全文获取类型
| 收费全文 | 1336篇 |
| 免费 | 426篇 |
| 国内免费 | 118篇 |
专业分类
| 测绘学 | 31篇 |
| 大气科学 | 10篇 |
| 地球物理 | 983篇 |
| 地质学 | 394篇 |
| 海洋学 | 37篇 |
| 天文学 | 6篇 |
| 综合类 | 53篇 |
| 自然地理 | 366篇 |
出版年
| 2024年 | 1篇 |
| 2023年 | 11篇 |
| 2022年 | 26篇 |
| 2021年 | 34篇 |
| 2020年 | 31篇 |
| 2019年 | 50篇 |
| 2018年 | 49篇 |
| 2017年 | 69篇 |
| 2016年 | 73篇 |
| 2015年 | 68篇 |
| 2014年 | 88篇 |
| 2013年 | 67篇 |
| 2012年 | 62篇 |
| 2011年 | 59篇 |
| 2010年 | 68篇 |
| 2009年 | 78篇 |
| 2008年 | 91篇 |
| 2007年 | 115篇 |
| 2006年 | 124篇 |
| 2005年 | 88篇 |
| 2004年 | 75篇 |
| 2003年 | 86篇 |
| 2002年 | 73篇 |
| 2001年 | 49篇 |
| 2000年 | 45篇 |
| 1999年 | 46篇 |
| 1998年 | 35篇 |
| 1997年 | 39篇 |
| 1996年 | 34篇 |
| 1995年 | 27篇 |
| 1994年 | 20篇 |
| 1993年 | 30篇 |
| 1992年 | 15篇 |
| 1991年 | 16篇 |
| 1990年 | 12篇 |
| 1989年 | 11篇 |
| 1988年 | 7篇 |
| 1987年 | 1篇 |
| 1986年 | 1篇 |
| 1979年 | 1篇 |
| 1954年 | 5篇 |
排序方式： 共有1880条查询结果，搜索用时 469 毫秒
91.
92.
早期左心功能不全的诊断目前仍较困难,作者在部分病人的胸部CT 图象上发现一种胸膜下区宽带状影与早期左心功能不全有密切关系,表现为两肺背部呈带状的模糊影象,这种胸膜下宽带状影对诊断早期左心功能不全,尤其是舒张功能有一定的临床意义。 相似文献
93.
作者参照国际放射防护委员会(ICRP)1991年60号出版物规定的实践中放射防护原则,设计CT扫描中患者及公众放射防护方策,通过优选予置扫描参数和适时扫描参数,降低CT 受检者的辐射剂量,并业已经过辐射测量实验结果的证实。对此,做一介绍。 相似文献
94.
95.
地震层析地球内部密度分布横向不均匀研究 总被引：2，自引：1，他引：2
本文使用Woodhouse和Dziewonski等分别用S波和P波层析分析提供的,以670km深度为分界的上地幔和下地幔地震波传播所反映出的横向不均匀的球谐函数系数,假定地幔内部密度与正常水平的偏离分别与P波和S波的速度Vp和Vs成比例（下地幔δρ=γδVp,上地幔δρ=σδVS2）,并用一组新的球谐函数系数表示全地幔密度分布模型.计算结果表明:1.比例系数γ=0.199,σ=3.13×10-5时,该全地幔密度模型可以70％恢复Dziewonski提供的下地幔密度模型,50％恢复Woodhouse提供的上地幔密度模型.合成密度分布基本上反映了分别由P波和S波速度异常所提供的地幔密度分布特征;2.所有穿过太平洋中部的剖面均明显地显示出一个低密度异常从地表一直延伸到核-幔边界;3.所有大陆下部均存在一个高密度异常区,而且深入到1200-1300km深度处;4.由密度异常的正、负交替基本上将部分地区地幔分成为3层,其深度依次为:第1层25-1300km左右,第2... 相似文献
96.
97.
98.
Wavepath traveltime tomography 总被引：1，自引：1，他引：1
The elastic-wave equation is used to construct sensitivity kernels relating perturbations in elastic parameters to traveltime deviations. Computation of the functions requires a correlation of the forward-propagating seismic wavefield with a backward propagation of the residual wavefield. The computation of the wavefields is accomplished using a finite difference algorithm and is efficiently executed on a CM-2 parallel processor. The source and receiver locations have maximum sensitivity to velocity structure. The sensitivity kernels or wavepaths are well suited for transmission traveltime inversion such as cross-borehole tomography and vertical seismic profiling. Conventional ray tomography and wavepath tomography are applied to a set of P -wave arrival times, from a cross-borehole experiment at Kesterson, California. Because the wavepaths have increased sensitivity near the source and receiver there are differences in resolution of the velocity structure. Both techniques recover the same relative variations in velocity where the coverage is adequate. The wavepath solution is more laterally continuous and the dominant variation is vertical, as is expected for the layered sediments in this region. 相似文献
99.
We invert ISC PcP and PKP absolute and differential traveltimes in an attempt to infer the long-wavelength topography of the core-mantle boundary (CMB). The data selection and processing methods are described and evaluated. These travel-time data are very noisy and the geographic distribution of the data is highly non-uniform, inhibiting reliable inference of CMB topography. Spatial averaging enhances the coherent component of the residual variance (related to heterogeneity), however, the random component of the variance is much larger than the coherent component. We show that for PcP data the coherent signal due to mantle heterogeneity overshadows that arising from the CMB, and that the effects of mantle heterogeneity are mapped into our inferred CMB solutions. The PcP data are not correlated across the spatial averaging bins and seem to have a strong bias due to small-scale structure and/or noise. The non-uniform geographic sampling of the data plays a role in the mapping of mantle heterogeneity onto the CMB. Spatial patterns of CMB models inferred from different phases do not agree. Amplitudes of seismically inferred CMB undulations vary greatly. The sensitivity of inferred CMB models to the processing, spatial averaging procedure, and inversion techniques are investigated. Topographic amplitudes increase strongly with increasing input residual variance. The power spectrum of inferred topography indicates that there are unmodelled heterogeneities that must be described with spherical harmonics of degree 6 and higher. Based on this work, we conclude that reliable inference of long-wavelength CMB topography is not likely with the current ISC data set or with a spherical harmonic expansion truncated to degree and order 6. 相似文献
100.