Suppression of seismic surface waves based on adaptive weighted super-virtual interferometry

For land seismic surveys, the surface waves are the dominant noises that mask the effective signals on seismograms. The conventional methods isolate surface waves from the effective signals by the differences in frequencies or apparent velocities, but may not perform well when these differences are not obvious. Since the original seismic interferometry can only predict inter-receiver surface waves, we propose the use of super-virtual interferometry (SVI), which is a totally data-driven method, to predict shot-to-receiver surface waves, since this method relieves the limitation that a real shot should collocate with one of the receivers for adaptive subtraction. We further develop the adaptive weighted SVI (AWSVI) to improve the prediction of dispersive surface waves, which may be generated from heterogeneous media at the near surface. Numerical examples demonstrate the effectiveness of AWSVI to predict dispersive surface waves and its applicability to the complex near surface. The application of AWSVI on the field data from a land survey in the east of China improves the suppression of the residual surface waves compared to the conventional methods.     

Analysis of effects of active sources on observed phase velocity based on the thin layer method

In the wave field induced by active sources, the observed phase velocity of surface waves is influenced by both mode incompatibility (i.e. non-planar spread of surface waves is idealized as plane waves) and body waves. Effects of sources are usually investigated based on numerical simulations and physical models. Several methods have been proposed to mitigate the effects. In application, however, these methods may also have difficulties since the energy of the body waves depends on soil stratification and parameters. There are multiple modes of surface waves in layered media, among which the higher modes dominate the wave field for soils with the irregular shear velocity profiles. Considering the mode incompatibility and the higher modes, we derive analytical expressions for the effective phase velocity of the surface waves based on the thin layer stiffness method, and investigate the effects of the body waves on the observed phase velocity through the phase analysis of the vibrations of both the surface waves and the body waves. The results indicate that the effective phase velocity of the surface waves in layered media varies with the frequency and the spread distance, and is underestimated compared to that of the plane surface waves in the spread range less than about one wavelength. The oscillations that appeared in the observed phase velocity are due to the involvement of the body waves. The mode incompatibility can be ignored in the range beyond one wavelength, while the influence range of the body waves is far beyond one wavelength. The body waves have a significant influence on the observed phase velocity of the surface waves in soils with a soft layer trapped between the first and the second layers because of strong reflections.     

Effect of topography and subsurface inhomogeneity on seismic rayleigh waves

The effect of topography and subsurface inhomogeneity on surface motion is investigated in the case of Rayleigh waves. In the previous paper, the same effect was investigated in the case of SV waves. Several types of topography, such as cliffs both with and without a soft layer at the foot of the slope, are considered. Computations are made using a new hybrid method combining a particle model with a finite element method. In cases of harmonic Rayleigh waves, surface motions with amplitudes as large as 1.5 to 5 times the horizontal surface displacement of the incident Rayleigh waves are produced near the slope and the sloping interface. When a Rayleigh wave propagating through a hard single-layered ground encounters a sloping interface where hard ground and soft ground make contact with each other, Rayleigh waves having two different, phase velocities are produced and they correspond to the fundamental mode, and the first mode determined by Haskell's method. In addition, the transient response when Rayleigh waves propagate through the cliff is also simulated. Assuming the vertical component of the Tokachi-oki Earthquake (1968) measured on the surface to be a Rayleigh wave, the incident Rayleigh wave can be obtained by a Fourier synthesis of eigenfunctions of Rayleigh waves.     

3D surface‐wave estimation and separation using a closed‐loop approach

Surface waves in seismic data are often dominant in a land or shallow‐water environment. Separating them from primaries is of great importance either for removing them as noise for reservoir imaging and characterization or for extracting them as signal for near‐surface characterization. However, their complex properties make the surface‐wave separation significantly challenging in seismic processing. To address the challenges, we propose a method of three‐dimensional surface‐wave estimation and separation using an iterative closed‐loop approach. The closed loop contains a relatively simple forward model of surface waves and adaptive subtraction of the forward‐modelled surface waves from the observed surface waves, making it possible to evaluate the residual between them. In this approach, the surface‐wave model is parameterized by the frequency‐dependent slowness and source properties for each surface‐wave mode. The optimal parameters are estimated in such a way that the residual is minimized and, consequently, this approach solves the inverse problem. Through real data examples, we demonstrate that the proposed method successfully estimates the surface waves and separates them out from the seismic data. In addition, it is demonstrated that our method can also be applied to undersampled, irregularly sampled, and blended seismic data.     

