全文获取类型
收费全文 | 6991篇 |
免费 | 1156篇 |
国内免费 | 1151篇 |
专业分类
测绘学 | 1821篇 |
大气科学 | 920篇 |
地球物理 | 2199篇 |
地质学 | 2203篇 |
海洋学 | 916篇 |
天文学 | 138篇 |
综合类 | 612篇 |
自然地理 | 489篇 |
出版年
2024年 | 22篇 |
2023年 | 76篇 |
2022年 | 228篇 |
2021年 | 318篇 |
2020年 | 375篇 |
2019年 | 322篇 |
2018年 | 288篇 |
2017年 | 374篇 |
2016年 | 376篇 |
2015年 | 415篇 |
2014年 | 457篇 |
2013年 | 520篇 |
2012年 | 497篇 |
2011年 | 493篇 |
2010年 | 400篇 |
2009年 | 406篇 |
2008年 | 440篇 |
2007年 | 438篇 |
2006年 | 412篇 |
2005年 | 365篇 |
2004年 | 293篇 |
2003年 | 255篇 |
2002年 | 241篇 |
2001年 | 194篇 |
2000年 | 164篇 |
1999年 | 132篇 |
1998年 | 147篇 |
1997年 | 116篇 |
1996年 | 115篇 |
1995年 | 71篇 |
1994年 | 84篇 |
1993年 | 83篇 |
1992年 | 58篇 |
1991年 | 26篇 |
1990年 | 19篇 |
1989年 | 18篇 |
1988年 | 14篇 |
1987年 | 16篇 |
1986年 | 7篇 |
1985年 | 8篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1972年 | 2篇 |
1954年 | 1篇 |
排序方式: 共有9298条查询结果,搜索用时 656 毫秒
41.
42.
43.
44.
Edward F. Roseman Christine A. Tomichek Tracy Maynard Jennifer A. Burton 《Journal of Sea Research》2005,53(4):309-318
Grubby (Myoxocephalus aenaeus, Cottidae) is a common benthic fish of inshore waters and estuaries of eastern Long Island Sound; however, little information exists on their life history or population demographics. This study utilised a long-term data series (1976–2002) to assess grubby life history and population demographics and explores trends in the Niantic River and Niantic Bay populations. In addition, we examined the age, size, and fecundity of adult grubby in 2001–02 to determine the population characteristics in the region. Mean grubby catch per unit effort (CPUE) in Niantic Bay ranged from 0.4 per trawl in 1976 to 2.9 per trawl in 1984 while river CPUE ranged from 0.4 per trawl in 1977 to 7.6 per trawl in 1989. Catch of grubby in bottom trawls varied seasonally with highest CPUE occurring in winter. Highest entrainment of grubby larvae occurred in 2001 while the lowest entrainment observed was in 1991. Four age classes, 0+ through III+, were derived from otolith analysis (N = 51) although length frequency analysis suggested the possibility of older fish in the population. The total number of eggs in ovaries ranged from 286 to 16 451 for grubby (N = 64) between 52 mm and 155 mm TL. Results of this study indicated a decline in abundance of adult grubby over the 26-year period, possibly related to concurrent declines in eelgrass (Zostera marina) abundance and/or increased water temperature. 相似文献
45.
结合近年来参与有关水下工程检测和项目监理的实践体验 ,从总结水下工程项目的基本特征着手 ,分析了水下工程项目监理的主要任务 ,并结合对于实施水下工程项目监理必备装备条件的探讨 ,介绍了国内外现有水下监测技术装备的开发应用状况 相似文献
46.
47.
The effect of the cone spacing of a conieal structure on the ice force is studied by model experiments. The ice force reduction coefficient presented in this paper expresses the relationship between the ice force and the arrangement of cones. The experiments prove that the mode of the ice failure before the boundary of upward-downward bending cone (UDBC) is crushing. A conclusion can also be drawn from the experiments that the ice force on the boundary of UDBC is by far less than that on a vertical pile with the same diameter. Moreover, the ice force frequencies on upright and inve-rted cones are obtained under the field condition of the platform JZ20-2, respectively. The results show that the alterna-tion of the ice force on UDBC can hardly induce resonance of platform JZ20-2. 相似文献
48.
On the basis of maps of sea level anomalies data set from October 1992 to January 2004, pronounced low frequency variations with periods of about 500 d are detected in the area near 20°N from 160°W to 130°E. A linear two-layer model is employed to explain the mechanism. It is found that the first-mode long baroclinic Rossby waves at 20°N in the northwest Pacific propagate westward in the form of free waves at a speed of about 10.3 cm/s. This confirms that the observed low frequency variabilities appear as baroclinic Rossby waves. It further shows that these low frequency variabilities around 20°N in the northwest Pacific can potentially be predicted with a lead up to 900 d. 相似文献
49.
Analysis of Dynamic Characteristics of Submarine Free Spanning Pipelines by Complex Damping Method 总被引:2,自引:0,他引:2
Considering the effect of the internal flowing fluid and the external marine environmental condition, the differential equation for the vortex-induced vibration (V1V) of the free spanning pipeline is derived and is discretized by the Hermit interpolation function. The free vibration equation with the damping term is solved by the complex damping method for the natural frequency, and then the effect of fluid damping on the natural frequency of the free spanning pipeline is analyzed.The results show that fluid damping has a significant influence on the damped natural frequency of the free spanning pipeline in the lock-in state, while it has little influence when the pipeline is out of the lock-in state. In the meantime,the change of the free span length has the same effect on the damped natural frequency and the undamped natural frequency. 相似文献
50.