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1.
短周期密集台阵的高频背景噪声互相关函数(NCF)是探查地球浅层精细结构的重要数据.然而高频背景噪声成分复杂且容易分布不均,分析其对NCF信号提取的影响,有助于获取可靠成像结果.本文基于布设于川滇地区盐源盆地的209个短周期台站组成的盐源台阵,利用密集台阵的噪声水平评估以及基于NCF的相干噪声分析两种方法,分析了其记录到的噪声波场特征及其对NCF的影响.结果表明,盐源台阵的整体噪声水平呈现北低南高的不均匀分布,高频噪声水平的强弱受控于当地的人类活动,亦受到浅部松散沉积层的影响.台阵垂直分量NCF中主要信号为基阶Rayleigh波,且产生该信号的相干噪声源的优势方位在不同频带具有较大区别:0.3～0.5Hz的噪声源强度较强且随时间变化较为稳定,主要能量来自台阵的南侧;0.5～1Hz的相干噪声源强度较低,有两个优势方向,其中较强的一个来自于台阵南侧,可能与0.3～0.5 Hz的噪声同源,较弱的一个来自于台阵北偏东方向;1～1.5Hz的背景噪声有四个较弱的优势方向,在台阵的不同区域有不同的优势方向,可能受到不同的局部噪声源的控制.垂向NCF中Rayleigh波的信噪比主要受控于波场的复杂程度,台阵南部受人文活动及沉积层影响,噪声水平较高,且由于盆山边缘复杂的反射、散射作用,其NCF波形复杂,信噪比偏低.受高频噪声源分布不均与及复杂地质结构的共同影响,盐源台阵的高频NCF中的信号复杂,后续对面波频散特征的提取应充分考虑噪声源对NCF的影响以获取可靠结果. 相似文献
2.
R. Maresca M. Castellano R. De Matteis G. Saccorotti P. Vaccariello 《Pure and Applied Geophysics》2003,160(9):1745-1764
3.
Many of the numerical techniques used for seismic zonation studies treat one-dimensional structural models and/or the incidence of plane polarized body waves. These techniques are often not adequate for laterally heterogeneous structures and for sources that are not located beneath the site of interest. In such cases a more rigorous treatment of the combined effects of the source, the path and the site response is needed. This can be accomplished with a hybrid approach combining modal summation and the finite-difference technique. To demonstrate the differences between these techniques, the ground motion in the city of Benevento (Italy) is modelled. We first compare the results obtained with one-and two-dimensional structural models for vertical incidence of plane polarized body waves. These results are then compared with those obtained with the hybrid approach for two-dimensional structural models.The comparisons have allowed us to find important differences in the response obtained with the different modelling techniques. For the same site, these differences consist of strong variations in amplitude and in the shape of the spectral amplifications. For a seismic source which is not located beneath the site, vertical incidence of waves significantly overestimates the local hazard in a laterally homogeneous structure. For a laterally heterogeneous area, we can conclude that one-dimensional modelling fails to estimate the seismic hazard, whereas for a seismic source which is not located beneath the site of interest, two-dimensional modelling with vertical incidence of plane polarized body waves may not allow reliable estimates to be made of the frequency bands at which amplifications occur. The results obtained for two-dimensional structural models are used for a zonation of the city of Benevento. 相似文献
4.
Phase and Correlation in `Random' Seismic Fields and the Reconstruction of the Green Function 总被引：3，自引：0，他引：3
Michel Campillo 《Pure and Applied Geophysics》2006,163(2-3):475-502
We first present a summary of recent results on coda interpretation. We emphasize the observation of the stabilization of
P to S energy ratio indicating the modal equipartition of the wavefield. This property clearly shows that the coda waves are
in the regime of multiple scattering. Numerical solutions of the elastic radiative transfer equation are used to illustrate
the evolution of the wave-field towards P-to-S energy stabilization, and asymptotically to complete isotropy. The energy properties
of the coda have been widely studied but the phase properties have often been neglected. The recently observed coherent backscattering
enhancement, an expression of the so-called `weak localization', demonstrates that interference effects still persist for
multiple diffracted waves. Another manifestation of the persistence of the phase is the possibility to reconstruct the Green
function between two stations by averaging the cross correlation of coda waves produced by distant earthquakes and recorded
at those two stations. This reconstruction is directly related to the properties of reciprocity and time reversal of any wavefield.
