共查询到20条相似文献，搜索用时 15 毫秒
1.
Detlef Wolf 《Geophysical Journal International》1985,80(1):275-279
Summary. Boussinesq's problem is solved for a uniform and incompressible Maxwell half-space subject to an external gravity field. The solution is based on momentum equations which account for stress advection in the hydrostatically pre-stressed continuum during its deformation. The analysis shows that disregarding the pre-stress term renders the theoretical stress distribution incorrect and the deformation singular in the inviscid limit of the Maxwell continuum. Our solution is contrasted with a recently published alternative solution of the same problem, where regularity in the inviscid limit was forced by modified boundary conditions. 相似文献
2.
利用宁夏25个常规地面观测站逐时资料和欧洲中期天气预报中心（ECMWF）ERA-Interim逐6 h 0.125º×0.125º分辨率再分析资料,对宁夏2016年5月11日大风扬沙天气过程的天气形势、影响系统及其热力、动力条件和形成机制进行了分析。结果表明：（1）大风和扬沙呈现出时间位相上的不一致性,沙尘超前大风约6 h。（2）200 hPa高空急流、500 hPa锋区、700 hPa低空急流和地面冷锋是此次过程的主要影响系统。（3）大风在不同阶段对扬沙起不同作用,在初期有利于扬沙的传输,后期对扬沙起抑制作用。（4）动量下传和变压风是大风形成和发展的直接原因,感热通量通过加强地面湍流形成混合层,从而引导动量下传是其间接原因,动量下传的重要机制是对流层高层高位涡的下传,过程风力最强时位涡高值区（≥2.0 PVU）由200 hPa下传至520 hPa。（5）扬沙的产生主要是冷平流和感热通量形成的热力不稳定共同作用的结果,变压风和动量下传大风是扬沙的输送机制,次级环流缺失和冷平流中心过低（750 hPa）对沙尘输送高度的抑制作用是沙尘天气偏弱的主要原因。 相似文献
3.
Bernard Valette 《Geophysical Journal International》1986,85(1):179-208
Summary. In this paper we examine the influence of the state of stress in the equilibrium configuration of the Earth (i.e. the pre-stress) upon its adiabatic perturbations. The equations governing these perturbations to the first order (Woodhouse & Dahlen; Dahlen) are re-derived using a Lagrangian approach. Different expressions of the sesquilinear form associated to the elastic-gravitational operator are given. One of these provides a way to extend to hydrostatically pre-stressed solids the criterion of local stability given by Friedman & Schutz for uniformly rotating fluids. Then the propagation in the Earth of seismic wavefronts is considered. It is shown that the nature of these different wavefronts is entirely determined by the quadratic coefficients of the development of the specific internal energy variation, corresponding to isentropic evolution, with respect to the Lagrangian finite deformation tensor. Expressions for the velocities of the various waves are given as functions of incidence angle and pre-stress for orthotropic elastic material. In the particular case where the elastic parameters depend only on one coordinate of a curvilinear system and the axis of orthotropy of the material coincides with the corresponding natural base vector, the elastodynamic equations are reduced to a simple system for a displacement stress vector, using surface operators. In particular for spherical geometry, equations are obtained which generalize to orthotropic pre-stress those given by Alterman et al. and Takeuchi & Saito. 相似文献
4.
A conservative staggered-grid finite difference method is presented for computing the electromagnetic induction response of an arbitrary heterogeneous conducting sphere by external current excitation. This method is appropriate as the forward solution for the problem of determining the electrical conductivity of the Earth's deep interior. This solution in spherical geometry is derived from that originally presented by Mackie et al. (1994 ) for Cartesian geometry. The difference equations that we solve are second order in the magnetic field H , and are derived from the integral form of Maxwell's equations on a staggered grid in spherical coordinates. The resulting matrix system of equations is sparse, symmetric, real everywhere except along the diagonal and ill-conditioned. The system is solved using the minimum residual conjugate gradient method with preconditioning by incomplete Cholesky decomposition of the diagonal sub-blocks of the coefficient matrix. In order to ensure there is zero H divergence in the solution, corrections are made to the H field every few iterations. In order to validate the code, we compare our results against an integral equation solution for an azimuthally symmetric, buried thin spherical shell model ( Kuvshinov & Pankratov 1994 ), and against a quasi-analytic solution for an azimuthally asymmetric configuration of eccentrically nested spheres ( Martinec 1998 ). 相似文献
5.
