全文获取类型
收费全文 | 34547篇 |
免费 | 18273篇 |
国内免费 | 43266篇 |
学科分类
地球科学 | 96086篇 |
出版年
2024年 | 157篇 |
2023年 | 398篇 |
2022年 | 635篇 |
2021年 | 916篇 |
2020年 | 2046篇 |
2019年 | 5478篇 |
2018年 | 5702篇 |
2017年 | 5389篇 |
2016年 | 5458篇 |
2015年 | 4802篇 |
2014年 | 4313篇 |
2013年 | 4866篇 |
2012年 | 4582篇 |
2011年 | 4349篇 |
2010年 | 4320篇 |
2009年 | 3617篇 |
2008年 | 2909篇 |
2007年 | 2913篇 |
2006年 | 2577篇 |
2005年 | 2459篇 |
2004年 | 2746篇 |
2003年 | 2466篇 |
2002年 | 2226篇 |
2001年 | 2023篇 |
2000年 | 1660篇 |
1999年 | 1746篇 |
1998年 | 1799篇 |
1997年 | 1879篇 |
1996年 | 1497篇 |
1995年 | 1456篇 |
1994年 | 1281篇 |
1993年 | 1218篇 |
1992年 | 1026篇 |
1991年 | 784篇 |
1990年 | 732篇 |
1989年 | 612篇 |
1988年 | 523篇 |
1987年 | 401篇 |
1986年 | 319篇 |
1985年 | 252篇 |
1984年 | 286篇 |
1983年 | 170篇 |
1982年 | 208篇 |
1981年 | 145篇 |
1980年 | 111篇 |
1979年 | 109篇 |
1978年 | 48篇 |
1977年 | 43篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
An elastoplastic model for sands is presented in this paper, which can describe stress–strain behaviour dependent on mean effective stress level and void ratio. The main features of the proposed model are: (a) a new state parameter, which is dependent on the initial void ratio and initial mean stress, is proposed and applied to the yield function in order to predict the plastic deformation for very loose sands; and (b) another new state parameter, which is used to determine the peak strength and describe the critical state behaviour of sands during shearing, is proposed in order to predict simply negative/positive dilatancy and the hardening/softening behaviour of medium or dense sands. In addition, the proposed model can also predict the stress–strain behaviour of sands under three-dimensional stress conditions by using a transformed stress tensor instead of ordinary stress tensor. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
2.
3.
Qian Huaisui 《中国地理科学(英文版)》1997,8(3):204-211
The crop estimates by remote sensing, developing quickly in recent decades, is a up-to-date technique. Regionalization for
large area crop estimates by remote sensing, a special applied regionalization, is the foundation of crop estimates in a large
area by remote sensing. According to the actual demands of wheat yield estimation by remote sensing and wheat agroclimatic
demarcation of China, this paper first puts forward some principles upheld in this regionalization and analyses its main bases.
Secondly, it works out the classificatory schemes about the optimum temporal for estimating wheat yield by remote sensing,
information sources of space remote sensing and landuse structure in China. Finally, According to the regionalization indices,
this study divides the wheat plantable region of China into 14 regions of crop yield estimates and 31 subregions of crop yield
estimates. 相似文献
4.
5.
Ren Tinawi Marc Sarrazin Andr Filiatrault 《Soil Dynamics and Earthquake Engineering》1993,12(8):469-477
The 1990 edition of the National Building Code of Canada (Associate Committee of the National Building Code, National Research Council, Ottawa, 1990) makes a clear distinction between eastern and western Canada in terms of seismic acceleration and velocity zones. While it is well established that ground motions can be amplified significantly through loose clay deposits, no results are available that take into consideration the typical high frequency content of ground motions in eastern Canada. This paper develops ground amplification curves for clays having depths between 10 and 70 m excited by typical eastern Canadian ground motions scaled to two different values of peak horizontal accelerations. Simplified free-field spectral design curves, which could be used by structural designers, are proposed. The curves show that maximum spectral accelerations occur for structural periods between 0.2 and 0.5 s. In addition, soil depth does not appear to be an important parameter controlling the response of typical clay deposits in eastern Canada. 相似文献
6.
This paper considers the problem of estimatingm, the number of components in a finite mixture of distributions from a parametric family. A step-up procedure using the bootstrap method is proposed. Some properties of the procedure are illustrated with simulation studies. An example of the method, applied to orientation of beach clasts, is given. 相似文献
7.
8.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
9.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
10.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献