全文获取类型
收费全文 | 687篇 |
免费 | 120篇 |
国内免费 | 312篇 |
专业分类
测绘学 | 47篇 |
大气科学 | 63篇 |
地球物理 | 131篇 |
地质学 | 345篇 |
海洋学 | 376篇 |
天文学 | 15篇 |
综合类 | 69篇 |
自然地理 | 73篇 |
出版年
2024年 | 4篇 |
2023年 | 9篇 |
2022年 | 53篇 |
2021年 | 33篇 |
2020年 | 39篇 |
2019年 | 57篇 |
2018年 | 37篇 |
2017年 | 37篇 |
2016年 | 40篇 |
2015年 | 42篇 |
2014年 | 50篇 |
2013年 | 61篇 |
2012年 | 62篇 |
2011年 | 42篇 |
2010年 | 38篇 |
2009年 | 45篇 |
2008年 | 46篇 |
2007年 | 50篇 |
2006年 | 55篇 |
2005年 | 45篇 |
2004年 | 26篇 |
2003年 | 29篇 |
2002年 | 32篇 |
2001年 | 29篇 |
2000年 | 28篇 |
1999年 | 27篇 |
1998年 | 16篇 |
1997年 | 12篇 |
1996年 | 11篇 |
1995年 | 10篇 |
1994年 | 15篇 |
1993年 | 4篇 |
1992年 | 11篇 |
1990年 | 2篇 |
1989年 | 6篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1983年 | 3篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1980年 | 1篇 |
排序方式: 共有1119条查询结果,搜索用时 343 毫秒
881.
Analysis of mechanical behaviour and internal stability of granular materials using discrete element method
下载免费PDF全文
![点击此处可从《国际地质力学数值与分析法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
N. Abdoulaye Hama T. Ouahbi S. Taibi H. Souli J.M. Fleureau A. Pantet 《国际地质力学数值与分析法杂志》2016,40(12):1712-1729
Granular materials like sand are widely used in civil engineering. They are composed of different sizes of grains, which generate a complex behaviour, difficult to assess experimentally. Internal instability of a granular material is its inability to prevent the loss of its fine particles under flow effect. It is geometrically possible if the fine particles can migrate through the pores of the coarse soil matrix and results in a change in its mechanical properties. This paper uses the three‐dimensional Particle Flow Code (PFC3D/DEM) to study the stability/instability of granular materials and their mechanical behaviour after suffusion. Stability properties of widely graded materials are analysed by simulating the transport of smaller particles through the constrictions formed by the coarse particles under the effect of a downward flow with uniform pressure gradient. A sample made by an initially stable material according to the Kenney & Lau geometrical criterion was divided into five equal layers. The classification of these layers by this criterion before and after the test shows that even stable granular materials can lose fine particles and present local instability. The failure criterion of eroded samples, in which erosion is simulated by progressive removal of fine particles, evolves in an unexpected way. Internal friction angle increases with the initial porosity, the rate of lost fine particles and the average diameter D50. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
882.
The air quality model system RAMS (Regional Atmospheric Modeling System)-CMAQ (Models-3 Community Multi-scale Air Quality) coupled with an aerosol optical/radiative module was applied to investigate the impact of different aerosol mixing states (i.e., externally mixed, half externally and half internally mixed, and internally mixed) on radiative forcing in East Asia. The simulation results show that the aerosol optical depth (AOD) generally increased when the aerosol mixing state changed from externally mixed to internally mixed, while the single scattering albedo (SSA) decreased. Therefore, the scattering and absorption properties of aerosols can be significantly affected by the change of aerosol mixing states. Comparison of simulated and observed SSAs at five AERONET (Aerosol Robotic Network) sites suggests that SSA could be better estimated by considering aerosol particles to be internally mixed. Model analysis indicates that the impact of aerosol mixing state upon aerosol direct radiative forcing (DRF) is complex. Generally, the cooling effect of aerosols over East Asia are enhanced in the northern part of East Asia (Northern China, Korean peninsula, and the surrounding area of Japan) and are reduced in the southern part of East Asia (Sichuan Basin and Southeast China) by internal mixing process, and the variation range can reach 5 W m-2. The analysis shows that the internal mixing between inorganic salt and dust is likely the main reason that the cooling effect strengthens. Conversely, the internal mixture of anthropogenic aerosols, including sulfate, nitrate, ammonium, black carbon, and organic carbon, could obviously weaken the cooling effect. 相似文献
883.
利用高分辨率电感耦合等离子体质谱仪,建立了内标法测试雪冰样品中Li,Sr,Sb,Pb,V,Mn,Fe,Cu,Zn痕量元素的方法,给出了方法的检测限、测试精密度和扩展不确定度.通过测试国际标准参考水样SLRS-5来控制方法的准确度,并以标准参考水样SLRS-5和枪勇冰川雪冰中铅为例,详细评定了标准参考水样与实际样品的不确... 相似文献
884.
885.
Diapycnal mixing plays an important role in the ocean circulation.Internal waves are a kind of bridge relating the diapycnal mixing to external sources of mechanical energy.Difficulty in obtaining eigen solutions of internal waves over curved topography is a limitation for further theoretical study on the generation problem and scattering process.In this study,a kind of transform method is put forward to derive the eigen solutions of internal waves over subcritical topography in twodimensional and linear framework.The transform converts the curved topography in physical space to flat bottom in transform space while the governing equation of internal waves is still hyperbolic if proper transform function is selected.Thus,one can obtain eigen solutions of internal waves in the transform space.Several examples of transform functions,which convert the linear slope,the convex slope,and the concave slope to flat bottom,and the corresponding eigen solutions are illustrated.A method,using a polynomial to approximate the transform function and least squares method to estimate the undetermined coefficients in the polynomial,is introduced to calculate the approximate expression of the transform function for the given subcritical topography. 相似文献
886.
