怀来、 延庆及其周边地区地壳 介质各向异性研究

提出了利用人工爆破P波走时反演地壳介质方位各向异性参数的方法. 在假定介质是弱各向异性介质的情况下, 使用扰动理论得到了线性化的反演公式, 其中待反演的弱各向异性参数是P波走时的线性函数. 如果在反演公式中参考走时取相同震中距接收点的P波平均走时, 那么所获得的弱各向异性参数与参考介质速度的选取无关. 反演得到的弱各向异性参数可以看作是不同震中距和不同深度范围内介质的等效弱各向异性参数. 等效弱各向异性参数在一定程度上反映了不同深度范围内水平方向相速度随方位的变化. 这种变化可能是不同时期构造应力作用的结果. 2007年中国地震局在首都圈怀来地区实施了一次大吨位人工爆破实验, 以爆破点为中心, 布设了高密度的地震观测台网和台阵. 台站相对于爆破点具有360°的全方位覆盖, 所得到的地震记录数据为研究怀来、 延庆地区地壳介质P波方位各向异性提供了必要条件. 我们通过走时反演获得了与水平方位相关的弱各向异性参数, 并对弱各向异性参数进行坐标变换, 得到了能够直观描述岩石弱各向异性的具有水平对称轴的横向各向同性介质, 给出了对应的3个独立弱各向异性参数及其对称轴方位, 讨论了介质各向异性与构造应力场的关系. 结果表明该地区地壳介质存在明显的方位各向异性, 其最大值约为4.6%.      

各向异性TI介质qP反射波走时层析成像

地震走时层析成像是反演地层各向异性参数分布的有效方法,但是关于地震各向异性介质走时层析成像的研究并不多,其技术远远没有达到成熟的阶段.在野外数据采集时,地表反射波观测方式相对井间和垂直地震剖面观测方式的成本更低,利用qP反射波走时反演各向异性参数具有更加广泛的实用价值.本文实现的TI介质地震走时层析成像方法结合了TI介质反射波射线追踪算法、走时扰动方程和非线性共轭梯度算法,它可以对任意强度的TI介质模型进行反演,文中尝试利用qP反射波走时重建TI介质模型的参数图像.利用qP反射波对层状介质模型和块状异常体模型进行走时反演,由于qP波相速度对弹性模量参数和Thomsen参数的偏微分不同,所以可以分别反演弹性模量参数和Thomsen参数.数值模拟结果表明:利用qP反射波可以反演出TI介质模型的弹性模量参数与Thomsen参数,不同模型的走时迭代反演达到了较好的收敛效果,与各向同性介质走时反演结果相比较,各向异性介质走时反演结果具有较好的识别能力.     

转换波方位各向异性裂缝检测技术研究及应用

HTI裂缝各向异性介质中,转换波随方位角的变化比较复杂,目前还没有解析公式可以表达其变化特征,只能通过物理实验或数值模拟来分析其应用的可能性.数值模拟结果表明,转换波在裂缝各向异性介质中传播时,其R分量和T分量的振幅属性都具有明显的方位各向异性特征,R分量振幅方位各向异性拟合椭圆的长轴方向指示裂缝方位,这与纵波方位各向异性特征相似;根据P波AVAZ方位各向异性分析原理,对转换波R分量振幅方位各向异性曲线进行方位椭圆拟合,寻找椭圆的长轴方向,即裂缝主方位,再由P波AVAZ技术中振幅响应与炮检方向和裂缝走向之间的夹角关系式得到裂缝的发育密度,从而构建转换波方位各向异性AVAZ裂缝检测技术.该技术已用于川西新场气田某区块的裂缝储层预测,取得了较好的应用效果.     

各向异性介质中S波接收函数成像方法

岩石圈各向异性主要由上地幔矿物晶格优势排列方向和上地壳内裂缝、裂隙的定向分布造成.在各向异性特征显著区域,利用SKS震相剪切波分裂获得的延迟时间高达1.5s以上.本文根据方位各向异性,利用广义反射透射系数矩阵方法正演S波接收函数,研究各向异性对不同反方位角接收函数转换震相走时的影响.我们发展了基于HTI模型各向异性走时校正的方法,成功的在单层和多层(快轴方向相同或不同)的各向异性介质中对齐不同反方位角接收函数的Moho面和LAB的转换震相走时.我们将该方法应用于在青藏高原东北缘的流动台站,试图在各向异性强度较大区域对实测数据转换震相走时的校正效果进行测试.结果表明:各向异性走时校正能够加强单台接收函数转换震相的可追踪性,能量增强的叠后转换震相在时深转换后更利于对界面深度的识别与判断;在考虑三维成像的情况下,我们的各向异性校正方法对提高成像结果的准确性有重要意义.     

