共查询到19条相似文献，搜索用时 500 毫秒
1.
可控源海洋电磁勘探(MCSEM)中空气波对海底电磁响应的影响已为业界所熟知,且已提出了多种压制方法.然而,由于空气波与海水层中其它信号相互作用的复杂性,至今其作用机理及对有效信号的影响方式尚未完全清楚,这也阻碍了浅水域MCSEM油气勘探的应用.本文在前人研究的基础上, 基于一维层状介质的电磁位和电磁场分析,采用电磁场的模式分解理论导出了空气、海水层和海底地层之间的相互作用关系.分析了MCSEM电磁响应的影响因素和空气波的作用机理.以无限水深假想模型、三层介质模型和四层介质模型为基础,导出了水平电偶极子(HED)的横电(TE)和横磁(TM)模式的电磁场关系,分析显示了前人提出的上下行波分离进行空气波压制所存在的缺陷.通过理论和数值模拟知道, 利用横电(TE)和横磁(TM)模式受空气相互作用影响的差异和电磁场水平分量与垂直分量受空气层相互作用影响程度的不同可减弱空气波对有效信号的影响,这将有助于实现浅水域海洋电磁勘探数据的有效利用. 相似文献
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拟解析近似方法是一种解决电磁场散射问题的快速求解积分方程的近似方法,它绕开了传统数值方法中的求解大型代数方程组或大型矩阵问题,适用于强散射和大扰动问题.本文应用孙建国提出的求解异常电场积分方程的张量拟解析近似理论公式,研究用其求解直流电场积分方程.利用接近实际的地电模型对异常电场进行模拟,研究了均匀场中异常球体的张量拟解析近似解;并对均匀场中的立方体异常体进行了数值计算.效果良好并具有很高的计算速度.研究结果为三维直流电场快速正反演模拟打下基础. 相似文献
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在海面波破碎区，假定随机粗糙的海面被两层不同散射性质的球形粒子覆盖，上层是由海面飞沫组成的球形Ｒａｙｌｅｉｇｈ粒子层，下层是由气泡组成的球形Ｍｉｅ粒子层，根据大量观测和实验的统计结果，确定了两层散射粒子的平均半径和单位体积的粒子数；并用迭代法求解随机粗糙海面上的两层粒子的矢量辐射传输方程．根据风波破碎时白冠覆盖率和海面飞沫、气泡的统计模型，给出了一个新的风驱海面的微波散射理论模型．利用这一模型定量地给出了海面后向散射系数和风速、视角、极化和白冠覆盖率等特征参量之间的函数关系，并研究了覆盖在波破碎区的散射粒子对海面后向散射系数影响程度． 相似文献
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提出了一种新的以层状半空间中周期分布斜线荷载动力格林函数为基本解的间接边界元方法,研究了周期分布凸起地形对平面SH波的散射问题.方法将散射波场分解为凸起内部散射波场和凸起外部散射波场.凸起内部散射波场通过在凸起闭合边界上施加虚拟斜线荷载产生的动力响应来模拟,而凸起外部散射波场则通过在凸起与半空间交界面上施加虚拟周期分布斜线荷载产生的动力响应来模拟.周期分布斜线荷载动力格林函数的引入,使得本文方法仅需针对一个凸起进行边界单元的离散和求解,便可完成问题的求解,避免了通过截断无限边界求解而引入的误差,方法具有较高精度的同时显著降低了求解自由度.文中通过与已有结果的比较,验证了方法的正确性,并以均匀半空间和基岩上单一土层中周期分布凸起为例进行了数值计算分析.研究表明,凸起间距对凸起地形间的动力相互作用有着显著的影响,同时层状半空间中周期分布凸起地形对SH波的散射与均匀半空间情况也有着显著的差别. 相似文献
5.
