Vibrations of square and circular foundations with concentric openings on elastic half space     

D.L. Karabalis  C.-F.D. Huang 《Soil Dynamics and Earthquake Engineering》2005,25(12):951-965

A study on the dynamic characteristics of rigid foundations with special geometries such as square or circular with concentric internal holes, is presented. The foundations are resting on a homogeneous, linear elastic halfspace and are subjected to external forces or seismic wave excitation. Both ‘relaxed’ and ‘non-relaxed’ boundary conditions at the interface between the foundation and the halfspace are considered, and several parametric studies are conducted to assess the influence of either type of boundary conditions upon each of the possible modes of vibration. Results for massive and massless foundations are presented in time and frequency domains for impulsive and harmonic excitations, respectively. A time domain boundary element method (BEM) developed by the authors for the solution of a class of 3-D soil-structure interaction (SSI) problems is used for all the analyses reported in this work. The accuracy and efficiency of the method and the BEM models developed in this work are assessed on the basis of comparison studies with published results.  相似文献   

Assessment of the cooling capacity of plate tectonics and flood volcanism in the evolution of Earth, Mars and Venus     

P. van Thienen  N.J. Vlaar  A.P. van den Berg 《Physics of the Earth and Planetary Interiors》2005,150(4):287-315

Geophysical arguments against plate tectonics in a hotter Earth, based on buoyancy considerations, require an alternative means of cooling the planet from its original hot state to the present situation. Such an alternative could be extensive flood volcanism in a more stagnant-lid like setting. Starting from the notion that all heat output of the Earth is through its surface, we have constructed two parametric models to evaluate the cooling characteristics of these two mechanisms: plate tectonics and basalt extrusion/flood volcanism. Our model results show that for a steadily (exponentially) cooling Earth, plate tectonics is capable of removing all the required heat at a rate of operation comparable to or even lower than its current rate of operation, contrary to earlier speculations. The extrusion mechanism may have been an important cooling agent in the early Earth, but requires global eruption rates two orders of magnitude greater than those of known Phanerozoic flood basalt provinces. This may not be a problem, since geological observations indicate that flood volcanism was both stronger and more ubiquitous in the early Earth. Because of its smaller size, Mars is capable of cooling conductively through its lithosphere at significant rates, and as a result may have cooled without an additional cooling mechanism. Venus, on the other hand, has required the operation of an additional cooling agent for probably every cooling phase of its possibly episodic history, with rates of activity comparable to those of the Earth.  相似文献   

西藏羌塘地区肖茶卡群与日干配错群层序及对比--兼论对羌南、羌北分区界线的质疑   总被引：1，自引：0，他引：1  

张树岐  王洪双  冯德臣  姜雪飞  李学彬  李庆武 《吉林地质》2005,24(2):16-20

西藏羌塘南部地区的晚三叠世地层，前人曾划分为羌南——保山地层区多玛分区日干配错群和羌北——昌都—思茅地层区赤布张错分区肖茶卡群。经过笔者研究后，根据宏观岩石组合特征将原日干配错群划分为鲁宗乃组、曲瑞恰乃组、亏莫塔雀组，其中鲁宗乃组为碳酸盐岩组合。曲瑞恰乃组为碎屑岩夹碳酸盐岩组合，亏莫塔雀组为碳酸盐岩夹少量碎屑岩组合，将原肖茶卡群划分为节拉日组、多涌组、扎那组、赛公药组。节拉日组为中基性火山岩组合，多涌组为碳酸盐岩夹碎屑岩组合，扎那组为泥岩、细碎屑岩夹中碎屑岩组合，赛公药组为中碎屑岩夹细碎屑岩组合。多涌组与鲁宗乃组可进行对比，扎那组和赛公药组与曲瑞恰乃组可对比。通过对比，认为区内晚三叠世的沉积环境基本相同，因而认为羌南、羌北的分区界线不在本研究区内。  相似文献   

Application of a long-range Terrestrial Laser Scanner to a detailed rockfall study at Vall de Núria (Eastern Pyrenees, Spain)     

