全文获取类型
收费全文 | 3717篇 |
免费 | 664篇 |
国内免费 | 843篇 |
专业分类
测绘学 | 220篇 |
大气科学 | 781篇 |
地球物理 | 776篇 |
地质学 | 2091篇 |
海洋学 | 429篇 |
天文学 | 168篇 |
综合类 | 412篇 |
自然地理 | 347篇 |
出版年
2024年 | 17篇 |
2023年 | 83篇 |
2022年 | 187篇 |
2021年 | 213篇 |
2020年 | 140篇 |
2019年 | 168篇 |
2018年 | 210篇 |
2017年 | 181篇 |
2016年 | 203篇 |
2015年 | 185篇 |
2014年 | 234篇 |
2013年 | 207篇 |
2012年 | 191篇 |
2011年 | 200篇 |
2010年 | 223篇 |
2009年 | 208篇 |
2008年 | 185篇 |
2007年 | 175篇 |
2006年 | 116篇 |
2005年 | 106篇 |
2004年 | 65篇 |
2003年 | 97篇 |
2002年 | 90篇 |
2001年 | 96篇 |
2000年 | 107篇 |
1999年 | 169篇 |
1998年 | 121篇 |
1997年 | 146篇 |
1996年 | 138篇 |
1995年 | 121篇 |
1994年 | 118篇 |
1993年 | 94篇 |
1992年 | 81篇 |
1991年 | 70篇 |
1990年 | 63篇 |
1989年 | 51篇 |
1988年 | 42篇 |
1987年 | 15篇 |
1986年 | 25篇 |
1985年 | 19篇 |
1984年 | 11篇 |
1983年 | 16篇 |
1982年 | 10篇 |
1981年 | 10篇 |
1980年 | 7篇 |
1978年 | 2篇 |
1977年 | 2篇 |
1975年 | 2篇 |
1969年 | 1篇 |
1958年 | 1篇 |
排序方式: 共有5224条查询结果,搜索用时 15 毫秒
461.
462.
463.
464.
This study focuses on the three‐dimensional (3‐D) characteristics of wave propagation in pipe‐pile using elastodynamic finite integration technique. First, a real 3‐D pile‐soil model in cylindrical coordinate system is presented. Then, the governing equations are established. With the boundary and initial conditions, the numerical solution is obtained. The accuracy and feasibility of the self‐written code are further verified via comparing with the measured data. Velocity histories at different angles of pile top and pile tip are illustrated, and the snapshots reflecting the 3‐D characteristics of wave propagation are given. It shows that the interferences of Rayleigh waves can confuse the result interpretation for pile integrity testing. The increase of hammer contact time can effectively mitigate the interferences, and the interferences of Rayleigh waves are weakest at an angle of 90° from where hammer hits. Besides, surrounding soil can partly mitigate the wave interferences. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
465.
河西地区重力动态变化的二维离散小波多尺度分解 总被引:7,自引:5,他引:2
采用二维离散小波的多尺度分解技术,对1994~1995年和2002~2003年的河西地区重力动态变化进行分解,提取该地区不同层次的重力动态变化异常,结果反映了不同深度在一定尺度意义上的介质密度的动态变化分布。 相似文献
466.
唐佑民 《成都信息工程学院学报》1991,(1)
本文从能量平衡角度,考虑海气相互作用和纬向间的差异,设计了一个纬向平均的海气耦合一维模式,进行了模式平衡态求解、敏感性试验以及随机分析。并对模式部分计算结果进行了与实测资料的比较,验证了模式的合理性和可信性。 相似文献
467.
The possibility of using a nonlinear empirical atmospheric model for hybrid coupled atmosphere-ocean modelling has been examined
by using a neural network (NN) model for predicting the contemporaneous wind stress field from the upper ocean state. Upper
ocean heat content (HC) from a 6-layer ocean model was a better predictor of the wind stress than the (observed or modelled)
sea surface temperature (SST). Our results showed that the NN model generally had slightly better skills in predicting the
contemporaneous wind stress than the linear regression (LR) model in the off-equatorial tropical Pacific and in the eastern
equatorial Pacific. When the wind stresses from the NN and LR models were used to drive the ocean model, slightly better SST
skills were found in the off-equatorial tropical Pacific and in the eastern equatorial Pacific when the NN winds were used
instead of the LR winds. Better skills for the model HC were found in the western and central equatorial Pacific when the
NN winds were used instead of the LR winds. Why NN failed to show more significant improvement over LR in the equatorial Pacific
for the wind stress and SST is probably because the relationship between the surface ocean and the atmosphere in the equatorial
Pacific over the seasonal time scale is almost linear.
Received: 2 March 1999 / Accepted: 13 July 2000 相似文献
468.
469.
自作者(1987)在辽东本溪群下部发现Sublepidodendron以来,又相继在吉南、辽南(范炳桓,1992)和辽西发现了Sub.sp.,Rhodeahsianghsiangensis等属于早石炭世的植物及维宪-纳缪尔A期孢粉化石,从而证实东北南部普遍存在早石炭世地层。本文记述了产于上述层位的植物化石30属79种,并归纳为Sublepidodendron-Archaeocalamites-Liopteris组合(带).东北南部早石炭世植物群中的一些属种,虽然显示了欧美植物群的特点,但其发生、演化又早于欧美植物群(分子),作者暂称为“东北南部早石炭世华夏植物群”。该植物群与东北北部安加拉植物群的界线,大致与槽台界线相吻合。 相似文献
470.
Dajun Tang Briggs K.B. Williams K.L. Jackson D.R. Thorsos E.I. Percival D.B. 《Oceanic Engineering, IEEE Journal of》2002,27(3):546-560
As part of the effort to characterize the acoustic environment during the high frequency sediment acoustics experiment (SAX99), fine-scale variability of sediment density was measured by an in situ technique and by core analysis. The in situ measurement was accomplished by a newly developed instrument that measures sediment conductivity. The conductivity measurements were conducted on a three-dimensional (3-D) grid, hence providing a set of data suited for assessing sediment spatial variability. A 3-D sediment porosity matrix is obtained from the conductivity data through an empirical relationship (Archie's Law). From the porosity matrix, sediment bulk density is estimated from known average grain density. A number of cores were taken at the SAX99 site, and density variations were measured using laboratory techniques. The power spectra were estimated from both techniques and were found to be appropriately fit by a power-law. The exponents of the horizontal one-dimensional (1-D) power-law spectra have a depth-dependence and range from 1.72 to 2.41. The vertical 1-D spectra have the same form, but with an exponent of 2.2. It was found that most of the density variability is within the top 5 mm of the sediment, which suggests that sediment volume variability will not have major impact on acoustic scattering when the sound frequency is below 100 kHz. At higher frequencies, however, sediment volume variability is likely to play an important role in sound scattering. 相似文献