空腔爆炸产生的地震波

本文首先讨论了压力空腔和爆炸中心的等价问题。计算层状介质中的空腔爆炸所产生的位移场时,可先求介质的单位脉冲响应,然后将其与等价的爆炸点源函数做褶积。对于均匀的半空间,自由面位移的暂态解是由三个无量纲的参数所决定的;由此可导出远场位移的精确解。本文也讨论了爆炸点附近的运动。最后还导出多层介质中空腔爆炸所产生的表面位移谱的形式解。     

时间域航空电磁法激电效应对电磁扩散的影响

由于激发极化效应的影响,时间域航空电磁晚期道信号经常会出现变号现象.基于电阻率的传统反演方法无法对变号数据进行正确反演,因此通常在数据处理中予以剔除.为深入了解极化介质的电磁扩散特征,认识航空瞬变电磁负响应的产生机理,本文研究时间域航空电磁系统的电磁扩散特征.我们以均匀极化、非极化半空间及层状介质模型为例,通过直接积分的方法求解频率域电场响应,并由欧姆定律得到电流响应,再经过汉克尔变换得到时间域电流响应.通过研究电流随时间在地下极化介质中的传播特征研究电磁扩散过程;通过对比不同激电参数对电磁扩散的影响,研究极化介质中感应电流与极化电流的扩散规律,从而合理地解释极化介质中负响应的产生机理.基于本文研究和分析结果,可加深对时间域航空电磁法中激电效应的认识.     

VERTICAL FREQUENCY EFFECT SOUNDING IN INDUCED POLARIZATION AND GALVANIC RESISTIVITY METHODS *

It has been shown that for a polarizable layer as a transmission medium there is an indirect proportionality between the frequency effect (fe) in induced polarization (IP) and the wave number of the electrical induced field. Making use of this relationship for a two layered earth a polarization transform function has been obtained. Since the mathematical expression for the polarization transform function is the same as that of the resistivity transform function, it is possible to make direct interpretation for IP frequency effect field curves. Thus, AA or QQ type resistivity sequences can be interpreted from induced polarization response of a horizontally stratified earth without resistivity extrema. A depth factor has been defined in order to obtain the true depth using the apparent depth. In this way, some electromagnetic effects between horizontal layers with different polarizabilities can partly be eliminated.     

Matrix polynomial representation of the anisotropic magnetotelluric impedance tensor

A formalism based upon the equal penetration depth stratification assumption is extended to a layered anisotropic medium, yielding a recursive algorithm for the computation of the magnetotelluric impedance tensor elements. The development of this new procedure requires an appropriate layer-discretization as well as an extension of the reflection coefficient definition for the anisotropic layered model.This method transfers the differential problem into an algebraic one, and is independent of the electric and magnetic field vectors. The technique is a recursive process which gives a rational matrix polynomial representation for the magnetotelluric impedance tensor.The procedure involves the use of second-order matrices, rather than the fourth-order ones normally used for such a case. Using this representation, a separation of the contribution of the model parameters from that of the frequency is achieved. Consequently the elements of the magnetotelluric impedance tensor are computed, for each frequency, using coefficients which are evaluated just once for a given model.     

Equivalent time-average and effective medium for periodic layers

The propagation of acoustic waves through a periodic layered medium is analyzed by an eigenvalue decomposition of the propagator matrix. This reveals how the velocity and attenuation of the layered medium vary as function of the periodic structure, material parameters and frequency. There are two important parameters which control the wave propagation in the periodic medium: the reflection coefficient and the ratio between one‐way traveltimes of the two parts of the cyclic layered medium. For low frequencies (large values of wavelength to layer thickness), the layered structure behaves as an effective medium, then there is a transition zone, and for higher frequencies (small values of wavelength to layer thickness) the medium is described by the time‐average velocity. In this paper we mostly concentrate on the transition zone between an effective medium and time‐average medium regimes. The width of the transition zone increases with larger values of the reflection coefficient. The transition zone corresponds to a blocking regime for which the transmission response of the layered structure is close to zero. For even higher frequencies, the time‐average medium is replaced by a new transition zone, and then again a time‐average medium. This pattern is periodically repeated with higher frequencies. For small values of the reflection coefficient, the transition between effective medium and time‐average medium occurs around a value of wavelength to layer thickness equal to 4.     

