全文获取类型
收费全文 | 40805篇 |
免费 | 7384篇 |
国内免费 | 8892篇 |
专业分类
测绘学 | 2621篇 |
大气科学 | 4260篇 |
地球物理 | 9343篇 |
地质学 | 23338篇 |
海洋学 | 7039篇 |
天文学 | 323篇 |
综合类 | 2843篇 |
自然地理 | 7314篇 |
出版年
2024年 | 261篇 |
2023年 | 730篇 |
2022年 | 1340篇 |
2021年 | 1740篇 |
2020年 | 1697篇 |
2019年 | 1879篇 |
2018年 | 1633篇 |
2017年 | 1773篇 |
2016年 | 1830篇 |
2015年 | 1953篇 |
2014年 | 2421篇 |
2013年 | 2590篇 |
2012年 | 2481篇 |
2011年 | 2547篇 |
2010年 | 2219篇 |
2009年 | 2551篇 |
2008年 | 2486篇 |
2007年 | 2753篇 |
2006年 | 2727篇 |
2005年 | 2406篇 |
2004年 | 2230篇 |
2003年 | 2015篇 |
2002年 | 1838篇 |
2001年 | 1532篇 |
2000年 | 1364篇 |
1999年 | 1218篇 |
1998年 | 1076篇 |
1997年 | 982篇 |
1996年 | 862篇 |
1995年 | 780篇 |
1994年 | 679篇 |
1993年 | 574篇 |
1992年 | 472篇 |
1991年 | 360篇 |
1990年 | 249篇 |
1989年 | 288篇 |
1988年 | 158篇 |
1987年 | 119篇 |
1986年 | 62篇 |
1985年 | 54篇 |
1984年 | 39篇 |
1983年 | 24篇 |
1982年 | 8篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 16篇 |
1978年 | 15篇 |
1977年 | 6篇 |
1971年 | 3篇 |
1954年 | 11篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
Historical data of total dissolved inorganic carbon (CT), together with nitrate and phosphate, have been used to model the evolution of these constituents over the year in the Atlantic water of the Norwegian Sea. Changes in nutrient concentration in the upper layer of the ocean are largely related to biological activity, but vertical mixing with the underlying water will also have an impact. A mixing factor is estimated and used to compute the entrainment of these constituents into the surface water from below. After taking the mixing contribution into account, the resulting nutrient concentration changes are attributed to biological production or decay. The results of the model show that the change in CT by vertical mixing and by biological activity based on nutrient equivalents needs another sink to balance the carbon budget. It cannot be the atmosphere as the surface water is undersaturated with respect to carbon dioxide and is, thus, a source of CT in this region. Inasmuch as the peak deficit of carbon is more than a month later than for the nutrients, the most plausible explanation is that other nitrogen and phosphate sources than the inorganic salts are used together with dissolved inorganic carbon during this period. As nitrate and phosphate show a similar trend, it is unlikely that the explanation is the use of ammonia or nitrogen fixation but rather dissolved organic nitrogen and phosphate, while dissolved organic carbon is accumulating in the water. 相似文献
42.
南海南部约30 ka来沉积有机质的生物输入特征 总被引:7,自引:4,他引:7
对位于南沙海区的1962柱状样中的有机质进行了热解色谱分析,估算了沉积有机质中水生生物输入和陆源生物输入的变化情况,得出了两种输入的高分辨率的堆积速率曲线,并依此探讨了有关的古海洋事件。发现Younger Dryas、Heinrich及Bond周期事件在本海区皆有表现,说明“西太平洋暖池”在末次冰期是不稳定的。 相似文献
43.
44.
董胜 《中国海洋大学学报(自然科学版)》1997,(2)
针对海上矿石运输,介绍了一种完成定量任务并能使投资费用最少为目标的最优船队组建方法,同时对易变参数进行了灵敏度分析,使决策更加科学化 相似文献
45.
A preliminary optical classification of lakes in Estonia and south Finland which can also be used for small bays of the Baltic Sea is elaborated. The classification is based on the optical properties of water (diffuse attenuation coefficient, diffuse reflectance) and parameters that are routinely monitored in water bodies (Secchi depth, concentration of chlorophyll-a, total suspended matter and yellow substance). The data complex used for our classification covers different types of water ecosystems (ranging from oligotrophic to hypertrophic) and the variability of water constituent concentrations in the ice-free period in Estonia and south Finland. Using cluster analysis, we found 5 optical classes of waters: clear (C), moderate (M), turbid (T), very turbid (V) and brown (B). There is satisfactory correspondence between class of water, shape of diffuse attenuation coefficient and diffuse reflectance spectra and trophic state of the lakes. 相似文献
46.
47.
本文根据1960~1994年千里岩和青岛月平均气温资料,分析了千里岩与青岛35年来气温的年、季变化特征及差别,发现35年来,千里岩和青岛的年、季平均气温均呈变暖趋势,但千里岩增温幅度较青岛小。从分析中发现,与气温变暖趋势相对应,西北太平洋副热带高压(以下简称副高)面积、强度指数的年、季变化也呈明显的增强、增大趋势。且年平均气温与冬季副高面积指数相关最显著。最后依此关系建立回归方程,并预测了1996年千里岩年平均气温的变化趋势。 相似文献
48.
二次特征值问题 (QEP)的主要的求解方法之一是转化为广义特征值问题 (GEP) ,然后用求解广义特征值的方法 (比如 QZ方法 )求解。本文研究由此获得的计算解的范数意义下的最佳向后扰动分析 ,所得结果是 Tisseur最近所得结果的加强。 相似文献
49.
Wave-induced seabed instability, either momentary liquefaction or shear failure, is an important topic in ocean and coastal engineering. Many factors, such as seabed properties and wave parameters, affect the seabed instability. A non-dimensional parameter is proposed in this paper to evaluate the occurrence of momentary liquefaction. This parameter includes the properties of the soil and the wave. The determination of the wave-induced liquefaction depth is also suggested based on this non-dimensional parameter. As an example, a two-dimensional seabed with finite thickness is numerically treated with the EFGM meshless method developed early for wave-induced seabed responses. Parametric study is carried out to investigate the effect of wavelength, compressibility of pore fluid, permeability and stiffness of porous media, and variable stiffness with depth on the seabed response with three criteria for liquefaction. It is found that this non-dimensional parameter is a good index for identifying the momentary liquefaction qualitatively, and the criterion of liquefaction with seepage force can be used to predict the deepest liquefaction depth. 相似文献
50.
AbstractWith the large-scale development and utilization of ocean resources and space, it is inevitable to encounter existing submarine facilities in pile driving areas, which necessitates a safety assessment. In this article, by referring to a wharf renovation project as a reference, the surrounding soil response and buried pipe deformation during pile driving in a near-shore submarine environment are investigated by three-dimensional (3D) numerical models that consider the pore water effect. Numerical studies are carried out in two different series: one is a case of a single pile focusing on the effect of the minimum plane distance of the pile–pipe, and the other is a case of double piles focusing on the effect of the pile spacing. 相似文献