全文获取类型
收费全文 | 10609篇 |
免费 | 2115篇 |
国内免费 | 2822篇 |
专业分类
测绘学 | 825篇 |
大气科学 | 2104篇 |
地球物理 | 2518篇 |
地质学 | 5797篇 |
海洋学 | 1200篇 |
天文学 | 574篇 |
综合类 | 1129篇 |
自然地理 | 1399篇 |
出版年
2024年 | 55篇 |
2023年 | 194篇 |
2022年 | 487篇 |
2021年 | 601篇 |
2020年 | 488篇 |
2019年 | 555篇 |
2018年 | 652篇 |
2017年 | 590篇 |
2016年 | 627篇 |
2015年 | 589篇 |
2014年 | 616篇 |
2013年 | 662篇 |
2012年 | 568篇 |
2011年 | 620篇 |
2010年 | 673篇 |
2009年 | 619篇 |
2008年 | 618篇 |
2007年 | 602篇 |
2006年 | 425篇 |
2005年 | 438篇 |
2004年 | 371篇 |
2003年 | 337篇 |
2002年 | 341篇 |
2001年 | 388篇 |
2000年 | 351篇 |
1999年 | 422篇 |
1998年 | 376篇 |
1997年 | 382篇 |
1996年 | 314篇 |
1995年 | 256篇 |
1994年 | 222篇 |
1993年 | 215篇 |
1992年 | 191篇 |
1991年 | 122篇 |
1990年 | 105篇 |
1989年 | 99篇 |
1988年 | 98篇 |
1987年 | 45篇 |
1986年 | 50篇 |
1985年 | 31篇 |
1984年 | 26篇 |
1983年 | 19篇 |
1982年 | 16篇 |
1981年 | 16篇 |
1980年 | 10篇 |
1979年 | 8篇 |
1978年 | 8篇 |
1977年 | 8篇 |
1975年 | 6篇 |
1958年 | 16篇 |
排序方式: 共有10000条查询结果,搜索用时 423 毫秒
61.
62.
63.
An offshore vessel with a dynamic positioning system (DP system) needs fast response to produce thrust to counteract the environmental forces acting on it for the purpose of maintaining its position and heading as close as possible to the working position. Therefore, quick and effective modulation of the thrust is the problem to determine the thrust and the rotation angle of the thruster devices of the ship. This paper presents an effective optimum control for a thruster system, using the sequential quadratic method to achieve economical and effective modulation of the thrust and the direction of the thruster. An optimum control study of a 2280 tons DP coring vessel with five rotary azimuth thruster marine positioning is studied in detail, which can quickly and exactly estimate the thrusts and angles of direction of all the thrusters. The results can provide a valuable thruster system for a dynamically positioned vessel. 相似文献
64.
A new set of equations of motion for wave propagation in water with varying depth is derived in this study. The equations expressed by the velocity potentials and the wave surface elevations include first-order non-linearity of waves and have the same dispersion characteristic to the extended Boussinesq equations. Compared to the extended Boussinesq equations, the equations have only two unknown scalars and do not contain spatial derivatives with an order higher than 2. The wave equations are solved by a finite element method. Fourth-order predictor–corrector method is applied in the time integration and a damping layer is applied at the open boundary for absorbing the outgoing waves. The model is applied to several examples of wave propagation in variable water depth. The computational results are compared with experimental data and other numerical results available in literature. The comparison demonstrates that the new form of the equations is capable of calculating wave transformation from relative deep water to shallow water. 相似文献
65.
河口环流和盐水入侵Ⅱ--径流量和海平面上升的影响 总被引:1,自引:0,他引:1
本文利用本序列上篇论文建立的理想河口数值模式,研究径流量和海平面上升对河口环流和盐水入侵的影响.在径流量增大的情况下,口门内表层向海的流速增大,底层向陆的密度流减弱,滞流点下移.口门外侧向口门的密度流增大,上升流趋于增强.口门内盐水入侵减弱,口外盐度减小、冲淡水扩展范围增大.在口门上游北岸底层盐度下降明显,口门处南岸表层盐度下降明显.径流量变化对盐水入侵影响十分巨大.在海平面上升的情况下,拦门沙区域向陆的密度流增强,滞流点上移,表层向海的流动增大.口门内盐水入侵增强,口外盐度增大,冲淡水扩展范围减小.海平面上升对盐水入侵影响十分明显,北岸底层盐度增大尤为特出. 相似文献
66.
This article describes absolute calibration results for both JASON-1 and TOPEX Side B (TSB) altimeters obtained at the Lake Erie calibration site, Marblehead, Ohio, USA. Using 15 overflights, the estimated JASON altimeter bias at Marblehead is 58 ± 38 mm, with an uncertainty of 19 mm based on detailed error analysis. Assuming that the TSB bias is negligible, relative bias estimates using both data from the TSB-JASON formation flight period and data from 48 water level gauges around the entire Great Lakes confirmed the Marblehead results. Global analyses using both the formation flight data and dual-satellite (TSB and JASON) crossovers yield a similar relative bias estimate of 146 ± 59 mm, which agrees well with open ocean absolute calibration results obtained at Harvest, Corsica, and Bass Strait (e.g., Watson et al. 2003). We find that there is a strong dependence of bias estimates on the choice of sea state bias (SSB) models. Results indicate that the invariant JASON instrument bias estimated oceanwide is 71 mm, with additional biases of 76 mm or 28 mm contributed by the choice of Collecte Localisation Satellites (CLS) SSB or Center for Space Research (CSR) SSB model, respectively. Similar analysis in the Great Lakes yields the invariant JASON instrument bias at 19 mm, with the SSB contributed biases at 58 mm or 13 mm, respectively. The reason for the discrepancy is currently unknown and warrants further investigation. Finally, comparison of the TOPEX/POSEIDON mission (1992-2002) data with the Great Lakes water level gauge measurements yields a negligible TOPEX altimeter drift of 0.1 mm/yr. 相似文献
67.
本文探讨西庄附近海域的波浪状况,分析由于人工挖沙影响,使得登州浅滩的防护作用发生变化,导致近岸波浪作用增强,造成海岸侵蚀.最后讨论设计波要素的确定,为防浪护坡工程提供必要的设计依据. 相似文献
68.
69.
于1992年4-10月在山东省青岛市黄岛前湾育苗场和胶南市红石崖镇邵家村养殖场,采用载玻片挂片和对中国对虾现场取样的方法,研究中国对虾育苗池和养成池载玻片挂 表面和对体表微型污着生物群落的组成和发展。 相似文献
70.
提出一种确定连续系统参数的时域方法。该方法利用方波输入和系统稳定性,离析出输出采样信号与结构参数的内在联系,推导出确定连续系统结构参数的算法.指出该算法与辨识方法相结合,可实现对含随机干扰的连续系统结构参数的估计. 相似文献