On Stress Distribution in Layered Rock Masses

Spatial distribution of principal stresses in a layered elastic medium is considered in the paper. Transformations of the principal stresses across an interface between the layers are analyzed. A method for reconstructing the spatial stress distributions in layered medium from several spot measurements is proposed. The introduced approach is capable of obtaining the stable solution of the stress reconstruction problem with respect to the noise in input data. A typical layered medium is considered for which the stresses are reconstructed from few independent spot measurements of the stress state. The proposed approach may be used for solving an inverse problem of tectonic stresses reconstruction from the borehole logging data.     

MT激电效应的模拟研究及在油气检测中的应用

本文引入Cole-Cole模型来模拟大地的激发极化效应,对三层水平地层且中间层为极化层的大地电磁测深的视电阻率进行了理论计算,分析了极化参数对视电阻率曲线的影响规律.采用广义逆方法对三层水平地层且中间层为极化层的模型进行了反演研究,结果表明该反演方法能够较好地确定地层电阻率的同时获得地层激电参数,应用于实际资料的反演时,反演结果与已知含油气地层的实际参数十分吻合.     

Dispersion splitting of Rayleigh waves in layered azimuthally anisotropic media

Rayleigh wave dispersion can be induced in an anisotropic medium or a layered isotropic medium. For a layered azimuthally anisotropic structure, traditional wave equation of layered structure can be modified to describe the dispersion behavior of Rayleigh waves. Numerical stimulation results show that for layered azimuthal anisotropy both the dispersion velocities and anisotropic parameters depend principally on anisotropic S-wave velocities. The splitting S-wave velocities may produce dispersion splitting of Rayleigh waves. Such dispersion splitting appears noticeable at azimuthal angle 45°. This feature was confirmed by the measured results of a field test. The fundamental mode splits into two branches at azimuthal angle 45° to the symmetry axis for some frequencies, and along the same direction the difference of splitting-phase velocities of the fundamental model reaches the maximum. Dispersion splitting of Rayleigh waves was firstly displayed for anisotropy study in dispersion image by means of multichannel analysis of surface waves, the image of which provides a new window for studying the anisotropic property of media.     

横观各向同性层状半空间中的弹性位错

本文在柱向量函数系下,利用传播矩阵法求解了层状横观各向同性半空间由内部点源位锚引起的变形;对六个基本点源位错,以等价体力法推出了横观各向同性情形下的点源函数,并且给出了内部任意剪切位错源引起的地表位移的积分表达式。为研究地球的层状结构,特别是其上部的横观各向同性对地表的地震位移、应变以及倾斜场的影响提供了计算公式。     

Electromagnetic fields in a steel-cased borehole

The development of an electromagnetic numerical modelling scheme for a magnetic dipole in an arbitrary casing segment in an inhomogeneous conductivity background has been difficult, due to the very high electrical conductivity and magnetic permeability contrasts between the steel casing and the background medium. To investigate the effect of steel casing efficiently, we have developed an accurate but simple finite‐element modelling scheme to simulate electromagnetic fields in a medium of cylindrically symmetric conductivity structures. In order to preserve the cylindrical symmetry in the resulting electromagnetic fields, a horizontal loop current source is used throughout. One of the main advantages of the approach is that the problem is scalar when formulated using the azimuthal electric field, even if the casing is both electrically conductive and magnetically permeable. Field calculations have been made inside the cased borehole as well as in another borehole which is not cased. Careful analyses of the numerical modelling results indicate that the anomaly observed in a cross‐borehole configuration is sensitive enough to be used for tomographic imaging.     

Low-frequency symmetric waves in fluid-filled boreholes and pipes with radial layering

Many tasks in geophysics and acoustics require estimation of mode velocities in cylindrically layered media. For example, acoustic logging or monitoring in open and cased boreholes need to account for radial inhomogeneity caused by layers inside the borehole (sand screen, gravel pack, casing) as well as layers outside (cement, altered and unaltered formation layers). For these purposes it is convenient to study a general model of cylindrically layered media with inner fluid layer and free surface on the outside. Unbounded surrounding media can be described as a limiting case of this general model when thickness of the outer layer is infinite. At low frequencies such composite media support two symmetric modes called Stoneley (tube) and plate (extensional) wave. Simple expressions are obtained for these two mode velocities valid at zero frequency. They are written in a general form using elements of a propagator matrix describing axisymmetric waves in the entire layered composite. This allows one to apply the same formalism and compute velocities for n -layered composites as well as anisotropic pipes. It is demonstrated that the model of periodical cylindrical layers is equivalent to a homogeneous radially transversely isotropic media when the number of periods increases to infinity, whereas their thickness goes to zero. Numerical examples confirm good validity of obtained expressions and suggest that even small number of periods may already be well described by equivalent homogeneous anisotropic media.     

