基阶与高阶瑞利波联合反演研究

研究了六层层状介质模型瑞利波基阶和高模式波相速度对横波速度、深度的敏感性,结果表明：基阶波较高模式波对7 m以内浅部地层的横波速度更敏感,敏感性频带在10～25 Hz范围内,峰值频带集中在18 Hz左右;高模式波较基阶波对深部地层的横波速度更敏感,敏感性频带宽,峰值分散.基阶波对浅层的敏感性和高模式波穿透深度更深的特点为近地表岩土层二维横波速度结构的联合反演提供了前提条件.利用阻尼最小二乘SVD（Singular Value Decomposition）算法联合基阶与高模式波对理论模型和实例数据进行横波速度反演,反演结果表明联合反演增强了反演的稳定性,提高了反演的精度.     

利用深反射地震数据构建的多阶面波频散曲线反演近地表横波速度结构——以跨班公湖—怒江缝合带深反射地震资料为例

深反射地震剖面法为了获取深部结构特征常常采取大的偏移距采集数据.目前公开发表的相关资料中,鲜有利用深反射地震炮集数据获取近地表的结构特征.为此,本文通过正演测试了相关数据处理流程,即利用有限差分正演了起伏地表模型的大偏移距地震单炮弹性波场特征,通过共检波点域面波信号F-K频谱叠加构建新方法,从深反射地震数据集中提取了高品质的多阶面波频散曲线,再利用多阶面波联合反演获得了近地表的结构特征.在前述正演流程基础上,利用跨越班公湖—怒江缝合带的SinoProbe深反射地震剖面中的实际炮集数据,求取了基阶和一阶瑞利波频散曲线,联合反演后得到近地表横波速度结构.该结果与初至波走时反演获取的纵波速度结构具有较好的一致性,且在近地表的浅层分辨率较纵波速度结构特征更高,而更与已有地质认识相吻合.本文提供的相关数据处理流程表明利用深反射地震炮集数据,也能够获取近地表浅层的横波速度结构.     

2015年8月12日天津化学爆炸产生的多模式面波分析及其应用研究

2015年8月12日天津滨海新区发生的强烈化学品爆炸造成了巨大的经济损失和社会影响.天津爆炸产生了清晰的大振幅面波信号,分析结果表明这组信号由基阶和高阶面波组成,可以追踪到约135 km外的远处台站.利用这组面波信号分别开展了以下研究：（1）利用附近三个台站记录的四个单频基阶Rayleigh波信号对爆破事件的绝对位置进行了网格搜索,结果与利用GPS测量的位置相差仅0.498 km;（2）分别利用网格搜索和主事件定位法,对两次子事件的相对位置进行了确定,距离约75 m左右,与前人研究结果吻合;（3）从面波记录中测量到36条基阶Rayleigh波、49条第一高阶Rayleigh波、9条基阶Love波和29条第一高阶Love波的频散曲线,并进一步反演获得研究区域地下4 km内的S波速度结构.反演结果显示地表处S波速度低至0.375 km·s^-1,在小于1 km的浅地表速度梯度较大,符合典型的盆地结构特征.本文的研究结果为类似爆炸等突发事件快速定位提供了新的思路,有助于灾后救援的迅速展开;同时得到天津滨海新区及周边浅层精细的速度结构,对于地震灾害评估有很大帮助.     

微动多阶瑞雷波SPAC系数反演方法及应用研究

为充分利用微动信号中基阶和高阶模式瑞雷波,本文研究了基于多阶瑞雷波SPAC系数直接反演的方法.该方法首先基于地层介质响应计算多阶瑞雷波的能量占比,考虑实际观测台阵有限台站个数对SPAC系数影响,正演计算多阶瑞雷波SPAC系数,再采用快速模拟退火算法对其反演以获得地下介质横波速度结构.在此基础上,本文通过数值模拟验证该方法的可靠性,分别选取三种典型地质模型,基于模式叠加算法合成理论微动信号,采用本文方法计算其理论多阶瑞雷波SPAC系数并反演,给出反演结果与真实模型对比.我们将该方法应用于上海中心城区的地质调查中,通过与钻探结果对比,进一步验证该方法的有效性.本文理论与实际应用研究表明,基于多阶瑞雷波SPAC系数直接反演的微动探测方法有助于提高反演结果的可靠性,尤其对含软硬夹层的复杂地层介质,可提高探测精度.     

