Multi-station analysis of surface wave dispersion

Methods based on multi-station recordings are presented for constructing the experimental dispersion curve of Rayleigh waves. Multi-station recording permits a single survey of a broad depth range, high levels of redundancy with a single field configuration, and the ability to adjust the offset, effectively reducing near field and far field effects. A method based on the linear regression of phase angles measured at multiple stations is introduced for determining data quality and filtering criteria. This method becomes a powerful tool for on site quality control in real time. The effects of multiple modes and survey line parameters, such as near offset, receiver spacing, and offset range, are investigated. Parametric studies result in general guidelines for the field data acquisition. A case study demonstrates how to easily deploy commonplace seismic refraction equipment to simultaneously record data for P-wave tomographic interpretation and multi-station analysis of surface wave.     

InterPACIFIC project: Comparison of invasive and non-invasive methods for seismic site characterization. Part II: Inter-comparison between surface-wave and borehole methods

The InterPACIFIC project was aimed at assessing the reliability, resolution, and variability of geophysical methods in estimating the shear-wave velocity profile for seismic ground response analyses. Three different subsoil conditions, which can be broadly defined as soft-soil, stiff-soil, and hard-rock, were investigated. At each site, several participants performed and interpreted invasive measurements of shear wave velocity (Vs) and compression wave velocity (Vp) in the same boreholes. Additionally, participants in the project analysed a common surface-wave dataset using their preferred strategies for processing and inversion to obtain Vs profiles. The most significant difference between the invasive borehole methods and non-invasive surface wave methods is related to resolution of thin layers and abrupt contrasts, which is inherently better for invasive methods. However, similar variability is observed in the estimated invasive and non-invasive Vs profiles, underscoring the need to account for such uncertainty in site response studies. V_S,30 estimates are comparable between invasive and non-invasive methods, confirming that the higher resolution provided by invasive methods is quite irrelevant for computing this parameter.     

面波频散测量的频时分析法

总结了面波频时分析(FTAN)的主要成果,如移动窗分析法、多重滤波法、残差频散测量法、时间变量滤波法,力图阐述各方法间的联系.对滤波参数的选择,详细介绍了常相对带宽、均等显示滤波、线性时间分辨、最佳滤波等,其中线性时间分辨是本文首次提出的新方法.对频时分析中的显示技术作了必要的讨论,最后介绍了如何利用频时分析方法作相速度测量.     

Global surface currents: a high-resolution product for investigating ocean dynamics

A global 1/4° resolution product of surface currents has been developed by the Centre de Topographie des Océans et de l’Hydrosphère. The surface current is calculated from a combination of Ekman currents derived from wind estimates from QuikSCAT satellite, geostrophic current anomalies derived from altimetry, and a mean geostrophic current derived from climatology. In the equatorial band, the currents are adjusted following the methodology proposed by Lagerloef et al. (J Geophys Res, 104(C10):22313–22326, 1999). These satellite-derived currents have been compared to different types of in situ current observations. A global validation is performed using Lagrangian surface drifting buoys and acoustic Doppler current profiler current observations along ship tracks. The comparison shows a very good agreement in the subtropical and mid-latitude bands. The correlation between the satellite-derived currents and the drifter currents in zonal mean bands is around 0.7 for most of the world oceans, both for the zonal and the meridional components. This correlation rises up to 0.8 in the regions of strong boundary currents. In the equatorial band, the correlation with the surface drifting buoys is reduced. A direct comparison with the TOGA/TAO moored current meter data at the equator shows that the low frequency currents are captured by the satellite current product, but there is a substantial high-frequency signal (<20 days), which is not reproduced. This is especially the case for the meridional component and is mainly related to the tropical instability waves. We also show that using daily QuikSCAT wind forcing improves the satellite current product, particularly in the high-latitude westerly wind belt and in the tropical Indian Ocean.     

Multichannel analysis of surface wave method with the autojuggie

The shear (S)-wave velocity of near-surface materials and its effect on seismic-wave propagation are of fundamental interest in many engineering, environmental, and groundwater studies. The multichannel analysis of surface wave (MASW) method provides a robust, efficient, and accurate tool to observe near-surface S-wave velocity. A recently developed device used to place large numbers of closely spaced geophones simultaneously and automatically (the ‘autojuggie’) is shown here to be applicable to the collection of MASW data. In order to demonstrate the use of the autojuggie in the MASW method, we compared high-frequency surface-wave data acquired from conventionally planted geophones (control line) to data collected in parallel with the automatically planted geophones attached to steel bars (test line). The results demonstrate that the autojuggie can be applied in the MASW method. Implementation of the autojuggie in very shallow MASW surveys could drastically reduce the time required and costs incurred in such surveys.     

