Synthetic Seismograms and Wide-angle Seismic Attributes from the Gaussian Beam and Reflectivity Methods for Models with Interfaces and Velocity Gradients

— The effects of interfaces and velocity gradients on wide-angle seismic attributes are investigated using synthetic seismograms. The seismic attributes considered include envelope amplitude, pulse instantaneous frequency, and arrival time of selected phases. For models with interfaces and homogeneous layers, head waves can propagate which have lower amplitudes, as well as frequency content, compared to the direct arrivals. For media with interfaces and velocity gradients, higher amplitude diving waves and interference waves can also occur. The Gaussian beam and reflectivity methods are used to compute synthetic seismograms for simple models with interfaces and gradients. From the results of these methods, seismic attributes are obtained and compared. It was found that both methods were able to simulate wide-angle seismic attributes for the simple models considered. The advantage of using the Gaussian beam method for seismic modeling and inversion is that it is fast and also asymptotically valid for laterally varying media.     

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.     

声波介质一次散射波场高斯束Born正演

Born正演是一种常用的地震波场正演模拟方法,也是线性化地震反演的理论基础.在实际应用时,Born正演通常结合常规的地震射线方法进行实现.为了克服常规地震射线方法的弊端,并且保证地震波场的模拟精度和计算效率,本文提出了一种基于高斯束的一阶散射波场Born正演方法.该方法分为两个环节:首先,我们利用高斯束的走时和振幅信息将地下散射点处的反射率映射为地表束中心位置处的局部平面波;然后,我们利用逆倾斜叠加将局部平面波转化为接收点处的时空域散射波场.在具体的实施过程中,我们提出一种以wavelet-bank方式实现的局部平面波合成方法,同现有的算法相比,可以在保持计算精度的同时,大大减少计算时间;此外,我们还利用最速下降法优化了高斯束的迭代循环过程,进一步提高了Born正演的计算效率.两个模型的应用效果证明,本文所提出的高斯束Born正演方法可以精确、高效的实现声波介质一次散射波场的正演模拟,为三维大规模地震波场的正演问题提供了一种切实可行的实现方案.     

基于Student′s t分布的不依赖子波最小二乘逆时偏移

最小二乘逆时偏移(Least-Squares Reverse Time Migration,LSRTM)与常规偏移相比具有更高的成像分辨率、振幅保真性及均衡性等优势,是当前研究的热点之一.震源子波的估计直接影响LSRTM结果的好坏,在实际情况下考虑到震源子波的空变特性,其估计十分困难.为了消除子波对LSRTM结果的影响,本文发展了基于卷积目标泛函的不依赖子波LSRTM算法.目标泛函由观测记录卷积模拟记录的参考道以及模拟记录卷积观测记录的参考道组成,由于观测子波和模拟子波在目标泛函的两项中同时存在,从而消除了子波的影响.此外,常用的基于L2范数拟合的LSRTM算法对噪声非常敏感,尤其是当地震数据中含有异常值时,常规LSRTM无法得到满意的结果.Student′s t分布相比L2范数具有更好的稳健性,本文将其推广到不依赖子波LSRTM中,提升了算法的稳健性,最后通过理论模型及实际资料试算验证了算法的有效性和对复杂模型的适应性.     

2D Laplace-Domain Waveform Inversion of Field Data Using a Power Objective Function