高频面波方法的若干新进展

面波多道分析方法(MASW)通过分析高频瑞雷波确定浅地表剪切波速度.在过去的20年中,由于该方法具有非侵入性、无损、高效及价格低的特点,越来越受到浅地表地球物理和地质工程学界的重视,视为未来最有希望的技术之一.这篇综述论文将介绍中国地质大学(武汉)浅地表地球物理团队近年来在研究高频面波的传播理论和应用中取得的部分成果.非几何波是一种仅存在于浅地表介质,尤其是未固结的沉积物中的独特的地震波.它的存在对快速而准确地获得表层S波速度有一定价值.我们的研究表明非几何波是一种具有频散特性的泄漏波.泄漏波的存在可能导致将其误认为瑞雷波的基阶或高阶能量,从而造成模式误判.这种模式误判会导致错误的反演结果.我们通过求取高基阶分离后的瑞雷波格林函数证明虚震源法瑞雷波勘探的可行性.这个结果将极大地降低野外瑞雷波勘探成本.勒夫波多道分析方法(MALW)中未知参数比瑞雷波的少,这使得勒夫波的频散曲线比瑞雷波的简单.因此,勒夫波反演更稳定,非唯一性更低.勒夫波数据生成的能量图像通常比瑞雷波的清晰,并具有更高的分辨率,从而可以更容易地拾取精确的勒夫波的相速度.利用雅克比矩阵分析波长与探测深度的关系表明对相同波长的基阶模式而言,瑞雷波的探测深度是勒夫波的1.3~1.4倍;而两种波的相同波长的高阶模式波的探测深度相同.我们也尝试了时间域勒夫波反演.按照勒夫波分辨率将地球模型剖分成了不同尺寸的块体,利用反卷积消除了地震子波对勒夫波波形的影响,通过更新每个块体的S波速度来拟合勒夫波波形,从而获得地下S波速度模型.该方法不基于水平层状模型假设,适用于任意二维介质模型.     

Inversion of Scholte wave dispersion and waveform modeling for shallow structure of the Ninetyeast Ridge

The construction of S-wave velocity models of marine sediments down to hundreds of meters below the seafloor is important in a number of disciplines. One of the most significant trends in marine geophysics is to use interface waves to estimate shallow shear velocities which play an important role in determining the shallow crustal structure. In marine settings, the waves trapped near the fluid–solid interface are called Scholte waves, and this is the subject of the study. In 1998, there were experiments on the Ninetyeast Ridge (Central Indian Ocean) to study the shallow seismic structure at the drilled site. The data were acquired by both ocean bottom seismometer and ocean bottom hydrophone. A new type of seafloor implosion sources has been used in this experiment, which successfully excited fast and high frequency (>500 Hz) body waves and slow, intermediate frequency (<20 Hz) Scholte waves. The fundamental and first higher mode Scholte waves have both been excited by the implosion source. Here, the Scholte waves are investigated with a full waveform modeling and a group velocity inversion approach. Shear wave velocities for the uppermost layers of the region are inferred and results from the different methods are compared. We find that the full waveform modeling is important to understand the intrinsic attenuation of the Scholte waves between 1 and 20 Hz. The modeling shows that the S-wave velocity varies from 195 to 350 m/s in the first 16 m of the uppermost layer. Depths levels of high S-wave impedance contrasts compare well to the layer depth derived from a P-wave analysis as well as from drilling data. As expected, the P- to S-wave velocity ratio is very high in the uppermost 16 m of the seafloor and the Poisson ratio is nearly 0.5. Depth levels of high S-wave impedance contrasts are comparable to the layer depth derived from drilling data.     

