由矩张量反演得到的北部湾地震的震源机制

1994年12月31日和1995年1月10日,在北部湾海域发生了MS=6.0和MS=6.1地震．运用广义反射-透射系数矩阵方法计算格林函数,利用中国数字地震台网(CDSN)记录的两次地震的长周期体波资料,用频率域矩张量反演方法得到了这两次地震的震源机制．反演结果表明,这两次地震都是以剪切位错为主的事件,它们都是在相同的构造环境下,受到同一断裂的构造运动所控制,在北西-南东向为主压应力、北东-南西向的主张应力的作用下发生的．连续发生的MS=6.0,6.1地震,与菲律宾海板块和欧亚板块沿近北西-南东向碰撞有关．MS=6.0地震发生后震源区应力场发生了明显的变化,MS=6.1地震的发生与MS=6.0地震的发生引起的应力场调整有一定的联系．     

含衰减地层微地震震源机制反演及其反演分辨率

微地震震源机制的反演对于非常规油气开发具有至关重要的作用.微地震信号主频高、能量小,容易受地层吸收衰减作用的影响使其波形发生畸变,本文提出了一种考虑地层吸收衰减作用的微震源机制反演方法,并利用费雷谢偏导矩阵的SVD分解（特征值分解）方法,分析研究了地层的吸收衰减因子的变化对于微地震震源机制反演分辨率的影响,根据理论计算给出了不同地震数据对各种震源机制反演的适用条件.理论计算证明,采用直达P波和S波数据联合反演震源的T值,单独利用直达P波反演震源的k值,可以有效降低地层吸收衰减作用对反演结果的影响.     

VTI介质速度与δ参数qP波全波形反演

获取准确的速度与各向异性参数模型,是全波形反演的主要目标.由于各向异性参数的敏感性较差,常规全波形反演方法很难同时得到满意的速度和各向异性参数模型.本文提出一种新的VTI介质qP波全波形反演方法.采用动校正速度参数化方式得到新的VTI介质qP波方程,改善各向异性参数δ对近偏移距数据的敏感性.在新的方程中,推导了VTI介质全波形反演梯度公式.基于敏感核分析设计了合理的分步反演策略,来缓解不同参数之间的串扰,进而提高速度和各向异性参数的反演精度.模型算例表明,新的反演策略能够有效提高速度和δ参数的反演精度.分步反演方法在非线性更强的各向异性反演中,能够得到更接近真实值的反演结果.     

2007年甘肃部分地震的震源参数反演

2007年甘肃测震台网记录到M_L>3.0的地震43次,本文选取了其中17例,利用垂直向PG波初动波形进行了地震矩张量反演,给出了最佳双力偶解,并对一些地震发震背景和发震机理进行了分析讨论.对地震机理研究及未来地震预报提供了参考依据.     

用波形反演确定的三维不均匀介质中地方震的震源机制

提出了一种用波形反演确定点源机制和时间函数的方法。这种方法允许应用振幅进行事件的再定位，也可以用最简单的方法优化结构模型。通过在资料拟合过程中状态空间的残差函数最小的方法可以做到这一点。     

甘肃岷县两次中强地震震源机制研究

利用地震震中周围7个台站的数字记录反演了甘肃岷县2003年11月13日ML5．5和2004年9月7日ML5．2地震的矩张量,得出了震源机制。将所得结果与用P波初动符号的结果进行了比较,二者具有较好的一致性。结合该地区的地震分布,对可能的发震断层进行了讨论。     

首都圈地区中小地震全波形震源机制反演

目前对首都圈地区中小地震的震源机制研究所用的方法主要有P波初动极性法、振幅比法及波形反演方法。前两种方法只利用了波形中很少量的信息,且对于稀疏台阵效果较差,而传统的波形反演法也仅利用了波形的部分信息,且对于震级、数据资料等有一定的要求。为克服上述传统方法中的局限并进一步提高震源机制解的准确性,本文采用一种新的综合利用上述各种信息的全波形匹配方法来反演首都圈地区中小地震的震源机制解,提高反演解的可靠性。为了检验所使用的全波形匹配震源机制反演方法的可靠性,我们首先进行了合成数据测试,结果表明全波形匹配方法可以得到稳定可靠的震源机制解。利用新的全波形匹配方法计算了2019年4月北京发生的两次地震震源机制解,并结合活动断裂进行分析。将新的全波形震源机制反演方法应用于首都圈地区2015年以来的22个地震(2.2≤M≤4.3),并对22个地震的震源机制解及应力轴分布进行了分析,与前人研究结果具有较好的一致性。     

