全文获取类型
收费全文 | 690篇 |
免费 | 101篇 |
国内免费 | 158篇 |
专业分类
测绘学 | 77篇 |
大气科学 | 79篇 |
地球物理 | 186篇 |
地质学 | 285篇 |
海洋学 | 67篇 |
天文学 | 147篇 |
综合类 | 55篇 |
自然地理 | 53篇 |
出版年
2024年 | 2篇 |
2023年 | 4篇 |
2022年 | 14篇 |
2021年 | 11篇 |
2020年 | 26篇 |
2019年 | 31篇 |
2018年 | 20篇 |
2017年 | 24篇 |
2016年 | 23篇 |
2015年 | 29篇 |
2014年 | 36篇 |
2013年 | 46篇 |
2012年 | 42篇 |
2011年 | 33篇 |
2010年 | 41篇 |
2009年 | 42篇 |
2008年 | 49篇 |
2007年 | 44篇 |
2006年 | 41篇 |
2005年 | 40篇 |
2004年 | 50篇 |
2003年 | 41篇 |
2002年 | 26篇 |
2001年 | 35篇 |
2000年 | 21篇 |
1999年 | 20篇 |
1998年 | 17篇 |
1997年 | 21篇 |
1996年 | 12篇 |
1995年 | 26篇 |
1994年 | 20篇 |
1993年 | 14篇 |
1992年 | 7篇 |
1991年 | 11篇 |
1990年 | 3篇 |
1989年 | 12篇 |
1988年 | 5篇 |
1987年 | 5篇 |
1986年 | 1篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有949条查询结果,搜索用时 62 毫秒
51.
本文对Lp估计的直接算法进行了详细的分析和讨论,由于直接算法效率低,文中提出了一种快速算法(残差逐步下降法),采用可变尺度因子来修正Huber的常数尺度因子。最后,以实例加以分析和说明。 相似文献
52.
53.
新疆构造运动期序及特征 总被引:6,自引:2,他引:6
新疆地域辽阔,地质构造运动复杂,特征各异。前人曾对新疆构造运动的划分及命名做了大量工作,但多有不一致,有些构造运动的划分或命名依据不足,因此失去了应有的代表性。现根据实际资料,对新疆地质构造运动重新归纳和认识,提出新疆境内构造运动可划分为11期32幕(次),其中前寒武纪6期、古生代2期17幕(次),中新生代3期9幕(次) 相似文献
54.
Kishor Panjabi Pradeep Goel Prasad Daggupati Narayan Kumar Shrestha Rituraj Shukla 《水文科学杂志》2020,65(13):2224-2232
ABSTRACT In humid regions, surface runoff is often generated by saturation-excess runoff mechanisms from relatively small variable source areas (VSAs). However, the majority of the current hydrologic models are based on infiltration-excess mechanisms. In this study, the AGricultural Non-Point Source Pollution (AGNPS) model was used to integrate the VSA concept using topographic wetness index (TWI). Both the original and AGNPS-VSA models were evaluated for a small agricultural field in Ontario, Canada. The results indicate that the AGNPS-VSA model performed better than original model. The AGNPS-VSA model predicted that only the saturated portion of the field with higher TWI values produced runoff, whereas the original AGNPS model showed uniform hydrologic response from the entire field. The results of this study are important for accurately mapping the locations of VSAs. This new model could be a powerful tool in identifying critical source areas for applying targeted best management practices to minimize pollutant loads to receiving waters. 相似文献
55.
56.
Observations with the 6-m telescope revealed eight new magnetic, chemically peculiar stars: HD 29925, HD 40711, HD 115606, HD 168796, HD 178892, HD 196691, HD 209051, and BD+32°2827. Zeeman observations of all these objects have been carried out for the first time. We selected candidates by analyzing the depression profile at a wavelength of λ5200 Å. This technique for selecting candidate magnetic stars was shown to be efficient: we found magnetic fields in 14 of the 15 objects that we selected for our observations with a Zeeman analyzer. A maximum longitudinal field strength B e exceeding 8 kG was found in HD 178892; in HD 209051 and HD196691, B e reaches 3.3 and 2.2 kG, respectively. For the remaining stars, we obtained lower limits of the longitudinal field (more than several hundred G). 相似文献
57.
58.
59.
We show that hydrostatically equilibrium models for the thin photospheres of helium stars based on new opacities κR (OPAL and OP) can be constructed only for masses M<5M ⊙. The parameter Г=κL/4πGMc, defined as the ratio of light pressure to gravity, exceeds a critical value of 1.0 for larger masses, which must result in mass outflow under light pressure. This mass limit matches the observed lower limit for the masses of Wolf-Rayet stars (M WR>5M ⊙)), which is an additional argument that the Wolf-Rayet stellar cores are actually helium stars. By solving the equation of radiative transfer in extended atmospheres, we construct a semiempirical model for a WN5 star (M WN5=10M ⊙)) with a helium core and an expanding envelope, whose physical and geometric parameters are known mainly from light-curve solution for the eclipsing binary V444 Cyg (WN5+06): outflow rate $\dot M \approx 1.0 \times 10^{ - 5} M_ \odot yr^{ - 1} $ , terminal velocity V ∞≈2000 km s?1, and expanding-envelope optical depth τenv≈25. The temperature at the outer boundary of the photosphere of a helium star surrounded by such an envelope is approximately 130 kK higher than that in the absence of an envelope, being T ph≈240 kK. Because of the high temperatures, the absorption coefficients at the corresponding photospheric levels are smaller than those in models with no envelope; therefore, the photosphere turns out to be in hydrostatic equilibrium and stable against light pressure (Гmax≈0.9). As a way out of this conflicting situation (an expanding envelope together with a hydrostatically equilibrium photosphere), we propose a model of discrete mass outflow, which is also supported by the observed cloudy structure of the envelopes in this type of stars. To quantitatively estimate parameters of the nonuniform outflow model requires detailed gas-dynamical calculations. 相似文献
60.