Feasibility of waveform inversion of Rayleigh waves for shallow shear-wave velocity using a genetic algorithm

Conventional surface wave inversion for shallow shear (S)-wave velocity relies on the generation of dispersion curves of Rayleigh waves. This constrains the method to only laterally homogeneous (or very smooth laterally heterogeneous) earth models. Waveform inversion directly fits waveforms on seismograms, hence, does not have such a limitation. Waveforms of Rayleigh waves are highly related to S-wave velocities. By inverting the waveforms of Rayleigh waves on a near-surface seismogram, shallow S-wave velocities can be estimated for earth models with strong lateral heterogeneity. We employ genetic algorithm (GA) to perform waveform inversion of Rayleigh waves for S-wave velocities. The forward problem is solved by finite-difference modeling in the time domain. The model space is updated by generating offspring models using GA. Final solutions can be found through an iterative waveform-fitting scheme. Inversions based on synthetic records show that the S-wave velocities can be recovered successfully with errors no more than 10% for several typical near-surface earth models. For layered earth models, the proposed method can generate one-dimensional S-wave velocity profiles without the knowledge of initial models. For earth models containing lateral heterogeneity in which case conventional dispersion-curve-based inversion methods are challenging, it is feasible to produce high-resolution S-wave velocity sections by GA waveform inversion with appropriate priori information. The synthetic tests indicate that the GA waveform inversion of Rayleigh waves has the great potential for shallow S-wave velocity imaging with the existence of strong lateral heterogeneity.     

Data-resolution Matrix and Model-resolution Matrix for Rayleigh-wave Inversion Using a Damped Least-squares Method

Inversion of multimode surface-wave data is of increasing interest in the near-surface geophysics community. For a given near-surface geophysical problem, it is essential to understand how well the data, calculated according to a layered-earth model, might match the observed data. A data-resolution matrix is a function of the data kernel (determined by a geophysical model and a priori information applied to the problem), not the data. A data-resolution matrix of high-frequency (≥2 Hz) Rayleigh-wave phase velocities, therefore, offers a quantitative tool for designing field surveys and predicting the match between calculated and observed data. We employed a data-resolution matrix to select data that would be well predicted and we find that there are advantages of incorporating higher modes in inversion. The resulting discussion using the data-resolution matrix provides insight into the process of inverting Rayleigh-wave phase velocities with higher-mode data to estimate S-wave velocity structure. Discussion also suggested that each near-surface geophysical target can only be resolved using Rayleigh-wave phase velocities within specific frequency ranges, and higher-mode data are normally more accurately predicted than fundamental-mode data because of restrictions on the data kernel for the inversion system. We used synthetic and real-world examples to demonstrate that selected data with the data-resolution matrix can provide better inversion results and to explain with the data-resolution matrix why incorporating higher-mode data in inversion can provide better results. We also calculated model-resolution matrices in these examples to show the potential of increasing model resolution with selected surface-wave data.     

Dipping-interface Mapping Using Mode-separated Rayleigh Waves

Multichannel analysis of surface waves (MASW) method is a non-invasive geophysical technique that uses the dispersive characteristic of Rayleigh waves to estimate a vertical shear (S)-wave velocity profile. A pseudo-2D S-wave velocity section is constructed by aligning 1D S-wave velocity profiles at the midpoint of each receiver spread that are contoured using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. Based on the assumption that a dipping-layer model can be regarded as stepped flat layers, high-resolution linear Radon transform (LRT) has been proposed to image Rayleigh-wave dispersive energy and separate modes of Rayleigh waves from a multichannel record. With the mode-separation technique, therefore, a dispersion curve that possesses satisfactory accuracy can be calculated using a pair of consecutive traces within a mode-separated shot gather. In this study, using synthetic models containing a dipping layer with a slope of 5, 10, 15, 20, or 30 degrees and a real-world example, we assess the ability of using high-resolution LRT to image and separate fundamental-mode Rayleigh waves from raw surface-wave data and accuracy of dispersion curves generated by a pair of consecutive traces within a mode-separated shot gather. Results of synthetic and real-world examples demonstrate that a dipping interface with a slope smaller than 15 degrees can be successfully mapped by separated fundamental waves using high-resolution LRT.     

