全文获取类型
收费全文 | 61855篇 |
免费 | 22559篇 |
国内免费 | 46604篇 |
专业分类
测绘学 | 4357篇 |
大气科学 | 25889篇 |
地球物理 | 18751篇 |
地质学 | 48139篇 |
海洋学 | 18533篇 |
天文学 | 2407篇 |
综合类 | 7297篇 |
自然地理 | 5645篇 |
出版年
2024年 | 237篇 |
2023年 | 681篇 |
2022年 | 1725篇 |
2021年 | 2196篇 |
2020年 | 3141篇 |
2019年 | 6453篇 |
2018年 | 6925篇 |
2017年 | 6563篇 |
2016年 | 6843篇 |
2015年 | 5884篇 |
2014年 | 5753篇 |
2013年 | 6302篇 |
2012年 | 5968篇 |
2011年 | 5811篇 |
2010年 | 5666篇 |
2009年 | 5091篇 |
2008年 | 4183篇 |
2007年 | 4213篇 |
2006年 | 3732篇 |
2005年 | 3498篇 |
2004年 | 3532篇 |
2003年 | 3174篇 |
2002年 | 2933篇 |
2001年 | 2746篇 |
2000年 | 2543篇 |
1999年 | 2969篇 |
1998年 | 2731篇 |
1997年 | 2842篇 |
1996年 | 2279篇 |
1995年 | 2181篇 |
1994年 | 1945篇 |
1993年 | 1796篇 |
1992年 | 1510篇 |
1991年 | 1139篇 |
1990年 | 957篇 |
1989年 | 858篇 |
1988年 | 735篇 |
1987年 | 576篇 |
1986年 | 452篇 |
1985年 | 340篇 |
1984年 | 358篇 |
1983年 | 218篇 |
1982年 | 273篇 |
1981年 | 181篇 |
1980年 | 140篇 |
1979年 | 125篇 |
1978年 | 62篇 |
1977年 | 53篇 |
1972年 | 48篇 |
1971年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 234 毫秒
101.
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
102.
Lin Liu 《Chinese Astronomy and Astrophysics》1997,21(4):488-500
With the aim of analyzing the errors in the radial position of satellites, we give in this paper the expressions for the radial perturbation which include the complete zeroth- and first-order terms in the orbital eccentricity. A simpler and faster method of calculating the perturbation is given. 相似文献
103.
1882年李希霍芬在太原西山建立石炭系的太原系含煤建造以来,石炭系与二叠系的分界意见,归纳起来有三:①以Pseudoschwagerina(s.1)带之底为界,下面为石炭系,上面为二叠系;②以晋东南的灰白色“佳祥砂岩”与太原西山黄绿色“骆驼脖子砂岩”对比,砂岩底面为石炭系与二叠系的分界线,下面是石炭系山西组,上面是二叠系石盒子组,山西组的地层时代定为晚石炭世;③以晋东南灰白色第三砂岩(陵川砂岩)与太原西山“北岔沟砂岩”对比,砂岩底面为石炭系与二叠系的分界线,下面是石炭系太原组,上面是二叠系山西组,山西组的地层时代为早二叠世。笔者认为石炭系与二叠系的划分,以第③方案较适宜。 相似文献
104.
THE SIMULATED STORAGE CAPACITY OF FLOOD AND WATERLOGGING IN THE TYPICAL AGRICULTURAL REGION IN JIANGHAN PLAIN 总被引:2,自引:0,他引:2
1INTRODUCTIONJianghanPlainisoneoftheimportantbasesofChina′sagriculturalproductivitylocatedinthemiddleChangjiangRivervalleyofcentralChina.TheJianghanplainapproximately6.6×104km2inareaandover30millioninpopulationistherenowned“countryoffishandrice"in… 相似文献
105.
In this paper, we idealize the actual solar atmosphere as a multi-isothermal-layer system so as to obtain the energy transmittance of the linear Alfvén wave that propagates through such a system in presence of a uniform oblique magnetic filed. The results indicate that the two-layer model is essentially different to the three-layer one. In the two-layer model, the temperature jump acts as a high pass filter. In the three-layer model, resonant transfer will take place and the transmittance undergoes oscillation as the trigonometric function terms dominate its behavior. For actual solar atmosphere, the result reveals that the lower parts of solar atmosphere are more suitable for those Alfvén waves with period of seconds to transfer their energy. 相似文献
106.
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
107.
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
108.
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
109.
Boli basin, between Yishu fracture belt and Dunmi fracture belt, is the biggest Mesozoic coal basin in the east of Heilongjiang Province. Now it is a fault - fold remnant basin. The basin' s shape is generally consistent with the whole distribution of the cover folds, an arc protruding southwards. The basement of the basin can be divided into three fault blocks or structural units. The formation and evoluation of the basin in Mesozoic was determined by the basement fault blocks' displacement features rusulted from by the movement of the edge faults and the main basement faults. 相似文献
110.