A. Abelln  J.M. Vilaplana  J. Martínez 《Engineering Geology》2006,88(3-4):136-148

In this study we show the application of a long-range Terrestrial Laser Scanner (TLS) to a detailed rockfall study in a test zone at Vall de Núria, located in the Eastern Pyrenees. Data acquisition was carried out using TLS-Ilris3D, the new generation of reflector-less laser scanners with a high range, accuracy and velocity of measurements. Eight scans were performed at 3 stations to acquire coordinates of almost 4 million points. The results from the acquired data are a high accuracy Digital Elevation Model (DEM) and the reconstruction of the joint geometry. The former is used for inventory of rockfalls and for more accurate rockfall simulation (trajectories and velocities). The latter allows us to model the geometry and volume of the source area in recent rockfalls. Our findings suggest that TLS technology could be a tool of reference in rockfall studies in the near future.  相似文献   

Effect of the selection of reference radio sources on geodetic estimates from VLBI observations     

O. Titov 《Journal of Geodesy》2007,81(6-8):455-468

This paper evaluates the effect of the accuracy of reference radio sources on the daily estimates of station positions, nutation angle offsets, and the estimated site coordinates determined by very long baseline interferometry (VLBI), which are used for the realization of the international terrestrial reference frame (ITRF). Five global VLBI solutions, based on VLBI data collected between 1979 and 2006, are compared. The reference solution comprises all observed radio sources, which are treated as global parameters. Four other solutions, comprising different sub-sets of radio sources, were computed. The daily station positions for all VLBI sites and the corrections to the nutation offset angles were estimated for these five solutions. The solution statistics are mainly affected by the positional instabilities of reference radio sources, whereas the instabilities of geodetic and astrometric time-series are caused by an insufficient number of observed reference radio sources. A mean offset of the three positional components (Up, North, East) between any two solutions was calculated for each VLBI site. From a comparison of the geodetic results, no significant discrepancies between the respective geodetic solutions for all VLBI sites in the Northern Hemisphere were found. In contrast, the Southern Hemisphere sites were more sensitive to the selected set of reference radio sources. The largest estimated mean offset of the vertical component between two solutions for the Australian VLBI site at Hobart was 4 mm. In the worst case (if a weak VLBI network observed a limited number of reference radio sources) the daily offsets of the estimated height component at Hobart exceeded 100 mm. The exclusion of the extended radio sources from the list of reference sources improved the solution statistics and made the geodetic and astrometric time-series more consistent. The problem with the large Hobart height component offset is magnified by a comparatively small number of observations due to the low slewing rate of the VLBI dish (1°/ s). Unless a minimum of 200 scans are performed per 24-h VLBI experiment, the daily vertical positions at Hobart do not achieve 10 mm accuracy. Improving the slew rate at Hobart and/or having an increased number of new sites in the Southern Hemisphere is essential for further improvement of geodetic VLBI results for Southern Hemisphere sites.  相似文献   

黄土震陷下限深度研究          下载免费PDF全文

徐舜华  秋仁东  王平 《地震工程学报》2007,29(3):240-244

利用兰州和西安地震小区划黄土震陷数据,获得了这两个地区黄土震陷下限深度预测的经验公式,与实验结果较为吻合。结果显示在一定的地震动条件下黄土的物理指标含水量、固结压力、剪切波速能够比较好的反映黄土震陷特性,通过研究它们与残余应变的关系可以预测黄土震陷下限深度。  相似文献   

不规则形状垂直井孔中波传播的2.5维边界元模拟     

盖增喜  王灿云  雷厅  陈晓非 《应用地球物理》2007,4(3):155-163

本文给出了求解声波测井工作中具有任意形状的垂直井孔中波传播的波数域边界元公式。我们将问题看成是垂向均匀的二维介质,并沿垂直方向变换到离散波数域。通过与有限差分计算的频散曲线的比较验证了算法和程序的正确性。边界元方法可以处理任意形状的边界,作为初步应用,我们研究了垂向裂纹对井孔中波传播的影响。  相似文献   

