基于波场垂向和水平方向外推的高陡构造偏移

常规的单程波波动方程偏移成像方法对大角度的高陡构造偏移成像存在内在的限制.根据波动方程在各个空间方向的数学特性和高陡构造反射地震波的传播特征,通过把地震波分解为垂向的上下行波、水平方向的前后行波和左右行波,提出基于波场垂向外推和水平方向外推相结合的单程波波动方程高陡构造偏移成像方法,即用波场垂向外推的单程波波动方程偏移成像方法解决中低角度平缓构造的偏移成像,用波场水平方向外推的单程波波动方程偏移成像方法解决中高角度陡倾构造的偏移成像.这种基于波场垂向和水平方向外推相结合的高陡构造偏移成像方法是常规单程波波动方程叠前深度偏移成像方法的补充和改进,它相对基于全波方程的逆时偏移具有计算效率上的优势.     

基于波场分离的弹性波逆时偏移

尽管叠前逆时偏移成像精度高,但仅针对单一纵波的成像也可能形成地下介质成像盲区,由于基于弹性波方程的逆时偏移成像可形成多波模式的成像数据,因此弹性波逆时偏移成像可提供更为丰富的地下构造信息.本文依据各向同性介质的一阶速度-应力方程组构建震源和检波点矢量波场,再利用Helmholtz分解提取纯纵波和纯横波波场,使用震源归一化的互相关成像条件获得纯波成像,避免了直接使用坐标分量成像而引起的纵横波串扰问题.针对转换波成像的极性反转问题,文中提出一种共炮域极性校正方法.为有效节约存储成本,也提出一种适用于弹性波逆时偏移的震源波场逆时重建方法,在震源波场正传过程中,仅保存PML边界内若干层的速度分量波场,进而逆时重建出所有分量的震源波场.本文分别对地堑模型和Marmousi2模型进行了弹性波逆时偏移成像测试,结果表明：所提出的共炮域极性校正方法正确有效,基于波场分离的弹性波逆时偏移成像的纯波数据能够对复杂地下构造准确成像.

     

基于Poynting矢量行波分离的最小二乘逆时偏移

因为在逆时偏移中基于双程波动方程构建震源波场和检波器波场,所以在波场延拓过程中地震波遇到波阻抗界面时,背向发育的反射波会与正常传播的波场互相关产生较强振幅的低频噪声.这一特点使得以逆时偏移为基础的最小二乘偏移方法在梯度计算时同样存在着低频噪声的干扰,从而导致反演收敛的速度减慢.考虑到计算量和存储成本的因素,本文借助Poynting矢量良好的方向指示性实现波场的上下行波分离,并在早期迭代的梯度计算中只保留震源波场和检波器波场沿不同垂直方向传播的组分之间的互相关,有效避免了成像噪声的干扰,提高了算法收敛的速率.数值算例验证了方案的有效性.

     

Extraction of amplitude-preserving angle gathers based on vector wavefield reverse-time migration

Angle-domain common-image gathers (ADCIGs) transformed from the shotdomain common-offset gathers are input to migration velocity analysis (MVA) and prestack inversion. ADCIGs are non-illusion prestack inversion gathers, and thus, accurate. We studied the extraction of elastic-wave ADCIGs based on amplitude-preserving elastic-wave reversetime migration for calculating the incidence angle of P-and S-waves at each image point and for different source locations. The P-and S-waves share the same incident angle, namely the incident angle of the source P-waves. The angle of incidence of the source P-wavefield was the difference between the source P-wave propagation angle and the reflector dips. The propagation angle of the source P-waves was obtained from the polarization vector of the decomposed P-waves. The reflectors’ normal direction angle was obtained using the complex wavenumber of the stacked reverse-time migration (RTM) images. The ADCIGs of P-and S-waves were obtained by rearranging the common-shot migration gathers based on the incident angle. We used a horizontally layered model, the graben medium model, and part of the Marmousi-II elastic model and field data to test the proposed algorithm. The results suggested that the proposed method can efficiently extract the P-and S-wave ADCIGs of the elastic-wave reverse-time migration, the P-and S-wave incident angle, and the angle-gather amplitude fidelity, and improve the MVA and prestack inversion.     

