Shallow velocity–depth model imaging by refraction tomography1

A tomographic imaging technique combined with coherence inversion is proposed for constructing a near-surface model from refraction events. A model obtained from coherence inversion serves as a good background model for the tomographic reconstruction. A simultaneous iterative reconstruction technique (SIRT) algorithm was used for this purpose. This is a simple algorithm and can be easily adapted to irregular acquisition geometry and limited angular aperture. Using synthetic data it was shown that the proposed procedure can be used for determination of local velocity anomalies in a shallow subsurface. The technique was also tested on a real data set.     

TI介质局部角度域射线追踪与叠前深度偏移成像

研究与实践表明,对于长偏移距、宽方位地震数据,忽略各向异性会明显降低成像质量,影响储层预测与描述的精度.针对典型的横向各向同性(TI)介质,本文面向深度域构造成像与偏移速度分析的需要,研究基于射线理论的局部角度域叠前深度偏移成像方法.它除了像传统Kirchhoff叠前深度偏移那样输出成像剖面和炮检距域的共成像点道集,还遵循地震波在成像点处的局部方向特征、基于扩展的脉冲响应叠加原理获得入射角度域和照明角度域的成像结果.为了方便快捷地实现TI介质射线走时与局部角度信息的计算,文中讨论和对比了两种改进的射线追踪方法:一种采用从经典各向异性介质射线方程演变而来的由相速度表征的简便形式;另一种采用由对称轴垂直的TI(即VTI)介质声学近似qP波波动方程推导出来的射线方程.文中通过坐标旋转将其扩展到了对称轴倾斜的TI(即TTI)介质.国际上通用的理论模型合成数据偏移试验表明,本文方法既适用于复杂构造成像,又可为TI介质深度域偏移速度分析与模型建立提供高效的偏移引擎.     

ZK03井水温异常与印尼8.7级地震的关系

分析认为该井水温出现的中、短、临异常可能与印尼8.7级地震有关.年日均值图显示,中期异常出现在震前11个月左右,表现为脉冲式向上突跳及阶跃式上升,最大异常量为日均正常上升速率的84倍;中短期异常出现在震前4个月和1个月,最大异常量为正常日均波动值的274倍;临震异常出现在震前的第9天,震前25小时达到最高值,最大异常量为正常日均波动值的2.5倍.水温异常出现的原因可能是由于地震前的长周期波通过上地幔或软流层传播,间接地导致了地下流体温度发生了微小的变化.     

2013年甘肃岷县MS6.6地震区域地震波传播模拟与地面运动特征研究

2013年7月22日,甘肃省定西市岷县发生M_S6.6地震,造成了严重的人员伤亡和经济损失.地震发生于青藏高原东北部边缘与甘肃东南部地区的交界处,是该区域一百多年以来发生的最大地震.分析本次地震观测数据显示,岷县M_S6.6地震产生的地震地面运动呈现出明显的区域变化特征.为了研究这种区域变化特征,本文使用全球CRUST1.0和SRTM30模型数据建立了甘肃岷县及周边地区的地下三维传播介质模型,使用并行的三维有限差分方法进行了岷县地震的区域地震波传播模拟,并与研究区内数字地震台网记录的地震观测资料进行对比.通过分析地面运动的峰值速度和持续时间的区域分布特征,发现：在青藏高原东北部边缘,较大的地形起伏对地面运动的峰值速度分布起主要影响作用;在青藏高原外围地区,地形起伏较小,而沉积层主要影响地面运动的峰值速度和持续时间,从而导致了明显的盆地效应,如地面运动的振幅放大和持续时间加长.因此,研究区剧烈的地形起伏和表层沉积层是影响地面运动的重要因素.     

APERTURE COMPENSATION TOMOGRAPHY1

Conventional velocity analysis can handle a horizontally stratified medium well. There is no indication, though, that it will be as successful when applied to a more complicated geological structure. In fact, a small angle of incidence may transform to a wide-angle reflection event for a dipping interface. In this case, conventional velocity analysis may lead to large errors and thus cannot be applied. Seismic tomography is attractive as it is virtually free from any restrictions imposed on the velocity distribution in the model space or on the setup of a seismic experiment. It is important, however, to recall that seismic tomography yields results of inferior quality compared to medical tomography. This paper investigates the reason for this and how to suppress a significant blurring of seismic tomograms. Unlike medical tomography, one cannot provide full angular coverage of the model space in a typical seismic experiment: the sources and the receivers cannot surround an unknown object inside the earth to provide a complete spectrum of view angles. Incomplete angular coverage may lead to the occurrence of large inaccuracies in the computed tomograms especially when the initial model is poorly chosen. We demonstrate a method of suppressing the adverse effects related to an incomplete angular recording. This is ‘compensation tomography’ which can be used efficiently in the case of a limited angular aperture. Numerical experiments illustrate the theory.     

