Equivalent linear method considering frequency dependent characteristics of stiffness and damping

Equivalent linear dynamic response analysis of ground is based on complex moduli and Fourier series expansion; therefore, it is not an equivalent method but an approximate method. Two deficiencies in the conventional equivalent linear method represented by SHAKE are described first. The maximum shear strength is overestimated, resulting in overestimation of the peak acceleration under a strong ground motion, and the amplification is underestimated at high frequency. The latter sometimes results in underestimation of the peak acceleration under weak ground shaking, and gives an incident wave with unrealistic large accelerations or a divergence of analysis in deconvolution analysis under strong ground motion. Both deficiencies are shown to come from the same cause, i.e. computing the effective strain as a constant fraction of the maximum strain. Since this is a key concept of the equivalent linear analysis, one cannot overcome both deficiencies at the same time in the conventional method. An apparent frequency dependence in stiffness and damping is shown to appear in the dynamic response, although soil itself does not show frequency dependent characteristics. Following this observation, the effective strain is expressed in terms of frequency from the similarity concept of the strain–frequency relationship between time domain and frequency domain. This enables the reduction of both deficiencies at the same time, resulting in a marked improvement in the equivalent linear analysis. The accuracy of the proposed method is examined by the simulations of three vertical array records during large earthquakes. The proposed method always gives much better prediction than conventional equivalent linear methods for both convolution and deconvolution analyses, and it is confirmed to be applicable at more than 1% shear strain.     

JCZ-1T超宽频带地震计LP垂直通道记录的固体潮特征

JCZ-1T超宽频带地震计具有全频带覆盖特性,其低频端输出延伸到固体潮汐,利用地震计LP通道的垂直分向观测数据,经过处理获得有关的固体潮资料,通过对记录信号的频谱分析对其有了初步的认知。JCZ-1T超宽频地震计LP加速度垂直通道记录的观测信号以固体潮汐信息为主,用该仪器观测可以获取与地震相关的综合观测数据,分析JCZ-1T记录波形叠加的的固体潮信息有着重要的实际意义,通过对资料处理运用,可以与传统前兆仪器结合观测,拓展其研究应用领域,更好的服务于地震科学研究。     

Determining the amplitude and timing of streamflow discontinuities: A cross wavelet analysis approach

Records of natural processes, such as gradual streamflow fluctuations, are commonly interrupted by long or short disruptions from natural non‐linear responses to gradual changes, such as from river‐ice break‐ups, freezing as a result of annual solar cycles, or human causes, such as flow blocking by dams and other means, instrument calibrations and failure. The resulting abrupt or gradual shifts and missing data are considered to be discontinuities with respect to the normal signal. They differ from random noise as they do not follow any fixed distribution over time and, hence, cannot be eliminated by filtering. The multi‐scale resolution features of continuous wavelet analysis and cross wavelet analysis were used in this study to determine the amplitude and timing of such streamflow discontinuities for specific wavebands. The cross wavelet based method was able to detect the strength and timing of abrupt shifts to new streamflow levels, gaps in data records longer than the waveband of interest and a sinusoidal discontinuity curve following an underlying modeled annual signal at ±0.5 year uncertainty. Parameter testing of the time‐frequency resolution demonstrated that high temporal resolution using narrow analysis windows is favorable to high‐frequency resolution for detection of waveband‐related discontinuities. Discontinuity analysis on observed daily streamflow records from Canadian rivers showed the following: (i) that there is at least one discontinuity/year related to the annual spring flood in each record studied, and (ii) neighboring streamflows have similar discontinuity patterns. In addition, the discontinuity density of the Canadian streamflows studied in this paper exhibit 11‐year cycles that are inversely correlated with the solar intensity cycle. This suggests that more streamflow discontinuities, such as through fast freezing, snowmelt, or ice break‐up, may occur during years with slightly lowered solar insolation. Copyright © 2013 John Wiley & Sons, Ltd.     