基于一步法波场延拓的正演模拟和逆时偏移成像

通常工业界实现逆时偏移算法时采用有限差分数值方法模拟地震波场,波场模拟常常受稳定性条件限制,且易产生数值频散,成像精度降低.本文引入了一步法波场延拓方法,首先构建声波传播算子,借助Chebyshev多项式和Jacobi-Anger展开式近似传播算子中的e指数项,进而实现波场递推,该方法时间步长的选取不受稳定性条件限制而且不存在空间频散现象.本文将一步法波场延拓方法用于逆时偏移成像的波场模拟,并提出双缓冲区存储策略,在不增加计算量的前提下,大幅降低了逆时偏移方法的波场存储量.波场模拟和逆时偏移成像测试表明,本文提出的一步法波场延拓方法模拟地震波场精度高,消除了频散影响,可在较大时间步长的情况下实现高精度波场模拟;提出的基于一步法波场延拓的逆时偏移方法成像质量好;基于双缓冲区存储策略的逆时偏移成像方法存储成本低.

     

基于Hilbert变换的全波场分离逆时偏移成像

逆时偏移方法利用双程波算子模拟波场的正向和反向传播,通常采用互相关成像条件获得偏移剖面,是一种高精度的成像方法.但是传统的互相关成像条件会在偏移结果中产生低频噪声;此外,如果偏移速度中存在剧烈速度变化还可能进一步产生偏移假象.为了提高逆时偏移的成像质量,可在成像过程中先对震源波场和检波点波场分别进行波场分离,然后选择合适的波场成分进行互相关成像.本文基于Hilbert变换,推导了可在偏移过程中进行上下行和左右行波场分离的高效波场分离公式以及相应的成像条件,结合Sigsbee 2B合成数据,给出了不同波场成分的互相关成像结果.数值算例结果表明,采用本文提出的高效波场分离算法以及合理的波场成分互相关成像条件可以获得高信噪比的成像结果.     

基于光流矢量行波分离的相关加权逆时偏移成像

逆时偏移由于基于双程波方程进行波场延拓, 应用互相关成像条件所得到的成像结果中包含了大量的低波数噪声.基于Poynting矢量行波分离的逆时偏移方法虽可有效压制这些噪声干扰, 但其也存在Poynting矢量计算不稳定、行波分离精度不高以及各行波分离成像剖面叠加权重难以确定等问题.本文基于光流矢量实现了震源正时波场和检波点逆时波场的高精度分离, 分别得到了左上、左下、右上、右下四个方向的行波, 进而由正时和逆时波场的每两个行波互相关得到16个成像剖面; 在此基础上, 将这16个成像剖面分别与参考剖面做相关计算, 并将相关值作为权重分别赋予各成像剖面, 最终实现了基于光流矢量行波分离的相关加权逆时偏移成像.理论模型数据与实际资料试算表明: 相比于常规基于Poynting矢量行波分离的逆时偏移方法, 本文方法可更精确、稳定地实现震源波场和检波点波场的行波分离, 同时该方法采取的相关加权成像方式, 避免了人为取舍或仅赋予各波场成像剖面经验权重的缺陷, 有效提高了逆时偏移的成像精度.

     

Reverse time migration of ground-penetrating radar with full wavefield decomposition based on the Hilbert transform

The conventional reverse time migration of ground-penetrating radar data is implemented with the two-way wave equation. The cross-correlation result contains low-frequency noise and false images caused by improper wave paths. To eliminate low-frequency noise and improve the quality of the migration image, we propose to separate the left-up-going, left-down-going, right-up-going and right-down-going wavefield components in the forward- and backward-propagated wavefields based on the Hilbert transform. By applying the reverse time migration of ground-penetrating radar data with full wavefield decomposition based on the Hilbert transform, we obtain the reverse time migration images of different wavefield components and combine correct imaging conditions to generate complete migration images. The proposed method is tested on the synthetic ground-penetrating radar data of a tilt-interface model and a complex model. The migration results show that the imaging condition of different wavefield components can highlight the desired structures. We further discuss the reasons for incomplete images by reverse time migration with partial wavefields. Compared with the conventional reverse time migration methods for ground-penetrating radar data, low-frequency noise can be eliminated in images generated by the reverse time migration method with full wavefield decomposition based on the Hilbert transform.     