基于各向异性全变分约束的多震源弹性波全波形反演

多震源编码技术可以提高全波形反演的计算效率,但同时会引入串扰噪声使反演结果质量降低. 全变分约束可以有效地压制层内噪声并突出模型界面,其与多震源技术的结合,能在大大提高弹性波全波形反演效率的同时提高反演质量. 本文提出了一种高效的动态多震源全波形反演策略,可以在离散串扰噪声的同时保证照明的均匀性. 根据残留串扰噪声的分布特征,构建了与之匹配的基于各向异性全变分约束的弹性波全波形反演方法. 为了减少周期跳跃效应,将基于稀疏约束的低频重构算法应用于弹性波地震记录,给出了利用快速梯度投影算法求解各向异性全变分约束的全波形反演流程. 模型数据测试结果表明本文的方法不仅能有效地抑制多震源方法导致的串扰噪声,同时也能降低观测数据中的噪声对反演结果的影响.

     

2003年11月13日岷县ML5.5级地震震源机制的矩张量反演

利用2003年11月13日岷县5.5级地震震中周围7个台站的数字记录反演了本次地震的矩张量,对所得结果与用P波初动符号的结果进行了比较,二者具有较好的一致性。结合该地区的地震分布和本次地震的余震分布,对可能的发震断层进行了讨论。     

黏弹性VTI介质微地震数据目标导向裂缝染色逆时偏移

水力压裂技术能够改善储层中裂缝的连通性,实现致密储层油气增产,利用压裂过程中产生的微地震事件进行裂缝偏移成像有助于评价水力压裂效果.水力压裂通常在具有黏弹性和各向异性的页岩油气储层中进行,地震波传播过程中会发生具有方向性的衰减,此外微地震震源及检波器通常分布在有限区域内,导致偏移剖面上存在远场照明较差和成像干扰的问题,难以有效进行裂缝描述.本文提出了一种黏弹性VTI介质目标导向逆时偏移裂缝成像方法,首先推导了黏弹性VTI介质染色波动方程并提出了多组偏移剖面互相关染色成像条件,然后通过裂缝逆时偏移成像测试分析了微地震数据信噪比、介质参数和事件定位精度对偏移结果的影响,最后结合地震干涉法减轻了定位误差造成的干扰.数值模拟实验表明,目标导向的染色逆时偏移成像算法能够有效压制成像剖面中的伪影干扰,改善目标区域照明,提高裂缝成像的空间分辨率.

     

Full waveform inversion using oriented time‐domain imaging method for vertical transverse isotropic media

Full waveform inversion for reflection events is limited by its linearised update requirements given by a process equivalent to migration. Unless the background velocity model is reasonably accurate, the resulting gradient can have an inaccurate update direction leading the inversion to converge what we refer to as local minima of the objective function. In our approach, we consider mild lateral variation in the model and, thus, use a gradient given by the oriented time‐domain imaging method. Specifically, we apply the oriented time‐domain imaging on the data residual to obtain the geometrical features of the velocity perturbation. After updating the model in the time domain, we convert the perturbation from the time domain to depth using the average velocity. Considering density is constant, we can expand the conventional 1D impedance inversion method to two‐dimensional or three‐dimensional velocity inversion within the process of full waveform inversion. This method is not only capable of inverting for velocity, but it is also capable of retrieving anisotropic parameters relying on linearised representations of the reflection response. To eliminate the crosstalk artifacts between different parameters, we utilise what we consider being an optimal parametrisation for this step. To do so, we extend the prestack time‐domain migration image in incident angle dimension to incorporate angular dependence needed by the multiparameter inversion. For simple models, this approach provides an efficient and stable way to do full waveform inversion or modified seismic inversion and makes the anisotropic inversion more practicable. The proposed method still needs kinematically accurate initial models since it only recovers the high‐wavenumber part as conventional full waveform inversion method does. Results on synthetic data of isotropic and anisotropic cases illustrate the benefits and limitations of this method.     