根据瞬变电磁场所满足的扩散方程与波动场所满足的波动方程之间的数学关系,运用扫时波场变换技术,得到稳定的虚拟波场.在虚拟波场的基础上,借用地震勘探中的Born近似逆散射技术实现对电性界面进行成像的目的.在理论推导中,直接从时间域波动方程出发,运用格林定理、引入均匀半空间的Green函数建立散射场关于速度扰动量的表达式;在数值求解中,运用奇异值分解的方法实现对第一类Fredholm积分方程的求解.通过对理论模型的计算以及实例应用,均表明瞬变电磁虚拟波场Born近似算法对地下电性界面具有良好的分辨能力. 相似文献
6.
针对大地电磁二维反演中TE、TM极化模式的选择问题,设计了多个二维、三维模型进行了正演计算,从数据对比和反演结果对比两方面入手,分析研究了三维模型和二维模型响应数据的差异、反演中极化模式的选择、以及三维模型数据的二维反演近似等问题.研究结果表明:在三维模型条件下,利用二维模型进行反演时,TE模式对模型的二维的近似程度要求远高于TM模式;当三维结构影响较明显时,利用TM模式数据进行二维反演比利用TE模式或利用TE+TM模式联合反演都更合理,反演结果中的虚假结构明显减少;对于TM模式,相位受三维畸变影响较小,视电阻率较大,所以二维反演中可适当加大相位的权;对于实测数据的二维反演,应优先考虑采用TM模式数据进行二维反演,其次是TM+TE模式,一般不要单独采用TE模式. 相似文献
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在层状半空间精确动力刚度矩阵和斜线荷载动力格林函数的基础上建立间接边界元方法,在频域内求解无破碎带断层场地对入射平面SH波的散射。为方便求解,将总波场分解为自由波场和散射波场,自由波场由直接刚度法求得,断层两侧的散射波场通过在断层面上分别对两侧施加均布斜线荷载产生的动力响应来模拟,虚拟荷载的密度可通过引入断层表面的边界条件确定,最后叠加自由波场和散射波场求得总波场。以有落差断层和无落差断层模型为例进行数值计算,分析断层落差、断层倾角以及断层两侧介质的刚度比对散射效应的影响。研究表明,断层落差与波长相当时,断层对SH波的放大作用最大;地表位移幅值随着断层倾角的增大逐渐增大;若断层无落差且其两侧刚度不同时,一般刚度较小一侧地表位移幅值较大且振荡更为剧烈,波从刚度较小一侧入射时位移幅值放大尤为显著。 相似文献
9.
基于自适应有限元的起伏地形MT二维正演模拟 总被引：1，自引：0，他引：1
本文利用自适应有限元方法模拟起伏地形对大地电磁数据的影响.其中非结构化网格的自动剖分采用基于后处理技术的后验误差方法(又称Z-Z方法)进行控制、加密;通过地堑、山脊、地垒、山谷四种模型讨论了起伏地形对大地电磁TE和TM两种极化模式的视电阻率和阻抗相位曲线的影响,其中仅取10 Hz(高频)和0.01 Hz(低频)两个频点进行分析.证实了复杂的地形条件对TM模式的影响要比对TE模式的影响大得多,在TM极化模式中,低频部分的视电阻率比高频部分的更容易受起伏地形影响,而在TE极化模式中,则刚好相反等重要结论,为实际工作提供了理论指导作用. 相似文献
10.
本文采用反射映像（即U/D成像）原理对大地电磁资料进行成像,将地面观测的波场分解成上行波和下行波,并分别向下延拓,利用上、下行波的时间一致性原理确定地下电性界面的位置.对TE和TM模式的MT响应分别处理可以得到两个深度剖面,两剖面的一致性便为实际反射界面的最佳估计.理论和实际资料的计算表明,MT偏移技术是比较有效的,它可获得地下界面直观的映像.和常规MT反演方法相比较,该方法具有算法简单等优点,并且还能获得表示地下地质构造真实映像的MT深度剖面. 相似文献
11.