Dispersion splitting of Rayleigh waves in layered azimuthally anisotropic media

Rayleigh wave dispersion can be induced in an anisotropic medium or a layered isotropic medium. For a layered azimuthally anisotropic structure, traditional wave equation of layered structure can be modified to describe the dispersion behavior of Rayleigh waves. Numerical stimulation results show that for layered azimuthal anisotropy both the dispersion velocities and anisotropic parameters depend principally on anisotropic S-wave velocities. The splitting S-wave velocities may produce dispersion splitting of Rayleigh waves. Such dispersion splitting appears noticeable at azimuthal angle 45°. This feature was confirmed by the measured results of a field test. The fundamental mode splits into two branches at azimuthal angle 45° to the symmetry axis for some frequencies, and along the same direction the difference of splitting-phase velocities of the fundamental model reaches the maximum. Dispersion splitting of Rayleigh waves was firstly displayed for anisotropy study in dispersion image by means of multichannel analysis of surface waves, the image of which provides a new window for studying the anisotropic property of media.     

Calculation of Rayleigh-wave phase velocities due to models with a high-velocity surface layer

Rayleigh-wave phase velocities have been utilized to determine shear (S)-wave velocities in near-surface geophysics since early 1980s. One of the key steps is to calculate theoretical dispersion curves of an earth model. When the S-wave velocity of the surface layer is higher than some of the layers below, however, the Rayleigh-wave phase velocity in a high-frequency range calculated by existing algorithms approaches the lowest S-wave velocity among the layers above the half-space, rather than a value related to the S-wave velocity of the surface layer. According to our numerical modeling results based on wave equation, trends of the Rayleigh-wave dispersive energy approach about a 91% of the S-wave velocity of the surface layer at a high-frequency range when its wavelength is much shorter than the thickness of the surface layer, which cannot be fitted by a dispersion curve calculated by existing algorithms. We propose a method to calculate Rayleigh-wave phase velocities of models with a high-velocity surface layer by considering its penetration depth. We build a substituted model that only contains the layer with the lowest S-wave velocity among the layers above the half-space and the layers above it. We use the substituted model to replace the original model to calculate phase velocities when the Rayleigh-wave wavelength is not long enough to penetrate the lowest S-wave velocity layer. Several synthetic models are used to verify fitness between the dispersion curve calculated by our proposed method and the trend of the highest dispersive energy. Examples of inversion also demonstrate high accuracy of using our method as the forward calculation method during the inversions.     

多极子阵列声波测井数据的纵横波慢度联合反演方法

多极子阵列声波测井仪器采集的单极和偶极数据受到地层、井孔、仪器测量系统的影响.在处理实际声波测井数据时,必须考虑多极子模式波的频散效应,以及测井仪器在其中的影响.根据仪器等效理论和相位匹配方法,本文提出了一种从多极子阵列声波测井数据中同时获得纵、横波慢度的联合反演方法.这种方法的关键在于利用相同仪器-地层模型计算多极子模式波频散曲线,以此来匹配频域内纵波与横波数据的相位.相对于将泄漏纵波和弯曲波频散效应分开处理的其他方法,该方法不仅可以减少纵横波速度反演的不确定性,而且还避免了从声波数据中提取频散数据的繁琐过程.通过理论分析和现场数据处理证明了本文联合反演方法的准确性和有效性.     

