全文获取类型
收费全文 | 17379篇 |
免费 | 3509篇 |
国内免费 | 5889篇 |
专业分类
测绘学 | 1206篇 |
大气科学 | 4415篇 |
地球物理 | 4655篇 |
地质学 | 7127篇 |
海洋学 | 5289篇 |
天文学 | 633篇 |
综合类 | 1109篇 |
自然地理 | 2343篇 |
出版年
2024年 | 132篇 |
2023年 | 374篇 |
2022年 | 658篇 |
2021年 | 810篇 |
2020年 | 855篇 |
2019年 | 1073篇 |
2018年 | 862篇 |
2017年 | 899篇 |
2016年 | 940篇 |
2015年 | 981篇 |
2014年 | 1127篇 |
2013年 | 1363篇 |
2012年 | 1141篇 |
2011年 | 1183篇 |
2010年 | 1006篇 |
2009年 | 1280篇 |
2008年 | 1144篇 |
2007年 | 1305篇 |
2006年 | 1196篇 |
2005年 | 1003篇 |
2004年 | 944篇 |
2003年 | 818篇 |
2002年 | 754篇 |
2001年 | 607篇 |
2000年 | 606篇 |
1999年 | 558篇 |
1998年 | 479篇 |
1997年 | 443篇 |
1996年 | 430篇 |
1995年 | 322篇 |
1994年 | 305篇 |
1993年 | 281篇 |
1992年 | 228篇 |
1991年 | 151篇 |
1990年 | 133篇 |
1989年 | 86篇 |
1988年 | 69篇 |
1987年 | 58篇 |
1986年 | 23篇 |
1985年 | 36篇 |
1984年 | 20篇 |
1983年 | 17篇 |
1982年 | 16篇 |
1981年 | 14篇 |
1980年 | 3篇 |
1979年 | 16篇 |
1978年 | 3篇 |
1977年 | 11篇 |
1976年 | 2篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
51.
52.
通过分析1980年以后的海冰资料和卫星云图照片,对黄、渤海沿岸11个海冰测点的历史变迁进行了评述,浅析了测点的优劣及使用资料时应注意的事项。 相似文献
53.
锶诱导的氧化胁迫对叉鞭金藻(Dicrateria inornata)的影响 总被引:3,自引:2,他引:3
采用不同浓度的锶胁迫方法研究了叉鞭金藻生长、叶绿素、MDA含量及三种抗氧化酶SOD、CAT、GPX活性的变化。结果表明,在实验设计的各种Sr^2 浓度内叉鞭金藻均能生长,但Sr^2 浓度较高时生长受到不同程度的抑制,细胞数量比对照组分别减少15.9%和51.0%;叉鞭金藻中叶绿素含量随着Sr^2 浓度的升高而降低,呈负相关关系;叉鞭金藻中MDA含量在较低Sr^2 浓度下稍呈下降趋势,但与对照组相比并无显著差异,而在23.04mmol/L Sr^2 浓度时是对照的1.25倍;低Sr^2 浓度时,叉鞭金藻中蛋白质含量没有大的变化,当浓度超过5.76mmol/L时则引起蛋白质含量下降;叉鞭金藻中抗氧化酶SOD在低Sr^2 时分别降低12.5%、7.5%和9.5%,高Sr^2 时显著升高,为对照组的85.7%;CAT、GPX活性在低Sr^2 浓度时差别不大,但当浓度为23.04mmol/L时升高显著,比对照组分别增加了19.8%和74.4%。 相似文献
54.
