全文获取类型
收费全文 | 56246篇 |
免费 | 21445篇 |
国内免费 | 46301篇 |
专业分类
测绘学 | 5035篇 |
大气科学 | 24289篇 |
地球物理 | 17408篇 |
地质学 | 45567篇 |
海洋学 | 18326篇 |
天文学 | 1673篇 |
综合类 | 6383篇 |
自然地理 | 5311篇 |
出版年
2024年 | 325篇 |
2023年 | 854篇 |
2022年 | 1701篇 |
2021年 | 2186篇 |
2020年 | 3044篇 |
2019年 | 6287篇 |
2018年 | 6592篇 |
2017年 | 6329篇 |
2016年 | 6478篇 |
2015年 | 5702篇 |
2014年 | 5464篇 |
2013年 | 6181篇 |
2012年 | 5756篇 |
2011年 | 5594篇 |
2010年 | 5533篇 |
2009年 | 4867篇 |
2008年 | 4014篇 |
2007年 | 4042篇 |
2006年 | 3477篇 |
2005年 | 3299篇 |
2004年 | 3371篇 |
2003年 | 3065篇 |
2002年 | 2870篇 |
2001年 | 2610篇 |
2000年 | 2290篇 |
1999年 | 2535篇 |
1998年 | 2397篇 |
1997年 | 2410篇 |
1996年 | 2022篇 |
1995年 | 1895篇 |
1994年 | 1673篇 |
1993年 | 1565篇 |
1992年 | 1317篇 |
1991年 | 989篇 |
1990年 | 905篇 |
1989年 | 730篇 |
1988年 | 624篇 |
1987年 | 472篇 |
1986年 | 401篇 |
1985年 | 309篇 |
1984年 | 322篇 |
1983年 | 212篇 |
1982年 | 242篇 |
1981年 | 165篇 |
1980年 | 145篇 |
1979年 | 132篇 |
1978年 | 59篇 |
1977年 | 53篇 |
1972年 | 45篇 |
1971年 | 54篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
42.
StudyonthecharacteristicsofthemarineboundarylayerintheEquatorialPacific¥ZhangZiyuandZhouMingyu(ReceivedAugust21,1993;accepted... 相似文献
43.
C. K. Paull W. Ussler H. G. Greene R. Keaten P. Mitts J. Barry 《Geo-Marine Letters》2003,22(4):227-232
A sediment gravity flow descended through the axis of Monterey Canyon on 20 December 2001 at 13:35 Pacific standard time.
The timing of this event is documented by a current-meter package which recorded an 11.9-dbar pressure increase in less than
10 min and was found 550 m down-canyon from its deployment site, buried completely within a >70-cm-thick gravity flow deposit.
This event is believed to have started in less than 290 m of water because an instrument at this location was also lost at
the same time. A 178-cm core collected after the event from the axis of the canyon at 1,297-m water depth contained fresh,
greenish, chlorophyll-rich organic material at 32-cm sub-bottom depth, suggesting the event extended to this water depth.
The only trigger identified for this mass movement event appears to be moderate sea and surf conditions. Thus, gravity flow
events of this magnitude do not require an exceptional triggering event. 相似文献
44.
Submersible pressure hulls with fiber-reinforced multilayer-sandwich constructions have been developed in recent years as substitutes for classical metallic ring-stiffened pressure hulls. This study aims to optimize the design of filament-wound multilayer-sandwich submersible pressure hulls, taking into consideration the shell buckling strength constraint, the angle-ply laminated facing failure strength constraint and the low-density isotropic core yielding strength constraint under hydrostatic pressure using the hybrid genetic algorithm (HGA). The thickness of the facing, the thickness of the core layer, the orientation angle of the fibers in the facings and the shear modulus of the core material are taken as design variables. A sensitivity analysis is performed to study the effects of the operational depths and the hull shell geometry parameter, the length-to-diameter ratio (L/D), on the optimal design of filament-wound multilayer-sandwich submersible pressure hulls with graphite/epoxy, glass/epoxy and boron/epoxy composite facings. The results reveal that the optimal weight of various sandwich pressure hulls increases linearly with the operational depth, but it is almost unchanged as the geometry paramter. Furthermore, Graphite/Epoxy is the best choice for the material of the facings in a light-weight design. With reference to wall design, Boron/Epoxy is the best choice for the material of the facing at shallow depths, but Graphite/Epoxy is the best choice at extreme depths. Results of this study provide a valuable reference for designers of underwater vehicles. 相似文献
45.
- In order to employ cost effective frequency domain analysis for off-shore structures treatment of hydrodynamic loading is essential. Drag and inertia dominated, resonating and antiresonating cases under random sea states are analyzed to highlight the implications and relative merits of four salient linearization techniques. 相似文献
46.
南海西沙西南海域表层沉积物特征 总被引:3,自引:1,他引:3
西沙西南海域表层沉积物样品矿物成分,地球化学,微体古生物分析测试结果表明:沉积物可分为7种类型,沉积环境主要是陆坡,部分为深海平原,海洋生物,海洋化学以及火山物质的沉积起到了积极的作用,陆源物质的影响较小,它们主要来源于北部大陆和南部岛礁等物源区。 相似文献
47.
The properties of salinity in the South China Sea (SCS), a significant marginal sea connecting the Pacific andIndian Oceans, are greatly influenced by the transport of fresh water flux between the two oceans. However, the long-termchanges in the intermediate water in the SCS have not been thoroughly studied due to limited data, particularly in relationto its thermodynamic variations. This study utilized reanalysis data products to identify a 60-year trend of freshening in theintermediate waters of the northern South China Sea (NSCS), accompanied by an expansion of low-salinity water. Thestudy also constructed salinity budget terms, including advection and entrainment processes, and conducted an analysis ofthe salinity budget to understand the impacts of external and internal dynamic processes on the freshening trend of theintermediate water in the NSCS. The analysis revealed that the freshening in the northwest Pacific Ocean and theintensification of intrusion through the Luzon Strait at intermediate levels are the primary drivers of the salinity changes inthe NSCS. Additionally, a weakened trend in the intensity of vertical entrainment also contributes to the freshening in theNSCS. This study offers new insights into the understanding of regional deep sea changes in response to variations in boththermodynamics and oceanic dynamic processes. 相似文献
48.
视电阻率定义在电法勘探中得到广泛的应用。目前在频率域电磁法中常常采用远区视电阻率定义。由于在频率域中收发距并非足够大,以致在低频段远区定义视电阻率往往不能反映地下电阻率值。我们定义全区电阻率及引入校正系数K,经二层三层模型及野外实测资料的计算,证明了方法的有效性。 相似文献
49.
地震瞬时属性分析技术在岩溶勘查中的应用 总被引:3,自引:1,他引:2
介绍地震瞬时属性分析技术的原理及处理方法,分别对岩溶在地震剖面、瞬时振幅剖面、瞬时频率剖面、瞬时相位剖面上的表现特征作了说明.并结合实际资料进行多参数解释,进而确定地下岩溶的发育情况、分布范围和规模,同时对异常进行钻井验证,结果表明,地震瞬时属性分析技术比常规地震处理方法能更好地排除干扰,确定岩溶异常. 相似文献
50.