伪二维弹性波联合反演近地表的速度和衰减

利用弹性波的初至波和面波,应用交叉梯度算子,联合反演了近地表的二维纵横波速度和衰减参数,并提出了采用一维弹性波正演模拟,应用二维Tikhonov正则化,同时反演出二维速度模型和衰减模型的方法。理论模型测试和实际数据应用结果均表明本文算法极大地提高了计算效率,同时能够反演出可靠的速度模型和衰减模型。      

Elastic full waveform inversion of multi‐component OBC seismic data

Elastic full waveform inversion of seismic reflection data represents a data‐driven form of analysis leading to quantification of sub‐surface parameters in depth. In previous studies attention has been given to P‐wave data recorded in the marine environment, using either acoustic or elastic inversion schemes. In this paper we exploit both P‐waves and mode‐converted S‐waves in the marine environment in the inversion for both P‐ and S‐wave velocities by using wide‐angle, multi‐component, ocean‐bottom cable seismic data. An elastic waveform inversion scheme operating in the time domain was used, allowing accurate modelling of the full wavefield, including the elastic amplitude variation with offset response of reflected arrivals and mode‐converted events. A series of one‐ and two‐dimensional synthetic examples are presented, demonstrating the ability to invert for and thereby to quantify both P‐ and S‐wave velocities for different velocity models. In particular, for more realistic low velocity models, including a typically soft seabed, an effective strategy for inversion is proposed to exploit both P‐ and mode‐converted PS‐waves. Whilst P‐wave events are exploited for inversion for P‐wave velocity, examples show the contribution of both P‐ and PS‐waves to the successful recovery of S‐wave velocity.     

Elimination of higher modes in dispersive in-seam multimode Love waves

A deterministic pure phase shift filter (PPSF) is developed to extract the fundamental mode from multimode surface Love waves. Because of different phase velocities of modes and hence different phase traveltimes for a fixed travel distance between source and receiver, the deterministic PPSF can be computed, provided that the dispersion relation of the medium is estimated from the existing transmission data. The process consists of (a) applying the deterministic PPSF to the multimode wave (this step of the filtering process results in a time series in which the amplitudes of the fundamental mode appear at acausal times and the amplitudes of higher modes appear at causal times); (b) setting amplitude values equal to zero for positive times; (c) applying the inverse PPSF to the filtered signal. By using such a deterministic PPSF process, the higher modes almost disappear. The method is applied to synthetic multimode data computed by the normal-mode summation method.     

基于邻域算法的瑞利面波垂直-水平振幅比及频散曲线联合反演及应用

瑞利面波垂直-水平振幅比(或ZH振幅比)是一个随频率变化的函数,对于台站下方浅层地壳结构非常敏感,且具有和频散资料不同的深度敏感核,是传统频散反演方法的一个很好的补充,从而可以将基阶瑞利面波的ZH振幅比和面波频散数据联合起来更好地反演获得观测台站下方的速度结构.本文提出了基于邻域算法的面波频散曲线与ZH振幅比联合反演方法,我们进行了基于理论模型的模拟测试,证明了联合反演是一种更为可靠的反演方法,且能更好地约束浅层地壳结构.相比于频散曲线单独反演,联合反演不仅可以精确反演获得地壳的Vs结构,对分层地壳的Vp/Vs也能很好地约束.然后我们将联合反演算法应用于实际测量数据,获得了中国西南昆明台(KMI)下方更为准确的地壳横波速度结构及Vp/Vs模型.     

Research on the middle-of-receiver-spread assumption of the MASW method

The multichannel analysis of surface wave (MASW) method has been effectively used to determine near-surface shear- (S-) wave velocity. Estimating the S-wave velocity profile from Rayleigh-wave measurements is straightforward. A three-step process is required to obtain S-wave velocity profiles: acquisition of a multiple number of multichannel records along a linear survey line by use of the roll-along mode, extraction of dispersion curves of Rayleigh waves, and inversion of dispersion curves for an S-wave velocity profile for each shot gather. A pseudo-2D S-wave velocity section can be generated by aligning 1D S-wave velocity models. In this process, it is very important to understand where the inverted 1D S-wave velocity profile should be located: the midpoint of each spread (a middle-of-receiver-spread assumption) or somewhere between the source and the last receiver. In other words, the extracted dispersion curve is determined by the geophysical structure within the geophone spread or strongly affected by the source geophysical structure. In this paper, dispersion curves of synthetic datasets and a real-world example are calculated by fixing the receiver spread and changing the source location. Results demonstrate that the dispersion curves are mainly determined by structures within a receiver spread.     

