全文获取类型
收费全文 | 14212篇 |
免费 | 2687篇 |
国内免费 | 3652篇 |
专业分类
测绘学 | 1188篇 |
大气科学 | 2783篇 |
地球物理 | 3637篇 |
地质学 | 7388篇 |
海洋学 | 1734篇 |
天文学 | 658篇 |
综合类 | 1394篇 |
自然地理 | 1769篇 |
出版年
2024年 | 68篇 |
2023年 | 237篇 |
2022年 | 704篇 |
2021年 | 820篇 |
2020年 | 692篇 |
2019年 | 853篇 |
2018年 | 870篇 |
2017年 | 823篇 |
2016年 | 908篇 |
2015年 | 799篇 |
2014年 | 963篇 |
2013年 | 928篇 |
2012年 | 894篇 |
2011年 | 930篇 |
2010年 | 882篇 |
2009年 | 801篇 |
2008年 | 736篇 |
2007年 | 802篇 |
2006年 | 603篇 |
2005年 | 486篇 |
2004年 | 395篇 |
2003年 | 380篇 |
2002年 | 422篇 |
2001年 | 365篇 |
2000年 | 407篇 |
1999年 | 510篇 |
1998年 | 454篇 |
1997年 | 419篇 |
1996年 | 353篇 |
1995年 | 326篇 |
1994年 | 360篇 |
1993年 | 267篇 |
1992年 | 212篇 |
1991年 | 164篇 |
1990年 | 132篇 |
1989年 | 104篇 |
1988年 | 121篇 |
1987年 | 66篇 |
1986年 | 55篇 |
1985年 | 52篇 |
1984年 | 38篇 |
1983年 | 38篇 |
1982年 | 40篇 |
1981年 | 28篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 8篇 |
1977年 | 4篇 |
1975年 | 3篇 |
1958年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 281 毫秒
51.
INTRODUCTIONTheOkinawaTrough (OT) ,locatednorthwestoftheRyukyuTrenchandtheRyukyugunto ,eastoftheEastChinaSeashelf,isaback arcbasinbulgedtotheRyukyuTrench .TheOTextendsfromKyushuinthesouthwestofJapantotheIlanPlaininthenortheasternTaiwan ,Chinainthesouthwest… 相似文献
52.
53.
54.
Jennifer L. Mercer Meixun Zhao Steven M. Colman 《Estuarine, Coastal and Shelf Science》2005,63(4):675-682
Alkenone unsaturation indices (UK37 and UK′37) have long been used as proxies for surface water temperature in the open ocean. Recent studies have suggested that in other marine environments, variables other than temperature may affect both the production of alkenones and the values of the indices. Here, we present the results of a reconnaissance field study in which alkenones were extracted from particulate matter filtered from the water column in Chesapeake Bay during 2000 and 2001. A multivariate analysis shows a strong positive correlation between UK37 (and UK′37) values and temperature, and a significant negative correlation between UK37 (and UK′37) values and nitrate concentrations. However, temperature and nitrate concentrations also co-vary significantly. The temperature vs. UK37 relationships (UK37=0.018 (T)−0.162, R2=0.84, UK′37=0.013 (T)−0.04, R2=0.80) have lower slopes than the open-ocean equations of Prahl et al. [1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta 52, 2303–2310] and Müller et al. [1998. Calibration of the alkenone paleotemperature index UK′37 based on core-tops from the eastern South Atlantic and the global ocean (60°N–60°S). Geochimica et Cosmochimica Acta 62, 1757–1772], but are similar to the relationships found in controlled studies with elevated nutrient levels and higher nitrate:phosphate (N:P) ratios. This implies that high nutrient levels in Chesapeake Bay have either lowered the UK37 vs. temperature slope, or nutrient levels are the main controller of the UK37 index. In addition, particularly high abundances (>5% of total C37 alkenones) of the tetra-unsaturated ketone, C37:4, were found when water temperatures reached 25 °C or higher, thus posing further questions about the controls on alkenone production as well as the biochemical roles of alkenones. 相似文献
55.
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
56.
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
57.
Full-rangenonlinearanalysisoffatiguebehaviorsofreinforcedconcretestructuresbyfiniteelementmethod¥SongYupu;ZhaoShunbo;WangRuim... 相似文献
58.
59.
60.