基于交错网格有限差分法的断面波叠前时间偏移成像影响因素正演分析

断层和断裂带的有效识别是地震资料解释中的重要环节,断层在地震信号响应中以断面波的形式体现,因此断面波成像的质量关系到断层的精细识别与刻画.本文利用精度较高的交错网格有限差分正演模拟方法对断面波成像的影响因素进行了正演研究,主要正演分析的参数包括采集因素中的电缆长度和采集方向,地质因素中的断层倾角、断距、反射系数,以及处理因素中的偏移方法等几个方面.通过正演论证得出：采用合理的采集参数能够提高断面波的照明度;有效结合地质因素能够提高断面波的解释精度;利用合理的偏移方法能够使断层归位更加准确,断面波有效成像.基于以上结论,对于断面波的精确识别与刻画,应综合采集因素,处理因素及地质因素,只有这样才能提高断层的解释精度,有效减小解释误差.     

Forward modeling of Rayleigh surface waves for analytical characterization of dominant dispersion trends

Forward modeling is of critical importance for inversion analysis of surface wave methods to obtain shear-wave velocity (V_S) profiles of soil sites. The dynamic stiffness matrix (DSM) method can provide forward modeling of Rayleigh surface waves to simulate complex wave propagation in layered soil sites. However, contamination from body waves and interference of multiple Rayleigh wave modes can reduce the accuracy of theoretical dispersion curves, especially at irregular soil sites with embedded low-velocity or high-velocity layers. An analytical method is developed herein to combine the techniques of the multichannel analysis of surface waves method with the DSM method to improve the accuracy of the theoretical dispersion analysis for soil sites. The proposed method implements multichannel analysis of the analytical displacement responses to capture dominant dispersion trends. Comparison of the results obtained with the new method against those from the transfer matrix method and the literature indicates that the new method can (1) effectively minimize the effects of contamination caused by body waves and interference from several Rayleigh wave modes, and (2) generate accurate dominant dispersion trends for soil sites with various stiffness profiles, especially for the high-frequency dispersion characteristics of the profiles with embedded low-velocity layers.     

Composition of high frequency ambient noise from cross-correlation:A case study using a small aperture array

Long-time cross correlation of ambient noise has been proved as a powerful tool to extract Green's function between two receivers.The study of composition of ambient noise is important for a better understanding of this method.Previous studies confirm that ambient noise in the long period (3 s and longer) mostly consists of surface wave,and 0.25-2.5 s noise consists more of body waves.In this paper,we perform cross correlation processing at much higher frequency (30-70 Hz) using ambient noise recorded by a small aperture array.No surface waves emerge from noise correlation function (NCF),but weak P waves emerge.The absence of surface wave in NCF is not due to high attenuation since surface waves are strong from active source,therefore probably the high ambient noise mostly consists of body wave and lacks surface wave.Origin of such high frequency body waves in ambient noise remains to be studied.     

偏离大圆路径传播对四川西部面波相速度成像的影响

面波成像是研究地壳上地幔横波速度结构的一种重要方法．通常,面波相速度或群速度成像假设面波沿大圆路径传播．但是,在地下介质速度结构变化较大时,面波会偏离大圆路径传播,从而导致基于大圆路径假设下的面波成像结果存在一定的误差. 我们采用基于射线追踪的面波成像方法,研究了面波的偏离大圆路径传播对四川西部地区面波相速度成像结果的影响．使用快速行进法(fast marching method)进行面波传播路径的射线追踪,采用子空间反演法(subspace inversion)进行迭代反演,对理论模型合成数据和川西台阵的短周期背景噪声相速度频散数据进行成像分析,并与使用大圆路径传播的成像结果进行对比．对理论模型的测试结果表明,当速度结构变化较大时,基于偏离大圆路径传播的面波成像能够更好地恢复模型异常．对川西台阵的真实数据反演结果显示:在短周期为6 s时,基于偏离大圆路径传播的反演方法较基于大圆路径传播的反演方法所获得的相速度异常的幅度更大些,在四川盆地区域两者的差异接近0.2 km/s;在周期为10 s时,两种反演方法的差异显著减小,基本都在0.1 km/s以内．这主要是因为6 s周期的面波相速度对复杂的上地壳浅层结构更为敏感,从而使得面波的偏离大圆路径传播效应对反演结果的影响更为显著．本文结果表明,当某一周期不同路径的面波相速度测量值变化较大,例如相对于平均相速度的异常超过10％时,则需考虑采用基于偏离大圆路径传播的面波成像方法,否则速度异常较大区域的反演结果可能会造成较大的偏差．      

Application of the time-frequency polarization analysis to study the complexity of surface wave propagation