The wavefield in the Laplace domain has a very small amplitude except only near the source point. In order to deal with this characteristic, the logarithmic objective function has been used in many Laplace domain inversion studies. The Laplace-domain waveform inversion using the logarithmic objective function has fewer local minima than the time- or frequency domain inversion. Recently, the power objective function was suggested as an alternative to the logarithmic objective function in the Laplace domain. Since amplitudes of wavefields are very small generally, a power <1 amplifies the wavefields especially at large offset. Therefore, the power objective function can enhance the Laplace-domain inversion results. In previous studies about synthetic datasets, it is confirmed that the inversion using a power objective function shows a similar result when compared with the inversion using a logarithmic objective function. In this paper, we apply an inversion algorithm using a power objective function to field datasets. We perform the waveform inversion using the power objective function and compare the result obtained by the logarithmic objective function. The Gulf of Mexico dataset is used for the comparison. When we use a power objective function in the inversion algorithm, it is important to choose the appropriate exponent. By testing the various exponents, we can select the range of the exponent from 5 × 10^?3 to 5 × 10^?8 in the Gulf of Mexico dataset. The results obtained from the power objective function with appropriate exponent are very similar to the results of the logarithmic objective function. Even though we do not get better results than the conventional method, we can confirm the possibility of applying the power objective function for field data. In addition, the power objective function shows good results in spite of little difference in the amplitude of the wavefield. Based on these results, we can expect that the power objective function will produce good results from the data with a small amplitude difference. Also, it can partially be utilized at the sections where the amplitude difference is very small.     

Joint viscoacoustic waveform inversion with the separated upgoing and downgoing wavefields in zero-offset vertical seismic profile data

The attenuation of seismic waves propagating in reservoirs can be obtained accurately from the data analysis of vertical seismic profile in terms of the quality-factor Q. The common methods usually use the downgoing wavefields in vertical seismic profile data. However, the downgoing wavefields consist of more than 90% energy of the spectrum of the vertical seismic profile data, making it difficult to estimate the viscoacoustic parameters accurately. Thus, a joint viscoacoustic waveform inversion of velocity and quality-factor is proposed based on the multi-objective functions and analysis of the difference between the results inverted from the separated upgoing and downgoing wavefields. A simple separating step is accomplished by the reflectivity method to obtain the individual wavefields in vertical seismic profile data, and then a joint inversion is carried out to make full use of the information of the individual wavefields and improve the convergence of viscoacoustic full-waveform inversion. The sensitivity analysis of the different wavefields to the velocity and quality-factor shows that the upgoing and downgoing wavefields contribute differently to the viscoacoustic parameters. A numerical example validates our method can improve the accuracy of viscoacoustic parameters compared with the direct inversion using full wavefield and the separate inversion using upgoing or downgoing wavefield. The application on real field data indicates our method can recover a reliable viscoacoustic model, which helps reservoir appraisal.     

双复杂条件下非倾斜叠加精确束偏移方法及应用I——声波方程

近年来,油气勘探的重心正转向具有复杂地表和复杂地质体的双复杂区域.本文发展了一种精确的双复杂条件下基于地表倾角信息的非倾斜叠加束偏移方法,相对于传统束成像方法无需进行三方面处理:(1)高程静校正;(2)相位校正;(3)束中心与接收点之间关于速度和束出射角的近似替换,因而具有更高的成像精度.通过加拿大逆掩断层模型、中原油田断层模型及实际资料的偏移试算,并与传统束偏移及波动方程偏移成像结果对比可知:本文非近似束偏移方法在近地表、高陡倾等构造处的成像精度、反射界面成像振幅等方面优于传统的偏移方法,以此验证了本文非倾斜叠加精确束偏移方法的正确性、优越性及适应性.     

多主频波场的时间阻尼全波形反演

低频成分缺失和地下速度强烈变化会导致严重的周期跳现象,是地震数据全波形反演的难题.通过对地震数据加时间阻尼和时间积分降主频处理,提出了一种可有效去除周期跳现象的多主频波场时间阻尼全波形反演方法.由浅到深的速度不准确会造成波形走时失配和走时失配的累积.浅部速度的准确反演可有效地减小深部波形走时失配与周期跳现象.对地震数据施加时间阻尼得到时间阻尼数据,利用不同阻尼值的时间阻尼地震数据实现由浅到深的全波形反演.低主频波场的周期跳现象相对高主频波场的要弱.对地震波场进行不同阶的时间积分以得到不同主频的波场,把低主频波场的全波形反演结果作为高主频波场全波形反演的初始模型.应用缺失4 Hz以下频谱成分的二维盐丘模型合成数据验证所提出的全波形反演方法的正确性和有效性,数值试验结果显示多主频波场的时间阻尼全波形反演方法对缺失低频成分地震数据和地下速度强烈变化具有很好的适应性.     

