全文获取类型
收费全文 | 6800篇 |
免费 | 1339篇 |
国内免费 | 1044篇 |
专业分类
测绘学 | 1408篇 |
大气科学 | 456篇 |
地球物理 | 1605篇 |
地质学 | 3578篇 |
海洋学 | 776篇 |
天文学 | 63篇 |
综合类 | 526篇 |
自然地理 | 771篇 |
出版年
2024年 | 19篇 |
2023年 | 92篇 |
2022年 | 201篇 |
2021年 | 268篇 |
2020年 | 280篇 |
2019年 | 331篇 |
2018年 | 236篇 |
2017年 | 374篇 |
2016年 | 342篇 |
2015年 | 352篇 |
2014年 | 491篇 |
2013年 | 505篇 |
2012年 | 484篇 |
2011年 | 530篇 |
2010年 | 391篇 |
2009年 | 511篇 |
2008年 | 510篇 |
2007年 | 455篇 |
2006年 | 450篇 |
2005年 | 371篇 |
2004年 | 349篇 |
2003年 | 310篇 |
2002年 | 255篇 |
2001年 | 197篇 |
2000年 | 208篇 |
1999年 | 135篇 |
1998年 | 120篇 |
1997年 | 85篇 |
1996年 | 65篇 |
1995年 | 61篇 |
1994年 | 43篇 |
1993年 | 30篇 |
1992年 | 29篇 |
1991年 | 14篇 |
1990年 | 18篇 |
1989年 | 12篇 |
1988年 | 10篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 7篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 5篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有9183条查询结果,搜索用时 31 毫秒
41.
42.
Inverse analysis is increasingly used in ecosystem modelling to objectively reconstruct a large number of unknown flows or interactions from a small number of observations. This type of analysis may be useful in relating observed regime shifts in ecosystem structure to underlying processes. Inversions of ecosystem flow networks currently use a constrained least-squares solution which at the same time minimizes the squared norm (the sum of squares) of the reconstructed flows. This minimum norm (MN) inversion is thought to be a parsimonious solution to the ecosystem flow inverse problem, but it may well not reflect how ecosystems are organised. It has been proposed instead that ecosystems evolve to maximize energy/mass flows or that they maximize the information content of the network weighted by ecosystem flows (ascendancy). We used simulated inverse experiments, where inverse analyses are applied to simulations of flow networks, to explore objective functions different than the MN generally used. We could not compute inverse solutions that maximize ascendancy because the objective function is unbounded. We could calculate inversions that maximize flows; however, these generally overestimated the simulated flows, even though the simulations were designed to maximize flows. It appears that the ecosystem flow inverse problem is too under-determined (too few data relative to the number of unknowns) to allow the use of these maximizing goal functions. We introduce a new minimization that simultaneously minimizes the squared flows and the squared differences between flows. This smoothing minimization makes the inverse flows as even as possible and it helps with some technical issues with MN inversions. The simulated inverse experiments indicated that this smoothed norm (SM) is the most robust in comparative analyses of contrasting ecosystem states, such as those that can be associated with regime shifts. Like the MN inversion, the SM inversion has no ecological basis. However, it is a conservative norm that is less likely to produce false differences between the dynamics of regimes. 相似文献
43.
44.
45.
46.
This paper outlines the structure of the institutions involved in scientific and technological development in France. It points to new trends in French marine policy, and gives details of the country's offshore petroleum technology and nodule mining. 相似文献
47.
A method is introduced to calculate and to account for the uncertainties in the predictions of oil spill trajectories using a classic oil spill model. The method considers the output of the oil spill model as a function of random variables, which are the input parameters, and calculates the standard deviation of the output results which gives a measure of the uncertainty of the model given the uncertainties of the input parameters.Instead of a single trajectory that is calculated by the oil spill model using the mean values of the parameters, a band of trajectories can be defined when various simulations are done taking into account the uncertainties of the input parameters. This band of trajectories defines envelopes of the trajectories that are likely to be followed by the spill given the uncertainties of the input.The method is applied to an oil spill that occurred in open sea near Madeira Islands, in the Atlantic Ocean, in December 1989. The simulations allow the understanding of how a change in the wind direction drove the spill towards the Islands.The envelope of likely trajectories that is obtained with the uncertainty modelling shows a band of trajectories that is in better agreement with the observations than the single trajectory simulated by the oil spill model, based on mean parameters. 相似文献
48.
49.
关于我国金枪鱼围网渔业发展问题的几点探讨 总被引:7,自引:1,他引:7
本文通过对联合国粮农组织公布的1989-1998年金枪鱼渔获量的分析,清晰了我国发展金枪鱼围网渔业的趋势,并着重对于围网生产密切相关的渔场选择、捕捞技术进行了几点探讨,还对开发科研工作谈了个人看法。 相似文献
50.