临夏台钻孔系统性质的论证

对中国临夏台2013年至2014年两年的井水位和四分量钻孔应变资料进行了预处理以消除趋势与突跳.根据文献(Means,1982;Young and Budynas,2005),相互正交的两条测线的应变观测值之和等于面应变.文献(刘序俨等,1988)证明近地表的面应变的2/3等于体应变,因此,可由4分量钻孔应变观测值得到钻孔体应变,然后根据体应变与井水位观测资料,从时域和频域对该钻孔系统的性质进行了论证.结果表明,在时域,体应变与井水位高度负相关.钻孔系统的灵敏度为—0.1620mm/10-9.把两年中的某两个月份的两者时间坐标轴和纵轴比例尺放大,发现井水位曲线的峰/谷与体应变观测曲线的谷/峰一一对应,两者的相位滞后非常小.在频域内,本文采用Venedikov调和分析方法分别取得了井水位与体应变9个月的半日波与全日波数个波群的逐月潮汐因子与相位滞后,然后作简单计算,得到了钻孔系统对上述波群的灵敏度与相位滞后.结果表明9个月中大多数波群的灵敏度不但十分相近,且非常接近由时域得到的周年频率分量的灵敏度,但相位滞后误差较大,本文对此进行了分析,认为由反正切得到的相位滞后受计算误差影响较大,应以时域经审视所得的相位滞后接近于零为准.通过时域与频域的分析,表明井水位对体应变的响应基本是线性时不变的,论证了临夏台钻孔系统基本上满足了叠加性、齐次性与时不变性,基本上为一线性时不变系统.     

RESIDUAL STATICS ESTIMATION BY STACK-POWER MAXIMIZATION IN THE FREQUENCY DOMAIN1

Traditionally, residual static corrections are based on timeshifts estimated for individual CMP sorted traces, which are later resolved into surface-consistent statics. This is a stable and attractive procedure because the data flow is simple and the memory storage required is limited. An alternative station-oriented method maximizing the stack-power estimates surface-consistent static corrections directly. The statics evaluation in this method involves several CMP gathers, which should improve the prediction of statics on noise-contaminated data. In this paper the performance of the above methods will be compared using synthetic as well as real seismic data. Neither method is capable of estimating large statics compared to the dominating period, because local optimization might fail. Global Monte Carlo search by, for instance, simulated annealing has been used to overcome the cycle-skipping problems when proper field statics are missing. Although this procedure is computationally very heavy, it may be the only way to deal with large residual statics. In order to enlarge the operational field for local optimization, it is suggested that the stack-power in the frequency domain is maximized. This makes it easy to change the frequency band during the optimization. Making use of the frequency domain will also normally be faster than the traditional time-domain optimization even for a limited number of iterations. Moreover, the main memory storage required can be significantly reduced, since it is only necessary to keep the frequency band in the memory, where the signal-to-noise ratio is good.     

Rayleigh-Wave Dispersive Energy Imaging Using a High-Resolution Linear Radon Transform

Multichannel Analysis of Surface Waves (MASW) analysis is an efficient tool to obtain the vertical shear-wave profile. One of the key steps in the MASW method is to generate an image of dispersive energy in the frequency-velocity domain, so dispersion curves can be determined by picking peaks of dispersion energy. In this paper, we propose to image Rayleigh-wave dispersive energy by high-resolution linear Radon transform (LRT). The shot gather is first transformed along the time direction to the frequency domain and then the Rayleigh-wave dispersive energy can be imaged by high-resolution LRT using a weighted preconditioned conjugate gradient algorithm. Synthetic data with a set of linear events are presented to show the process of generating dispersive energy. Results of synthetic and real-world examples demonstrate that, compared with the slant stacking algorithm, high-resolution LRT can improve the resolution of images of dispersion energy by more than 50%.     

地震台站标准化中地震计防震加固装置试验分析

本文根据地震观测台站标准化对地震计等专业设备开展防震加固的要求,对地震传感器加装防震装置后开展振动台测试与台站观测数据一致性分析.振动台测试结果表明在振动台面出现PGA大于103.2 cm·s-2时,地震计出现了明显位移现象,因此,建议地处场地烈度大于Ⅶ度区台站地震计须加装防震装置;我们采用平方相干方式对台站观测数据进...     

Application of spectral analysis to determine hydraulic diffusivity of a sandy aquifer (Pingtung County,Taiwan)

The study demonstrates spectral relationships in the time–frequency domain for one‐dimensional groundwater flow in aquifers bounded by fluctuating boundaries. By nature, the solutions of spectral equations are non‐linear complex functions. To determine hydraulic diffusivity in the governing equations, it is required that the data are collected from the spectra of water levels at the fluctuating boundaries and observation wells. Hydraulic diffusivity thus can be obtained by an iterative inverse approach. This paper presents an application in Pingtung County of Taiwan to determine the hydraulic diffusivity of a sandy aquifer under confined conditions. Spectral density function of water level obtained from tidal boundaries and observation wells are used to approximate hydraulic diffusivity, which yields an averaged value of 1·26 × 10⁶ m²/h. Copyright © 2004 John Wiley & Sons, Ltd.     