Simulation and analysis of point-source surface wave fields

The complexity of near surface intensifies the diversity of seismic wave fields, which makes study on near surface wavefields important in many aspects. The strong absorption of low velocity layer can affect the resolution of seismic data, and free boundary can cause surface wave. Considering the above problems, we focus on the Rayleigh wavefields simulation using finite-difference wave equation of higher-order staggered grids and PML boundary conditions. Free boundary, buried source and overlying low velocity layer are taken into consideration and point explosion source is adopted. Through some numerical simulation with different parameters, we quantitatively analyze relationship between wave intensity and source depth, as well as the energy variation with propagation and obtain some practical knowledge and conclusions.     

SWprocess: a workflow for developing robust estimates of surface wave dispersion uncertainty

Journal of Seismology - Non-invasive surface wave methods are increasingly being used as the primary technique for estimating a site’s small-strain shear wave velocity (Vs). Yet, in...     

Uncertainty of moment tensor determination from surface wave analysis for shallow earthquakes

We treat the problem of the non-uniqueness of the moment tensor inversion for shallow earthquakes from long period surface wave data. We give an existence condition for double couples radiating the same long period surface waves as the deviatoric moment tensor obtained by such inversion. We describe the family of such double couples and show that they may provide better estimates of double couple mechanisms than the traditional “best double-couple” solution. We demonstrate the results of application to four large shallow earthquakes with different source signatures.     

Multi-offset phase analysis of surface wave data (MOPA)

Inaccuracy in the shear wave velocity profile inverted from surface wave data manifests from both modelling error and data uncertainty. An alternative method for dispersion curve evaluation by weighted linear regression of phase-offset data can be applied to both equispaced and non-equispaced data for objective identification of these often overlooked error sources.From field data, near-field effects are noted to at most half a wavelength and lateral discontinuities identified by marked changes in wavenumber with offset. Transition frequencies to dominant higher modes appear lower than when identified from standard plane-wave transform methods. Effects can be discriminated by their frequency, position or offset dependence.When a non-corrupt dispersion curve is extracted, the errors are up to 5% at low frequency. Through theoretical Gaussian error propagation analysis, the resulting shear wave velocity profile shows up to 18% uncertainty at depth.     

基于面波频散曲线聚类分析的近地表横波速度反演

获取准确的近地表横波速度对复杂地表条件下弹性波地震数据处理和成像非常重要.在浅层面波工程勘探中通过反演提取的频散曲线可以获得近地表横波速度结构.在多道面波频散曲线分析中, 频散关系拾取的精度直接影响速度反演结果的可靠性.本文在多道面波叠加及自动拾取频散曲线基础上, 提出了基于面波频散曲线聚类分析的近地表横波速度反演方法.该方法充分考虑了低信噪比条件下面波频散曲线的不确定性, 通过在频散曲线拾取中引入曼哈顿距离K-Means聚类算法提高频散曲线拾取的准确性.采用多道多窗口叠加技术提高了面波反演对横向速度变化的适应性, 通过聚类算法和多窗口叠加提高反演的可靠性, 聚类算法获得较准确的频散曲线更利于后续横波速度反演过程.模拟数据算例对比表明本文提出的方法比常规算法效果更好, 精度更高.将提出的方法应用于工程勘探和油气勘探的面波数据反演中, 结果也验证了该方法的有效性.

     

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.     

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

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区域数字地震台网面波震级Ms测定与系统误差分析

以国家地震台网测定的面波震级为标准,利用福建区域数字地震台网测定的近距离面波震级Ms结果与国家地震台网测定的面波震级结果进行比较和统计分析。根据两者之间的相对差量,主要从数据采集器的最大动态放大倍数分贝数、FBS-3宽频带反馈式速度平坦型地震计的幅频特性和测定面波震级展开公式推导3方面进行分析。结果表明：震中距△、数据采集器的动态范围和地震计记录地面运动振动频率的有效频带宽度是影响面波震级测定的主要因素,进而提出适合于福建区域台网测定近距离面波震级Ms的台基校正值及标准偏差。     

Implications of surface wave data measurement uncertainty on seismic ground response analysis

The extent of data uncertainty of surface-wave measurements may have significant consequences on 1D seismic response analysis and it may lead to inaccurate estimate of design ground motion. In this paper, an attempt has been made to quantify the data measurement uncertainty from a large repetition of the field data at two test sites and to determine bounds of data uncertainty. In the inversion with neighborhood algorithm, we generated profiles below the misfit value calculated from the uncertainty bound. Equivalent profiles have been selected covering the whole misfit value range and these profiles are subjected to equivalent linear 1D ground response analysis. The results of seismic response analysis are presented in the form of amplification spectra and response spectra which show remarkable variations. Significant variation is observed in peak frequency and peak amplification and it differs from one site to the other based on their measured data uncertainty bound. Peak Ground Acceleration (PGA) and peak spectral acceleration at both the sites reflect very high Coefficient of Variation (COV).     

