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   70081篇
  免费   697篇
  国内免费   1240篇
测绘学   2229篇
大气科学   4926篇
地球物理   13540篇
地质学   28290篇
海洋学   5102篇
天文学   12106篇
综合类   2233篇
自然地理   3592篇
  2022年   334篇
  2021年   547篇
  2020年   559篇
  2019年   632篇
  2018年   5964篇
  2017年   5166篇
  2016年   3889篇
  2015年   973篇
  2014年   1373篇
  2013年   2369篇
  2012年   2473篇
  2011年   4615篇
  2010年   3748篇
  2009年   4515篇
  2008年   3774篇
  2007年   4405篇
  2006年   1976篇
  2005年   1501篇
  2004年   1693篇
  2003年   1665篇
  2002年   1475篇
  2001年   1173篇
  2000年   1036篇
  1999年   877篇
  1998年   880篇
  1997年   863篇
  1996年   669篇
  1995年   650篇
  1994年   594篇
  1993年   503篇
  1992年   503篇
  1991年   440篇
  1990年   484篇
  1989年   449篇
  1988年   378篇
  1987年   475篇
  1986年   414篇
  1985年   535篇
  1984年   568篇
  1983年   556篇
  1982年   510篇
  1981年   495篇
  1980年   479篇
  1979年   455篇
  1978年   457篇
  1977年   394篇
  1976年   390篇
  1975年   392篇
  1974年   353篇
  1973年   393篇
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
92.
93.
94.
The evolution of views and practical decisions regarding the problem of collecting payment for the use of surface waters with regard to their quality in the former USSR and present-day Ukraine is discussed. A simple and feasible method is proposed for differentitation of regional (basin) rates of payment for the use of surface water in Ukraine with allowance made for its quality assessed based on a system of environmental classifications.  相似文献   
95.
96.
The subsurface geometry of the Sebastián Vizcaíno Basin is obtained from the 2D inversion of gravity data, constrained by a density-versus-depth relationship derived from an oil exploration deep hole. The basin accumulated a thick pile of marine sediments that evolved in the fore-arc region of the compressive margin prevalent along western North America during Mesozoic and Tertiary times. Our interpretation indicates that the sedimentary infill in the Sebastián Vizcaíno Basin reaches a maximum thickness of about 4 km at the centre of a relatively symmetric basin. At the location of the Suaro-1 hole, the depth to the basement derived from this work agrees with the drilled interface between calcareous and volcaniclastic members of the Alisitos Formation. A sensitivity analysis strongly suggests that the assumed density function leads to a nearly unique solution of the inverse problem.  相似文献   
97.
98.
The Lower Carboniferous conglomerate at Lligwy Bay, Anglesey, is shown to be underlain as well as overlain by limestones of D1 age so that the conglomerate is of intra-D1 age. It is considered to have been deposited very close to a coastline.  相似文献   
99.
Non-reflectionsymmetric turbulent motions in an electrically conducting fluid provide for an average electromotoric force α parallel to the average magnetic field . This effect – furthermore called α-effect – can lead to a self-excited built-up of magnetic fields by dynamo action. Since by actions of Coriolis forces turbulent motions in rotating systems generally are non-reflectionsymmetric, the explanation of stellar and planetary magnetic fields becomes possible. – In this first part spherical alternating field dynamos are calculated on conditions similar to those at the sun; α-effect and differential rotation provide for dynamo action. – The basic equations, which form a system of partial differential equations, are treated by a separation method; in this way our problem is transformed into a two-parametric eigenvalue problem for an infinite system of ordinary differential equations. The two eigenvalues determine the frequency and the amount of the induction actions necessary for the field maintenance. The solution of this problem was made by an electronic computer. – The results give evidence for the possibility of magnetohydrodynamic alternating field dynamos. Furthermore the magnitude of the calculated eigenvalues and the similarities between the calculated field and the observed solar magnetic field are showing that the sun realizes a dynamo of this kind.  相似文献   
100.
The study of REE distribution in the clay fraction of sedimentary rocks from two coalfields made it possible to distinguish several types of REE distribution, which correlate with their mineral composition. It is shown that the REE fractionation was related to the mineral reconstruction of the primary clay fraction and some detrital minerals in the course of postsedimentary transformations of rocks during diagenesis, early catagenesis, and beginning of late catagenesis. These transformations were governed by several factors, such as the composition of sediments; hydrochemical features of accumulation environment; the chemical composition, dynamics, and feeding sources of pore solutions; the porosity and permeability of sediments and rocks; and the content of organic matter and its reaction ability.  相似文献   
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