Using broadband seismic coda waves, we show that the dominant phases of the Green function in the band 2 s–10 s, namely fundamental
mode Rayleigh and Love waves, are reconstructed. We analyze the time symmetry of the cross correlation and show how the level
of symmetry evolves with the isotropization of the diffuse field with lapse time. Similarly we investigate the correlation
in continuous ambient noise records. Whereas the randomness of the coda results from multiple scattering by randomly distributed
scatterers, we assume that the seismic noise is random mostly because of the distribution of sources at the surface of the
Earth. Surface waves can be extracted from long time series. The dispersion curves of Rayleigh waves are deduced from the
correlations. On paths where measurements from earthquake data are also available, we show that they are in good agreement
with those deduced from noise correlation. The measurement of velocities from correlation of noise along paths crossing different
crustal structures opens the way for a `passive imaging' of the Earth's structure. 相似文献
5.
By exploiting the capability of identifying and extracting surface waves existing in a seismic signal, we can proceed to estimate the angular displacement (rotation about the horizontal axis normal to the direction of propagation of the wave; rocking) associated with Rayleigh waves as well as the angular displacement (rotation about the vertical axis; torsion) associated with Love waves.For a harmonic Rayleigh (Love) wave, rocking (torsion) would be proportional to the harmonic vertical (transverse horizontal) velocity component and inversely proportional to the phase velocity corresponding to the particular frequency of the harmonic wave (a fact that was originally exploited by Newmark (1969) [15] to estimate torsional excitation). Evidently, a reliable estimate of the phase velocity (as a function of frequency) is necessary. As pointed out by Stockwell (2007) [17], because of its absolutely referenced phase information, the S-Transform can be employed in a cross-spectrum analysis in a local manner. Following this suggestion a very reliable estimate of the phase velocity may be obtained from the recordings at two nearby stations, after the dispersed waves have been identified and extracted. Synthesis of the abovementioned harmonic components can provide a reliable estimate of the rocking (torsional) motion induced by an (extracted) Rayleigh (Love) wave.We apply the proposed angular displacement estimation procedure for two well recorded data sets: (1) the strong motion data generated by an aftershock of the 1999 Chi-Chi, Taiwan earthquake and recorded over the Western Coastal Plain (WCP) of Taiwan, and (2) the strong motion data generated by the 2010 Darfield, New Zealand earthquake and recorded over the Canterbury basin. The former data set is dominated by basin-induced Rayleigh waves while the latter contains primarily Love waves. 相似文献
6.
On the repeatability and consistency of three-component ambient vibration array measurements 总被引：1，自引：0，他引：1
Brigitte Endrun Matthias Ohrnberger Alexandros Savvaidis 《Bulletin of Earthquake Engineering》2010,8(3):535-570
Ambient vibration measurements with small, temporary arrays that produce estimates of surface wave dispersion have become
increasingly popular as a low-cost, non-invasive tool for site characterisation. An important requirement for these measurements
to be meaningful, however, is the temporal consistency and repeatability of the resulting dispersion and spatial autocorrelation
curve estimates. Data acquired within several European research projects (NERIES task JRA4, SESAME, and other multinational
experiments) offer the chance to investigate the variability of the derived data products. The dataset analysed here consists
of repeated array measurements, with several years of time elapsed between them. The measurements were conducted by different
groups in different seasons, using different instrumentations and array layouts, at six sites in Greece and Italy. Ambient
vibration amplitude spectra and locations of dominant sources vary between the two measurements at each location. Still, analysis
indicates that this does not influence the derived dispersion information, which is stable in time and neither influenced
by the instrumentation nor the analyst. The frequency range over which the dispersion curves and spatial autocorrelation curves
can be reliably estimated depends on the array dimensions (minimum and maximum aperture) used in the specific deployment,
though, and may accordingly vary between the repeated experiments. The relative contribution of Rayleigh and Love waves to
the wavefield can likewise change between repeated measurements. The observed relative contribution of Rayleigh waves is generally
at or below 50%, with especially low values for the rural sites. Besides, the visibility of higher modes depends on the noise
wavefield conditions. The similarity of the dispersion and autocorrelation curves measured at each site indicates that the
curves are stable, mainly determined by the sub-surface structure, and can thus be used to derive velocity information with
depth. Differences between velocity models for the same site derived from independently determined dispersion and autocorrelation
curves—as observed in other studies—are consequently not adequately explained by uncertainties in the measurement part. 相似文献
7.
