全文获取类型
收费全文 | 45545篇 |
免费 | 20313篇 |
国内免费 | 45410篇 |
专业分类
测绘学 | 4656篇 |
大气科学 | 22306篇 |
地球物理 | 14679篇 |
地质学 | 41144篇 |
海洋学 | 18049篇 |
天文学 | 1207篇 |
综合类 | 5217篇 |
自然地理 | 4010篇 |
出版年
2024年 | 327篇 |
2023年 | 785篇 |
2022年 | 1405篇 |
2021年 | 1841篇 |
2020年 | 2727篇 |
2019年 | 6082篇 |
2018年 | 6168篇 |
2017年 | 5903篇 |
2016年 | 5915篇 |
2015年 | 5404篇 |
2014年 | 4914篇 |
2013年 | 5567篇 |
2012年 | 5325篇 |
2011年 | 5203篇 |
2010年 | 5134篇 |
2009年 | 4423篇 |
2008年 | 3754篇 |
2007年 | 3759篇 |
2006年 | 3305篇 |
2005年 | 3081篇 |
2004年 | 3232篇 |
2003年 | 2832篇 |
2002年 | 2585篇 |
2001年 | 2414篇 |
2000年 | 1918篇 |
1999年 | 1843篇 |
1998年 | 1830篇 |
1997年 | 1888篇 |
1996年 | 1465篇 |
1995年 | 1410篇 |
1994年 | 1245篇 |
1993年 | 1213篇 |
1992年 | 1051篇 |
1991年 | 771篇 |
1990年 | 742篇 |
1989年 | 623篇 |
1988年 | 532篇 |
1987年 | 401篇 |
1986年 | 334篇 |
1985年 | 260篇 |
1984年 | 287篇 |
1983年 | 174篇 |
1982年 | 217篇 |
1981年 | 152篇 |
1980年 | 115篇 |
1979年 | 119篇 |
1978年 | 56篇 |
1977年 | 49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
61.
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
62.
以N_(263)浸渍在DA_(201)大孔吸附树脂上制得了N_(263)浸渍树脂,测定了静态时树脂Pb(Ⅱ)的吸附容量。通过柱层析法,试验了酸度,流速和干扰离子等因素对N_(263)浸渍树脂吸附和洗脱Pb的影响。建立了N_(263)浸渍树脂分离─富集后衔接原子吸收和比色法测定Pb(Ⅱ)的方法,测定了地质样品中Pb(Ⅱ),测试结果理想。 相似文献
63.
64.
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
65.
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
66.
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
67.
68.
69.
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
70.
北天山东段康古尔塔格带是晚古生代塔里木板块和准噶尔板块碰撞的结果。它是一条复杂的、强烈的高应变带.并具有独特的变形机制、应变序列以及构造变形。本文运用构造-地层研究方法对该碰撞带的构造特征加以分析和研究。 相似文献