首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   256309篇
  免费   4295篇
  国内免费   3332篇
测绘学   6837篇
大气科学   18973篇
地球物理   53531篇
地质学   88738篇
海洋学   21184篇
天文学   55888篇
综合类   994篇
自然地理   17791篇
  2021年   2223篇
  2020年   2583篇
  2019年   2834篇
  2018年   3283篇
  2017年   3043篇
  2016年   5606篇
  2015年   4181篇
  2014年   6894篇
  2013年   14201篇
  2012年   6402篇
  2011年   7614篇
  2010年   6729篇
  2009年   9355篇
  2008年   8180篇
  2007年   7589篇
  2006年   9582篇
  2005年   7617篇
  2004年   7541篇
  2003年   7022篇
  2002年   6664篇
  2001年   5962篇
  2000年   5923篇
  1999年   5186篇
  1998年   5215篇
  1997年   5008篇
  1996年   4666篇
  1995年   4415篇
  1994年   4101篇
  1993年   3838篇
  1992年   3617篇
  1991年   3586篇
  1990年   3754篇
  1989年   3500篇
  1988年   3293篇
  1987年   3842篇
  1986年   3406篇
  1985年   4213篇
  1984年   4726篇
  1983年   4404篇
  1982年   4313篇
  1981年   3920篇
  1980年   3644篇
  1979年   3506篇
  1978年   3478篇
  1977年   3273篇
  1976年   3039篇
  1975年   2956篇
  1974年   2920篇
  1973年   3071篇
  1972年   2025篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A methodology has been developed to calculate the dynamic probabilistic movement and resulting stresses for marine pipelines subjected to storm waves. A directional wave spectrum is used with a Fourier series expansion to simulate short-crested waves and calculate their loads on the pipeline. The pipeline displacements resulting from these loads are solutions to the time-dependent beam-column equation which also includes the soil resistance as external loading. The statistics of the displacements for individual waves are combined with the wave statistics for a given period of time, e.g. pipeline lifetime, to generate probabilistic estimates for net pipeline movement. On the basis of displacements for specified probability levels the pipeline configuration is obtained from which pipeline stresses can be estimated using structural considerations, e.g. pipeline stiffness, end restraints, etc.  相似文献   
993.
In this paper the peculiarities of compiled oceanographic data sets, which consist primarily in the presence of some characteristic information as well as in the planned incomplete filling of the files with observations, are discussed. A number of special ways are suggested which essentially improve the packing density of computerized data without applying special archiving algorithms. The effectiveness of these methods has been tested by treating thein situ hydrological/chemical data file compiled in the Black Sea in 1988.Translated by Vladimir A. Puchkin.  相似文献   
994.
A finite-difference scheme and a modified marker-and-cell (MAC) algorithm have been developed to investigate the interactions of fully nonlinear waves with two- or three-dimensional structures of arbitrary shape. The Navier–Stokes (NS) and continuity equations are solved in the computational domain and the boundary values are updated at each time step by the finite-difference time-marching scheme in the framework of a rectangular coordinate system. The fully nonlinear kinematic free-surface condition is implemented by the marker-density function (MDF) technique developed for two fluid layers.To demonstrate the capability and accuracy of the present method, the numerical simulation of backstep flows with free-surface, and the numerical tests of the MDF technique with limit functions are conducted. The 3D program was then applied to nonlinear wave interactions with conical gravity platforms of circular and octagonal cross-sections. The numerical prediction of maximum wave run-up on arctic structures is compared with the prediction of the Shore Protection Manual (SPM) method and those of linear and second-order diffraction analyses based on potential theory and boundary element method (BEM). Through this comparison, the effects of non-linearity and viscosity on wave loading and run-up are discussed.  相似文献   
995.
996.
997.
998.
X-ray images of sediment core allow recognition of fine-scaled sedimentary structures which traditional epoxy casts or lacquer peels are unable to resolve. The X-ray imaging system SCOPIX has the additional advantage of acquiring high-resolution digital pictures suitable for further analysis by advanced image processing techniques. Because of the frequent presence of both physical and biogenic sedimentary structures in undisturbed sediment cores, two image analysis procedures have been established which simplify the information contained in the images (in term of structures and facies), and allow a quantitative characterization of the observed structures. This form of parametrization of the image structure constitutes an additional source of information for a better interpretation of the physical processes responsible for the generation of the sedimentary structures.  相似文献   
999.
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号