Scattering of elastic waves by two dimensional multilayered dipping sediments of arbitrary shape embedded in an elastic half-sapce is investigated by using a bondary method. The displancement field is evaluated throughout the elastic media for both steady state and transient incident SH waves. The unknown scattered field is expressed in terms of wave functions which satisfy the equation of motion, traction-free boundary condition and appropariate radiation conditions. The transient response is constructed from the steady state solution by using the fast Fourier transform technique. The numerical results presented demonstrate that scattering of waves by subsurface irregularities may cause locally very large amplification of surface ground motion. The motion can be affected greatly by the scattered surface waves in the sediments. The results clearly indicate that the surface ground motion depends upon a number of parameters present in the problem, such as frequency and the angle of incidence of the incoming wave, impedance contrast between the layers and location of the observation point. 相似文献
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为了了解地震震源和地球介质的性质,很有必要对地震波的辐射、传播和衰减问题作仔细的分析。作为一种近似,可以暂且忽略地球的曲率,把传播地震波的地球介质视为多层半空间。为简便起见,地震波的衰减问题另作考虑。这样,便需要研究多层、均匀、各向同性和完全弹性半空间中地震震源辐射的地震波传播问题。 用哈斯克尔（Haskell）矩阵法解多层介质中弹性波的传播问题是很方便的。如果 相似文献
14.
一、引言 地震面波的频散性质、地震波辐射的方向性等特性已经广泛地用于地壳和上地幔结构以及震源机制的研究中,并且取得了许多有用的成果.研究地震波如何从震源辐射出来、如何在实际介质中传播和衰减的这一问题,对于利用地震波确定地壳和上地幔结构以及震源的参数,是很有必要的.关于这一问题的研究,已经作过许多工作。已往的工作中,为了分析方便,往往采用简单的地壳-上地幔模型或简单的震源模型,或两者都相 相似文献
15.
Arthur Frankel 《Pure and Applied Geophysics》1989,131(4):639-685
This paper reviews applications of the finite-difference and finite-element methods to the study of seismic wave scattering in both simple and complex velocity models. These numerical simulations have improved our understanding of seismic scattering in portions of the earth where there is significant lateral heterogeneity, such as the crust. The methods propagate complete seismic wavefields through highly complex media and include multiply scattered waves and converted phases (e.g.,P toSV, SV toP, body wave to surface wave). The numerical methods have been especially useful in cases of moderate and strong scattering in complex media where multiple scattering becomes important. Progress has been made with numerical methods in understanding how near-surface, low-velocity basin structures scatter surface waves and vertically-incident body waves. The numerical methods have proven useful in evaluating scattering of surface waves and body waves from topography of both the free surface and interfaces buried at depth. Numerical studies have demonstrated the importance of conversions from body waves to surface waves (andvice versa) when lateral heterogeneities and topographic relief are present in the uppermost crust. Recently, several investigations have applied numerical methods to study seismic wave propagation in velocity models which vary randomly in space. This stochastic approach seeks to understand the effects of small-scale complexity in the earth which cannot be resolved deterministically. These experiments have quantified the relationships between the statistical properties of the random heterogeneity and the measurable properties of high-frequency (1 Hz) seismograms. These simulations have been applied to the study of many features observed in actual high-frequency seismic waves, including: the amplitude and time decay of seismic coda, the apparent attenuation from scattering, the dispersion of waveforms, and the travel time and waveform variations across arrays of receivers. 相似文献
16.
This paper presents a semi-analytical method for studying the two-dimensional problem of elastic wave scattering by surface irregularities in a half-space. The new method makes use of the member of a c-completeness family of wave functions to construct the scattering fields, and then applies equal but opposite tractions to those of the foregoing constructed scattering fields on the horizontal surface of the half-space to produce additional scattering fields. These additional scattering fields are a series of Lamb's solutions. Thus the whole scattering field constructed in the series automatically satisfies the Navier equations, the condition of zero traction on the half-space surface, and the radiation boundary conditions at infinity. Using the traction-free conditions along the canyon surface, the coefficients of the series solutions are determined via a least-squares method. For incident P, SV, and Rayleigh waves, the numerical results are presented for the scattering displacements in the vicinity of a semi-circular canyon in the half-space. 相似文献
17.