Optimum pilot sweep

The successful application of high-resolution seismic methods requires evaluating each element in the seismic system and ensuring that each part of the system contributes optimally to the success of the method. Unfortunately, unlike data processing, seismic signal generation is not carefully optimized. The purpose of our study was to optimize the source signal in order to better coordinate field operations with subsequent data processing to achieve their common objective. We developed an iterative method for a rational frequency distribution of the energy of a seismic source. The method allows the optimum amplitude spectrum of a source signal to be calculated, thus providing the best data quality at the end of the processing. We assume that the source signal is affected by a total transfer function, by the reflectivity function of a target interval, and by ambient noise, whose characteristics, if not known, can be estimated or measured in practice. The transfer function includes data processing other than the correlation stage and the final trace-optimizing filter. The variance of a reflectivity estimate is considered to be a measure of the data quality and improvement of the characteristic corresponds to a decrease in the variance. For this reason, a constrained Wiener deconvolution filter is used as the final trace-optimizing filter. It not only minimizes the variance of a reflectivity estimate but also ensures a specific signal-to-noise ratio. The method is made feasible by following the Vibroseis technique, primarily because of the versatility of the technique in controlling the signal spectrum. With the optimum amplitude spectrum obtained, the corresponding optimum pilot sweep can be readily calculated. Examples using synthetic data are presented to illustrate the method.     

Fractured reservoir delineation using multicomponent seismic data

The characteristic seismic response to an aligned-fracture system is shear-wave splitting, where the polarizations, time-delays and amplitudes of the split shear waves are related to the orientation and intensity of the fracture system. This offers the possibility of delineating fractured reservoirs and optimizing the development of the reservoirs using shear-wave data. However, such applications require carefully controlled amplitude processing to recover properly and preserve the reflections from the target zone. Here, an approach to this problem is suggested and is illustrated with field data. The proposed amplitude processing sequence contains a combination of conventional and specific shear-wave processing procedures. Assuming a four-component recording (two orthogonal horizontal sources recorded by two orthogonal horizontal receivers), the split shear waves can be simulated by an effective eigensystem, and a linear-transform technique (LTT) can be used to separate the recorded vector wavefield into two principal scalar wavefields representing the fast and slow split shear waves. Conventional scalar processing methods, designed for processing P-waves, including noise reduction and stacking procedures may be adapted to process the separated scalar wavefields. An overburden operator is then derived from and applied to the post-stacked scalar wavefields. A four-component seismic survey with three horizontal wells drilled nearby was selected to illustrate the processing sequence. The field data show that vector wavefield decomposition and overburden correction are essential for recovering the reflection amplitude information in the target zone. The variations in oil production in the three horizontal wells can be correlated with the variations in shear-wave time-delays and amplitudes, and with the variations in the azimuth angle between the horizontal well and the shear-wave polarization. Dim spots in amplitude variations can be correlated with local fracture swarms encountered by the horizontal wells. This reveals the potential of shear waves for fractured reservoir delineation.     

利用基于子波估计的频谱校正方法提高Q值估计精度(英文)

用Q值刻画的地震衰减在地震信号处理和解释中具有很广泛的应用。利用反射地震资料进行Q值估计需要解决地震子波和反射系数序列耦合的问题。从反射地震资料中去除反射系数序列的影响,这个过程称为频谱校正。本文提出了一种基于子波估计的求取Q值的方法,进而设计了一个反Q滤波器。该方法利用反射地震资料的高阶统计量进行子波估计,并利用所估计子波实现频谱校正。我们利用合成数据实验给出了质心频移法与频谱比法这两种常用的Q值估计方法在不同参数设置下的性能。人工合成数据和实际数据处理表明,利用本文提出的方法进行频谱校正后,可以得到可靠的Q值估计。经过反Q滤波,地震数据的高频部分得到了有效地恢复。     

