全文获取类型
收费全文 | 7891篇 |
免费 | 1119篇 |
国内免费 | 1266篇 |
专业分类
测绘学 | 218篇 |
大气科学 | 237篇 |
地球物理 | 819篇 |
地质学 | 4859篇 |
海洋学 | 325篇 |
天文学 | 2789篇 |
综合类 | 371篇 |
自然地理 | 658篇 |
出版年
2024年 | 41篇 |
2023年 | 125篇 |
2022年 | 209篇 |
2021年 | 220篇 |
2020年 | 190篇 |
2019年 | 325篇 |
2018年 | 185篇 |
2017年 | 204篇 |
2016年 | 291篇 |
2015年 | 265篇 |
2014年 | 445篇 |
2013年 | 381篇 |
2012年 | 379篇 |
2011年 | 449篇 |
2010年 | 394篇 |
2009年 | 644篇 |
2008年 | 572篇 |
2007年 | 666篇 |
2006年 | 615篇 |
2005年 | 512篇 |
2004年 | 459篇 |
2003年 | 453篇 |
2002年 | 353篇 |
2001年 | 333篇 |
2000年 | 266篇 |
1999年 | 268篇 |
1998年 | 262篇 |
1997年 | 143篇 |
1996年 | 124篇 |
1995年 | 102篇 |
1994年 | 88篇 |
1993年 | 72篇 |
1992年 | 64篇 |
1991年 | 38篇 |
1990年 | 30篇 |
1989年 | 22篇 |
1988年 | 17篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 7篇 |
1984年 | 8篇 |
1983年 | 1篇 |
1982年 | 6篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1977年 | 1篇 |
1971年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 531 毫秒
101.
102.
103.
104.
胶莱盆地发育演化及其油气前景探讨 总被引:5,自引:0,他引:5
根据胶莱盆地区域地质特征,地球动力学背景及盆地中、新生代沉积特征分析,论述盆地的发育和演化阶段,并对胶莱盆地含油气前景进行探讨。 相似文献
105.
波浪作用下海床的有效应力分析 总被引:4,自引:0,他引:4
波浪作用下海床的稳定性分析是海洋工程地质评价的重要内容。海床的稳定性可通过计算分析其随时间变化的有效应力场来评估。本文建议了一个周期荷载作用下土体的本构模型 ,并用于计算波浪作用下海床的应力与变形。采用Biot固结理论和有限单元法 ,分析了海床的动态应力场与孔隙水压力场。波浪作用下两种渗透系数时有效应力的动态变化过程结果对比 ,反映了渗透消散作用对海床有效应力变化的影响 相似文献
106.
This paper presents the results of a numerical performance analysis to demonstrate the worthiness of a recently patented new concept propulsor, the so-called “thrust-balanced propeller (TBP)”. The main advantage of this unconventional propulsor is its inherent ability to reduce the unsteady effect of blade forces and moments when it is operating in a non-uniform wake flow. The propulsor comprises a pair of diametrically opposed blades that are connected to one another and mounted so as to be rotatable together through a limited angle about their spindle axis. A quasi-hydrodynamic approach is described and applied to perform the numerical analysis using a state-of-the-art lifting surface procedure for conventional propellers. Performance comparisons with a conventional fixed-pitch propeller are made for the blade forces and moments, efficiency, cavitation extents and fluctuating hull pressures. Bearing in mind the quasi-static nature of the analyses, the results present favourable performance characteristics for the thrust-balanced propeller and support the worthiness of the concept. However, the concept needs to be proved through physical model tests, which are planned to take in a cavitation tunnel. 相似文献
107.
108.
哈维氏弧菌的生物学特性、流行病学及检测技术 总被引:4,自引:0,他引:4
哈维氏弧菌是1种革兰氏阴性、发光的海洋细菌,广泛分布于海洋环境中。它是最近10多年才被认识到的水产养殖动物的重要致病菌。文中综述哈维氏弧菌的生物学特性、流行病学、致病机制、密度感应系统、检测技术、疾病的防治及其应用。 相似文献
109.
An applied Fourier transform computation for the hydrodynamic wave-resistance coefficient is shown, oriented to potential flows with a free surface and infinity depth. The presence of a ship-like body is simulated by its equivalent pressure disturbance imposed on the un-perturbed free surface, where a linearized free surface condition is used. The wave-resistance coefficient is obtained from the wave-height downstream. Two examples with closed solutions are considered: a submerged dipole, as a test-case, and a parabolic pressure distribution of compact support. In the three dimensional case, a dispersion relation is included which is a key resource for an inexpensive computation of the wave pattern far downstream like fifteen ship-lengths. 相似文献
110.
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