利用物理模型三维纵波数据分析HTI介质的方位各向异性

本文通过理论计算、数值模拟与穹窿物理模型三维数据对比分析的方法,对HTI介质中纵波方位各向异性现象进行研究.主要是进行目的层动校正速度以及走时的分析.结果显示,理论数值与实验数值耦合较好,HTI介质会引起动校正速度以及走时随方位角呈现椭圆形的变化;同时发现,观测系统中最大偏移距与目的层深度的比值以及方位角分布对各向异性分析有较大影响. 三维纵波方位各向异性分析对于数据的观测系统设计以及数据质量有较高的要求.     

基于碳酸盐岩裂缝岩石物理模型的横波速度和各向异性参数预测

高角度缝隙充填的碳酸盐岩储层可以等效为具有水平对称轴的横向各向同性介质.本文提出了适用于裂缝型碳酸盐岩的岩石物理模型构建流程,重点介绍了在碳酸盐岩各向同性背景中,综合利用微小裂隙模型和线性滑动模型添加缝隙系统,并分析了当缝隙充填不同流体时,各向异性参数随纵横波速比的变化特征.同时本文讨论了裂缝密度和缝隙充填流体对地震反射系数的影响,推导了不同类型流体充填时储层反射系数与裂缝密度的近似关系式,阐述了各向异性流体替换理论,最终实现饱含流体碳酸盐岩裂缝储层的纵横波速度和各向异性参数的估测.选取某碳酸盐岩工区A井对该方法进行试算,结果表明基于碳酸盐岩裂缝岩石物理模型估算的纵横波速度值与测井值吻合较好,而且估测所得的各向异性参数值也能够较好地反映出裂缝储层位置.     

TI介质中的P波衰减各向异性及其在裂缝参数反演中的应用（英文）

由于裂缝及所含流体的综合作用,含裂缝介质不仅具有速度各向异性,还具有显著的衰减各向异性。衰减各向异性分析可应用于裂缝产状、裂缝密度和尺度等的估计,为储层识别提供重要信息。本文从理论上推导了ATI介质(对称轴沿任意方向的横向各向同性介质)中P波衰减各向异性关系,并据此改进了ATI介质中P波衰减各向异性的测量方法,避免了现有的基于泰勒展开改进的谱比法在应用于宽方位或全方位数据时误差较大的问题,同时基于P波衰减各向异性分析可对裂缝倾角和方位角进行初步估计。结合P波衰减各向异性分析和S波速度各向异性分析,可实现小尺度裂缝的裂缝密度和裂缝尺度联合反演,相比于单独使用横波速度各向异性分析方法或单独使用P波衰减各向异性分析方法,其反演结果更准确、收敛性更好。本文将衰减各向异性分析应用于中国东部某油田的微地震数据,通过衰减倾向各向异性分析提取的裂缝倾角,与微地震监测结果相符。     

小江断裂带北段地壳浅层P波速度各向 异性观测的原理与数值试验

以二维情形下观测速度场为各向同性场和各向异性场的叠加为前提, 提出了一种利用走时残差估算地震波速度各向异性的方法, 即剩余慢度矢量法. 利用小江断裂带北段巧家流动地震台阵24个台站记录的3181次地震事件的P波走时残差, 采用剩余慢度矢量法计算了各观测台站周围水平方向上尺度为0.5°×0.5°, 震源深度为0—5 km的剩余慢度矢量, 由此得到了P波快波和慢波方向. 计算结果表明, 大部分观测台站周围的P波速度方向性较为一致, 快波方向为ESE向, 慢波方向为NNE向. 快波方向与小江断裂带北段应力场P轴方向较为一致, 而慢波方向与应力场T轴方向一致, 表明应力的长期作用可能是导致P波速度各向异性的重要原因.      