Reverse-time migration using multidirectional wavefield decomposition method

Reverse-time migration has attracted more and more attention owing to the advantages of high imaging accuracy, no dip restriction, and adaptation to complex velocity models. Cross-correlation imaging method is typically used in conventional reverse-time migration that produces images with strong low-frequency noise. Wavefield decomposition imaging can suppress such noise; however, some residual noise persists in the imaging results. We propose a 2D multidirectional wavefield decomposition method based on the traditional wavefield decomposition method. First, source wavefields and receiver wavefields are separated into eight subwavefields, respectively. Second, cross-correlation imaging is applied to selected subwavefields to produce subimages. Finally, the subimages are stacked to generate the final image. Numerical examples suggest that the proposed method can eliminate the low-frequency noise effectively and produce high-quality imaging profiles.     

基于Poynting矢量的归一化波场分离互相关逆时偏移成像条件(英文)

叠前逆时偏移是目前精度较高的成像方法,然而严重的低频噪声降低了逆时偏移的构造成像精度,偏移噪声的压制是逆时偏移必需要考虑的问题。分析了低频噪声的产生机理,并根据声波方程Poynting矢量的方向指示地震波场的传播方向的原理,分离出上、下、左、右行波,该方法计算量和存储量都远远小于常用的二维傅里叶变换分离方法,进而提出归一化的波场分离互相关成像条件,以压制逆时偏移低频噪声,提高成像精度。实现了Marmousi模型的试算,表明在波场延拓过程中利用Poynting矢量能够较好的分离上、下、左、右行波,与常规方法、拉普拉斯滤波、二维傅里叶变换波场分离的成像结果对比表明,成像时使用归一化的波场分离互相关成像条件能更好的压制偏移噪声,得到精度更高的逆时偏移成像结果。     

基于解析时间波场外推与波场分解的逆时偏移方法研究

双程波方程逆时深度偏移是复杂介质高精度成像的有效技术, 但其结果中通常包含成像方法引起的噪音和假象, 一般的滤波方法会破坏成像剖面上的振幅, 其中的假象也会给后续地质解释带来困扰.将波场进行方向分解然后实现入射波与反射波的相关成像能够有效地消除这类成像噪音, 并提高逆时偏移成像质量.波传播方向的分解通常在频率波数域实现, 它会占用大量的存储和计算资源, 不便于在沿时间外推的逆时深度偏移中应用.本文提出解析时间波场外推方法, 可以在时间外推的每个时间片上实现波传播方向的显式分解, 逆时深度偏移中利用分解后的炮检波场进行对应的相关运算, 实现成像噪音和成像信号的分离.在模型和实际数据上的测试表明, 相比于常规互相关逆时偏移成像结果, 本文方法能够有效地消除低频成像噪音和特殊地质构造导致的成像假象.     

基于波场重构的早至波波形反演

初至波走时反演难以得到复杂的地下构造,而早至波波形反演在处理远偏移距地震记录时易产生周期跳跃现象,只能利用近偏移距数据进行反演.但近偏移距的早至波所携带的信息较少,难以实现对地下介质的高精度速度建模.为解决上述问题,并充分利用远偏移距的早至波信息,本文提出基于初至波波场重构的早至波反演方法.该方法将早至波波动方程作为惩...     

