全文获取类型
收费全文 | 1238篇 |
免费 | 215篇 |
国内免费 | 341篇 |
专业分类
测绘学 | 73篇 |
大气科学 | 193篇 |
地球物理 | 494篇 |
地质学 | 559篇 |
海洋学 | 250篇 |
天文学 | 2篇 |
综合类 | 58篇 |
自然地理 | 165篇 |
出版年
2024年 | 6篇 |
2023年 | 18篇 |
2022年 | 41篇 |
2021年 | 49篇 |
2020年 | 56篇 |
2019年 | 78篇 |
2018年 | 67篇 |
2017年 | 64篇 |
2016年 | 51篇 |
2015年 | 46篇 |
2014年 | 90篇 |
2013年 | 85篇 |
2012年 | 75篇 |
2011年 | 96篇 |
2010年 | 61篇 |
2009年 | 90篇 |
2008年 | 102篇 |
2007年 | 98篇 |
2006年 | 94篇 |
2005年 | 69篇 |
2004年 | 73篇 |
2003年 | 58篇 |
2002年 | 22篇 |
2001年 | 33篇 |
2000年 | 48篇 |
1999年 | 40篇 |
1998年 | 40篇 |
1997年 | 29篇 |
1996年 | 24篇 |
1995年 | 24篇 |
1994年 | 12篇 |
1993年 | 10篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 7篇 |
1989年 | 3篇 |
1988年 | 5篇 |
1987年 | 3篇 |
1986年 | 1篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有1794条查询结果,搜索用时 15 毫秒
91.
从提高动测桩的功能和精度,深入研究桩-土相互作用机理出发,作者开展了波动方程分析程序的编制和研究。作者采用了凯普威普(CAPWAP C)程序中的一些基本假设和基本模型,而在一些模型和方法上作了某些改进,在此基础上提出了“新编凯斯波动分析程序”。该程序可以解决一般凯斯法(Case method)不能解决的问题:变断面桩;桩身沿长度方向上材料不同的桩;桩身较长的桩(25m 以上)。可以提供较为深入的更全面的数据:单桩总静阻力;桩侧静摩阻力;桩端端承力;各土层的摩阻力,…,等。本文叙述了“新编凯斯波动分析程序”的编制目的,基本原理与模型,计算步骤。并提供了在某工程实例中的应用情况。 相似文献
92.
本文采用类比法和定量化方法对上海宝山区村镇民房震害损失进行了预测。在此基础上,对上海已有的和新建的民房提出抗震对策和措施。 相似文献
93.
本文通过考虑地震烈度的随机性和模糊性,以及石结构强度破坏等级界限的模糊性,研究多层石结构抗震抗剪能力的可靠性问题.将地震地面运动模拟为含有模糊烈度参数的平稳过滤有色噪声的随机过程,引入抗剪强度破坏指数,建立了石结构抗震抗剪能力的模糊安全准则,为研究多层石结构的抗震抗剪能力可靠性提供了一个合理的评定方法。 相似文献
94.
WANG Guixian WANG Xingkui GUO Jiachao Professor Sediment Research Laboratory Dept. of Hydraulic Engineering Tsinghua University Beijing China. Associate Professor Dept. of Hydraulic Engineering Tsinghua University. Assistant Dept. of Hydraulic Engineering Tsinghua University. 《国际泥沙研究》1993,(3)
I. INTRODUCTIONResistance to flow in alluvial streams is one of the crucial problems in fluvial hydraulics. Despite that large amount of studies have been conducted for centuries, no perfectmethod describing and calculating the resistance to flow in alluvial streams is availablehll now. Investigation on bed forms and resistance properties of light--weight bed materials is even more scarce. USWES (1936) has conducted systematic experiments onlight--weight materials in a flume & m long an… 相似文献
95.