System parameters of soil foundations for time domain dynamic analysis

Soil-structure interaction analysis is usually carried out in the frequency domain, because the compliance functions of the half-space are known only in the frequency domain. Since non-linear analysis cannot be carried out in the frequency domain, a system with frequency independent parameters is used to represent the half-space soil medium so that a nonlinear analysis in the time domain becomes possible. The objective of this paper is to propose a system with lumped parameters, which are independent of frequency, to represent the half-space soil medium. The proposed frequency independent system consists of a number of real discrete structure elements; thus the existing dynamic analysis programs may be adoptable with little modification. In this paper, the parameters are found by minimizing the sum of the squares of deviations between the steady-state responses of the theoretical half-space model and those of the lumped parameter system over a specified frequency range. Once the parameters have been found, the lumped parameter system can be used in practical applications for time domain dynamic analysis of either linear or non-linear structures. In comparison with the dynamic response of the theoretical half-space model, the lumped parameter system yields satisfactory results.     

内蒙古中部地区形变主要干扰的时频响应特征分析

选取内蒙古中部地区定点形变观测中受降雨、大风、气压干扰以及地震波影响的典型数据,采用S变换,对该地区形变观测中4种主要干扰信号的时频响应特征进行分析与讨论。结果表明,降雨干扰信号主要集中在低频区域,其频率先增大后减小;大风干扰表现为高频干扰,时频域能量强度与观测曲线受风扰影响变化幅度成正比;气压干扰信号的优势频率分布在低频段内;地震波影响中VP垂直摆和JCZ地震计记录的地震波较为相似,但VP垂直摆记录的频段要小于JCZ地震计,VP垂直摆主要记录的是低频地震波。同时,4种干扰信号的时频域频率、能量强度随时间变化趋势与数据观测时域变化趋势相一致。     

Using the Wiener–Levinson algorithm to suppress ground-roll

In land seismic surveys, the seismic data are mostly contaminated by ground-roll noise, high amplitude and low frequency. Since the ground-roll is coherent with reflections and depends on the source, the spectral band of seismic signal and ground-roll always overlap, which can be clearly seen in the spectral domain. So, separating them in time or frequency domain commonly causes waveform distortions and information missing due to cut-off effects. Therefore, the combination of these factors leads to search for alternative filtering methods or processes. We applied the conventional Wiener–Levinson algorithm to extract ground-roll from the seismic data. Then, subtracting it from the seismic data arithmetically performs the ground-roll suppression. To set up the algorithm, linear or nonlinear sweep signals are used as reference noise trace. The frequencies needed in creating a reference noise trace using analytical sweep signal can be approximately estimated in spectral domain. The application of the proposed method based on redesigning of Wiener–Levinson algorithm differs from the usual frequency filtering techniques since the ground-roll is suppressed without cutting signal spectrum. The method is firstly tested on synthetics and then is applied to a shot data from the field. The result obtained from both synthetics and field data show that the ground-roll suppression in this way causes no waveform distortion and no reduction of frequency bandwidth of the data.     

Stabilized finite elements for harmonic soil dynamics problems near the undrained-incompressible limit

Simple Finite Element models for soil dynamics and earthquake engineering problems in the frequency domain are a fast and valuable tool providing a first approximation before a full non-linear analysis in the time domain is performed.Quite often the problem concerns saturated soils with very small permeability and pore fluid of neglectable compressibility. In the limit, the permeability is assumed to be zero and the pore fluid incompressible. Here, engineers use standard finite element codes formulated in terms of displacements but incompressibility may result in volumetric locking of the mesh with a severe loss of accuracy.The purpose of this paper is to present a simple mixed finite element formulation in the frequency domain based on displacements and pore pressures as main variables. A suitable stabilization technique allowing for equal order interpolation of displacements and pressures has been introduced for incompressible and zero permeability limits.Of course, the range of application is limited to those problems in which the behaviour of the material can be approximated by linear models, and therefore modelling of phenomena such as liquefaction, cyclic mobility or cavitation occur is excluded.The paper shows as well an extremely simple way of coupling solid and water domains as it occurs for instance in quay walls under dynamic loading.     