Poynting and polarization vectors based wavefield decomposition and their application on elastic reverse time migration in 2D transversely isotropic media

With the progress in computational power and seismic acquisition, elastic reverse time migration is becoming increasingly feasible and helpful in characterizing the physical properties of subsurface structures. To achieve high-resolution seismic imaging using elastic reverse time migration, it is necessary to separate the compressional (P-wave) and shear (S-wave) waves for both isotropic and anisotropic media. In elastic isotropic media, the conventional method for wave-mode separation is to use the divergence and curl operators. However, in anisotropic media, the polarization direction of P waves is not exactly parallel to the direction of wave propagation. Also, the polarization direction of S-waves is not totally perpendicular to the direction of wave propagation. For this reason, the conventional divergence and curl operators show poor performance in anisotropic media. Moreover, conventional methods only perform well in the space domain of regular grids, and they are not suitable for elastic numerical simulation algorithms based on non-regular grids. Besides, these methods distort the original wavefield by taking spatial derivatives. In this case, a new anisotropic wave-mode separation scheme is developed using Poynting vectors. This scheme can be performed in the angle domain by constructing the relationship between group and polarization angles of different wave modes. Also, it is performed pointwise, independent of adjacent space points, suitable for parallel computing. Moreover, there is no need to correct the changes in phase and amplitude caused by the derivative operators. By using this scheme, the anisotropic elastic reverse time migration is more efficiently performed on the unstructured mesh. The effectiveness of our scheme is verified by several numerical examples.     

一种稳定的波场延拓反Q滤波算法

反Q滤波算法中稳定性是关键。针对地下介质为层状Q模型结构,本文提出一种稳定的反Q滤波算法,基于波场延拓原理,对于每个常Q层反Q滤波算法分两步完成:(1)利用一种稳定的波场延拓算法,将地表波场记录直接延拓到当前层上;(2)在当前层内进行常Q反滤波。在步骤(1)中,利用稳定的波场延拓算法并结合稳定因子,将地表波场记录直接延拓到当前层上,避免了上覆层反Q滤波误差的累积。在步骤(2)中,对于当前层内的反Q滤波,通过对该层地震信号的Gabor谱分析,拾取时变的限幅频率,并确定相应于该频率值的限幅增益值来实现反Q滤波算法的稳定性。最后通过理论模型与实际数据验证了算法的优越性。     

基于自适应变步长波场延拓的可控层分阶层间多次波模拟

地下低速夹层的存在导致地震数据中包含较强能量的层间多次波,有效识别和预测深部储层上覆地层产生的层间多次波是提高深部储层解释精度的重要环节,而准确模拟层间多次波是辅助识别地震数据中层间多次波的一种非常有效的方法.本文提出了一种基于自适应变步长波场延拓的可控地层分阶层间多次波模拟方法,该方法基于自适应变步长波场延拓,以递归循环的方式实现分阶层间多次波的模拟.通过对模型添加双重层位约束,可以模拟指定地层产生的各阶层间多次波.利用二维反周期延拓方法压制波场延拓的边界反射优于传统方法,例如吸收边界法.提出自适应变步长波场延拓技术,大大提升了波场模拟的效率.理论和数值例子表明,本文方法模拟的一次波和各阶层间多次波与常用的有限差分方法模拟结果具有很好的一致性,且克服了有限差分方法无法分阶模拟波场的不足,显著提升了层间多次波识别的效率.

     

抛物Radon变换法近偏移距波场外推

本文给出了抛物Radon变换的基本原理,以及部分动校正后的CMP道集抛物线近似有效性的证明,基于带限正反最小平方抛物Radon变换的Levinson递推算法,对缺失的近偏移距地震波场进行叠前重建和外推.给出了抛物Radon变换法地震道重建外推的基本原理和叠前地震数据规则化的处理流程,另外对于Radon域均匀采样的情形,本文给出了均匀层状介质和Marmousi模型的近偏移距外推结果,计算结果验证了算法的稳定性和适用性.     