流体饱和孔隙定向裂缝储层中地震波衰减的模拟分析

在含有定向排列裂缝组的多孔岩石地层中,特别是碳酸盐岩储集层,弹性波的传播和衰减,因受诸如裂缝闭合、尺寸、及其填充的流体和走向等因素影响,具有各向异性特征。在本次研究中,我们采用了基于Chapman等效介质模型,利用数值实验方法研究了这类储层中P波传播特性诸如速度、衰减和横波分裂等各向异性随频率和方位的变化规律。模拟结果表明,当一组裂缝闭合与张开时随方位变化的速度、衰减和各向异性是有所不同的。随方位变化衰减最小值或P波速度最大值与张开裂缝的走向趋于一致,闭合裂缝的P波速度大于张开裂缝,而衰减和各向异性则张开裂缝的大于闭合裂缝;不同尺度裂缝,速度的最大值和衰减最小值与裂缝组平均方位对应,小尺度裂缝对波的传播有小的影响,方位依赖的各向异性相比其他裂缝属性有更小的影响;裂缝密度对P波速度、衰减和各向异性有更大的影响,而衰减比速度和各向异性更敏感裂缝尺寸;在地震勘探频段油、气饱和的衰减与盐水饱和不同,充填油和气随方位变化值比较低。两组裂缝有相同的密度,快横波偏振方位线性的决定于一组裂缝的方位。     

非均匀正交各向异性特征参数地震散射反演方法研究

在长波长假设条件下,水平层状地层中发育一组垂直排列的裂缝构成了等效正交各向异性介质.各向异性参数与裂缝弱度参数的估算有助于非均匀各向异性介质的各向异性特征描述,而弹性逆散射理论是非均匀介质参数反演的有效途径.基于地震散射理论,我们首先推导了非均匀正交介质中纵波散射系数方程,并通过引入正交各向异性特征参数,提出了一种新颖的正交各向异性方位弹性阻抗参数化方法.为了提高反演的稳定性与横向连续性,我们发展了贝叶斯框架下的正交各向异性方位弹性阻抗反演方法,同时考虑了柯西稀疏约束正则化和平滑模型约束正则化,最终使用非线性的迭代重加权最小二乘策略实现了各向异性特征参数的稳定估算.模型和实际资料处理表明,反演结果与测井解释数据相吻合,证明了该方法能够稳定可靠地从方位叠前地震资料中获取各向异性特征参数,减小参数估算的不确定性,为非均匀正交介质的各向异性预测提供了一种高可靠性的地震反演方法.     

VTI介质P波非双曲时差分析

具有垂直对称轴的横向各向同性介质模型(VTI)是目前各向异性理论研究和多波多分量地震资料叠前成像处理中最常用的一种各向异性模型.VTI介质中反射 P波时距曲线一般不再是双曲线.基于不同的相速度近似公式会得到不同的时距关系式.文中对几种典型的非双曲时距曲线与射线追踪得到的准确时距曲线在不同各向异性强度下进行了对比，结果表明Muir等和Stovas等提出的非双曲时距公式由于过高地考虑了横波垂直速度的影响与精确的时距曲线有很大偏差；Tsvankin等提出的弱各向异性非双曲时距公式在ε-δ<0时误差增大；Alkhalifah等提出的非双曲时距公式在大炮检距任意各向异性强度下都具有较高的精度，适于在实际资料处理中应用.     

Transmission + reflection anisotropic wave-equation traveltime and waveform inversion

A transmission + reflection wave-equation traveltime and waveform inversion method is presented that inverts the seismic data for the anisotropic parameters in a vertical transverse isotropic medium. The simultaneous inversion of anisotropic parameters and ε is initially performed using transmission wave-equation traveltime inversion method. Transmission wave-equation traveltime only provides the low-intermediate wavenumbers for the shallow part of the anisotropic model; in contrast, reflection wave-equation traveltime estimates the anisotropic parameters in the deeper section of the model. By incorporating a layer-stripping method with reflection wave-equation traveltime, the ambiguity between the background-velocity model and the depths of reflectors can be greatly mitigated. In the final step, multi-scale full-waveform inversion is performed to recover the high-wavenumber component of the model.  We use a synthetic model to illustrate the local minima problem of full-waveform inversion and how transmission and reflection wave-equation traveltime can mitigate this problem. We demonstrate the efficacy of our new method using field data from the Gulf of Mexico.     