层状介质对破裂过程的影响

震源动力学中破裂产生的地震动在层状介质中的传播模拟,是地震学以及地震工程学研究的前沿课题之一。本文通过建立精确的三维模型,选取具备灵活网格、高精度高效率计算性能的谱元法,利用有效抑制伪震荡的时间域离散方法——加权速度Newmark方法以及多次透射人工边界条件,进行了SCEC/USGS基准项目中TPV5模型的地震破裂过程模拟,得到基于层状介质模型和均匀介质模型（后者采用相同破裂模型）的埋深2km的震源参数结果。将二者进行对比,并具体分析破裂面位错、地震矩、破裂传播时间、上升时间和地表位移,发现层状介质对破裂过程的传播影响较为明显:① 层状介质的存在整体增加了破裂面上的位错,在层状介质模型下计算得到的地震矩约是均匀介质模型结果的1.3倍,因此认为层状介质增强了地震破裂过程中的能量释放;② 层状介质的存在使得破裂传播至地表的速度减慢,并缩短了地表各点的上升时间,增强了地表的地震动响应;③ 层状介质对于地表位移有着明显的增加作用,同时协同破裂面上的初始应力异常区域对位移峰值中心的改变有显著影响。④ 介质分异面附近地震动强烈。对结果进行整理后发现,在具有地下层状介质的地区要充分考虑层状介质产生的场地效应,否则可能会低估该地区的地震危险性。     

One Solution of the Forward Problem of DC Resistivity Well Logging by the Method of Volume Integral Equations with Allowance for Induced Polarization

For theoretically studying the intensity of the influence exerted by the polarization of the rocks on the results of direct current (DC) well logging, a solution is suggested for the direct inner problem of the DC electric logging in the polarizable model of plane-layered medium containing a heterogeneity by the example of the three-layer model of the hosting medium. Initially, the solution is presented in the form of a traditional vector volume-integral equation of the second kind (IE2) for the electric current density vector. The vector IE2 is solved by the modified iteration–dissipation method. By the transformations, the initial IE2 is reduced to the equation with the contraction integral operator for an axisymmetric model of electrical well-logging of the three-layer polarizable medium intersected by an infinitely long circular cylinder. The latter simulates the borehole with a zone of penetration where the sought vector consists of the radial J_r and J_z axial (relative to the cylinder’s axis) components. The decomposition of the obtained vector IE2 into scalar components and the discretization in the coordinates r and z lead to a heterogeneous system of linear algebraic equations with a block matrix of the coefficients representing 2x2 matrices whose elements are the triple integrals of the mixed derivatives of the second-order Green’s function with respect to the parameters r, z, r', and z'. With the use of the analytical transformations and standard integrals, the integrals over the areas of the partition cells and azimuthal coordinate are reduced to single integrals (with respect to the variable t = cos ? on the interval [?1, 1]) calculated by the Gauss method for numerical integration. For estimating the effective coefficient of polarization of the complex medium, it is suggested to use the Siegel–Komarov formula.     

弹性半空间位错内源的数值解

本文首先将一般剪切位错点源表示成四种基本点位错的组合,断层面倾角和滑动系数仅与组合系数有关.利用将三维动力学问题化为两维的处理方法,将每一基本点源问题化成求解位移函数的轴对称问题,最后将人工透射边界的处理方法同有限差分结合起来,提出一种简便、节省的计算理论地震图的方法.利用这种方法获得了四种基本点位错源的典型近场理论地震图,并初步讨论了近场地面运动的若干特征,表明浅源地震近场地面位移中震相不能分离,且点位错源在近场产生大小相当的水平与垂直位移.本文的方法可用于研究复杂的近场地震波传播问题.      

