全文获取类型
收费全文 | 26837篇 |
免费 | 3883篇 |
国内免费 | 5413篇 |
专业分类
测绘学 | 3345篇 |
大气科学 | 4087篇 |
地球物理 | 6877篇 |
地质学 | 11313篇 |
海洋学 | 3537篇 |
天文学 | 1098篇 |
综合类 | 1794篇 |
自然地理 | 4082篇 |
出版年
2024年 | 139篇 |
2023年 | 443篇 |
2022年 | 798篇 |
2021年 | 1054篇 |
2020年 | 1196篇 |
2019年 | 1399篇 |
2018年 | 975篇 |
2017年 | 1300篇 |
2016年 | 1296篇 |
2015年 | 1330篇 |
2014年 | 1697篇 |
2013年 | 1993篇 |
2012年 | 1672篇 |
2011年 | 1776篇 |
2010年 | 1400篇 |
2009年 | 1789篇 |
2008年 | 1755篇 |
2007年 | 1812篇 |
2006年 | 1734篇 |
2005年 | 1381篇 |
2004年 | 1291篇 |
2003年 | 1097篇 |
2002年 | 862篇 |
2001年 | 754篇 |
2000年 | 681篇 |
1999年 | 628篇 |
1998年 | 639篇 |
1997年 | 538篇 |
1996年 | 451篇 |
1995年 | 403篇 |
1994年 | 329篇 |
1993年 | 322篇 |
1992年 | 230篇 |
1991年 | 177篇 |
1990年 | 142篇 |
1989年 | 139篇 |
1988年 | 109篇 |
1987年 | 61篇 |
1986年 | 55篇 |
1985年 | 52篇 |
1984年 | 29篇 |
1982年 | 24篇 |
1981年 | 19篇 |
1980年 | 21篇 |
1979年 | 13篇 |
1978年 | 13篇 |
1977年 | 20篇 |
1976年 | 26篇 |
1973年 | 17篇 |
1971年 | 14篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
71.
Spatial pattern and regional types of rural settlements in Xuzhou City, Jiangsu Province, China 总被引:3,自引:1,他引:3
This paper principally focuses on the morphological differences,spatial pattern and regional types of rural settlements in Xuzhou City of Jiangsu Province in China.Using satellite images of Xuzhou City taken in 2007 and 2008 and models of exploratory spatial data analysis(ESDA) and spatial metrics,the paper conducts a quantitative analysis of the morphological pattern of rural settlements,and finds significant characteristics.First,rural settlements in Xuzhou City are significantly agglomerated in terms of their spatial distribution;meanwhile,there is significant variation in the geographical density distribution.Second,the scale of rural settlements in Xuzhou City is larger than the average in Jiangsu Province,and the histogram of the scale data is more even and more like a gamma distribution.There are a significant high-value cluster in the scale distribution,and local negative correlation between the scale and density distribution of rural settlements in Xuzhou City.Third,the morphology of rural settlements in Xuzhou City shows relative regularity with good connection and integrity,but the spatial variation of the morphology is anisotropic.Finally,according to the characteristics of density,scale,and form of rural settlements,the rural settlements of Xuzhou City are divided into three types:A high-density and point-scattered type,a low-density and cluster-like type and a mass-like and sparse type.The research findings could be used as the scientific foundation for rural planning and community rebuilding,particularly in less-developed areas. 相似文献
72.
Multivariate numerical analyses (DCA, CCA) were used to study the distribution of chironomids from surface sediments of 100 lakes spanning broad ecoclimatic conditions in northern Swedish Lapland. The study sites range from boreal forest to alpine tundra and are located in a region of relatively low human impact. Of the 19 environmental variables measured, ordination by CCA identified mean July air temperature as one of the most significant variables explaining the distribution and the abundance of chironomids. Lossonignition (LOI), maximum lake depth and mean January air temperature also accounted for significant variation in chironomid assemblages. A quantitative transfer function was created to estimate mean July air temperature from sedimentary chironomid assemblages using weightedaveraging partial least squares regression (WAPLS). The coefficient of determination was relatively high (r2 = 0.65) with root mean squared error of prediction (RMSEP, based on jack-knifing) of 1.13 °C and maximum bias of 2.1 °C, indicating that chironomids can provide useful quantitative estimates of past changes in mean July air temperature. The paper focuses mainly on the relationship between chironomid composition and July air temperature, but the relationship to LOI and depth are also discussed. 相似文献
73.
KONG Desen LUAN Maotian and WANG Weiming School of Civil Engineering Harbin Institute of Technology Harbin P.R.China School of Civil Architectural Engineering Shandong University of Science Technology Qingdao P.R.China Institute of Geotechnical Engineering Dalian University of Technology Dalian P.R.China 《中国海洋大学学报(英文版)》2006,(1)
The study on the earthquake-resistant performance of a pile-soil-structure interaction system is a relatively complicated and primarily important issue in civil engineering practice. In this paper, a computational model and computation procedures for pile-supported structures, which can duly consider the pile-soil interaction effect, are established by the finite element method. Numerical implementation is made in the time domain. A simplified approximation for the seismic response analysis of pile-soil-structure systems is briefly presented. Then a comparative study is performed for an engineering example with numerical results computed respectively by the finite element method and the simplified method. Through comparative analysis, it is shown that the results obtained by the simplified method well agree with those achieved by the finite element method. The numerical results and findings will offer instructive guidelines for earthquake-resistant analysis and design of pile-supported structures. 相似文献
74.