A new technique called the time-frequency polarization analysis was introduced in this paper. The technique combined the traditional surface wave analysis techniques (moving window and multi-filter) with the singular value decomposition method to measure the incidence azimuth of surface waves with different wavelengths. It was applied to study the progagation paths of surface waves across the different blocks of Chinese continent and different zones of Qinghai-Xizang (Tibet) plateau. The results show that the method can make full use of the differences in frequency compositions and arrival times of different surface wave modes, and give better polarization analysis results. The analysis by actual data shows that the lateral heterogeneity of the lithospheric structure influences the propagation paths of surface waves severely. Deviations of the paths across the Qinghai-Xizang plateau from great circle paths are great. Deviations of the surface waves across the different zones in Qinghai-Xizang plateau are different. This project was sponsored by the Joint Earthquake Science Foundation.     

浅层地震反射资料的多阶振型面波反演

浅层地震反射波法和面波方法是两种相互独立发展的地震勘探方法,在各自的数据采集和处理中,对方都是作为干扰信号而存在. 本文利用浅层地震反射资料中被视为干扰的面波信号,通过成熟的多道面波勘探技术处理浅层地震反射资料,在频率-波数域中提取多阶振型面波的频散曲线,并基于该曲线反演浅地表S波速度结构. 这种方法充分开发利用了已有数据,无需单独的面波数据采集系统,同时为解释浅层地震反射资料提供了额外的信息约束. 结果表明:浅层地震反射资料中可提取出可靠的多阶振型面波频散曲线,并能给出稳定的反演结果,同时,面波反演的多解性可以通过高阶振型反演得以进一步约束;低速层的存在是观测频散曲线出现振型跳跃或呈“之”字形回折的必要条件而非充分条件.      

Scattering and diffraction of earthquake motions in irregular,elastic layers,II: Rayleigh and body P and SV waves

We present our study of the wave propagation in an irregularly layered, elastic wave-guide excited by incoming Rayleigh surface waves and P and SV body waves. Our aim is to show examples of applying a method that will make it possible to analyze the distribution and amplification of displacements, rotations, curvatures, strains, and stresses on or below the ground surface during passage of strong earthquake ground motion. We employ the weighted-residuals method, which makes it possible to calculate the scattered and diffracted waves, and then we illustrate the amplification of motions in the vicinity of inhomogeneity.     

Signal reconstruction of surface waves on SASW measurement using Gaussian Derivative wavelet transform

Surface wave method consists of measurement and processing of the dispersive Rayleigh waves recorded from two or more vertical transducers. The dispersive phase data are inverted and the shear wave velocity versus depth is obtained. However, in case of residual soil, the reliable phase spectrum curve is difficult to be produced. Noises from nature and other human-made sources disturb the generated surface wave data. In this paper, a continuous wavelet transform based on mother wavelet of Gaussian Derivative was used to analyze seismic waves in different frequency and time. Time-frequency wavelet spectrum was employed to localize the interested seismic response spectrum of generated surface waves. It can also distinguish the fundamental mode of the surface wave from the higher modes of reflected body waves. The results presented in this paper showed that the wavelet analysis is able to determine reliable surface wave spectrum of sandy clayey residual soil.     

地下夹塞断面形状随机性对平面SH波散射的影响

采用波函数展开法及边界离散的方法给出了任意断面形状的地下夹塞对平面SH波散射的半解析解; 利用蒙特卡罗方法随机模拟产生了30组夹塞断面样本, 通过对该样本在平面SH波入射下地表位移幅值的统计分析, 研究了夹塞断面形状随机性对平面SH波散射的影响. 结果表明, 夹塞断面形状随机性对平面SH波的散射具有重要影响. 以圆形夹塞为例, 当断面半径的变异系数为0.1时, 地表位移响应幅值变异系数可达0.71. 随着入射频率的升高, 变异系数逐渐增大; 随着夹塞刚度的降低, 变异系数逐渐增大; 随着夹塞埋深的增加, 变异系数逐渐减小.      

Fault detection by reflected surface waves based on ambient noise interferometry

Detecting subsurface fault structure is important for evaluating potential earthquake risks associated with active faults. In this study, we propose a new method to detect faults using reflected surface waves observed in ambient noise cross correlation functions. Ambient noise tomography using direct surface waves obtained from ambient noise interferometry has been widely used to characterize active fault zones. In cases where a strong velocity contrast exists across the fault interface, fault-reflected surface waves are expected. We test this idea using a linear array deployed in the Suqian segment of Tanlu fault zone in Eastern China. The fault-reflected surface waves can be clearly seen in the cross-correlation functions of the ambient noise data, and the spatial position of the fault on the surface is close to the stations where the reflected signals first appear. Potentially reflected surface waves could also be used to infer the dip angle, fault zone thickness and the degree of velocity contrast across the fault by comparing synthetic and observed waveforms.     

