关问题,认为甘肃山丹青羊泉下二叠统剖面是北祁连山—河西走廊地区的重要代表剖面之一。以该剖面研究为依据,提出走廊东部地区下二叠统的进一步划分方案,分析该区早二叠世地层岩性特征差异的原因,并与走廊西部地区同期地层进行对比,进一步明确了大黄沟组的含意。 相似文献
52.
Formation of saline groundwaters in the Baltic region through freezing of seawater during glacial periods 总被引:1,自引:0,他引:1
Typical Ca---Cl brines occur in crystalline and metamorphic rocks below freshwater horizons at various localities in Sweden and Finland. Total dissolved solids (TDS) range in concentration between 2 and 120 gl−1 and have long been thought to derive from water-rock interactions. The relationships between Na, Cl and Br in these brines suggest, however, that they were derived from freezing of seawater during glacial periods. The brines were subsequently diluted by meteoric waters and their Ca/Mg ratio was increased through water-rock interactions in the subsurface. The hydrogeological model for both the formation of freeze-derived marine brines and their lateral intrusion involves restricted inland marine basins in recent and subrecent polar climatic belts. Seawater in such basins gradually freezes in response to glaciation. The solutes which concentrate in the remaining water body are residual after precipitation of a sequence of minerals which include carbonates, mirabilite and hydrohalite. Hydraulic pressure of the growing ice sheet over the frozen seas is gradually added to the ambient hydrostatic pressure exerted on the brines. This, together with their increased density, increases the intrusional potential of the brines. As the land ice cannot exert hydraulic pressure on continental groundwater in the aquifers, the balance of pressure favours deep landward intrusion of brines. Post-glacial processes cause the subsurface dilution and replacement of the brines both by seawater and fresh waters. The presence of such brines also far from present-day coastal settings reflects the shifting of coastlines as a result of isostatic movements and eustatic sea-level changes associated with glaciation and deglaciation. 相似文献
53.
Shi Zhi-dong Li Zhong-yuanUniversity of Science Technology of China Hefei PRC Advanced Centre for Earth Science Astronomy Third World Academy of Science 《紫金山天文台台刊》1996,(2)
Areview about K-H instability related to comet tail is presented.The effect of dust graing on KHI is theoretically analysed and relations of critical shear to excit KHI with the properties of dust grains both for sheared ion flow and for sheared dust flow re put out.Their possible application in comet tail is suggested. 相似文献
54.
介绍了自动高精度测距系统。该系统能以5×10-7的测距精度测定多个观测点的形变,可作为跨断层动态连续观测的新仪器。 相似文献
55.
Ren Tinawi Marc Sarrazin Andr Filiatrault 《Soil Dynamics and Earthquake Engineering》1993,12(8):469-477
The 1990 edition of the National Building Code of Canada (Associate Committee of the National Building Code, National Research Council, Ottawa, 1990) makes a clear distinction between eastern and western Canada in terms of seismic acceleration and velocity zones. While it is well established that ground motions can be amplified significantly through loose clay deposits, no results are available that take into consideration the typical high frequency content of ground motions in eastern Canada. This paper develops ground amplification curves for clays having depths between 10 and 70 m excited by typical eastern Canadian ground motions scaled to two different values of peak horizontal accelerations. Simplified free-field spectral design curves, which could be used by structural designers, are proposed. The curves show that maximum spectral accelerations occur for structural periods between 0.2 and 0.5 s. In addition, soil depth does not appear to be an important parameter controlling the response of typical clay deposits in eastern Canada. 相似文献
56.
57.
58.
BIAN Jian-min LIN Nian-feng TANG Jie College of Environment Resources Jilin University Changchun Jilin China 《东北亚地学研究》2005,(Z1)
Desertification is an environmental issue in the world. The salt-alkalization desertification land area formed by both primary and secondary salt-alkalization has extended in a large scale, which has become a significant eco-environmental problem. Based on the characteristics of eco-environment and the situation of desertification in western Songnen plain, this paper reports the analyzes of its formation in background and cause. An early warning system on the salt-alkalization desertification is established and the GIS technology is used to abstract the information of desertification evaluation index. Supported by the integrated technology of the GIS and ANN, the orientation and quantitative result of desertification are gained, which is helpful to the eco-environment protection and resource development in western Songnen Plain. 相似文献
59.
秦岭凤太矿田层控铅锌(铜)矿床的金属物质、硫和成矿溶液主要来自深部基底的岩石,属海底喷流—沉积成因矿床。从这一理论认识出发可进一步分析控矿地质条件及今后在该区有效地寻找同类矿床。 相似文献
60.
The energy flow ofBranchiura sowerbyi was studied for the first time in China in a shallow macrophytic lake, Biandantang Lake, Hubei Province. The energy flow
was calculated from the measurement of flesh production (12.5241kJ/m2a), egestion (517.7302kJ/m2a), metabolism (38.3273 kJ/m2a), and excretion (4.3798kJ/m2a). The net growth efficiency of the species is about 22.7%, which accords well with the generally reported value for oligochaetes.
In addition, the relationship between starvation respiration (R, mgO2/ind·d), wet weight (Ww, mg) and temperature (T, °C) were also measured, with the regression function beingR=0.008Ww0.736 e0.050T.
Project supported by NSFC (30270278, 3960019), the foundation of the government of Hubei Province (No. 2000J109), and the
foundation of Ecological Station, CAS in the Institute of Hydrobiology. 相似文献