一阶多次波聚焦变换成像

将多次波转换成反射波并按传统反射波偏移算法成像,是多次波成像的一种方法.聚焦变换能准确的将多次波转换为纵向分辨率更高的新波场记录,其中一阶多次波转换为反射波.本文对聚焦变换提出了两点改进:1)提出局部聚焦变换,以减小存储量和计算量,增强该方法对检波点随炮点移动的采集数据的适应性;2)引入加权矩阵,理论上证明原始记录的炮点比检波点稀疏时,共检波点道集域的局部聚焦变换可以将多次波准确转换成炮点与检波点有相同采样频率的新波场记录.本文在第一个数值实验中对比了对包含反射波与多次波的原始记录做局部聚焦变换和直接对预测的多次波做局部聚焦变换两种方案,验证了第二种方案转换得到的波场记录信噪比更高且避免了第一个方案中切聚焦点这项比较繁杂的工作.第二个数值实验表明:在炮点采样较为稀疏时,该方法能有效的将一阶多次波转换成反射波;转换的反射波能提供更丰富的波场信息,成像结果更均衡、在局部有更高的信噪比,以及较高的纵向分辨率.     

Noise transfer in variable‐depth streamer deghosting

Single‐component towed‐streamer marine data acquisition records the pressure variations of the upgoing compressional waves followed by the polarity‐reversed pressure variations of downgoing waves, creating sea‐surface ghost events in the data. The sea‐surface ghost for constant‐depth towed‐streamer marine data acquisition is usually characterised by a ghost operator acting on the upgoing waves, which can be formulated as a filtering process in the frequency–wavenumber domain. The deghosting operation, usually via the application of the inverse Wiener filter related to the ghost operator, acts on the signal as well as the noise. The noise power transfer into the deghosted data is proportional to the power spectrum of the inverse Wiener filter and is amplifying the noise strongly at the notch wavenumbers and frequencies of the ghost operator. For variable‐depth streamer acquisition, the sea‐surface ghost cannot be described any longer as a wavenumber–frequency operator but as a linear relationship between the wavenumber–frequency representation of the upgoing waves at the sea surface and the data in the space–frequency domain. In this article, we investigate how the application of the inverse process acts on noise. It turns out that the noise magnification is less severe with variable‐depth streamer data, as opposed to constant depth, and is inversely proportional to the local slant of the streamer. We support this statement via application of the deghosting process to real and numerical random noise. We also propose a more general concept of a wavenumber–frequency ghost power transfer function, applicable for variable‐depth streamer acquisition, and demonstrate that the inverse of the proposed variable‐depth ghost power transfer function can be used to approximately quantify the action of the variable‐depth streamer deghosting process on noise.     

基于逆重复m序列的岩矿石标本电性参数测量方法研究

岩矿石标本电性参数实验中,常使用一个或者多个频率激励的变频法测量电阻率,存在观测效率低、可用信息少、抗干扰能力弱等实际问题.本文将变频法改进为伪随机电流信号法进行观测,提出激励场源使用逆重复m序列伪随机电流信号的观测方法,推导出抑制工频干扰的参数选取条件,进而针对岩矿石标本电性参数实验中激励场源的特殊要求,详细分析了观测系统中信号产生与接收面临的问题,并给出了解决方法:采用高驱动能力设计及阻抗匹配技术解决岩矿石标本电阻率分布跨度大(n×10ⁿ×10⁶Ωm)导致的发送端驱动能力不足以及接收端阻抗不匹配等问题;采用系统频率响应校正技术修正系统存在的观测误差;采用高频微弱信号产生与拾取的相关技术,保证电流密度处于岩矿石标本的线性响应区间,避免非线性效应,实现宽频率分布范围观测(10-3~10⁴kHz).采用所研发的可编码、高精度、小电流(10^-6A)、连续可调输出的精密电流源,使用阻容模型进行实验测试,结果表明:(1)该方法可在10^-3~10⁴Hz频带内产生90个有效测量频点,可满足频带宽、频点密、压制工频干扰的观测要求;(2)实验中恒流源输出稳定,不同负载下直流输出稳定性误差小于0.5%,精度满足实验中对输出电流的要求;(3)标定后测量误差小于0.5%,系统具有较高的观测精度与抗干扰能力;(4)与变频法相比观测效率提高2.3倍.通过该方法开发的观测装置可有效提高工作效率和抗干扰能力,提高观测数据的准确性,适用于各种类型岩矿石标本电性参数观测工作.     