Specific features of surface wave radiation by a shallow source

The uncertainty of the seismic moment tensor inversion from records of long period surface waves radiated by a shallow source is considered. The special cases of sources of pure thrust, normal, and strike-slip faults are examined in detail. Results of numerical modeling of Love and Rayleigh wave radiation patterns and frequency dependences of the radiation intensity are presented for these types of sources. It is shown that, in the general case, the focal mechanism and seismic moment of a shallow double couple can be uniquely determined from long period surface waves only if one of the nodal planes is subhorizontal. The threshold (maximum) value of the dip angle of this plane is determined by the source depth, the spectral range of observed surface waves, and the model structure in the vicinity of the source.     

The free surface response of a subsurface anomaly to an SH wave with source on the surface

The response of a shallow subsurface anomaly to an SH wave with source on the surface is computed by using a two dimensional finite element technique. It is demonstrated that the effects of the depth and relative material composition of the anomaly can be identified in the power spectra. The behaviour of the response agrees with principles established from investigating cases where the SH wave is incident from below.     

Effect of lateral heterogeneity on surface wave testing: Numerical simulations and a countermeasure

The surface wave (S-wave) method has gained popularity in engineering practice for determining S-wave velocity depth profiles. A growing trend is towards the application of S-wave testing for spatially 2-D S-wave velocity tomography, ignoring the assumption of horizontally layered medium. A fourth-order velocity-stress finite difference method is used to perform numerical simulations of S-wave testing in earth models with lateral variation. Results show that the lateral heterogeneity induces a non-stationary property in the space domain, resulting in false depth-related dispersion or higher modes if conventional approach based on stationary assumption is used for the dispersion analysis. Artifacts maybe introduced in spatially 2-D S-wave velocity imaging if the effect of lateral heterogeneity is not accounted for. As a potential countermeasure, a high-lateral-resolution S-wave method is proposed to reduce the effect of lateral heterogeneity while maintaining the resolution and depth range of dispersion analysis. It consists of a walk-away survey and a phase-seaming procedure when synthesizing seismograms with different nearest source-to-receiver offset, allowing wide-wavelength dispersion analysis within a small spatial range. The proof of concept is given with several numerical examples. They show that the high-resolution S-wave method can greatly alleviate the effect of lateral heterogeneity and increase spatial resolution.     

Numerical method for calculating the surface wave field in the presence of caustics

Due to the lateral heterogeneity of the upper layers of the Earth, paths of surface waves deviate from arcs of great circles. Because of the sphericity of the Earth, the paths intersect on a hemisphere opposite to the epicenter and form caustics consisting of two branches, with their tangent point being a cusp. For this reason, the field of surface waves cannot be analyzed in terms of the ray theory at distances larger than 90°. The asymptotic approach to the analysis of the field in the vicinity of such caustics is very ill-suited for numerical implementation. The difficulties of such an approach to the field calculation are aggravated by the fact that such caustics are superimposed in some regions. Therefore, it is suggested to use the theorem of representation, according to which the field within a certain contour is expressed as an integral whose integrand contains values of the function itself, its derivative along the normal to the contour, and Green’s function. The field on the contour (the circle bounding a hemisphere centered at the epicenter) is calculated by the ray method because rays do not intersect on this hemisphere. These data are used for the construction of the field on the opposite hemisphere assumed to be homogeneous, which enables the construction of Green’s function for this hemisphere. This limitation is not very stringent because the configuration of rays and caustics on this hemisphere is mainly determined by the field on the circle. The integral in the representation theorem is calculated numerically. Numerical examples are presented for models in which one caustic or two superimposed caustics form. These calculations yield constraints on variations in the amplitude and phase of the wave. Rayleigh wave fields are also calculated for a model of the real Earth. It is shown that, at some points, the Rayleigh wave spectrum can be strongly distorted because caustics corresponding to different periods differ in shape.     

利用地震噪声准实时监测短周期面波波速变化

依据由噪声信号提取面波格林函数的原理,利用福建省地震台网25个宽频带台站2007年7月2日—8月29日的脉动观测资料,得到了瑞雷波群速度在福建地区的分布,并将该结果作为下一步相对变化动态成像的背景速度分布．分布结果表明,福建地区周期为3—5s的瑞雷波群速度大致在2.9—3.1km/s之间,平均速度为3.0km/s,瑞雷波群速度分布呈北高南低的现象,这与福建地区北部多山、南部多平原盆地的地理环境有很好的吻合．而且,该群速度分布图在漳州盆地地区表现出一个非常明显的低速,这主要是因为受到盆地沉积层的影响．通过滑动窗（窗长为20d,步长1d）技术得到了观测区内周期约为3-5s的瑞雷波波速分布变化的连续图像;再利用扣除背景影响的技术,得到了2007年8月14日—2008年7月1日福建地区瑞雷波波速的相对变化时空动态图像．通过分析相对变化时空动态图像与该时间范围内发生的地震的对应关系,表明福建地区瑞雷波波速在多次网内中等强度的地震或震群（ML＞3.0）中均表现出震前波速升高,震后下降恢复的变化趋势．初步分析认为,这可能与震前整个地区受到的应力增大震后应力得到释放所导致的介质变化有关．