Frequency dependent polarization analysis of ambient seismic noise recorded at a broadband seismometer in the central United States 
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下载免费PDF全文 We present a new approach to polarization analysis of seismic noise recorded by three-component seismometers.It is based on statistical analysis of frequency-dependent particle motion properties determined from a large number of time windows via eigenanalysis of the 3-by-3,Hermitian,spectral covariance matrix.We applied the algorithm to continuous data recorded in 2009 by the seismic station SLM,located in central North America.A rich variety of noise sources was observed.At low frequencies (0.05 Hz) we observed a tilt-related signal that showed some elliptical motion in the horizontal plane.In the microseism band of 0.05-0.25 Hz,we observed Rayleigh energy arriving from the northeast,but with three distinct peaks instead of the classic single and double frequency peaks.At intermediate frequencies of 0.5-2.0 Hz,the noise was dominated by non-fundamental-mode Rayleigh energy,most likely P and Lg waves.At the highest frequencies (3 Hz),Rayleigh-type energy was again dominant in the form of Rg waves created by nearby cultural activities.Analysis of the time dependence of noise power shows that a frequency range of at least 0.02-1.0 Hz (much larger than the microseism band) is sensitive to annual,meteorologically induced sources of noise. 相似文献
8.
Rayleigh面波地震背景噪声成像技术已被成功运用到全球范围不同尺度的地球内部结构的研究中,并以背景噪声场是时空均匀分布为前提假设.然而真实的噪声源分布的时空非均匀性将导致经验格林函数提取存在偏差,最终影响噪声成像结果的精准性.近年来,噪声源分布特征研究逐步成为提高噪声成像精准度、深化地震背景噪声成像的关键问题.本文利用频率-波束域分析法对中国西北地区的一个大孔径台阵(WuTan Array,简称WTA)在2014全年的垂直分量连续记录做了聚束分析,研究了Rayleigh波噪声源分布特征.结果显示:WTA台阵成功探测到了10～20s周期范围的来自于全球不同方位的Rayleigh波噪声信号,其源区分布具有明显的季节变化特征:冬季集中分布在北大西洋方位,而夏季则转为印度洋方位噪声信号最强.此外,Rayleigh波噪声源区空间分布还表现出一定的频率依赖性,即在较低频段(0.0488～0.0635Hz)在北大西洋、北太平洋、印度洋及西太平洋四个方位均有分布;而在频率较高频段(0.0928～0.1025Hz)则集中分布于西太平洋方位.Rayleigh波噪声源时空分布特征和频率依赖性与海洋活动本身的季节性变化和频谱特征有关.并初步推测本文所观测到的Rayleigh波是由加剧的海浪运动直接作用于海岸、大陆架或海底而激发产生的第一类地脉动噪声信号. 相似文献
9.
10.
Brigitte Endrun 《Journal of Seismology》2011,15(3):443-472
This study applies array methods to measure the relative proportions of Love and Rayleigh waves in the ambient vibration wavefield.
Information on these properties is of special relevance for frequencies around the horizontal-to-vertical (H/V) spectral amplitude
ratio peak. The analysis of H/V curves, a popular technique in site characterisation, commonly assumes that the curves represent
the frequency-dependent Rayleigh wave ellipticity. For the detailed interpretation of amplitudes or the inversion of the curves,
it is therefore necessary to estimate and correct for the contribution of other wave types to the ambient vibration wavefield.
