全文获取类型
收费全文 | 6999篇 |
免费 | 1074篇 |
国内免费 | 1103篇 |
专业分类
测绘学 | 508篇 |
大气科学 | 1265篇 |
地球物理 | 2298篇 |
地质学 | 2473篇 |
海洋学 | 655篇 |
天文学 | 521篇 |
综合类 | 364篇 |
自然地理 | 1092篇 |
出版年
2024年 | 44篇 |
2023年 | 106篇 |
2022年 | 241篇 |
2021年 | 286篇 |
2020年 | 284篇 |
2019年 | 346篇 |
2018年 | 272篇 |
2017年 | 261篇 |
2016年 | 278篇 |
2015年 | 275篇 |
2014年 | 431篇 |
2013年 | 465篇 |
2012年 | 376篇 |
2011年 | 449篇 |
2010年 | 389篇 |
2009年 | 450篇 |
2008年 | 455篇 |
2007年 | 436篇 |
2006年 | 455篇 |
2005年 | 407篇 |
2004年 | 342篇 |
2003年 | 323篇 |
2002年 | 246篇 |
2001年 | 220篇 |
2000年 | 225篇 |
1999年 | 171篇 |
1998年 | 170篇 |
1997年 | 125篇 |
1996年 | 140篇 |
1995年 | 135篇 |
1994年 | 107篇 |
1993年 | 70篇 |
1992年 | 65篇 |
1991年 | 35篇 |
1990年 | 18篇 |
1989年 | 15篇 |
1988年 | 16篇 |
1987年 | 4篇 |
1986年 | 8篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 6篇 |
排序方式: 共有9176条查询结果,搜索用时 93 毫秒
41.
42.
采用金属潜在生态危害系数和危害指数的评估方法,评价了大亚湾海域重金属对海洋生态系统的潜在危害。结果表明,大亚湾海域重金属潜在生态危害轻微,而养殖海区和近岸海区的重金属潜在生态危害则相对较重,大亚湾海域重金属的潜在生态危害较珠江口外浅海轻。 相似文献
43.
44.
介绍了Ekman螺线的动力学结构 ,分析研究了风应力及海水内部的动量传递形式 .结果表明 :动量传递方向与海流方向成 45°左偏 .这对于研究海洋内部的动力结构是有意义的 . 相似文献
45.
工程风险分析中的风险当量及其评价标准 总被引:7,自引:2,他引:7
目前,量化、半量化风险分析是工程风险分析的主流。在对工程进行量化和半量化风险分析时,无论采用什么方法,风险评价指标和评价标准都是关键的重要问题。只有在建立了统一的评价指标和评价标准体系的基础上,对工程进行的风险分析能够得出正确和一致的结论。文章通过对国内外风险分析理论和实践的研究和总结,提出了在工程风险分析中确定风险评价指标和评价标准的原则和方法。 相似文献
46.
Cheng-Chien Liu Richard L. Miller Kendall L. Carder Zhongping Lee Eurico J. D’Sa James E. Ivey 《Journal of Oceanography》2006,62(3):235-248
We present a new approach that incorporates two models to estimate the underwater light field from remote sensing of ocean
color. The first employs a series of analytical, semi-analytical, and empirical algorithms to retrieve the spectrum of inherent
optical properties (IOPs), including the absorption and the backscatter coefficients, from the spectrum of remote sensing
reflectance. The second model computes the profile of photosynthetically available radiation E
0,PAR
(z) for a vertically homogeneous water column using the information of the retrieved IOPs and the ambient optical environment.
This computation is based on an improved look-up table technology that possesses high accuracy, comparable with the full solution
of the radiative transfer equation, and meets the computational requirement of remote sensing application. This new approach
was validated by in situ measurements and an extensive model-to-model comparison with a wide range of IOPs. We successfully mapped the compensation
depth by applying this new approach to process the SeaWiFS imagery. This research suggests that E
0,PAR
(z) can be obtained routinely from ocean-color data and may have significant implications for the estimation of global heat
and carbon budget. 相似文献
47.
Shigenao Maruyama Koutaro Tsubaki Keisuke Taira Seigo Sakai 《Journal of Oceanography》2004,60(3):563-568
Deep seawater in the ocean contains a great deal of nutrients. Stommel et al. have proposed the notion of a “perpetual salt fountain” (Stommel et al., 1956). They noted the possibility of a permanent upwelling of deep seawater with no additional external energy source. If
we can cause deep seawater to upwell extensively, we can achieve an ocean farm. We have succeeded in measuring the upwelling
velocity by an experiment in the Mariana Trench area using a special measurement system. A 0.3 m diameter, 280 m long soft
pipe made of PVC sheet was used in the experiment. The measured data, a verification experiment, and numerical simulation
results, gave an estimate of upwelling velocity of 212 m/day.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
48.
Abstract. A component analysis of the sugar and lipid fraction in foam layers around kelp beds on the South African west coast showed the following trends: In the residue of foam drainage of both 12 h and 120 h old foam the sugar fraction is represented by mannose (33–41 %), galactose (4–30%), fucose (3–29%) and glucose (19–26%). With the exception of fucose, the same components could be found in foam filtration. In the lipid fraction the main fatty acids are palmitic (23.3–27.3%), oleic (14.0–5.5%) and timnodonic acid (11.2–9.1%). Selacholeinic acid was only found in 12 h old foam while lignoceric acid could not be located in foam filtration. Against expectation mannitol was not identified in quantifiable amounts in foam samples although it is the dominant component of kelp mucilage, whose presence is of major importance for foam formation. Hypotheses explaining the lack of mannitol are forwarded. Component sugar and lipid content for 11 of foam was calculated and estimation of sugar and lipid content in an average foam patch (volume of 30001) is given. The importance of foam formation as a mechanism for binding and transfer of energy in the marine environment is emphasized. 相似文献
49.
50.
研究协方差阵Σ的二次型容许估计问题。设 y1,y2 ,… ,yniid,n≥ 2 ,y1与 p维正态分布N (β,Σ )有相同的前四阶矩。其中β =(β1,β2 ,… ,βp)′∈ Rp与Σ =(σij) p× p >0均未知。记 y =△ (y1,y2 ,… ,yn)′。在二次损失 L (d ,Σ ) =tr(d -Σ) 2下给出Σ的二次型估计 a S2 + nby-y-′是容许估计的必要条件为 :(n - 1) a + b + 2 max(a,b)≤ 1。此必要条件比张立振等协方差阵的二次型容许估计中的必要条件有了明显的加强 相似文献