全文获取类型
收费全文 | 303篇 |
免费 | 51篇 |
国内免费 | 66篇 |
专业分类
测绘学 | 41篇 |
大气科学 | 26篇 |
地球物理 | 124篇 |
地质学 | 118篇 |
海洋学 | 54篇 |
天文学 | 19篇 |
综合类 | 23篇 |
自然地理 | 15篇 |
出版年
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 8篇 |
2020年 | 14篇 |
2019年 | 9篇 |
2018年 | 10篇 |
2017年 | 16篇 |
2016年 | 10篇 |
2015年 | 23篇 |
2014年 | 15篇 |
2013年 | 13篇 |
2012年 | 21篇 |
2011年 | 22篇 |
2010年 | 15篇 |
2009年 | 36篇 |
2008年 | 20篇 |
2007年 | 14篇 |
2006年 | 20篇 |
2005年 | 20篇 |
2004年 | 17篇 |
2003年 | 9篇 |
2002年 | 13篇 |
2001年 | 12篇 |
2000年 | 12篇 |
1999年 | 13篇 |
1998年 | 8篇 |
1997年 | 9篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 7篇 |
1993年 | 1篇 |
1992年 | 2篇 |
1989年 | 2篇 |
1986年 | 1篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有420条查询结果,搜索用时 37 毫秒
91.
92.
针对传统的边缘检测方法将灰度变化较为平缓的区域误判为边缘的问题,提出了基于二阶局部纹理熵的边缘检测方法。利用了二阶局部纹理熵能表示局部区域灰度变化大小的性质,并采用灰度错位压缩法消除灰度值压缩造成的条纹现象。实验表明,本文提出的方法能有效地检测出各类边缘,解决了上述的误判问题;通过不同大小窗口的边缘检测效果比较,建议在采用基于二阶局部纹理熵的边缘检测方法时将窗口设置为3×3。 相似文献
93.
海岸线是分形领域最传统的研究课题 ,但是在目前的海岸线研究中却忽略了一个重要且根本性的问题 ,那就是海岸线分维数不同计算方法之间是否存在着一定的差异 ,根据不同计算方法计算所得的分维数之间是否具有可比性。在概述海岸线分维数计算方法的基础上 ,分析了它们之间的异同 ,并进行了实证研究。指出 :使用量规法、网格法计算所得海岸线分维数结果之间存在着客观的差异 ,文中利用量规法计算所得的海岸线分维数结果大于使用网格法计算所得的海岸线分维数结果。 相似文献
94.
James T. Waples Kent A. Orlandini Kim M. Weckerly David N. Edgington J. Val Klump 《Marine Chemistry》2003,80(4):661
234Th/238U disequilibria have been used extensively in studies of particle dynamics and the fate and transport of particle-reactive matter in marine environments. Similar work in low salinity, estuarine, and freshwater systems has not occurred primarily because the lower concentrations of both parent and daughter nuclides that are typical of these systems often render established methods for the analysis of 234Th inadequate. The application of this radionuclide tracer technique to these systems, however, has great potential. To this end, we present a method for measuring low activities of 234Th in relatively small samples (<200 l) using low background gas-flow proportional counters, a 229Th yield monitor, and empirical corrections for the interferences from real and apparent betas that are emitted by other thorium isotopes and their progeny. For samples with low 234Th/228Th activity ratios, we improve upon current beta counting methodologies that rely on immediate sample counting, weak beta absorption, or multiple beta counts so that, using the analytical approach outlined here, it should be possible to measure 234Th activities (i) as low as 1.5 dpm/total sample, (ii) up to 2 weeks after radiochemical purification of thorium, and (iii) with only one sample count for alpha and beta activity. 相似文献
95.
97.
介绍了一种对2台步进电机相互间采取等差角同步旋转的控制方案.根据这个方案设计和制作的机电控制电路实现了同步旋转快门功能,最终的同步精度控制在步进电机的性能1.8度/步的范围内,应用在激光测距系统中起到了安全保护单光子探测器的作用. 相似文献
98.
99.
《国际泥沙研究》2020,35(6):587-599
Existing layer-averaged numerical models for turbidity currents have mostly adopted the global minimum time step (GMiTS) for solution updating, which confines their computational efficiency and limits their attractiveness for field applications. This paper presents a highly efficient layer-averaged numerical model for turbidity currents by implementing the combined approach of the local graded-time-step (LGTS) and the global maximum-time-step (GMaTS). The governing equations are solved for unstructured triangular meshes by the shock-capturing finite volume method along with a set of well-balanced evaluations of the numerical flux and geometrical slope source terms. The quantitative accuracy of the model, given reasonably estimated empirical and model parameters (e.g., bed friction, water entrainment, sediment deposition and erosion coefficients), is demonstrated by comparing the numerical solutions against laboratory data of the current front positions and deposition profiles, as well as field data of the current front positions. The improved computational efficiency is demonstrated by comparing the computational cost of the present model against that of a traditional model that uses a GMiTS. For the present simulated cases, the maximum reduction of the computational cost is approximately 80% (e.g., a simulation that cost 1 h before will only require 12 min with the new model). 相似文献
100.
Present‐day understanding of rill dynamics is hampered by a lack of detailed data on velocity distributions in rills. The latter are dif?cult to collect with traditional techniques due to the very low water depths and the relatively high ?ow velocities in rills. The objectives of this paper were to investigate the feasibility of miniaturized acoustic Doppler velocimeter (mADV) measurements in rill ?ow and to explore longitudinal variations in ?ow velocities and their relationship with rill bed morphology. Detailed data on longitudinal ?ow velocity were required to achieve these objectives. A 1·8 m long rill was formed freely in a ?ume at 5° slope and 0·001 m3 s?1 discharge. Rill topography was characterized by an alternation of steps and pools. The ?ume surface was then ?xed to preserve rill roughness. A topographical scanning of the entire ?ume surface was made. Velocity was measured with a mADV along the rill, and at different depths. Flow depth in a longitudinal direction was also measured using an elevation gauge. A strong relationship exists between rill topography and ?ow hydraulics. Over steps, ?ow was unidirectional and rapidly accelerating until a threshold Froude number (Fn) value between 1·3 and 1·7 was reached and a hydraulic jump occurred leading to the formation of a pool. In the pool, the ?ow pattern was multidirectional and complex. The ?ow was subcritical when leaving the pool and accelerated over the next step until the threshold Froude number value was again reached. Energy loss in the rill was concentrated in the pools, mainly due to the action of a hydraulic jump. This mechanism of energy dissipation appeared to be an essential factor in rill formation and bedform evolution. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献