声发射矩张量反演

地震矩张量反演是获取震源过程的有效方法.岩石变形过程中的声发射与地震类似,均是弹性应变能快速释放.如假设条件得到满足,矩张量反演方法同样可用于了解声发射震源过程.声发射矩张量反演可使用P波位移进行计算.当样品尺寸远大于声发射波长,且声发射由微破裂产生,声发射源的尺度很小时,P波矩张量反演可采用远场近似.本文首先针对声发射的特点,实现根据远场P波反演声发射矩张量的算法,并通过人工声发射实验对算法的正确性和可靠性进行了检验.最后,用声发射矩张量反演方法对花岗岩单轴压缩实验的声发射源特征进行了分析.结果表明：对于纯剪切破裂模式,声发射矩张量反演可得断层面;对于非纯剪切破裂模式,如纵向挤压导致的横向张性劈裂,由于存在多解性不能得到断层面,但可通过矩张量的迹区分破裂模式.     

Analysis of different parameterisations of waveform inversion of compressional body waves in an elastic transverse isotropic Earth with a vertical axis of symmetry

In a multi‐parameter waveform inversion, the choice of the parameterisation influences the results and their interpretations because leakages and the tradeoff between parameters can cause artefacts. We review the parameterisation selection when the inversion focuses on the recovery of the intermediate‐to‐long wavenumbers of the compressional velocities from the compressional body (P) waves. Assuming a transverse isotropic medium with a vertical axis of symmetry and weak anisotropy, analytical formulas for the radiation patterns are developed to quantify the tradeoff between the shear velocity and the anisotropic parameters and the effects of setting to zero the shear velocity in the acoustic approach. Because, in an anisotropic medium, the radiation patterns depend on the angle of the incident wave with respect to the vertical axis, two particular patterns are discussed: a transmission pattern when the ingoing and outgoing slowness vectors are parallel and a reflection pattern when the ingoing and outgoing slowness vectors satisfy Snell's law. When the inversion aims at recovering the long‐to‐intermediate wavenumbers of the compressional velocities from the P‐waves, we propose to base the parameterisation choice on the transmission patterns. Since the P‐wave events in surface seismic data do not constrain the background (smooth) vertical velocity due to the velocity/depth ambiguity, the preferred parameterisation contains a parameter that has a transmission pattern concentrated along the vertical axis. This parameter can be fixed during the inversion which reduces the size of the model space. The review of several parameterisations shows that the vertical velocity, the Thomsen parameter δ, or the Thomsen parameter ε have a transmission pattern along the vertical axis depending on the parameterisation choice. The review of the reflection patterns of those selected parameterisations should be done in the elastic context. Indeed, when reflection data are also inverted, there are potential leakages of the shear parameter at intermediate angles when we carry out acoustic inversion.     

3D waveform inversion of downhole microseismic data for transversely isotropic media

3D anisotropic waveform inversion could provide high-resolution velocity models and improved event locations for microseismic surveys. Here we extend our previously developed 2D inversion methodology for microseismic borehole data to 3D transversely isotropic media with a vertical symmetry axis. This extension allows us to invert multicomponent data recorded in multiple boreholes and properly account for vertical and lateral heterogeneity. Synthetic examples illustrate the performance of the algorithm for layer-cake and ‘hydraulically fractured’ (i.e. containing anomalies that simulate hydraulic fractures) models. In both cases, waveform inversion is able to reconstruct the areas which are sufficiently illuminated for the employed source-receiver geometry. In addition, we evaluate the sensitivity of the algorithm to errors in the source locations and to band-limited noise in the input displacements. We also present initial inversion results for a microseismic data set acquired during hydraulic fracturing in a shale reservoir.     

利用矩张量反演法研究江苏高邮—宝应M_S4.9级地震震源机制解和震源深度

基于江苏、安徽、山东和浙江等省区域台网共19个宽频带数字台站的地震波形,采用HypoDD双差定位方法确定了2012年7月20日江苏高邮—宝应MS4.9级地震震中位置,再利用时间域矩张量反演法TDMT_INV获得了其震源机制解和震源深度.反演结果显示:最佳双力偶解为节面Ⅰ走向290°,倾角88°,滑动角-21°;节面Ⅱ走向21°,倾角69°,滑动角-177°,地震矩震级为MW4.95,震源深度约为7~9km.利用滑动时窗相关法提取sPn震相测定震源深度为8.95km,两者一致性较好.随后不同地壳模型和不同震中定位误差对反演结果的影响试验揭示了反演结果具有稳定性.通过以下几种分析:1与利用CAP(Cut and Paste)矩张量反演法得到的结果进行对比;2P波初动投影;3正反演试验探求反演结果不稳定的影响因素等方法,验证了反演结果的可靠性.综合本文研究成果、震后科学考察结果(包括重力测量和地震烈度分布图)及现有的地质构造资料,推测此次地震的发震构造为杨汊仓—桑树头断裂,节面Ⅱ为断层面,是一个右旋走滑兼有少量正断层性质的错动.     