稳定渠道几何形态解析 总被引：1，自引：0，他引：1
本文基于临界理论,同时引入合理的水流切应力表达式,建立起稳定渠道断面形态微分方程,据此获得断面曲线、过水面积及湿周的全部分析解。在此基础上,结合现有基本力学方程,严格从理论上导出稳定渠道的河相关系,并用实测资料作初步验证,结果较为满意。还运用积分中值定理给出断面宽度的另一形式的分析解,以便于水力计算中应用。 相似文献
6.
David Al-Attar 《Geophysical Journal International》2007,171(2):755-760
A formal solution to the elastodynamic equation in an anelastic earth model is presented. The derivation also incorporates the effects of aspherical structure, rotation, self-gravitation, and pre-stress. It is found that the solution can be expressed as a sum of the normal modes of the earth model along with additional terms accounting for anelastic relaxation processes. However, the derivation does not assume that such an eigenfunction expansion is possible, and so avoids difficulties previously encountered due to the non self-adjointness of the problem. 相似文献
7.
Within the mushroom-shaped head of a cylindrical mantle plume melting occurs, the melt segregates from the matrix, and the matrix deforms and spreads laterally. These processes have been studied with a model of two-phase flow with melting. After characteristics of the solution near the axis of symmetry of the plume were found, a set of asymptotic relations for the variables along the symmetry axis was derived from McKenzie's equations for conservation of mass, momentum and energy of a two-phase system. The distribution of porosity along the plume axis and the vertical and radial segregation velocity of the melt in the vicinity of the axis of symmetry were obtained as functions of depth. Our analytic results show that within the head of a cylindrical mantle plume the contribution of the deformation of the matrix to the total non-hydrostatic pressure gradient cannot be neglected, and melt convergence or divergence is controlled by the radial scale of the upward velocity profile at the depth of the beginning of melting. 相似文献
8.
Jacques Hinderer Hilaire Legros Martine Amalvict 《Geophysical Journal International》1982,71(2):303-332
Summary. The basic equations describing the dynamical effects of the Earth's fluid core (Liouville, Navier-Stokes and elasticity equations) are derived for an ellipsoidal earth model without axial symmetry but with an homogeneous and deformable fluid core and elastic mantle.
We develop the balance of moment of momentum up to the second order and use Love numbers to describe the inertia tensor's variations. The inertial torque takes into account the ellipticity and the volume change of the liquid core. On the core—mantle boundary we locate dissipative, magnetic and viscous torques. In this way we obtain quite a complete formulation for the Liouville equations.
These equations are restricted in order to obtain the usual Chandler and nearly diurnal eigenfrequencies.
Then we propose a method for calculating the perturbations of these eigenfrequencies when considering additional terms in the Liouville equations. 相似文献
We develop the balance of moment of momentum up to the second order and use Love numbers to describe the inertia tensor's variations. The inertial torque takes into account the ellipticity and the volume change of the liquid core. On the core—mantle boundary we locate dissipative, magnetic and viscous torques. In this way we obtain quite a complete formulation for the Liouville equations.
These equations are restricted in order to obtain the usual Chandler and nearly diurnal eigenfrequencies.
Then we propose a method for calculating the perturbations of these eigenfrequencies when considering additional terms in the Liouville equations. 相似文献
9.
Synchronous or quasi-synchronous stereoscopic sea-ice-air comprehensive observation was conducted during the First China Arctic Expedition in summer of 1999. Based on these data, the role of sea ice in sea-air exchange was studied. The study shows that the kinds, distribution and thickness of sea ice and their variation significantly influence the air-sea heat exchange. In floating ice area, the heat momentum transferred from ocean to atmosphere is in form of latent heat; latent heat flux is closely related to floating ice concentration; if floating ice is less, the heat flux would be larger. Latent heat flux is about 21 23 6 W·m -2, which is greater than sensible heat flux. On ice field or giant floating ice, heat momentum transferred from atmosphere to sea ice or snow surface is in form of sensible heat. In the floating ice area or polynya, sea-air exchange is the most active, and also the most sensible for climate. Also this area is the most important condition for the creation of Arctic vapor fog. The heat exchange of a large-scale vapor fog process of about 500000 km 2 on Aug. 21 22,1999 was calculated; the heat momentum transferred from ocean to air was about 14 8×10 9 kW. There are various kinds of sea fog, radiation fog, vapor fog and advection fog, forming in the Arctic Ocean in summer. One important cause is the existence of sea ice and its resultant complexity of both underlying surface and sea-air exchange. 相似文献
10.