瞬时偏振滤波技术压制X分量中的面波(英文)

瑞雷面波与体波在偏振上的差异使得利用偏振特征分离这两种波场成为可能。为克服滑动时窗法在应用中存在的一些问题,本文将工程地震和天然地震信号分析中的复数道技术与瞬时偏振分析技术引进到能源领域多分量地震勘探转换波记录中的面波压制处理。理论模拟与野外实测数据的处理试验显示了较好的压制面波、保留有效信号的去噪效果,证明了该技术值得在多分量数据处理中推广应用。     

Instantaneous polarization filtering focused on suppression of surface waves

The different characteristics of polarization of body and Rayleigh waves make it possible to separate these two types of waves by their characteristics and suppress the latter. The moving time-window analysis often is used in polarization filtering but it is difficult to determine a suitable time-window length, resulting in some problems, such as complex eigenvalues and non-convergence. For overcoming these disadvantages, in this paper, we introduce the concept of complex-trace analysis and conduct de-noise processing to suppress undesirable surface waves by instantaneous polarization analysis in the case of horizontal and vertical component seismic recordings from the Hauinan coal mine. The performance of the method is illustrated by examples with synthetic and field data and its effectiveness to remove surface waves from multi-component seismic data is demonstrated.     

Effect of topography and subsurface inhomogeneities on seismic SV waves

The effect of topography and subsurface inhomogeneity on surface motion is investigated in the case of incident SV waves. Several types of topography, such as a cliff, a cliff with a soft layer and filled land, are considered. Computations are made using a new method combining a particle model with a finite element method. The accuracy of this method is discussed through comparisons with Wong's solutions, which are in good agreement. It is found that the surface displacement is very much influenced by surface irregularities when the incident wavelengths are comparable to the size of the topographic features. Rayleigh waves are strongly produced in the neighbourhoods of the slopes of a cliff and a cliff with a soft layer, the latter being a cliff adjacent to and covered at its foot by a soft layer. Thus, a zone of large amplification takes place near a slope, combining incident SV waves and Rayleigh waves. A large displacement also occurs at the upper corner of the slope. In filled land, vertical and horizontal displacements are produced, which are 3 times larger than those at a distance. The present results are considered to be significant from the viewpoint of engineering seismology.     

弹性介质中瑞雷面波有限差分法正演模拟

为研究瑞雷面波的形成机理及传播规律,促进瑞雷面波资料处理方法的发展,本文根据弹性波方程,采用交错网格有限差分数值求解算法,对浅层各向同性弹性介质进行了包括瑞雷面波和体波在内的全波场模拟. 提出了变系数吸收边界条件并将之应用于正演模拟,使边界条件的处理简单而高效,同时给出了角点的处理方法. 对工程勘察中常见的连续和层状介质模型进行了模拟,获得了更加接近实际情况的地震记录. 结合模拟记录,探讨了瑞雷面波的形成条件,同时讨论了震源埋深对面波能量的影响.     

求解二维弹性半空间散射问题的波函数组合法

提出了一种新的波函数组合法,将体波与面波特征函数组合求解不规则河谷地形在不同波型以不同角度入射的非一致位移输入问题.新方法严格满足不规则河谷散射表面自由条件,克服了目前波函数函数展开法不能完备表达二维不规则地形散射问题的缺点.数值算例表明波函数组合方法具有精确、稳定、计算量小的特点,可以应用于结构-地基动力相互作用的非一致输入问题.     

Transient Scattering of Elastic Waves by Dipping Layers of Arbitrary Shape Part 1: Antiplane Strain Model

Scattering of elastic waves by two dimensional multilayered dipping sediments of arbitrary shape embedded in an elastic half-sapce is investigated by using a bondary method. The displancement field is evaluated throughout the elastic media for both steady state and transient incident SH waves. The unknown scattered field is expressed in terms of wave functions which satisfy the equation of motion, traction-free boundary condition and appropariate radiation conditions. The transient response is constructed from the steady state solution by using the fast Fourier transform technique. The numerical results presented demonstrate that scattering of waves by subsurface irregularities may cause locally very large amplification of surface ground motion. The motion can be affected greatly by the scattered surface waves in the sediments. The results clearly indicate that the surface ground motion depends upon a number of parameters present in the problem, such as frequency and the angle of incidence of the incoming wave, impedance contrast between the layers and location of the observation point.