全文获取类型
收费全文 | 59071篇 |
免费 | 9893篇 |
国内免费 | 14462篇 |
专业分类
测绘学 | 5515篇 |
大气科学 | 11726篇 |
地球物理 | 11289篇 |
地质学 | 27499篇 |
海洋学 | 8525篇 |
天文学 | 5291篇 |
综合类 | 3800篇 |
自然地理 | 9781篇 |
出版年
2024年 | 407篇 |
2023年 | 1054篇 |
2022年 | 2071篇 |
2021年 | 2508篇 |
2020年 | 2552篇 |
2019年 | 2952篇 |
2018年 | 2162篇 |
2017年 | 2540篇 |
2016年 | 2581篇 |
2015年 | 2889篇 |
2014年 | 3416篇 |
2013年 | 3990篇 |
2012年 | 3600篇 |
2011年 | 3655篇 |
2010年 | 3186篇 |
2009年 | 3959篇 |
2008年 | 3975篇 |
2007年 | 4265篇 |
2006年 | 4059篇 |
2005年 | 3684篇 |
2004年 | 3213篇 |
2003年 | 2978篇 |
2002年 | 2628篇 |
2001年 | 2178篇 |
2000年 | 2165篇 |
1999年 | 1950篇 |
1998年 | 1697篇 |
1997年 | 1284篇 |
1996年 | 1108篇 |
1995年 | 894篇 |
1994年 | 837篇 |
1993年 | 712篇 |
1992年 | 541篇 |
1991年 | 436篇 |
1990年 | 298篇 |
1989年 | 276篇 |
1988年 | 197篇 |
1987年 | 139篇 |
1986年 | 96篇 |
1985年 | 67篇 |
1984年 | 39篇 |
1983年 | 29篇 |
1982年 | 24篇 |
1981年 | 16篇 |
1980年 | 29篇 |
1979年 | 9篇 |
1978年 | 23篇 |
1977年 | 11篇 |
1976年 | 4篇 |
1954年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 5 毫秒
41.
V. J. Mclntyre A. C. Gilmore J. B. Hearnshaw 《Journal of Astrophysics and Astronomy》1990,11(1):81-123
Photographic spectra of SN1987A in the LMC have been obtained from 1987 February 25 to 1988 June 30. Microdensitometer tracings
of these have been reduced to intensity and corrections for instrumental response have been applied to the spectra. This paper
presents these data in an atlas format, discusses the reduction procedures in detail, and presents radial velocity measurements
of selected lines in the spectra 相似文献
42.
In the upper Chesapeake Bay (Maryland, U.S.A.) field surveys were conducted at 18 multiple longshore sand bar sites. The multiple bar systems were found in water depths less than approximately 2 m (mean sea level), and exhibited mild bottom slopes of 0·0052 or less. The number of bars composing each system ranged from four to 17 and the spacing between the crests typically increased in the offshore direction, ranging from 12 to 70 m. Bar height also typically increased with distance offshore and ranged from 0·03 to 0·61 m. A grain size analysis of crest and trough sediment did not reveal any significant differences and the sediment was categorized as ‘fine sand’. A review of the literature data indicated that the Chesapeake Bay multiple bars possessed similar characteristics to those found in Gelding Bay (Baltic Sea); similarities in fetch, wave height and tidal range between the two bays may account for this finding. The surf-scaling parameter indicated that the multiple bar systems were extremely dissipative with regard to wave energy, and wave height appeared to be an important factor in controlling bar spacing and bar height. A multiple wave break point hypothesis was discussed as a possible mechanism for the formation of Chesapeake Bay multiple longshore bars, and limited observational evidence appeared to support such a mechanism. 相似文献
43.
同时采用4个台站的国际超导重力仪长期连续重力观测资料和国际地球自转服务中心提供的同步地球自转参数,研究了极移引起的地球重力场变化特征。利用自回归模型估计了各序列的功率谱密度和积谱密度,结果表明极移导致的重力效应的主要能量集中在Chandler摆动和周年项附近,叠积后实际重力观测与极移重力信号理论值之间的差异分别为0.4%和3.9%,说明超导重力仪可有效监测极移导致的重力变化。 相似文献
44.
空间数据仓库的认知过程 总被引:3,自引:3,他引:0
从理论上探讨了空间数据仓库的认知过程.主要是十一个层次的世界模型通过十个算子转换的过程,对十一个层次世界及十个算子概念进行了较详细地讨论,并用代数系统对其进行了定义。 相似文献
45.
针对多通道滤光器太阳磁场望远镜的磁场观测定标及掌握(所采用的)谱线特征之需要,取VAL宁静太阳大气模型计算了7条Fel光球线的Stokes轮廓、形成深度、贡献函数分布,从而较为系统地对多条反常及正常Zeeman线的特征及性质作出了分析与总结,解释了在磁光效应影响下Stokes Q,U参量的形成深度曲线在近线心区域处出现陡峭峰值的原因。 相似文献
46.
S. A. Lamzin 《Astrophysics and Space Science》1995,224(1-2):211-214
I argue that temperatures of spots, responsible for observed periodical light variations of T Tauri stars (TTS), are not known with reliable accuracy to discriminate between chromospheric and accretion theories of TTS 's phenomenon. The hypothesis is set up that spots on classical TTS (CTTS) are due to heating of stellar surface by radiation from a collisional accretion shock, whereas spots on weak line TTS (WTTS), at least in some cases, are connected with a collisionless accretion shock rather than chromospheric activity. Possible scenarios of WTTS interaction with circumstellar matter are discussed. 相似文献
47.
北部湾北部海上油田的开发工作始于50年代,60~70年代作了大量地质工作。改革开放后,油气勘查取得突破性进展。累计发现油气构造6个,含油面积超过40km2,石油地质储量约4亿t,已开发油田2个,揭开我国南方油气资源开发序幕 相似文献
48.
Numerical simulations of the evolving activity of comet Hale-Bopp are presented, assuming a porous, spherical nucleus, 20
km in radius, made of dust and gas-laden amorphous ice. The main effects included are: crystallization of amorphous ice and
release of occluded gas, condensation, sublimation and flow of gases through the pores, changing pore sizes, and flow of dust
grains. The model parameters, such as initial pore size and porosity, emissivity, dust grain size, are varied in order to
match the observed activity. In all cases, a sharp rise in the activity of the nucleus occurs at a large heliocentric distance
pre-perihelion, marked by a few orders of magnitude increase in the CO and the CO2 fluxes and in the rate of dust emission. This is due to the onset of crystallization, advancing down to a few meters below
the surface, accompanied by release of the trapped gases. A period of sustained, but variable, activity ensues. The emission
of water molecules is found to surpass that of CO at a heliocentric distance of 3 AU. Thereafter the activity is largely determined
by the behaviour of the dust. If a dust mantle is allowed to build up, the water production rate does not increase dramatically
towards perihelion; if most of the dust is ejected, the surface activity increases rapidly, producing a very bright comet.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
49.
Zhang Chuanling 《地球科学与环境学报》1997,(1)
文中详细地论述了新疆阿尔泰哈巴河一带古生代火山岩的地质、岩石化学、稀土元素地球化学特征。将其分为三个类型,认为火山岩是在陆缘裂陷槽的区域构造背景下发生和演化的。对各类型火山岩岩浆来源,上升模式也做了初步讨论。 相似文献
50.
概略地叙述了西宁地区的泥石流形成条件和形成机理,并将西宁地区泥石流划分为稀性泥石流类及泥石质泥石流种类。在此基础上提出防治泥石流灾害的意见。 相似文献