Dynamical rupture process on a three-dimensional fault with non-uniform frictions and near-field seismic waves 总被引：1，自引：0，他引：1
Summary. Dynamical rupture process on the fault is investigated in a quasi-three-dimensional faulting model with non-uniform distributions of static frictions or the fracture strength under a finite shearing pre-stress. The displacement and stress time functions on the fault are obtained by solving numerically the equations of motion with a finite stress—fracture criterion, using the finite difference method.
If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage.
The stick—slip faulting and irregular rupture propagation radiate high-frequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults. 相似文献
If static frictions are homogeneous or weakly non-uniform, the rupture propagates nearly elliptically with a velocity close to that of P waves along the direction of pre-stress and with a nearly S wave velocity in the direction perpendicular to it. The rise time of the source function and the final displacements are larger around the centre of the fault. In the case when the static frictions are heavily non-uniform and depend on the location, the rupture propagation becomes quite irregular with appreciably decreased velocities, indicating remarkable stick-slip phenomena. In some cases, there remain unruptured regions where fault slip does not take place, and high stresses remain concentrated up to the final stage. These regions could be the source of aftershocks at a next stage.
The stick—slip faulting and irregular rupture propagation radiate high-frequency seismic waves, and the near-field spectral amplitudes tend to show an inversely linear frequency dependence over high frequencies for heavily non-uniform frictional faults. 相似文献
11.
Summary. This paper describes a method that enables one to calculate the effects of localized heterogeneities on the wavefield in an otherwise regular medium. It does so by connecting a finite element solution for a heterogeneous inclusion to any type of solution for the regular medium, e.g. a reflectivity solution for a layered medium or an analytical solution for a simple half-space. Once the Green's functions for the regular medium are determined, the method reduces to a coupled set of algebraic equations for the wavefield, with the incident field and/or body forces as known variables. An efficient numerical scheme is derived for the solution of these equations. 相似文献
12.
Reflection coefficients for weak anisotropic media 总被引：1，自引：0，他引：1
Matthias Zillmer Dirk Gajewski Boris M. Kashtan 《Geophysical Journal International》1997,129(2):389-398
The interaction of plane elastic waves with a plane boundary between two anisotropic elastic half-spaces is investigated. The anisotropy dealt with in this study is of a general type. Explicit expressions for energy-related reflection and transmission coefficients are derived. They represent an approximation which is valid for a small deviation of the elastic parameters from isotropy.
Classical perturbation theory is applied on a 6times6 non-symmetric real eigenvalue problem to calculate first-order corrections for the polarization and stress of the plane waves. The explicit solution of the isotropic problem is used as a reference case. Degenerate perturbation theory is used to consider the splitting of the isotropic S -wave into two anisotropic qS-waves. The boundary conditions for two half-spaces in welded contact lead to a 6times6 system of linear equations. A correction to the isotropic solution is calculated by linearization. The resultant coefficients are functions of horizontal slowness, Lamé parameters and densities of the reference media, and of the perturbation of the elasticity tensors from isotropy. 相似文献
Classical perturbation theory is applied on a 6times6 non-symmetric real eigenvalue problem to calculate first-order corrections for the polarization and stress of the plane waves. The explicit solution of the isotropic problem is used as a reference case. Degenerate perturbation theory is used to consider the splitting of the isotropic S -wave into two anisotropic qS-waves. The boundary conditions for two half-spaces in welded contact lead to a 6times6 system of linear equations. A correction to the isotropic solution is calculated by linearization. The resultant coefficients are functions of horizontal slowness, Lamé parameters and densities of the reference media, and of the perturbation of the elasticity tensors from isotropy. 相似文献
13.