I use available ambient vibration array measurements to determine the relative amount of Love and Rayleigh waves on the horizontal
components by frequency-dependent analysis of the main propagation and polarisation directions, with a special emphasis on
the H/V peak frequency as determined from the same recordings. Tests with synthetic data demonstrate the feasibility of this
approach, at least in the presence of dominant source regions. Analysis of the data from 12 measurements at nine European
sites, which include shallow as well as deep locations that span a wide range of impedance contrasts at the sediment-bedrock
interface, indicates that the relative contribution of Rayleigh waves varies widely with frequency, from close to 0% to more
than 70%. While most data sets show relative Rayleigh wave contributions between 40% and 50% around the H/V peak, there are
also examples where Love waves clearly dominate the wavefield at the H/V peak, even for a site with a low impedance contrast.
Longer-term measurements at one site indicate temporal variations in the relative Rayleigh wave content between day- and nighttime.
Results calculated with the method introduced herein generally compare well with results of modified spatial autocorrelation
analysis. These two methods might be used in a complimentary fashion, as both rely on different properties of the ambient
vibration wavefield. This study illustrates that it is possible to measure the relative Rayleigh wave content of the noise
wavefield from array data. Furthermore, the examples presented herein indicate it is important to estimate this property,
as the assumption that there are an equal proportion of Love and Rayleigh waves is not always correct. 相似文献
11.
利用地震噪声准实时监测短周期面波波速变化 总被引：8，自引：0，他引：8
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． 相似文献
12.
Variations of local seismic response in Benevento (Southern Italy) using earthquakes and ambient noise recordings 总被引：1，自引：0，他引：1
The city of Benevento (Southern Italy) has been repeatedly struck by large historical earthquakes. A heterogeneous geologic structure and widespread soft soil conditions make the estimation of site effects crucial for the seismic hazard assessment of the city. From 2000 until 2004, we installed seismic stations to collect earthquake data over zones with different geological conditions. Despite the high level of urban noise, we recorded more than 150 earthquakes at twelve sites. This data set yields the first, well documented experimental evidence for weak to moderate local amplifications. We investigated site effects primarily by the classical spectral ratio technique (CSR) using a rock station placed on the Benevento hill as reference. All sites in the Calore river valley and in the eastern part of the Benevento hill show a moderate high-frequency (f > 4 Hz) amplification peak. Conversely, sites in the Sabato river valley share weak-to-moderate amplification in a wide frequency band (from 1–2 to 7–10 Hz), without evident frequency peaks. Application of no-reference-site techniques to earthquake and noise data confirms the results of the CSRs in the sites of the Calore river valley and of the eastern part of the Benevento hill, but fails in providing indications for site effects in the Sabato river valley, being the H/V ratios nearly flat. One-dimensional modeling indicates that the ground motion amplification can be essentially explained in terms of a vertically varying geologic structure. High-frequency narrow peaks are caused by the strong impedance contrast existing between near-surface soft deposits and stiff cemented conglomerates. Conversely, broad-band amplifications in the Sabato river valley are likely due to a more complex layering with weak impedance contrasts both in the shallow and deep structure of the valley. 相似文献
13.
Surface wave tomography of the crust and upper mantle of Chinese mainland and its neighboring region
Introduction The three-dimensional S wave velocity of Chinese mainland and its neighboring region in-verted by surface wave dispersion data plays an important role in studying the lateral variation of lithosphere and geodynamic process, and understanding the forming and evolution of Chinese mainland and the relationship between shallow and deep structures. The three-dimensional veloc-ity structures of China and its major tectonic blocks were respectively studied by SONG, et al (1993), ZHOU… 相似文献
14.