Full‐wavefield migration: using surface and internal multiples in imaging

Multiple scattering is usually ignored in migration algorithms, although it is a genuine part of the physical reflection response. When properly included, multiples can add to the illumination of the subsurface, although their crosstalk effects are removed. Therefore, we introduce full‐wavefield migration. It includes all multiples and transmission effects in deriving an image via an inversion approach. Since it tries to minimize the misfit between modeled and observed data, it may be considered a full waveform inversion process. However, full‐wavefield migration involves a forward modelling process that uses the estimated seismic image (i.e., the reflectivities) to generate the modelled full wavefield response, whereas a smooth migration velocity model can be used to describe the propagation effects. This separation of modelling in terms of scattering and propagation is not easily achievable when finite‐difference or finite‐element modelling is used. By this separation, a more linear inversion problem is obtained. Moreover, during the forward modelling, the wavefields are computed separately in the incident and scattered directions, which allows the implementation of various imaging conditions, such as imaging reflectors from below, and avoids low‐frequency image artefacts, such as typically observed during reverse‐time migration. The full wavefield modelling process also has the flexibility to image directly the total data (i.e., primaries and multiples together) or the primaries and the multiples separately. Based on various numerical data examples for the 2D and 3D cases, the advantages of this methodology are demonstrated.     

数据域特征波反射走时反演

地震波的运动学信息（走时、斜率等）通常用于宏观速度建模.针对走时反演方法,一个基本问题是走时拾取或反射时差的估计.对于成像域反演方法,可以通过成像道集的剩余深度差近似计算反射波时差.在数据域中,反射地震观测数据是有限频带信号,如果不能准确地确定子波的起跳时间,难以精确地确定反射波的到达时间.另一方面,如果缺乏关于模型的先验信息,则很难精确测量自地下同一个反射界面的观测数据同相轴和模拟数据同相轴之间的时差.针对走时定义及时差测量问题,首先从叠前地震数据的稀疏表达出发,利用特征波场分解方法,提取反射子波并估计局部平面波的入射和出射射线参数.进一步,为了实现自动和稳定的走时拾取,用震相的包络极值对应的时间定义反射波的到达时,实现了立体数据中间的自动生成.理论上讲,利用包络极值定义的走时大于真实的反射波走时,除非观测信号具有无限带宽（即delta脉冲）.然而,走时反演的目的是估计中-大尺度的背景速度结构,因此走时误差导致的速度误差仍然在可以接受的误差范围内.利用局部化传播算子及特征波聚焦成像条件将特征波数据直接投影到地下虚拟反射点,提出了一种新的反射时差估计方法.既避免了周期跳跃现象以及串层等可能性,又消除了振幅因素对时差测量的影响.最后,在上述工作基础之上,提出了一种基于特征波场分解的新型全自动反射走时反演方法（CWRTI）.通过对泛函梯度的线性化近似,并用全变差正则化方法提取梯度的低波数部分,实现了背景速度迭代反演.在理论上,无需长偏移距观测数据或低频信息、对初始模型依赖性低且计算效率高,可以为后续的全波形反演提供可靠的初始速度模型.理论和实际资料的测试结果证明了本文方法的有效性.

     

Seismic wavefield modeling based on time-domain symplectic and Fourier finite-difference method

Seismic wavefield modeling is important for improving seismic data processing and interpretation. Calculations of wavefield propagation are sometimes not stable when forward modeling of seismic wave uses large time steps for long times. Based on the Hamiltonian expression of the acoustic wave equation, we propose a structure-preserving method for seismic wavefield modeling by applying the symplectic finite-difference method on time grids and the Fourier finite-difference method on space grids to solve the acoustic wave equation. The proposed method is called the symplectic Fourier finite-difference (symplectic FFD) method, and offers high computational accuracy and improves the computational stability. Using acoustic approximation, we extend the method to anisotropic media. We discuss the calculations in the symplectic FFD method for seismic wavefield modeling of isotropic and anisotropic media, and use the BP salt model and BP TTI model to test the proposed method. The numerical examples suggest that the proposed method can be used in seismic modeling of strongly variable velocities, offering high computational accuracy and low numerical dispersion. The symplectic FFD method overcomes the residual qSV wave of seismic modeling in anisotropic media and maintains the stability of the wavefield propagation for large time steps.     

基于源波场重构的子波自适应频移算子

地震信号中的多尺度信息对于分辨率、成像精度和反演结果有非常重要的意义,本文提出一种基于直达波模拟波场重建震源子波的地震数据频移算子,以期能应用于基于波动的地震信号多尺度分频.频移过程主要分为两步,第一步利用直达波反传构建震源子波,第二步借助震源子波和频移算子进行多尺度分频.与常规数字滤波器相比,频移算子突破了滤波造成信...