Inversion for elliptically anisotropic velocity using VSP reflection traveltimes

This paper presents a traveltime inversion approach, using the reflection traveltimes from offset VSP data, to reconstruct the horizontal and vertical velocities for stratified anisotropic media. The inverse problem is reduced to a set of linear equations, and solved by the singular value decomposition (SVD) technique. The validity of this inversion scheme is verified using two sets of synthetic data simulated using a finite‐difference method, one for an isotropic model and the other for an elliptically anisotropic model. The inversion result demonstrates that our anisotropic velocity inversion scheme may be applied to both isotropic and anisotropic media. The method is finally applied to a real offset VSP data set, acquired in an oilfield in northwestern China.     

由VSP反射波旅行时重建各向异性层速度结构

本文提出-种利用有偏VSP资料反射波旅行时信息重建椭圆各向异性介质中水平向与垂直向速度的方法。其中,地下介质假定为层状椭圆各向异性介质。反射波旅行时间采用射线追踪理论及几何关系计算得到,反演中的线性方程组采用奇异值分解(SVD)技术进行求解。 方法检测时,我们对各向同性介质及椭圆各向异胜介质情况下有限差分法正演模拟的深井有偏移距VSP地震资料分别进行各向同性和各向异性方法反演成像。结果表明,本文所述方法较之各向同性介质模型反演方法对介质类型有很好的适用性,同时也说明了本方法的司行性。最后,我们分别介绍了对实际有偏VSP资料反演得到的地下介质的速度结构图像。     

Ray tracing for continuously rotated local coordinates belonging to a specified anisotropy

Conventional ray tracing for arbitrarily anisotropic and heterogeneous media is expressed in terms of 21 elastic moduli belonging to a fixed, global, Cartesian coordinate system. Our principle objective is to obtain a new ray-tracing formulation, which takes advantage of the fact that the number of independent elastic moduli is often less than 21, and that the anisotropy thus has a simpler nature locally, as is the case for transversely isotropic and orthorhombic media. We have expressed material properties and ray-tracing quantities (e.g., ray-velocity and slowness vectors) in a local anisotropy coordinate system with axes changing directions continuously within the model. In this manner, ray tracing is formulated in terms of the minimum number of required elastic parameters, e.g., four and nine parameters for P-wave propagation in transversely isotropic and orthorhombic media, plus a number of parameters specifying the rotation matrix connecting local and global coordinates. In particular, we parameterize this rotation matrix by one, two, or three Euler angles. In the ray-tracing equations, the slowness vector differentiated with respect to traveltime is related explicitly to the corresponding differentiated slowness vector for non-varying rotation and the cross product of the ray-velocity and slowness vectors. Our formulation is advantageous with respect to user-friendliness, efficiency, and memory usage. Another important aspect is that the anisotropic symmetry properties are conserved when material properties are determined in arbitrary points by linear interpolation, spline function evaluation, or by other means.     

Generalized Dix equation and analytic treatment of normal-moveout velocity for anisotropic media*

Despite the complexity of wave propagation in anisotropic media, reflection moveout on conventional common-midpoint (CMP) spreads is usually well described by the normal-moveout (NMO) velocity defined in the zero-offset limit. In their recent work, Grechka and Tsvankin showed that the azimuthal variation of NMO velocity around a fixed CMP location generally has an elliptical form (i.e. plotting the NMO velocity in each azimuthal direction produces an ellipse) and is determined by the spatial derivatives of the slowness vector evaluated at the CMP location. This formalism is used here to develop exact solutions for the NMO velocity in anisotropic media of arbitrary symmetry. For the model of a single homogeneous layer above a dipping reflector, we obtain an explicit NMO expression valid for all pure modes and any orientation of the CMP line with respect to the reflector strike. The contribution of anisotropy to NMO velocity is contained in the slowness components of the zero-offset ray (along with the derivatives of the vertical slowness with respect to the horizontal slownesses) — quantities that can be found in a straightforward way from the Christoffel equation. If the medium above a dipping reflector is horizontally stratified, the effective NMO velocity is determined through a Dix-type average of the matrices responsible for the ‘interval’ NMO ellipses in the individual layers. This generalized Dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous, arbitrarily anisotropic media. For models with a throughgoing vertical symmetry plane (i.e. if the dip plane of the reflector coincides with a symmetry plane of the overburden), the semi-axes of the NMO ellipse are found by the more conventional rms averaging of the interval NMO velocities in the dip and strike directions. Modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one (zero-offset) ray. Remarkably, the expressions for geometrical spreading along the zero-offset ray contain all the components necessary to build the NMO ellipse. This method is orders of magnitude faster than multi-azimuth, multi-offset ray tracing and, therefore, can be used efficiently in traveltime inversion and in devising fast dip-moveout (DMO) processing algorithms for anisotropic media. This technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces. The high accuracy of our NMO expressions is illustrated by comparison with ray-traced reflection traveltimes in piecewise-homogeneous, azimuthally anisotropic models. We also apply the generalized Dix equation to field data collected over a fractured reservoir and show that P-wave moveout can be used to find the depth-dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy.     