多分量地震波波场分解研究

提出了一种有效的多分量地震波波场分解方法, 该方法假设在相邻道集波场的振幅、相位变化不大的情况下, 根据纵波和转换波在传播中相对于不同的极化矢量和慢度得到的垂向和水平向分量, 在最小方差约束下从多分量输入数据中获取纵波和转换波的偏振方向和振幅并重构纵波和转换波, 从而分离纵波和转换波. 这种方法精度较高, 不仅能有效分解一次反射波的多分量波场, 多次反射的纵波和转换波也能被恰当地分解. 该方法不需要输入地层速度等参数信息, 减少了人为误差的影响. 通过对合成资料的处理, 验证了方法的可行性和正确性.     

不依赖源子波的跨孔雷达时间域波形反演

波形反演是近年来较热门的反演方法,其分辨率可以达到亚波长级别.在波形反演的实际应用中,源子波的估计十分重要.传统方法使用反褶积来估计源子波并随着反演过程更新,该方法在合成数据波形反演中效果较好,但在实际数据反演过程中存在一系列的问题.由于实际数据信噪比较低,在源子波估计过程中需要大量的人为干涉,且结果并不一定可靠.本文使用一种基于褶积波场的新型目标函数,令反演过程不再依赖源子波.详细推导了针对跨孔雷达波形反演的梯度及步长公式,实现介电常数和电导率的同步反演.针对一个合成数据模型同时反演介电常数和电导率,结果表明该方法能够反演出亚波长尺寸异常体的形状和位置.接着,将该方法应用到两组实际数据中,并与基于估计源子波的时间域波形反演结果进行比较.结果表明不依赖源子波的时间域波形反演结果分辨率更高,也更准确.     

各向异性介质qP波传播描述II:分离纯模式标量波

在各向异性地震波场中,qP波与qS波常常是耦合在一起的.多分量地震数据处理中一个关键环节就是波型分离(即模式解耦),以纵波成分为主的常规单分量地震数据的成像则需要合理描述标量qP波的传播算子.本文作者曾构建了在运动学上同弹性波动方程等价,动力学上突出标量qP波的伪纯模式波动方程.为了彻底消除qS波残余,本文根据波矢量与qP波偏振矢量之间的偏差,提出从伪纯模式波场提取纯模式标量qP波的方法.数值分析展示了投影偏差算子在波数域和空间域的特征.基于不同复杂程度理论模型的试验结果表明,联合"伪纯模式传播算子"与"投影偏差校正"可为各向异性介质分离模式波场传播过程提供一种简便的描述工具.     

相对保幅的角度域VSP逆时偏移(英文)

本文介绍了一种改进的角度域VSP逆时偏移方法。对VSP逆时偏移中的逆推公式进行了改进,为方便数值计算出相对保幅的角度域共成像点道集(ADCIGs)。此外VSP记录到的波场信息丰富,包括上行波场、下行波场和直达波场等,本文分析了这些波场的响应特征,发现直达波和下行波在角度域共成像点道集(ADCIGs)上都产生了成像噪音,直达波产生的噪音尤为严重。把该方法用于我国西部地区实际观测的VSP资料,不仅获得相对保幅角度域共成像点道集(ADCIGs),而且压制了成像噪音。通过数值模型试算,实际资料的应用验证了该方法的实用性与有效性,从而为VSP偏移速度分析、VSP AVA/AVO分析和反演等提供可靠的基础资料。     

检波点与震源能量比值在偏移成像条件中的作用分析

利用检波点能量与震源能量比值构造衰减系数,研究波场各频率成分在自相关与反褶积两类成像条件中偏移成像的作用.分析检波点能量与震源能量比值分布特征,有利于克服反褶积型成像条件中分母趋于零时计算不稳定现象,也有助于平滑检波点波场与震源波场;检波点能量与震源能量比值超出正常范围容易导致偏移结果中出现照明不均匀现象.本文方法有助于改善反褶积型成像条件的偏移计算,也有助于应用照明技术指导偏移成像结果的振幅校正.     