利用鄂西地区长时间段宽频地震台站的三分量背景噪声记录,采用波形互相关方法得到台站对间的互相关函数,并通过聚束分析获得瑞雷波和勒夫波的慢度谱,研究鄂西地区背景噪声源的时空分布特征。结果表明,5~10 s周期范围,背景噪声来源于南太平洋且没有季节变化;10~20 s周期范围,慢度谱上显示明显的能量环,表明噪声源来源于多个方向,且表现出强烈和急剧的季节变化;20~40 s周期范围,慢度谱上也存在明显的能量环,其产生机制可能与此周期下提出的次重力波机制相似。在不同的周期范围内,噪声源分布方位有所不同,但在周期10~40 s范围噪声源在各方向均有分布。因此,利用长时间段连续噪声数据计算的互相关函数在周期10~40 s范围内满足背景噪声面波层析成像的理论前提。 相似文献
15.
Effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves 下载免费PDF全文
下载免费PDF全文 With the development of the dense array, the surface wave velocity and azimuthal anisotropy under the array can be directly obtained by beamforming the noise cross-correlation functions (NCFs). However, the retrieval of the Green's function by cross-correlating the seismic noise requires that the noise source has a uniform distribution. For the case with uneven noise source, the azimuthal dependence on the sources in the expression for the spatial coherence function, which corresponds to the NCF in the time domain, has the same form as the azimuthal dependence of the surface wave velocity in weakly anisotropic media. Therefore, the uneven noise source will affect the surface wave anisotropy extraction. In this study, three passive seismic methods, i.e., beamforming, SPAC (spatial autocorrelation), and NCF, are compared to demonstrate that an uneven source distribution and uneven station distribution have equivalent effects on the outcome from each method. A beamforming method is proposed to directly extract the velocity and azimuthal anisotropy of surface waves. The effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves was investigated using data from the ChinArray Phase II. A method for correcting the apparent anisotropy in beamforming results caused by an uneven station distribution is suggested. 相似文献
16.
Marc Wathelet Denis Jongmans Matthias Ohrnberger Sylvette Bonnefoy-Claudet 《Journal of Seismology》2008,12(1):1-19
Valuable information about one-dimensional soil structures can be obtained by recording ambient vibrations at the surface,
in which the energy contribution of surface waves predominates over the one of other types of waves. The dispersion characteristics
of surface waves allow the retrieval of the shear-wave velocity as a function of depth. Microtremor studies are usually divided
in two stages: deriving the dispersion (or auto-correlation) curve from the recorded signals and inverting it to obtain the
site velocity profile. The possibility to determine the dispersion curve over the adequate frequency range at one site depends
on the array aperture and on the wavefield spectra amplitude that can be altered by filtering effects due to the ground structure.
Microtremors are usually recorded with several arrays of various apertures to get the spectral curves over a wide frequency
band, and different methods also exist for processing the raw signals. With the objective of defining a strategy to achieve
reliable results for microtremor on a shallow structure, we analyse synthetic ambient vibrations (vertical component) simulated
with 333 broadband sources for a 25-m deep soil layer overlying a bedrock. The first part of our study is focused on the determination
of the reliable frequency range of the spectral curves (dispersion or auto-correlation) for a given array geometry. We find
that the wavenumber limits deduced from the theoretical array re sponse are good estimates of the valid spectral curve range.
In the second part, the spectral curves are calculated with the three most popular noise-processing techniques (frequency–wavenumber,
high-resolution frequency–wavenumber and spa tial auto-correlation methods) and inverted indi vidually in each case. The inversions are
performed with a tool based on the neighbour hood algorithm that offers a better estimation of the global uncertainties than
classical linearised methods, especially if the solution is not unique. Several array apertures are necessary to construct
the dispersion (auto-correlation) curves in the appropriate frequency range. Considering the final velocity profiles, the
three tested methods are almost equivalent, and no significant advantage was found for one particular method. With the chosen
model, all methods exhibit a penetration limited to the bedrock depth, as a consequence of the filtering effect of the ground
structure on the vertical component, which was observed in numerous shallow sites. 相似文献
17.