Scattering of an arbitrary elastic wave incident upon a spherically symmetric inclusion is considered and solutions are developed in terms of the spherical vector system of Petrashen, which produces results in terms of displacements rather than displacement potentials and in a form suitable for accurate numerical computations. Analytical expressions for canonical scattering coefficients are obtained for both the cases of incidentP waves and incidentS waves. Calculations of energy flux in the scattered waves lead to elastic optical theorems for bothP andS waves, which relate the scattering cross sections to the amplitude of the scattered fields in the forward direction. The properties of the solutions for a homogeneous elastic sphere, a sphere filled by fluid, and a spherical cavity are illustrated with scattering cross sections that demonstrate important differences between these types of obstacles. A general result is that the frequency dependence of the scattering is defined by the wavelength of the scattered wave rather than the wavelength of the incident wave. This is consistent with the finding that the intensity of thePS scattering is generally much stronger than theSP scattering. When averaged over all scattering angles, the mean intensity of thePS converted waves is2V
p
2
/V
s
4
times the mean intensity of theSP converted waves, and this ratio is independent of frequency. The exact solutions reduce to simple and easily used expressions in the case of the low frequency (Rayleigh) approximation and the low contrast (Rayleigh-Born) approximation. The case of energy absorbing inclusions can also be obtained by assigning complex values to the elastic parameters, which leads to the result that an increase in attenuation within the inclusion causes an increased scattering cross section with a marked preference for scatteredS waves. The complete generality of the results is demonstrated by showing waves scattered by the earth's core in the time domain, an example of high-frequency scattering that reveals a very complex relationship between geometrical arrivals and diffracted waves. 相似文献
18.
O. N. Savina P. A. Bespalov V. O. Rapoport N. A. Ryzhov 《Geomagnetism and Aeronomy》2006,46(2):247-253
The nonlinear equation of the first order of the Riccati type has been obtained for the wave impedance of acoustic gravity waves in the nonisothermal atmosphere. The vertically nonuniform horizontal wind and the effect of viscosity on the horizontal components of the velocity field have been taken into account in the calculations. The boundary-value problem for the Riccati equation is defined by the boundary emission condition at high altitudes. Upon finding the wave impedance along with the generalized polarization relationship, all remaining disturbances of the atmospheric parameters related to acoustic gravity waves are found with the help of a simple integration. The results of using a developed formalism are illustrated by the numerical computation of acoustic gravity wave fields in the atmosphere with real vertical profiles of temperature and horizontal field velocity. 相似文献
19.
Yuehua Zeng 《Pure and Applied Geophysics》2006,163(2-3):533-548
One of the many important contributions that Aki has made to seismology pertains to the origin of coda waves (Aki, 1969; Aki
and Chouet, 1975). In this paper, I revisit Aki's original idea of the role of scattered surface waves in the seismic coda.
Based on the radiative transfer theory, I developed a new set of scattered wave energy equations by including scattered surface
waves and body wave to surface wave scattering conversions. The work is an extended study of Zeng et al. (1991), Zeng (1993) and Sato (1994a) on multiple isotropic-scattering, and may shed new insight into the seismic coda wave
interpretation. The scattering equations are solved numerically by first discretizing the model at regular grids and then
solving the linear integral equations iteratively. The results show that scattered wave energy can be well approximated by
body-wave to body wave scattering at earlier arrival times and short distances. At long distances from the source, scattered
surface waves dominate scattered body waves at surface stations. Since surface waves are 2-D propagating waves, their scattered
energies should in theory follow a common decay curve. The observed common decay trends on seismic coda of local earthquake
recordings particular at long lapse times suggest that perhaps later seismic codas are dominated by scattered surface waves.
When efficient body wave to surface wave conversion mechanisms are present in the shallow crustal layers, such as soft sediment
layers, the scattered surface waves dominate the seismic coda at even early arrival times for shallow sources and at later
arrival times for deeper events. 相似文献