刘权威 《大地测量与地球动力学》1996,(2)
经典最小二乘准则下的方差分析要求观测值的正态随机误差之间是相互独立的,而实际遇到的测量数据经常并不满足这一条件。为了从广泛意义上更科学地研究和应用方差分析这一理论,提出了加权方差理论,导出和论证了相应的公式,并以实例讨论了它在地壳垂直运动分析中的应用。 相似文献
75.
76.
A method that estimates mass and stiffness matrices of shear building from modal test data is presented in this paper. The method depends on only measurable points that are less in number than the total structural degrees of freedom, and on the first two orders of structural mode measured. So it is applicable to most of the general test. Based on this method modal data of unmeasurable points are estimated, then global mass and stiffness matrices of structure are obtained by using the first two orders of modal data. Taking advantage of iteration the optimum global mass and stiffness matrices are gained. Finally, an example is studied in this paper. Its result shows that this method is reliable. © 1998 John Wiley & Sons, Ltd. 相似文献
77.
西秦岭北带泥盆纪Nereites遗迹相及其环境分析 总被引:3,自引:1,他引:3
西秦岭北带泥盆系舒家坝组下一中上部的巨厚陆源碎屑岩系中含有极丰富的遗迹化石,它们可为解释该组沉积环境提供重要信息:(1)由大量典型深水遗迹化石组成的Nereites遗迹相,可与世界各地浊积岩系和复理石相中的Nereites遗迹相进行对比,该遗迹相主要发育在浊流沉积区直至深海平原,其沉积水深最大可超过2000m;(2)舒家坝组的遗迹群落中,除大量属深水型分子外,还见有典型的浅水分子,两种水深性质不同的遗迹组合共生在一个沉积剖面,正是浊积岩系和复理石相才具有的独特分布特征;(3)根据沉积层序和遗迹组合的对比确认舒家坝组不存在风暴岩。 相似文献
78.
This study tests the assumption that the characteristics of channels within multiple channel rivers are different from those of single channel rivers. Some river restoration approaches propose radical transformation of river patterns, from multiple to single channels, based on the link between river patterns and their in-channel characteristics. Determining the links between river patterns and their in-channel characteristics is complicated by differences in geology, history, climate and discharge among rivers. Furthermore, multiple channel rivers are composed of a mosaic of channel types with a range of in-channel characteristics. This study minimizes these problems by analysing a single river containing neighbouring single and multiple channel patterns with little change in discharge downstream, and by analysing all channel types. The study addressed two objectives: to determine the hydraulic geometry, energy, and sediment mobility characteristics of neighbouring single and multiple channel river patterns, and to test for statistical differences in these characteristics between patterns. The Renous River shows a wandering pattern for 11.5 km, with multiple channels around semipermanent islands and abandoned channels in the flood plain. The river displays a single channel river pattern where channels are confined by their valley walls, upstream and downstream of wandering. The analysis was conducted at three scales. First, the confined single channel and wandering multiple channel patterns were compared (pattern scale). Second, the confined channel pattern was compared to single and multiple channel sections within the wandering pattern (section scale). Third, all channel types were compared (channel type scale). Multi response permutation procedure (MRPP) and analysis of variance (ANOVA) were used to analyze differences between channels. Difference tests found no simple discrimination between the single and multiple channel river patterns of the Renous River. Tests between the single confined and multiple wandering channel patterns found few differences in the in-channel variables. The tests did find differences between multiple channel sections within the wandering pattern and confined single channels; however, a greater number of differences were found between multiple channel and single channel sections within the wandering pattern, highlighting the variability within the wandering pattern. Two groups emerged when all channel types were tested for differences: perennial main-channels containing the thalweg, and ephemeral side-channels. Therefore, side-channels define the in-channel characteristics of wandering rivers because few differences were found among main-channels in either pattern. This analysis suggests that all channel types, not just main-channels, should be investigated to obtain a complete picture of a river pattern prior to any restoration efforts. Engineers must exercise caution when applying the link between river patterns and in-channel characteristics to river restoration efforts. 相似文献
79.
在无气象数据的条件下,提出一种基于集合经验模态分解(EEMD)和季节性自回归移动平均模型(SARIMA)的对流层延迟(ZTD)预报新方法,并分别选取长春、上海、乌鲁木齐3个地区4个季节的ZTD数据进行预测分析。结果表明,基于EEMD-SARIMA的ZTD改正预报模型能够满足不同地区、不同季节下的ZTD估计需求,是一种高精度的ZTD预报方法。 相似文献
80.