全文获取类型
收费全文 | 1198篇 |
免费 | 205篇 |
国内免费 | 158篇 |
专业分类
测绘学 | 103篇 |
大气科学 | 172篇 |
地球物理 | 434篇 |
地质学 | 200篇 |
海洋学 | 166篇 |
天文学 | 185篇 |
综合类 | 33篇 |
自然地理 | 268篇 |
出版年
2024年 | 3篇 |
2023年 | 8篇 |
2022年 | 20篇 |
2021年 | 20篇 |
2020年 | 27篇 |
2019年 | 42篇 |
2018年 | 23篇 |
2017年 | 45篇 |
2016年 | 41篇 |
2015年 | 44篇 |
2014年 | 41篇 |
2013年 | 71篇 |
2012年 | 39篇 |
2011年 | 52篇 |
2010年 | 59篇 |
2009年 | 70篇 |
2008年 | 83篇 |
2007年 | 69篇 |
2006年 | 113篇 |
2005年 | 76篇 |
2004年 | 66篇 |
2003年 | 44篇 |
2002年 | 53篇 |
2001年 | 59篇 |
2000年 | 49篇 |
1999年 | 48篇 |
1998年 | 36篇 |
1997年 | 20篇 |
1996年 | 42篇 |
1995年 | 21篇 |
1994年 | 24篇 |
1993年 | 33篇 |
1992年 | 21篇 |
1991年 | 15篇 |
1990年 | 20篇 |
1989年 | 14篇 |
1988年 | 22篇 |
1987年 | 5篇 |
1986年 | 7篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 2篇 |
1981年 | 4篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有1561条查询结果,搜索用时 15 毫秒
121.
高层建筑地震反应最优多重TLD控制 总被引:3,自引:1,他引:3
本文以坑层建筑地震反应进行了最优多重TLD(MTLD)s控制研究。文中阐述了TLD系统的工作原理笔MTLDs系统的参数;建立了多层结构-TMLDs系统耦联体系的运动方程;分析了MTLDs系统的参数对在谐波作用下多层结构动力反应的影响以及各参数之间的关系。 相似文献
122.
Numerical modelling ofSH wave seismograms in media whose material properties are prescribed by a random distribution of many perfectly elastic cavities and by intrinsic absorption of seismic energy (anelasticity) demonstrates that the main characteristics of the coda waves, namely amplitude decay and duration, are well described by singly scattered waves in anelastic media rather than by multiply scattered waves in either elastic or anelastic media. We use the Boundary Integral scheme developed byBenites
et al. (1992) to compute the complete wave field and measure the values of the direct waveQ and coda wavesQ in a wide range of frequencies, determining the spatial decay of the direct wave log-amplitude relation and the temporal decay of the coda envelope, respectively. The effects of both intrinsic absorption and pure scattering on the overall attenuation can be quantified separately by computing theQ values for corresponding models with (anelastic) and without (elastic) absorption. For the models considered in this study, the values of codaQ
–1 in anelastic media are in good agreement with the sum of the corresponding scatteringQ
–1 and intrinsicQ
–1 values, as established by the single-scattering model ofAki andChouet (1975). Also, for the same random model with intrinsic absorption it appears that the singly scattered waves propagate without significant loss of energy as compared with the multiply scattered waves, which are strongly affected by absorption, suggesting its dominant role in the attenuation of coda waves. 相似文献
123.
唐志宽 《武汉大学学报(信息科学版)》1992,(4)
本文介绍了一个以单片机为核心的电话单机计时计费器的设计。提出了一种识别脉冲拨号和双音多频拨号的方法,同时在设计中充分利用了单片机的资源,注意发挥软件功能。因此使该设计具有结构合理,使用方便,性能价格比高的特点。 相似文献
124.
125.
V. Krishan 《Journal of Astrophysics and Astronomy》1988,9(4):231-236
The occurrence of superluminal motion in extragalactic radio sources is believed to be quite common. Among others, the geometrical
scattering of radio radiation can also cause superluminal expansion and or motion and halo formation, In this paper, the effectiveness
of the stimulated Raman scattering in producing these features is investigated. The scattering medium is a plasma whose position,
density and temperature decide the rate and angle of scattering. When the radiation from a stationary and constant source
gets scattered from a stationary plasma, a halo is formed around the source. However, the scattering of a rotating radiation
beam does produce superluminal motion of the virtual source. It is found that the plasma should have the characteristics of
the emission-line regions and the intercloud medium in order to Raman scatter the radiation. Since the scattering is polarization
dependent, it is possible to estimate the rotation of the electric vector along the direction of the apparent motion of a
radio source. 相似文献
126.
In this paper, the dynamic stress concentration and scattering of SH-waves by bi-naterial structures that possess an interface elliptic cavity are investigated. First, by using the complex function method, the Green‘s function is constructed. This yields the solution of the displacement field for an elastic half space with a semi-elliptic canyon impacted by an anti-plane harmonic line source loading on the horizontal surface. Then, the problem is divided into an upper and lower half space along the horizontal interface, regarded as a harmony model. In order to satisfythe integral continuity condition,the unknown anti-plane forces are applied to the interface. The integral equations with unknown forces can be established through the continuity condition, and after transformation, the algebraic equations are solved numerically. Finally, the distribution of the dynamic stress concentration factor (DSCF) around the elliptic cavity is given and the effect of different parameters on DSCF is discussed. 相似文献
127.
128.
129.
测量了不同人土雾浓度下垂直偏振入射光的透射光强。利用实验数据,分析了垂直、平行方向上透射光强的变化趋势。得出平行偏振分量透射光强大致呈指数下降的趋势,垂直偏振分量透射光强先略微上升而后下降。由退偏振度随雾浓度的变化讨论了退偏产生的原因,指出雾浓度较低时,雾滴可近似视为球形。基于光的偏振特性,从理论上分析并计算得到光在雾中传输时透射、散射光强。实验结果对研究光在雾中传输及分析大气中雾对透射光偏振态的影响有一定的参考价值。 相似文献
130.
Multiple Parameter Regularization: Numerical Solutions and Applications to the Determination of Geopotential from Precise Satellite Orbits 总被引:1,自引:0,他引:1
Kaula’s rule of thumb has been used in producing geopotential models from space geodetic measurements, including the most recent models from satellite gravity missions CHAMP. Although Xu and Rummel (Manuscr Geod 20 8–20, 1994b) suggested an alternative regularization method by introducing a number of regularization parameters, no numerical tests have ever been conducted. We have compared four methods of regularization for the determination of geopotential from precise orbits of COSMIC satellites through simulations, which include Kaula’s rule of thumb, one parameter regularization and its iterative version, and multiple parameter regularization. The simulation results show that the four methods can indeed produce good gravitational models from the precise orbits of centimetre level. The three regularization methods perform much better than Kaula’s rule of thumb by a factor of 6.4 on average beyond spherical harmonic degree 5 and by a factor of 10.2 for the spherical harmonic degrees from 8 to 14 in terms of degree variations of root mean squared errors. The maximum componentwise improvement in the root mean squared error can be up to a factor of 60. The simplest version of regularization by multiplying a positive scalar with a unit matrix is sufficient to better determine the geopotential model. Although multiple parameter regularization is theoretically attractive and can indeed eliminate unnecessary regularization for some of the harmonic coefficients, we found that it only improved its one parameter version marginally in this COSMIC example in terms of the mean squared error. 相似文献