Correction of highly noisy strong motion records using a modified wavelet de-noising method

During the past earthquakes, many valuable acceleration time histories were recorded by analog and digital accelerometers. These records are important sources of information in the field of earthquake engineering and engineering seismology. However, a large number of these records are contaminated by noise and it is necessary to correct them for practical applications. On the other hand, only a few records can be corrected using conventional filtering because of mathematical limitations of the method. However, advances in the field of time–frequency analysis and wavelet transform theory provide useful non-linear and adaptive de-noising methods for removing of non-stationary and high-energy noise from the recorded signals. In this paper, the characteristics and capabilities of the modified non-linear adaptive wavelet de-noising method are examined for correction of highly noisy strong motion records. In the frequency domain, it is shown that this method can attenuate the noise in the whole frequency range of engineering interest while in the time domain it can detect and remove non-stationary noise. In addition, the displacement response spectra of these wavelet de-noised records are more stable than conventional filtered records with respect to different correction functions. It is found that a large number of noisy acceleration records that are usually discarded from sets of records used for estimating the ground motions can be corrected using this new method.     

基于POCS方法指数阈值模型的不规则地震数据重建(英文)

不规则地震数据会对地震多道处理技术的正确运行造成不良影响,降低地震资料的处理质量。本文将广泛用于图形图像重建的凸集投影方法应用到地震数据重建领域,实现规则样不规则道缺失数据的插值重建。对于整道缺失地震数据,将POCS迭代重建过程由时间域转移到频率域实现,避免每次迭代都对时间做正反Fourier变换,节约了计算量。在迭代过程中,阈值参数的选择方式对重建效率有重要影响。本文设计了两种阈值集合模型进行重建试验,试验结果表明:在相同重建效果下,指数型阈值集合模型可以有效减少迭代次数,提高重建效率。此外,分析了POCS重建方法的抗噪性能和抗假频性能。最后,理论模型和实际资料处理效果验证了本文重建方法的正确性和有效性。     

Practical flood frequency estimates

The best information on which to base estimates of future flood frequencies is records of past flood events. Where there is a substantial record at the location for which estimates are desired the estimation process is generally straighforward, although a variety of methods is used and there is major uncertainty in the estimates. In general, the frequency of future events is assumed to be indicated by the observed frequency of past events under constant controlling watershed conditions.Techniques are available for using information on historical (pre-record) flood data to improve the reliability of flood frequency estimates. There are methods for detecting and managing extremely unusual actual events (outliers) and for improving the reliability of short-record estimates based on long-record data at related locations. Regional correlation analysis is usable for establishing flood frequency estimates for locations where records are not available.Detailed hydrologic analysis, usually involving rainfall-runoff studies, is required for establishing flood frequency relationships for modified conditions of the watershed or, in many cases, for establishing flood frequency estimates for newly formed drainage systems such as in urban areas and airports.The principal use of flood frequency functions is to compare expected changes in flood damages (due to a contemplated action) with the economic and social costs or benefits of the contemplated action.     

Comparison between seismic vulnerability models and experimental dynamic properties of existing buildings in France

Elastic fundamental frequency is a key-parameter of simplified seismic design and vulnerability assessment methods. Empirical relationships exist in codes to estimate this frequency but they miss experimental data to validate them accounting for national feature of building design and, above all, corresponding uncertainties. Even if resonance frequency extracted from ambient vibrations may be larger than the elastic frequency (at yield) generally used in earthquake engineering, ambient vibration recordings may provide a large set of data for statistical analysis of periods versus building characteristics relationships. We recorded ambient vibrations and estimated the fundamental frequency of about 60 buildings of various types (RC and masonry) in Grenoble City (France). These data complete the set existing yet, made of 26 RC-buildings of Grenoble (Farsi and Bard 2004) and 28 buildings in Nice (France) (Dunand 2005). Statistical analysis of these experimental data was performed for fundamental frequencies of RC shear wall structures and the results are compared with existing relationships. Only building height or number of stories has a statistical relevancy to estimate the resonance frequency but the variability associated to the proposed relationships is large. Moreover, we compared the elastic part of capacity curves of RC and masonry buildings used in the European Risk-UE method for vulnerability assessment with the experimental frequencies. The variability is also large and the curves may not be consistent with French existing buildings.     

Applications of wavelet analysis to the investigation of the dynamic behaviour of geotechnical structures

Harmonic wavelet analysis is a tool that has been developed over the past 15 years for the analysis of non-stationary signals in the time–frequency domain. This paper discusses the use of this technique and its application to the problems of soil dynamics and earthquake engineering. Specific reference is made to the use of this technique in investigating the performance of liquefiable slopes in centrifuge model earthquakes and the investigation of earthquake accelerograms.

Harmonic wavelet analysis will be shown to be a versatile tool that can reveal information unavailable traditional time or frequency domain analysis.