Reflected waves in finely layered tilted orthorhombic media

Upscaling in seismics is a homogenization of finely layered media in the zero-frequency limit. An upscaling technique for arbitrary anisotropic layers has been developed by Schoenberg and Muir. Applying this technique to a stack of layers of orthorhombic (ORT) symmetry whose vertical symmetry planes are aligned, results in an effective homogeneous layer with orthorhombic symmetry. If the symmetry planes in a horizontal orthorhombic layer are rotated with respect to vertical, the medium is referred to as tilted orthorhombic (TOR) medium, and the stack composed of TOR layers in zero-frequency limit will produce an effective medium of a lower symmetry than orthorhombic. We consider a P-wave that propagates through a stack of thin TOR layers, then it is reflected (preserving the mode) at some interface below the stack, and then propagates back through the same stack. We propose to use a special modified medium for the upscaling in case of this sequential down- and up-propagation: each TOR layer in the stack is replaced by two identical TOR layers whose tilt angles have the opposite algebraic sign. In this modified medium, one-way propagation of a seismic wave (any wave mode) is equivalent to propagation of a pure-mode reflection in the original medium. We apply this idea to study the contribution from an individual layer from the stack and show how the approach can be applied to a stack of TOR layers. To demonstrate the applicability of the model, we use well log data for the upscaling. The model we propose for the upscaling can be used in well-seismic ties to correct the effective parameters obtained from well log data for the presence of tilt, if latter is confirmed by additional measurements (for example, borehole imaging).     

Azimuthal Seismic Amplitude Variation with Offset and Azimuth Inversion in Weakly Anisotropic Media with Orthorhombic Symmetry

Seismic amplitude variation with offset and azimuth (AVOaz) inversion is well known as a popular and pragmatic tool utilized to estimate fracture parameters. A single set of vertical fractures aligned along a preferred horizontal direction embedded in a horizontally layered medium can be considered as an effective long-wavelength orthorhombic medium. Estimation of Thomsen’s weak-anisotropy (WA) parameters and fracture weaknesses plays an important role in characterizing the orthorhombic anisotropy in a weakly anisotropic medium. Our goal is to demonstrate an orthorhombic anisotropic AVOaz inversion approach to describe the orthorhombic anisotropy utilizing the observable wide-azimuth seismic reflection data in a fractured reservoir with the assumption of orthorhombic symmetry. Combining Thomsen’s WA theory and linear-slip model, we first derive a perturbation in stiffness matrix of a weakly anisotropic medium with orthorhombic symmetry under the assumption of small WA parameters and fracture weaknesses. Using the perturbation matrix and scattering function, we then derive an expression for linearized PP-wave reflection coefficient in terms of P- and S-wave moduli, density, Thomsen’s WA parameters, and fracture weaknesses in such an orthorhombic medium, which avoids the complicated nonlinear relationship between the orthorhombic anisotropy and azimuthal seismic reflection data. Incorporating azimuthal seismic data and Bayesian inversion theory, the maximum a posteriori solutions of Thomsen’s WA parameters and fracture weaknesses in a weakly anisotropic medium with orthorhombic symmetry are reasonably estimated with the constraints of Cauchy a priori probability distribution and smooth initial models of model parameters to enhance the inversion resolution and the nonlinear iteratively reweighted least squares strategy. The synthetic examples containing a moderate noise demonstrate the feasibility of the derived orthorhombic anisotropic AVOaz inversion method, and the real data illustrate the inversion stabilities of orthorhombic anisotropy in a fractured reservoir.