Site characterization using Gauss–Newton inversion of 2-D full seismic waveform in the time domain

Site characterization for design of deep foundations is very crucial, as unanticipated site conditions still represent significant problems and disputes occur during construction. Traditional surface-based geophysical methods, which use wave velocity dispersion or first-arrival times, have been widely used recently to assess spatial variation; however they cannot well characterize reverse profiles or buried low-velocity zones. For better characterization of these challenging site conditions, a full waveform inversion based on Gauss–Newton method is presented. The inversion scheme is based on a finite-difference solution of the 2-D elastic wave equation in the time domain. The strength of this approach is the ability to generate all possible wave types of seismic wavefields that are then compared with observed data to infer complex subsurface properties. Virtual sources and reciprocity of wavefields are used for calculation of partial derivative wavefields to reduce computer time. Cross convolution between observed and estimated wavefields are also employed to allow the technique to be independent of the source signatures. The capability of the presented technique is tested with both synthetic and real experimental data sets. The inversion results from synthetic data show the ability of characterizing anomalies of low- and high-velocity zones, and the inversion results from real data are generally consistent with SPT N-value, including the identification of a buried low-velocity layer.     

地震相干偏移与数据自参照偏移的关系

地震相干偏移和数据自参照偏移是最近几年发展起来的地震成像新方法,并且已在地震勘探研究领域开始应用.该方法在VSP（垂直地震剖面）中的应用尤其值得关注,因为它部分地解决了一些在地面反射地震方法中难以解决的问题.地震相干偏移是用地震数据的互相关处理后的数据进行偏移,而数据自参照偏移则不必在偏移前对数据进行互相关处理,而是通过记录数据中的两个不同部分波场的延拓来互相参照进行偏移.二者的优点都是在偏移运算中可以不考虑原始震源位置、激发时间以及地震波初始传播路径.本文从偏移成像的理论出发分析了这两类算法,发现二者在成像原理上是一致的.本文还通过一个二维VSP多次波偏移的数值模拟实验,比较和归纳了这两类算法在实际应用中的特点.     

基于局部斜率属性的VSP波场分离研究

基于垂直地震剖面(VSP)资料中上、下行波视速度的差异,利用地震剖面同相轴局部斜率属性参数,提出了一种分离上、下行波场的新方法.首先利用Fourier变换初步分离上、下行波场,然后利用平面波分解滤波器(Plane Wave Destruction (PWD) Filter)技术估计初始分离波场的同相轴局部斜率属性参数,在此基础上对VSP原始资料波场分离.该方法是一种时间域最小平方优化分离波场的方法,不存在其他滤波方法阈值滤波器边界的影响,减少了因镶边问题带来的假象.模拟和实际资料处理结果表明,该方法与传统方法相比,分离出的上、下行波噪声假象少,振幅保持好,更好地消除了上、下行波的相互影响.     

A central difference method with low numerical dispersion for solving the scalar wave equation

In this paper, we propose a nearly‐analytic central difference method, which is an improved version of the central difference method. The new method is fourth‐order accurate with respect to both space and time but uses only three grid points in spatial directions. The stability criteria and numerical dispersion for the new scheme are analysed in detail. We also apply the nearly‐analytic central difference method to 1D and 2D cases to compute synthetic seismograms. For comparison, the fourth‐order Lax‐Wendroff correction scheme and the fourth‐order staggered‐grid finite‐difference method are used to model acoustic wavefields. Numerical results indicate that the nearly‐analytic central difference method can be used to solve large‐scale problems because it effectively suppresses numerical dispersion caused by discretizing the scalar wave equation when too coarse grids are used. Meanwhile, numerical results show that the minimum sampling rate of the nearly‐analytic central difference method is about 2.5 points per minimal wavelength for eliminating numerical dispersion, resulting that the nearly‐analytic central difference method can save greatly both computational costs and storage space as contrasted to other high‐order finite‐difference methods such as the fourth‐order Lax‐Wendroff correction scheme and the fourth‐order staggered‐grid finite‐difference method.