The impact of boundary on the fractional advection–dispersion equation for solute transport in soil: Defining the fractional dispersive flux with the Caputo derivatives     

Xiaoxian Zhang  Mouchao Lv  John W. Crawford  Iain M. Young 《Advances in water resources》2007

The inherent heterogeneity of geological media often results in anomalous dispersion for solute transport through them, and how to model it has been an interest over the past few decades. One promising approach that has been increasingly used to simulate the anomalous transport in surface and subsurface water is the fractional advection–dispersion equation (FADE), derived as a special case of the more general continuous time random walk or the stochastic continuum model. In FADE, the dispersion is not local and the solutes have appreciable probability to move long distances, and thus reach the boundary faster than predicted by the classical advection–dispersion equation (ADE). How to deal with different boundaries associated with FADE and their consequent impact is an issue that has not been thoroughly explored. In this paper we address this by taking one-dimensional solute movement in soil columns as an example. We show that the commonly used FADE with its fractional derivatives defined by the Riemann–Liouville definition is problematic and could result in unphysical results for solute transport in bounded domains; a modified method with the fractional dispersive flux defined by the Caputo derivatives is presented to overcome this problem. A finite volume approach is given to numerically solve the modified FADE and its associated boundaries. With the numerical model, we analyse the inlet-boundary treatment in displacement experiments in soil columns, and find that, as in ADE, treating the inlet as a prescribed concentration boundary gives rise to mass-balance errors and such errors could be more significant in FADE because of its non-local dispersion. We also discuss a less-documented but important issue in hydrology: how to treat the upstream boundary in analysing the lateral movement of tracer in an aquifer when the tracer is injected as a pulse. It is shown that the use of an infinite domain, as commonly assumed in literature, leads to unphysical backward dispersion, which has a significant impact on data interpretation. To avoid this, the upstream boundary should be flux-prescribed and located at the upstream edge of the injecting point. We apply the model to simulate the movement of Cl⁻ in a tracer experiment conducted in a saturated hillslope, and analyse in details the significance of upstream-boundary treatments in parameter estimation.  相似文献   

Variabilité de la densité spectrale de la température et du vent moyen dans la couche limite planétaire     

H. Bardeau 《Pure and Applied Geophysics》1982,120(1):95-107

We have carried out a series of recordings reproducing the small scale structures of temperature and windspeed within the boundary layer. The results obtained have been processed by computer and a direct comparison made between the slope of the spectral density functions and the theoretical slopen=5/3 defining the turbulence in the region of the inertial subrange.The measurements made at various altitudes on thin atmospheric samples demonstrate the variability of the spectral density slope with altitude. The hypotheses put forward by A. M. Obukhov, R. Bolgiano, G. H. Shur, P. Misme, A. Monin or S. Panchev, for example, enable the spectral variations and certain discontinuities in the turbulence observed during the experiments, within and beyond the boundary layer, to be explained.

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Les continents péri-atlantiques au crétacé supérieur: Migrations des faunes continentales et problèmes paléogéographiques     

J.-C. Rage 《Cretaceous Research》1981,2(1):65-84

Terrestrial vertebrates offer possibilities of reconstructing the migrations by land-routes followed during the late Cretaceous on the peri-Atlantic continents (North America, South America, Europe, Africa). South America and Africa were not separated before the Aptian. Later, migrations could still have occurred between Africa and South America during the late Cretaceous by a land-route (probably discontinuous) situated on the Rio Grande Rise-Walvis Ridge barrier; it is not impossible that some amphibia used this route. In Laurasia, two provinces were largely separated during the early part of the late Cretaceous: Euramerica and Asiamerica in the terminology of Cox (1974), (that is, Europe plus eastern North America and Asia plus western North America). During the latest Cretaceous, western North America became connected with Euramerica, but probably separated from Asia. During the latest Cretaceous, a route, probably terrestrial, permitted important faunal exchanges between South America and Laurasia; this connection was situated in the Caribbean region, perhaps where Central America is today. Limited faunal exchange occurred between Euramerica and North Africa.  相似文献