887.
研究一类受已知动态特性的外部扰动作用的非线性时滞系统的扰动抑制问题。基于内模原理构造能够抵消扰动影响的伺服补偿器,然后利用极大值原理及灵敏度法设计带有补偿器的最优控制律,使得系统在无扰动作用时以1种最优方式运行,而有扰动作用时,控制律能够消除扰动引起的稳态误差,实现无静差扰动抑制。最后,通过仿真算例验证提出算法的有效性。 相似文献
888.
利用我国地震台站记录的瑞利波观测资料,通过适配滤波频时分析技术进行数据处理,获得了穿越我国东南及陆缘地区的瑞利波频散.使用随机反演理论取得了东南大陆及陆缘地带4°×4°网格的纯路径频散数据.在网格反演的基础上使用Harkrider的面波反演程序求得了该区剪切波的三维速度结构.结果表明:1.华南大陆Moho界面埋深为30-40km,并由西向东逐渐减薄,在陆缘与浅海地域为25-28km,具有明显的分区特征.2.上地幔低速层埋深为60-0km,变化幅度较大,这与深部断裂分布及深层过程有关,但NS向剖面上各界面的起伏变化均比EW向剖面平缓.3.东南陆缘是东亚大陆的海陆过渡带,在深部表现为Moho界面埋深和地壳平均速度降低的地带,地幔深部界面的起伏形态充分表明,深浅介质结构和物质耦合的不均匀性. 相似文献
889.
Amer Ali Al-Rawas Abdulwahid Hago Hector H. Sutherland Ali A. Yousif Mohammed Al-Shihi Badr Al-Shihi 《Geotechnical and Geological Engineering》2001,19(1):69-84
Two methods for the quantification of clay minerals using X-ray diffraction techniques: the Constant Mineral Standards Method and the Constant Clay Method were applied to soil samples from Al-Khod (Northern Oman). The first method was based on the addition of different concentrations of clay to constant mineral standards while the latter was based on the additions of known internal standards to the clay sample. The clay investigated in this study contained montmorillonite, palygorskite, illite and kaolinite. The areas of the reflection peaks were found to relate linearly with additions of different proportions of clay or mineral standards. Using the two methods, the concentrations of the different components and their experimental errors were determined. From the errors and self consistency of the results obtained, and the practicality of sample preparation, it is proposed to use the Constant Mineral Standards Method. 相似文献
890.
SUMMARY
This work quantifies the extent to which disturbance phenomena contribute to the observed geomagnetic field on the time-scale of months to years. A deterministic approach was adopted in which geomagnetic monthly means were analysed in the time domain. By presupposing the time dependence of the external and induced field variations, which we assume to vary as the aa geomagnetic index, we arrived at an estimate of the relative amplitude of the disturbance to each component of the field at a number of observatories. In all some 58 000 geomagnetic monthly means from 59 observatories operating this century were analysed. Special attention was paid to the selection and validation of the data as otherwise the numerical computations would have been unduly compromised.
The results consist of 177 estimates of the relative amplitude of disturbance, one for each of three orthogonal components north ( X ), east ( Y ) and vertically down ( Z ) at each observatory. The disturbance to the X component was found to be consistently negative over the whole of the Earth's surface with an intensification in auroral regions. The disturbances to the Z component were found to be smaller than that for X except in high latitudes. Mean disturbances to the Y component were smaller still. Results were in general consistent with the dipole field of the magnetospheric ring current, aligned with, though of opposite polarity to, the Earth's main field. Typical amplitudes of the mean disturbance field from month to month were of the order 10 nT.
The results can be used to estimate the variation of the disturbance field. Subtracting this from both monthly and annual means yields an improved estimate of the field originating in the core and its secular variation. Some examples are presented. 相似文献
This work quantifies the extent to which disturbance phenomena contribute to the observed geomagnetic field on the time-scale of months to years. A deterministic approach was adopted in which geomagnetic monthly means were analysed in the time domain. By presupposing the time dependence of the external and induced field variations, which we assume to vary as the aa geomagnetic index, we arrived at an estimate of the relative amplitude of the disturbance to each component of the field at a number of observatories. In all some 58 000 geomagnetic monthly means from 59 observatories operating this century were analysed. Special attention was paid to the selection and validation of the data as otherwise the numerical computations would have been unduly compromised.
The results consist of 177 estimates of the relative amplitude of disturbance, one for each of three orthogonal components north ( X ), east ( Y ) and vertically down ( Z ) at each observatory. The disturbance to the X component was found to be consistently negative over the whole of the Earth's surface with an intensification in auroral regions. The disturbances to the Z component were found to be smaller than that for X except in high latitudes. Mean disturbances to the Y component were smaller still. Results were in general consistent with the dipole field of the magnetospheric ring current, aligned with, though of opposite polarity to, the Earth's main field. Typical amplitudes of the mean disturbance field from month to month were of the order 10 nT.
The results can be used to estimate the variation of the disturbance field. Subtracting this from both monthly and annual means yields an improved estimate of the field originating in the core and its secular variation. Some examples are presented. 相似文献