全文获取类型
收费全文 | 7486篇 |
免费 | 1349篇 |
国内免费 | 2398篇 |
专业分类
测绘学 | 1093篇 |
大气科学 | 51篇 |
地球物理 | 2592篇 |
地质学 | 5632篇 |
海洋学 | 482篇 |
天文学 | 26篇 |
综合类 | 894篇 |
自然地理 | 463篇 |
出版年
2024年 | 41篇 |
2023年 | 120篇 |
2022年 | 206篇 |
2021年 | 255篇 |
2020年 | 313篇 |
2019年 | 371篇 |
2018年 | 331篇 |
2017年 | 331篇 |
2016年 | 393篇 |
2015年 | 414篇 |
2014年 | 460篇 |
2013年 | 479篇 |
2012年 | 493篇 |
2011年 | 530篇 |
2010年 | 424篇 |
2009年 | 576篇 |
2008年 | 545篇 |
2007年 | 575篇 |
2006年 | 570篇 |
2005年 | 467篇 |
2004年 | 480篇 |
2003年 | 396篇 |
2002年 | 337篇 |
2001年 | 259篇 |
2000年 | 269篇 |
1999年 | 249篇 |
1998年 | 219篇 |
1997年 | 210篇 |
1996年 | 177篇 |
1995年 | 141篇 |
1994年 | 137篇 |
1993年 | 116篇 |
1992年 | 97篇 |
1991年 | 55篇 |
1990年 | 50篇 |
1989年 | 41篇 |
1988年 | 35篇 |
1987年 | 27篇 |
1986年 | 16篇 |
1985年 | 5篇 |
1984年 | 6篇 |
1983年 | 7篇 |
1979年 | 5篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1954年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 21 毫秒
91.
Traction image method for irregular free surface boundaries in finite difference seismic wave simulation 总被引:16,自引:1,他引:16
In this study, we propose a new numerical method, named as Traction Image method, to accurately and efficiently implement the traction-free boundary conditions in finite difference simulation in the presence of surface topography. In this algorithm, the computational domain is discretized by boundary-conforming grids, in which the irregular surface is transformed into a 'flat' surface in computational space. Thus, the artefact of staircase approximation to arbitrarily irregular surface can be avoided. Such boundary-conforming gridding is equivalent to a curvilinear coordinate system, in which the first-order partial differential velocity-stress equations are numerically updated by an optimized high-order non-staggered finite difference scheme, that is, DRP/opt MacCormack scheme. To satisfy the free surface boundary conditions, we extend the Stress Image method for planar surface to Traction Image method for arbitrarily irregular surface by antisymmetrically setting the values of normal traction on the grid points above the free surface. This Traction Image method can be efficiently implemented. To validate this new method, we perform numerical tests to several complex models by comparing our results with those computed by other independent accurate methods. Although some of the testing examples have extremely sloped topography, all tested results show an excellent agreement between our results and those from the reference solutions, confirming the validity of our method for modelling seismic waves in the heterogeneous media with arbitrary shape topography. Numerical tests also demonstrate the efficiency of this method. We find about 10 grid points per shortest wavelength is enough to maintain the global accuracy of the simulation. Although the current study is for 2-D P-SV problem, it can be easily extended to 3-D problem. 相似文献
92.
93.
94.
95.
We have found that the extensively used finite difference scheme time3d produces time fields which are 'asymmetric' in the sense that traveltimes computed to the right of the source are faster than traveltimes computed to the left. All finite difference schemes will, as they are approximations to the wave equation, to some extent fail to obey reciprocity perfectly. We show, however, that the errors in time3d may be significant—and unnecessarily large. An asymmetry in the point source initialization has been identified, and after correction time3d produces time fields with an improved reciprocity. 相似文献
96.
Jacques De Medina Laura M. G. Motta João D. G. Dos Santos 《Geotechnical and Geological Engineering》2006,24(4):949-971
This paper summarizes the resilient and permanent deformation characteristics of laterite gravels, determined by cyclic loading in triaxial tests of compacted specimens. A study of laterite pavements was carried out on eight paved road sections in the state of Mato Grosso. Also reported is the influence of fine sand admixture, specimen size and soaking in two laterite samples from Roraima and Rondônia in the North of Brazil. The experimental relationships of resilient moduli against applied stresses are interpreted empirically for models what may be incorporated in a structural analysis of full depth of laterite pavement. 相似文献
97.
基于有限差分法的抗滑桩计算机辅助设计 总被引:3,自引:0,他引:3
基于地基系数“m-m”法、“m-k”法、“k-k”法的原理,考虑桩顶和桩底边界条件以及桩在滑动面处位移、转角、弯矩和剪力的连续条件,可解得桩身各节点的位移和内力,提出了进行抗滑桩全桩内力计算的有限差分法。根据差分方程并用VB6.0编制了实用的计算程序,既可避免繁琐的查表计算,提高计算速度,又可提高计算精度,直观生动,真正实现了人机交互,在界面的引导下,设计人员可完成全部计算,并绘出内力图形和抗滑桩截面配筋图,使设计更方便快捷,该软件可以极大地提高生产效率,降低工程造价,从而实现抗滑桩的优化设计。最后采用上述方法对某滑坡的悬臂抗滑桩进行了设计与计算。 相似文献
98.
采矿引起的边坡倾倒滑移变形机理与变形安全性分析研究—以抚顺西露天矿边坡为例 总被引:1,自引:0,他引:1
在矿山开发、水电建设过程中倾倒变形问题日渐突出。倾倒变形边坡一般具有反倾边坡结构,变形的过程和机理比较复杂、涉及的变形岩体范围较大、危害严重。论文以抚顺西露天矿边坡为例,通过岩体位移监测资料的跟踪分析、底面摩擦模拟试验,以岩体力学、松散介质力学原理为理论基础,探讨了采矿影响下边坡倾倒滑移体的变形机理,分析认为采矿引起的倾倒滑移变形问题是在特定的地质构造和采矿工程条件下产生的,并提出了岩体变形安全性分析方法。通过工程实例,验证了方法的实用性,并针对实际问题提出了变形整治对策。 相似文献
99.
介绍了精密全站仪进行结构测试的原理和方法,通过对某大型游泳跳水馆钢结构网架在张拉和加载试验中的变形测量实践,证明其具有作业简便、能够全面反映结构变形情况之优点,且精度较高,成果可靠。 相似文献
100.