基于贝叶斯理论的叠前多波联合反演弹性模量方法

AVO反演可以获得地层岩性和流体信息,而叠前反演问题都是高维的和非适定的,因此获得可靠稳定的解对叠前反演至关重要. 本文给出了一种基于贝叶斯理论的纵波和转换波联合反演密度比和模量比的方法. 鉴于剪切模量比、体积模量比可以更好地指示油气,基于岩石物理中速度比与模量比之间的关系,将此关系式代入Zoeppritz方程的近似形式Aki-Richards公式中,得到与模量比有关的反射系数近似公式. 联合纵波和转换波,利用最小二乘准则构建目标函数,最终反演出密度比、剪切模量比、体积模量比三个参数. 在反演过程中引入贝叶斯理论,假定先验信息服从高斯分布,待求参数服从改进的Cauchy分布,并去除待求参数之间的相关性. 利用模型数据和实际数据对本文方法进行测试,并与常规的单独利用纵波数据来反演方法进行比较,结果表明联合反演稳定性更好、精度更高、抗噪音能力更强,验证了本文方法的可行性和有效性.     

S-wave Velocity Profiles for Earthquake Engineering Purposes for the Cologne Area (Germany)

Local S-wave velocity-depth profiles are a key factor in seismic hazard assessment, as they allow the amplification potential of the sedimentary cover to be evaluated. Ambient seismic noise is mainly composed of surface waves, and therefore contains vital information about the S-wave velocity structure, allowing polarization or dispersion curves to be obtained from single station or array noise recordings. At two sites in the area of Cologne, Germany, the extended spatial correlation method was applied to such recordings and apparent phase velocity curves in the frequency range of interest for earthquake engineering were obtained. Using this data, a linearized inversion, the simplex downhill method, and a genetic algorithm yielded similar S-wave profiles. However, the latter method is recommended since it is less dependent upon a good starting model. Importantly, the presence of low-velocity layers in the Cologne area made it necessary to consider in the frequency range of interest higher modes in the inversion procedures. Finally, independent information on the total thickness of the sedimentary cover permitted the estimation of a 2D S-wave velocity profile crossing the Cologne area. Here, the H/V ratio inversion using 20 single-station noise recordings was used, with the results in good agreement with a geological profile.     

基于精确Zoeppritz方程的非线性AVO三参数反演

常规AVO三参数反演通常存在密度反演不准确的问题,而密度参数对常规油气藏中的流体识别、流体饱和度计算、孔隙度计算以及非常规油气藏中TOC含量计算、裂缝预测等都至关重要,因此对于研究如何利用大偏移距振幅信息和富含密度信息的PS波地震资料来提高密度反演结果的稳定性和精度显得尤为重要.研究基于贝叶斯反演理论框架,引入三变量Cauchy分布先验约束,利用精确Zoeppritz方程构建了AVO三参数联合反演的目标函数,对目标函数进行Taylor二阶非线性简化,得到模型参数的迭代更新公式,实现了大偏移距地震振幅信息的利用和PP波、PS波联合反演.合成数据和实际地震数据的方法测试结果表明,新方法不仅可以直接反演纵波速度、横波速度和密度,而且还具有很高的精度,尤其是密度反演结果.基于合成数据的PP波、PS波单独反演结果与PP波和PS波联合反演结果对比显示,联合反演稳定性更好,精度更高,抗噪能力更强,验证了该方法的可行性和有效性.与基于Aki-Richards近似公式的反演结果对比表明,该反演方法具有更高的反演精度和更好的抗噪性.