Determining the focal mechanisms and depths of relatively small earthquakes using a few stations by full‐waveform modelling

Determining the focal mechanism of earthquakes helps us to better define faults and understand the stress regime. This technique can be helpful in the oil and gas industry where it can be applied to microseismic events. The objective of this paper is to find double couple focal mechanisms, excluding scalar seismic moments, and the depths of small earthquakes using data from relatively few local stations. This objective is met by generating three‐component synthetic seismograms to match the observed normalized velocity seismograms. We first calculate Green's functions given an initial estimate of the earthquake's hypocentre, the locations of the seismic recording stations and a 1D velocity model of the region for a series of depths. Then, we calculate the moment tensor for different combinations of strikes, dips and rakes for each depth. These moment tensors are combined with the Green's functions and then convolved with a source time function to produce synthetic seismograms. We use a grid search to find the synthetic seismogram with the largest objective function that best fits all three components of the observed velocity seismogram. These parameters define the focal mechanism solution of an earthquake. We tested the method using three earthquakes in Southern California with moment magnitudes of 5.0, 5.1 and 4.4 using the frequency range 0.1–2.0 Hz. The source mechanisms of the events were determined independently using data from a multitude of stations. Our results obtained, from as few as three stations, generally match those obtained by the Southern California Earthquake Data Center. The main advantage of this method is that we use relatively high‐frequency full‐waveforms, including those from short‐period instruments, which makes it possible to find the focal mechanism and depth of earthquakes using as few as three stations when the velocity structure is known.     

层状TI介质中微地震定位和各向异性速度结构同时反演

微地震监测被广泛应用于非常规油气资源的水力压裂作业、油藏描绘和水驱前缘监测工程中.微地震定位采用的初始速度模型一般是基于地震测井记录和射孔数据建立,该速度模型的不准确性易引起定位误差.为降低这种定位误差,本文发展了一种微地震定位和各向异性速度结构同时反演的方法.研究对象为1-D的层状TI介质,其中对称轴方向任意.利用改进的分区多步最短路径算法计算qP、qSV和qSH波的到达时间和射线路径,结合共轭梯度法求解带约束的阻尼最小二乘问题.数值模拟结果表明,该算法能同时进行各向异性速度结构模型（每层的Thomsen参数和界面深度）和微震震源参数（空间坐标和发震时刻）的反演,并且对随机噪声不敏感,有利于实际工程应用.

     

岩石裂纹扩展微观机制声发射定量反演

岩石受载内部微裂纹扩展及其震源机制反演有助于认识宏观裂纹扩展过程的非线性断裂力学行为.借助声发射监测手段,本文建立了仅涉及微裂纹张开/闭合和剪切滑移的位移不连续震源模型,通过各位置处传感器耦合质量标定及点源远场P波矩张量反演获得了含预制裂纹砂岩受载过程的震源机制解及时变响应特征,在全局坐标系下分析了微裂纹的三种断裂力学行为.结果表明:在位移不连续模型中,震源矩张量特征值与试样泊松比之间必须满足特定约束条件,该约束条件下的优化问题可采用拉格朗日乘子和Levenberg-Marquardt迭代法求解;受载砂岩裂纹扩展过程中,声发射震源以剪切滑移机制占优,微裂纹空间取向及运动方向与试样宏观主裂纹夹角平均值分别为40.9°和17.7°;对微裂纹体积分解表明岩体微观破裂机制以沿X方向I型张开为主,而沿Y方向的II型断裂滑移方向与试样全局变形方向相一致,由于试样内部晶粒分布非均质性造成了少量沿Z方向的III型平面外微裂纹滑移行为;受载砂岩裂纹扩展过程中微裂纹模式角与震源极性值变化趋势一致、利用震源震级评估的局部应力降值与实验观测结果相吻合,这两者均表明了位移不连续模型在震源机制定量反演中的适用性.