电性任意各向异性且分块连续变化CSAMT三维有限元数值模拟

考虑地球介质电导率任意各向异性且随空间位置连续变化的情况,本文实现了直接求解电磁场的可控源音频大地电磁测深（CSAMT）三维有限元数值模拟.首先给出了电导率任意各向异性介质中CSAMT二次电场满足的控制方程及其相应变分问题,然后采用任意六面体单元对研究区域进行剖分,在网格单元中对任意各向异性电导率进行线性插值,解决了实际工作中岩矿石电导率各向异性且连续变化的情况,将变分问题转化为线性代数方程组的求解.电导率各向异性且连续变化一维模型三维有限元数值模拟结果与电导率各向异性且分层均匀渐进模型解析解结果对比验证了方法的有效性;三维地电模型电导率随位置线性变化且各向同性、主轴各向异性、方位各向异性和倾斜各向异性的数值模拟结果表明,电导率各向异性且连续变化对CSAMT视电阻率和相位数据均有明显的影响.     

层状多孔介质的地震电磁异常数值模拟方法研究

本文针对电激发模型在各向同性分层介质中利用传播矩阵方法对电磁异常产生过程的数值模拟.层状模型中一部分为饱和多孔介质,另一部分为普通线弹性介质,数值模拟过程中将固体场作为主要场单独进行计算,而将其形变作为等效流体源计算耦合电磁场.模拟结果显示,地震波在经过饱和多孔介质边界的时候会产生电磁场,而其在流体层内部传播的过程中则未产生电磁异常.模拟结果可以用于解释一些伴随地震产生的电磁异常,包括地震发生时以及地震波到达时所产生的电磁异常现象,另一方面对于前震及其它一些无感地震所产生的电磁异常的解释也具有一定的参考价值.      

Pseudo-3D constrained inversion of transient electromagnetic data for a polarizable SMS hydrothermal system in the Deep Sea

Seafloor massive sulfide (SMS) deposits are generated by high-temperature hydrothermal systems. Their precious resources have attracted global interest. A number of investigations with controlled-source electromagnetic (CSEM) methods have been implemented in recent years. There are three major problems with SMS surveying using EM methods. First, SMS imaging techniques for hydrothermal systems have a limited range. Simulations and applications have validated only simple layered models. Second, their inversion efficiencies must be improved further. Laterally constrained inversions and spatially constrained inversions are usually used to map geological structures. However, choosing their suitable weighting parameters is inefficient. Third, the effects of induced polarization (IP) on ore deposits are not considered in such inversions. A non-polarizable model is unable to accurately depict a polarizable model. To resolve these problems, an advanced strategy is used to improve the efficiency of the pseudo-3D inversion process. The proposed imaging method has the ability to map complex 3D geoelectrical structures, and therefore, it can both obtain information regarding surface ore deposits and distinguish between active and inactive hydrothermal systems. However, this method can also be used to depict the distributions of alteration zones and buried deposits. Furthermore, the influences of IP on the inversion are discussed with respect to the Cole- Cole model, and it is shown that the effects of IP on polarizable deposits cannot be ignored during the inversion.     

Transhorizon propagation of decameter waves

Solutions to the problem of the point source field in a spherically layered medium are analyzed. For a three-layer waveguide model, a solution in the form of the Watson integral was used. A consideration of the singularities in the plane of the integration variable made it possible to represent the integral as a superposition of three waves. Two of them are connected with the interaction of the primary spherical wave with the lower convex and upper concave interfaces. The third wave is connected with the alternate action with both interfaces. The fourth wave is caused by the interaction between the primary wave and random inhomogeneities of the external medium (the ionosphere). Here, simulation was carried out based on Green equations. The considered unique data of flight measurements of the point source field strength indicate the efficiency of simulating the transhorizon propagation of decameter waves based on the superposition of all four aforesaid wave packets.     

二维频率测深边界单元法正演计算

在二维层状介质中含有横向非均匀体的地电断面情况下，对电偶源的偶极长度作了延长，并选取了适当的基本解及无穷远边界条件，因而将相应的含源谐变电磁场的边值问题转化为较简单的边界积分方程．用边界单元法求出了相应的电磁场分量，进而计算阻抗视电阻率，并对这一算法作了相应的检验．