An introduction to Maslov's asymptotic method 总被引：3，自引：0，他引：3
Summary. Familiar concepts such as asymptotic ray theory and geometrical spreading are now recognized as an asymptotic form of a more general asymptotic solution to the non-separable wave equation. In seismology, the name Maslov asymptotic theory has been attached to this solution. In its simplest form, it may be thought of as a justification of disc-ray theory and it can be reduced to the WKBJ seismogram. It is a uniformly valid asymptotic solution, though. The method involves properties of the wavefronts and ray paths of the wave equation which have been established for over a century. The integral operators which build on these properties have been investigated only comparatively recently. These operators are introduced very simply by appealing to the asymptotic Fourier transform of Ziolkowski & Deschamps. This leads quite naturally to the result that phase functions in different domains of the spatial Fourier transform are related by a Legendre transformation. The amplitude transformation can also be inferred by this method. Liouville's theorem (the incompressibility of a phase space of position and slowness) ensures that it is always possible to obtain a uniformly asymptotic solution. This theorem can be derived by methods familiar to seismologists and which do not rely on the traditional formalism of classical mechanics. It can also be derived from the sympletic property of the equations of geometrical spreading and canonical transformations in general. The symplectic property plays a central role in the theory of high-frequency beams in inhomogeneous media. 相似文献
14.
陇中黄土高原垂直感热平流输送对LAS和EC观测感热通量差异的影响 总被引：1，自引：0，他引：1
利用"黄土高原陆面过程试验研究(LOPEX)"2010年6月定西站的陆面过程综合观测资料,以造成大孔径闪烁仪(LAS)和涡动相关仪(EC)测量感热通量的差异为切入点,从中尺度垂直感热平流输送过程入手,将垂直感热平流输送的贡献作为地表有效能量的一部分引入EC直接观测的感热通量,对比引入前后LAS和EC测量感热通量的差异大小.结果表明:黄土高原定西观测站近地层大气垂直方向主要表现为向上运动,峰值达到0.074 m·s-1,为垂直感热平流输送提供了必要的动力条件.尤其该地区处在半干旱区,地表受太阳辐射加热比较显著,近地层温度梯度最大可达0.39 K·m-1,为产生垂直感热平流提供了能量基础.加入垂直感热平流输送的贡献后,EC地表能量不闭合度的日均值由直接观测的0.30缩小到0.24.修正后,LAS和EC测量的感热通量拟合的线性趋势系数由直接观测时的1.258缩小到1.186,两者差异得以缩小. 相似文献
15.
When interpreting electromagnetic fields observed at the Earth's surface in a realistic geophysical environment it is often necessary to pay special attention to the effects caused by inhomogeneities of the subsurface sedimentary and/or water layer and by inhomogeneities of the Earth's crust. The inhomogeneities of the Earth's crust are expected to be especially important when the electromagnetic field is generated by a source located in a magma chamber of a volcano. The simulation of such effects can be carried out using generalized thin-sheet models, which were independently introduced by Dmitriev (1969 ) and Ranganayaki & Madden (1980 ). In the first part of the paper, a system of integral equations is derived for the horizontal current that flows in the subsurface inhomogeneous conductive layer and for the vertical current crossing the inhomogeneous resistive layer representing the Earth's mantle. The terms relating to the finite thickness of the laterally inhomogeneous part of the model are retained in the equations. This only marginally complicates the equations, whilst allowing for a significant expansion of the approximation limits.
The system of integral equations is solved using the iterative dissipative method developed by the authors in the period from 1978 to 1988. The method can be applied to the simulation of the electromagnetic field in an arbitrary inhomogeneous medium that dissipates the electromagnetic energy. When considered on a finite numerical grid, the integral equations are reduced to a system of linear equations that possess the same contraction properties as the original equations. As a result, the rate at which the iterative-perturbation sequence converges to the solution remains independent of the numerical grid used for the calculations. In contrast to previous publications on the method, aspects of the algorithm implementation that guarantee its effectiveness and robustness are discussed here. 相似文献
The system of integral equations is solved using the iterative dissipative method developed by the authors in the period from 1978 to 1988. The method can be applied to the simulation of the electromagnetic field in an arbitrary inhomogeneous medium that dissipates the electromagnetic energy. When considered on a finite numerical grid, the integral equations are reduced to a system of linear equations that possess the same contraction properties as the original equations. As a result, the rate at which the iterative-perturbation sequence converges to the solution remains independent of the numerical grid used for the calculations. In contrast to previous publications on the method, aspects of the algorithm implementation that guarantee its effectiveness and robustness are discussed here. 相似文献
16.