In regions where active source seismic exploration is constrained by limitations of energy penetration and recovery, cost and logistical concerns, or regulatory restrictions, analysis of natural source seismic data may provide an alternative. In this study, we investigate the feasibility of using locally‐generated seismic noise in the 2–6 Hz band to obtain a subsurface model via interferometric analysis. We apply this technique to three‐component data recorded during the La Barge Passive Seismic Experiment, a local deployment in south‐western Wyoming that recorded continuous seismic data between November 2008 and June 2009. We find traffic noise from a nearby state road to be the dominant source of surface waves recorded on the array and observe surface wave arrivals associated with this source up to distances of 5 kms. The orientation of the road with respect to the deployment ensures a large number of stationary points, leading to clear observations on both in‐line and cross‐line virtual source‐receiver pairs. This results in a large number of usable interferograms, which in turn enables the application of standard active source processing methods like signal processing, common offset stacking and traveltime inversion. We investigate the dependency of the interferograms on the amount of data, on a range of processing parameters and on the choice of the interferometry algorithm. The obtained interferograms exhibit a high signal‐to‐noise ratio on all three components. Rotation of the horizontal components to the radial/transverse direction facilitates the separation of Rayleigh and Love waves. Though the narrow frequency spectrum of the surface waves prevents the inversion for depth‐dependent shear‐wave velocities, we are able to map the arrival times of the surface waves to laterally varying group and phase velocities for both Rayleigh and Love waves. Our results correlate well with the known geological structure. We outline a scheme for obtaining localized surface wave velocities from local noise sources and show how the processing of passive data benefits from a combination with well‐established exploration seismology methods. We highlight the differences with interferometry applied to crustal scale data and conclude with recommendations for similar deployments. 相似文献
18.
We present the results of the analysis of array recorded microtremors at 14 sites, close to the edges of the Mygdonian basin in northern Greece (Euroseistest). These measurements were made in order to better constrain the geometry and velocity structure of the basin as the soil layers taper out close to rock outcrop, where geology is complex and we may expect significant changes of the subsoil structure over short distances. The data were analysed using the SPAC method and HVSR. The first interprets the measurements as Rayleigh waves (for the vertical component we analysed) and allows to invert a phase velocity dispersion curve from computed correlation coefficients. The second estimates a local transfer function directly, from ratios of Fourier amplitude spectra. A phase velocity dispersion curve could be derived for 12 of the 14 measurement sites, and at three of the sites no resonant frequency was observed in the HVSR. It is encouraging that we obtained good results at most of our sites, in spite of the lateral heterogeneity expected close to the edges of Euroseistest. Our results allow us to obtain shear wave velocity models at most of the measurement sites (12 out of 14). They are also useful to explore the relation between size of the array and wavelength range for which a dispersion curve may be estimated, which in our case has strong limitations. We identify the frequency of resonance of the sediments as a small loss of coherency in SPAC’s correlation coefficients. Finally, we also consider the applicability of the joint inversion of the resonance frequency determined using HVSR and the phase velocity dispersion curve obtained from SPAC. 相似文献
19.
用单孔检层法测定山西临汾流动地震观测台址两个钻孔的P／S波速度结构,为研究地震观测台址速度结构对地震波接收效果的影响提供基础资料。与常规P／S波测井不同的是,在P波速度测试中采用高频24道水听器串共炮点接收,有效降低单点多炮接收因重复性问题带来的零时误差,提高P波速度测定的精度。该方法可为类似工程场址的高精度P／S波速度测试提供新途径。 相似文献
20.
This paper uses the 8 broad-band stations＇ microseism data recorded by the Seismic Monitoring Network of Fujian Province to calculate the vertical correlation coefficient between two stations at intervals of 5 minutes. According to the time intervals technique we obtain the different coefficients and then add the correlative coefficients. Depending on this, we extract the group velocity of Rayleigh waves from the cross correlation of the ambient seismic noise between two seismic stations and figure out the group velocity＇ spatial distribution. The results show that the signal noise ratio （SNR） increases proportionally to the superposition times, but the results from different days are similar to one another. Synchronously, the arrival-time is also stable and there is no obvious change when coming across typhoons. It is found the velocity of the surface wave is 2.9 - 3.1 km/s in Fujian Province, which is close to the observationally attained value. 相似文献