The North Pacific has the world's largest groundfish fisheries, with a value in excess of $1 billion annually. The North Pacific also is home to the largest population of Steller sea lions (SSL), an ancient species that is now endangered. Decline of the western stock of SSL in the 1970–1990s coincided with the US nationalization and exponential growth in the Gulf of Alaska and Bering Sea/Aleutian Islands groundfish fishery.The article treats five aspects of the competition between the fishery and protection of the endangered species. First, we analyze competition of interests in the use, management, and preservation of the North Pacific commons, including offshore and near-shore fisheries, environmental organizations, governments (federal, state, and local), and the scientific community. Second, the article compares and contrasts provisions of the National Environmental Policy Act, the Marine Mammal Protection Act, the Endangered Species Act, and the Magnuson–Stevens Act (as amended in 1996 by the Sustainable Fisheries Act).Third, we examine the regulatory environment with a focus on the National Marine Fisheries Service (NMFS) and its relationship to the North Pacific Fishery Management Council. We illustrate how NMFS establishes fishery management plans and the methods, such as crafting biological opinions and environmental-impact-statements, it uses to resolve conflicts. Fourth, we investigate how the dual pressures on the agency—to increase fisheries production for the benefit of the industry and to preserve endangered marine species—have affected its culture (behavioral norms and operational codes). Then, the article explains agency changes in terms of crises, competition among affected interests, and development of new scientific knowledge.The article concludes by drawing lessons from the SSL controversy in the North Pacific which may pertain to national and global biodiversity conflicts. These concern the role of law in biodiversity protection, the role of science, and agency capacity for change. 相似文献
55.
56.
提出了海上浮动软管的三维静态分析模型。在四级海情下,对输送柴油的软管求得了解析解,得出了软管在侧向和垂直方向的偏移及其所受的拉力,认为此时的软管是不安全的。 相似文献
57.
Modelling of flow around a near-bed pipeline with a spoiler 总被引:1,自引:0,他引:1
Flow around a pipeline with and without a spoiler near a smooth wall is simulated by solving the Navier–Stokes equations. Finite-difference formulation with a second-order upwind scheme in a curvilinear coordinate system is employed. The influences of the spoiler on hydrodynamic forces, pressure distribution, vortex shedding frequency, velocity profile under the pipe, as well as shear stress on the wall are investigated. The attachment of a spoiler significantly increases drag, root-mean-square (RMS) lift, flow through the gap between the pipe and the wall and shear stress on the seabed around the pipe. The spoiler also generates a non-zero mean downward force on the pipeline, which may enhance the self-burial of the pipeline. 相似文献
58.
We have examined wind-induced circulation in the Sea of Okhotsk using a barotropic model that contains realistic topography
with a resolution of 9.25 km. The monthly wind stress field calculated from daily European Centre for Medium-Range Weather
Forecasting (ECMWF) Re-Analysis data is used as the forcing, and the integration is carried out for 20 days until the circulation
attains an almost steady state. In the case of November (a representative for the winter season from October to March), southward
currents of velocity 0.1–0.3 m s−1 occur along the bottom contours off the east of Sakhalin Island. The currents are mostly confined to the shelf (shallower
than 200 m) and extend as far south as the Hokkaido coast. In the July case (a representative for the summer season from April
to September), significant currents do not occur, even in the shallow shelves. The simulated southward current over the east
Sakhalin shelf appears to correspond to the near-shore branch of the East Sakhalin Current (ESC), which was observed with
the surface drifters. These seasonal variations simulated in our experiments are consistent with the observations of the ESC.
Dynamically, the simulated ESC is interpreted as the arrested topographic wave (ATW), which is the coastally trapped flow
driven by steady alongshore wind stress. The volume transport of the simulated ESC over the shelf reaches about 1.0 Sv (1
Sv = 106 m3s−1) in the winter season, which is determined by the integrated onshore Ekman transport in the direction from which shelf waves
propagate.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
59.
60.
In order to accurately design a sand compaction pile (SCP) with low replacement area ratio, it is important to understand the mechanical interaction between the sand pile and clay ground and its mechanism during consolidation process in composite ground. In this article, therefore, a series of numerical analyses on composite ground improved by SCP with low replacement area ratio were carried out. The applicability of numerical analyses, in which an elasto-viscoplastic consolidation finite element method was applied, were confirmed by comparing the results obtained from a series of laboratory model tests with the composite ground improved by SCP. Through the results of the numerical analyses, mechanical behavior of the sand pile and clay in composite ground during consolidation is elucidated, together with a stress sharing mechanism between sand pile and clay. 相似文献