引发强沙尘暴的蒙古气旋的动力特征分析 总被引：10，自引：6，他引：4
对造成2002年4月5—9日东亚地区一次引发强沙尘暴天气过程的蒙古气旋发生发展过程进行了动力学分析。结果发现,这次气旋发展发生在斜压区中,500 hPa涡度平流和对流层下部的温度平流、大气加热率均对气旋的发生有贡献;气旋的发展阶段以温度平流为主,在气旋发展最强盛阶段对流层下部温度平流的作用大于其他项;气旋减弱阶段大气加热率所起的作用占主导地位,涡度平流和温度平流作用较小;Q矢量锋生函数分析表明大气斜压性的作用不可忽视。位涡的垂直输送作用明显,并形成了对流层内的垂直位涡柱。在气旋的发生发展过程中,始终伴有高空急流,它不仅有利于气旋的发展,同时还激发了低空急流的出现,因此引发该次强沙尘暴的蒙古气旋发生发展过程中对流层内有高低空急流耦合作用。 相似文献
17.
D. W. Vasco 《Geophysical Journal International》2000,142(3):970-990
Many geophysical inverse problems derive from governing partial differential equations with unknown coefficients. Alternatively, inverse problems often arise from integral equations associated with a Green's function solution to a governing differential equation. In their discrete form such equations reduce to systems of polynomial equations, known as algebraic equations. Using techniques from computational algebra one can address questions of the existence of solutions to such equations as well as the uniqueness of the solutions. The techniques are enumerative and exhaustive, requiring a finite number of computer operations. For example, calculating a bound to the total number of solutions reduces to computing the dimension of a linear vector space. The solution set itself may be constructed through the solution of an eigenvalue problem. The techniques are applied to a set of synthetic magnetotelluric values generated by conductivity variations within a layer. We find that the estimation of the conductivity and the electric field in the subsurface, based upon single-frequency magnetotelluric field values, is equivalent to a linear inverse problem. The techniques are also illustrated by an application to a magnetotelluric data set gathered at Battle Mountain, Nevada. Surface observations of the electric ( E y ) and magnetic ( H x ) fields are used to construct a model of subsurface electrical structure. Using techniques for algebraic equations it is shown that solutions exist, and that the set of solutions is finite. The total number of solutions is bounded above at 134 217 728. A numerical solution of the algebraic equations generates a conductivity structure in accordance with the current geological model for the area. 相似文献
18.
19.
In modelling complex a-centric phenomena which evolve through local interactions within a discrete time-space, cellular automata (CA) represent a valid alternative to standard solution methods based on differential equations. Flow-type phenomena (such as lava flows, pyroclastic flows, earth flows, and debris flows) can be viewed as a-centric dynamical systems, and they can therefore be properly investigated in CA terms.SCIDDICA S4a is the last release of a two-dimensional hexagonal CA model for simulating debris flows characterised by strong inertial effects. S4a has been obtained by progressively enriching an initial simplified model, originally derived for simulating very simple cases of slow-moving flow-type landslides.Using an empirical strategy, in S4a, the inertial character of the flowing mass is translated into CA terms by means of local rules. In particular, in the transition function of the model, the distribution of landslide debris among the cells is obtained through a double cycle of computation. In the first phase, the inertial character of the landslide debris is taken into account by considering indicators of momentum. In the second phase, any remaining debris in the central cell is distributed among the adjacent cells, according to the principle of maximum possible equilibrium.The complexities of the model and of the phenomena to be simulated suggested the need for an automated technique of evaluation for the determination of the best set of global parameters. Accordingly, the model is calibrated using a genetic algorithm and by considering the May 1998 Curti–Sarno (Southern Italy) debris flow.The boundaries of the area affected by the debris flow are simulated well with the model. Errors computed by comparing the simulations with the mapped areal extent of the actual landslide are smaller than those previously obtained without genetic algorithms. As the experiments have been realised in a sequential computing environment, they could be improved by adopting a parallel environment, which allows the performance of a great number of tests in reasonable times. 相似文献