The Control Of Coherent Eddies In Vegetation Canopies: Streamwise Structure Spacing, Canopy Shear Scale And Atmospheric Stability 总被引:9,自引:9,他引:0
An analogy has been established between a plane mixing layer and the atmospheric flow near the top of a vegetation canopy. It is based on a common feature, a strong inflection in the mean velocity profile, responsible for hydrodynamical instabilities that set the pattern for the coherent eddies and determine the turbulence length scales. In an earlier study, this analogy was tested using a small data set from thirteen experiments, all in near-neutral conditions. It provided a good prediction of the streamwise spacing w of the dominant canopy eddies (evaluated from time series of vertical velocity) that appears to depend on a shear length scale Ls = U(h)/U'(h), where h is canopy height, U is mean velocity and U' the vertical gradient dU/dz. The present analysis utilizes an extensive data set of approximately 700 thirty-minute runs, from six experiments on two forest sites and a maize crop, with a large range of stability conditions. w was estimated for each run using the wavelet transform as an objective, automated detection method. First, the variations of w and Ls with atmospheric stability are discussed. Neutral and unstable values exhibit a large scatter whereas in stable conditions both variables decrease with increasing stability. It is subsequently found that w is directly related to Ls, in a way close to the neutral prediction w /h = 8.1Ls/h.The Strouhal number Str = Ls /w is then shown to vary with atmospheric stability, weakly in unstable conditions, more significantly in stable conditions. Altogether these results suggest that, to some extent, the plane mixing-layer analogy can be extended to non-neutral conditions. It is argued that the primary effect of atmospheric stability, at least in stable conditions, is to modify the shear length scale Ls through changes in U(h) and U'(h), which in turn determines the streamwise spacing of the active, coherent motions. 相似文献
96.
《中国科学D辑(英文版)》2006,(Z2)
An open-path eddy covariance system was set up in Damxung rangeland station to measure the carbon flux from July to October, 2003. The canopy quantum yield (α) of alpine meadow was calculated by the linear function between the net ecosystem carbon dioxide exchange (NEE) and the photosynthetic active radiation (PAR) under low light, and how it was influenced by the temperature was also discussed. Results showed that the canopy or decreased almost linearly with temperature, with the decrease in every 1℃increase of temperature similar to those measured on leaf level of C3 plant. At the beginning, the decrease of canopyαwith temperature was 0.0005 umol CO2·μmol-1 PAR; while it increased to 0.0008μmol CO2·μmol-1 PAR in September, showing a rising trend with plant growing stages. Compared with the canopy a calculated with rectangular hyperbola function, the value in the paper was lower. However, the method advanced here has the advantages in examining the relationship betweenαand the key environmental factors, such as temperature. 相似文献
97.
Remotely sensed estimation of forest canopy density: A comparison of the performance of four methods
Chudamani Joshi Jan De Leeuw Andrew K. Skidmore Iris C. van Duren Henk van Oosten 《International Journal of Applied Earth Observation and Geoinformation》2006
In recent years, a number of alternative methods have been proposed to predict forest canopy density from remotely sensed data. To date, however, it remains difficult to decide which method to use, since their relative performance has never been evaluated. In this study the performance of: (1) an artificial neural network, (2) a multiple linear regression, (3) the forest canopy density mapper and (4) a maximum likelihood classification method was compared for prediction of forest canopy density using a Landsat ETM+ image. Comparison of confusion matrices revealed that the regression model performed significantly worse than the three other methods. These results were based on a z-test for comparison of weighted kappa statistics, which is an appropriate statistic for analysis of ranked categories. About 89% of the variance of the observed canopy density was explained by the artificial neural networks, which outperformed the other three methods in this respect. Moreover, the artificial neural networks gave an unbiased prediction, while other methods systematically under or over predicted forest canopy density. The choice of biased method could have a high impact on canopy density inventories. 相似文献
98.
99.
100.
山西中部地区农村民居震害特征及抗震对策 总被引:1,自引:0,他引:1
收集整理山西中部地区历史地震灾害资料,总结了山西中部地区农村民居由地震引起的地基震害、建筑结构震害、建筑材料与施工震害三大特征,讨论了今后农村民居和村镇公共建筑的抗震对策。为将地震造成的民居震害损失降到最低程度,提出要实行总体规划,增强防震减灾意识,加强对农村民居抗震工作的指导的建议。 相似文献