全文获取类型
收费全文 | 26616篇 |
免费 | 5479篇 |
国内免费 | 7136篇 |
专业分类
测绘学 | 1316篇 |
大气科学 | 9123篇 |
地球物理 | 6145篇 |
地质学 | 9171篇 |
海洋学 | 3937篇 |
天文学 | 5577篇 |
综合类 | 1309篇 |
自然地理 | 2653篇 |
出版年
2024年 | 86篇 |
2023年 | 349篇 |
2022年 | 836篇 |
2021年 | 1011篇 |
2020年 | 1050篇 |
2019年 | 1266篇 |
2018年 | 1022篇 |
2017年 | 1033篇 |
2016年 | 1085篇 |
2015年 | 1250篇 |
2014年 | 1765篇 |
2013年 | 1806篇 |
2012年 | 1824篇 |
2011年 | 1868篇 |
2010年 | 1790篇 |
2009年 | 2355篇 |
2008年 | 2109篇 |
2007年 | 2281篇 |
2006年 | 2011篇 |
2005年 | 1735篇 |
2004年 | 1583篇 |
2003年 | 1326篇 |
2002年 | 1160篇 |
2001年 | 953篇 |
2000年 | 862篇 |
1999年 | 808篇 |
1998年 | 704篇 |
1997年 | 506篇 |
1996年 | 467篇 |
1995年 | 426篇 |
1994年 | 365篇 |
1993年 | 334篇 |
1992年 | 234篇 |
1991年 | 213篇 |
1990年 | 152篇 |
1989年 | 100篇 |
1988年 | 119篇 |
1987年 | 73篇 |
1986年 | 49篇 |
1985年 | 55篇 |
1984年 | 33篇 |
1983年 | 24篇 |
1982年 | 29篇 |
1981年 | 18篇 |
1980年 | 26篇 |
1979年 | 21篇 |
1978年 | 14篇 |
1977年 | 20篇 |
1954年 | 17篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
This paper presents a second-order work analysis in application to geotechnical problems by using a novel effective multiscale approach. To abandon complicated equations involved in conventional phenomenological models, this multiscale approach employs a micromechanically-based formulation, in which only four parameters are involved. The multiscale approach makes it possible a coupling of the finite element method (FEM) and the micromechanically-based model. The FEM is used to solve the boundary value problem (BVP) while the micromechanically-based model is utilized at the Gauss point of the FEM. Then, the multiscale approach is used to simulate a three-dimensional triaxial test and a plain-strain footing. On the basis of the simulations, material instabilities are analyzed at both mesoscale and global scale. The second-order work criterion is then used to analyze the numerical results. It opens a road to interpret and understand the micromechanisms hiding behind the occurrence of failure in geotechnical issues. 相似文献
12.
采用香港11个GPS测站的观测资料进行1 h、2 h、3 h和4h静态PPP解算,获得4组PPP坐标序列,利用调和分析求取11个测站处8个主要分潮的负荷位移参数(振幅和相位),将其与海潮模型计算的负荷位移参数进行对比,并比较分析PPP反演值与海潮模型值改正海潮负荷信号的效果。结果表明,垂直和水平方向上,不同PPP结果反演8个分潮的负荷位移分别具有约5 mm和7 mm的差异;PPP反演8个分潮垂向负荷位移优于全球海潮模型,但水平方向上的反演效果稍弱。 相似文献
13.
利用东北地区近几年流动重力观测数据,分析前郭5.8级震群周边重力点值的时序变化特征,并基于重力段差变化对东北地区整体和局部重力变化引入能够反映异常显著性程度的指标量G和C值,最后对前郭5.8级震群前的重力场动态变化作三维密度反演。结果表明:1)前郭5.8级震群发震构造两侧的局部重力变化具有较明显的差异性,其变化趋势可作为识别构造活化或解耦运动的标志;2)显著性指标量G和C值能够为地震重力前兆的定量描述提供新思路;3)震前重力变化的反演结果显示地震发生在质量运移的过渡区域,垂向反演结果对本次地震的震源深度有一定的揭示意义。 相似文献
14.
敦煌-格尔木铁路沿线地形复杂、起沙因素多变、沙源丰富,沙害问题日益严重。目前对其风沙活动规律还未有研究,不利于防沙工作的开展。为此,通过对自北向南的5个观测点(S1、S2、S3、S4、S5)风速和风向的观测、计算和分析,利用平均风速、起沙风况及输沙势对敦格铁路沿线的风动力环境特征进行研究。结果表明:S5、S4和S3的风况对铁路风沙灾害防治意义较大。S5年平均风速、起沙风频率和输沙势最大,春季风沙活动最为强烈,且风向单一、风力强劲,风沙运动方向基本与铁路垂直,沙粒易在铁路附近堆积。S4夏季风沙活动最为强烈;S3春季风沙活动最为强烈,且风向单一,S4和S3的风沙运动方向与铁路夹角小于90°,附近沙源广阔,铁路易受风沙侵蚀,阻碍交通运营。 相似文献
15.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
16.
17.
18.
19.
20.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献