Simulation of Near-Fault Strong Ground Motion in the Beijing Region

On the basis of previous study of the 1679 Sanhe-Pinggu(M8.0) earthquake,the biggest event in history ever recorded in Beijing and its adjacent area,we made a 3-D strong ground motion simulation utilizing the staggered-grid finite differences method to study the distributions of peak ground velocity with different earthquake source models in the Beijing region.In the paper,earthquake source models and a transmission medium velocity model are established and the corresponding parameters are given in accordance to the results from a related previous study.Then,using a three-dimensional finite difference computing program of near-fault strong ground motion developed by Graves,the peak ground velocity caused by a destructive earthquake in the Beijing area is simulated.In our computation model,the earthquake source is 3km in depth,and a total number of 21,679 observation points on the ground surface are figured out.The transmission medium velocity model is composed of four stratums which are the Quaternary deposit,the upper crust,the upper part of the middle crust and the lower part of the middle crust.With the minimum grid spacing of 0.15km,a total of 2.28×106 grids are generated.Using a time step of 0.02 seconds we calculated the peak ground velocity for a duration of 8 seconds.After the analysis of the simulation results,we observed some basic characteristics of near-fault strong ground motion such as the concentration effect of near-fault peak ground velocity,rupture directivity effect,hanging wall effect,and basin effect.The results from our simulation and analysis suggest that the source and transmitting medium parameters in our model are suitable and the finite difference method is applicable to estimate the distribution of strong ground motion in the study region.     

Cross-spectral analysis of rainfall and runoff for Raritan and Mullica river basins in New Jersey

Cross-correlation and cross-spectral analysis were employed in the analysis of rainfall and runoff in two river basins: the Raritan and Mullica River basins in New Jersey. Cross-covariance and coherence were studied in the correlograms for the following correlation cases: (a) rainfall-runoff for each one basin separately; (b) rainfall-rainfall analysis for two main meteorological stations in each one of the basins; (c) runoff-runoff for two main gaging stations in each one of the basins. From the estimates of the coherence at various frequencies the cross-spectral analysis shows a highly nonlinear relationship between rainfall and runoff in Raritan and Mullica River basins. A poor coherence observed at the annual cycles for each basin makes it difficult to predict the annual oscillations of runoff from those of rain-fall by a linear regression model. The high coherence between rainfall (or runoff) at the first station and rainfall (or runoff) at the second station within the same basin at almost all frequencies establishes an accurate prediction on a linear basis.     

震后废墟中Wi-Fi信号场强传输模型研究

针对震后废墟中无法精确检测出电磁波的传输特性问题,本文提出了一种在震后废墟中Wi-Fi信号场强的检测方法。首先是对Wi-Fi信号场强在组成震后废墟主要介质中的传输特性进行检测推导。然后通过测量在废墟不同距离下场强的大小,即可推导出废墟下电磁波的场强与传输距离的关系,最后得出场强在震后废墟中的传输模型。实验表明,该传输模型的计算结果准确度高,对震后废墟中电磁波传输特性的检测具有重要意义。     

Scattering and attenuation of elastic waves in random media

In seismic exploration, elastic waves are sent to investigate subsurface geology. However, the transmission and interpretation of the elastic wave propagation is complicated by various factors. One major reason is that the earth can be a very complex medium. Nevertheless, in this paper, we model some terrestrial material as an elastic medium consisting of randomly distributed inclusions with a considerable concentration. The waves incident on such an inhomogeneous medium undergo multiple scattering due to the presence of inclusions. Consequently, the wave energy is redistributed thereby reducing the amplitude of the coherent wave.The coherent or average wave is assumed to be propagating in a homogeneous continuum characterized by a bulk complex wavenumber. This wavenumber depends on the frequency of the probing waves; and on the physical properties and the concentration of discrete scatterers, causing the effective medium to be dispersive. With the help of multiple scattering theory, we are able to analytically predict the attenuation of the transmitted wave intensity as well as the dispersion of the phase velocity. These two sets of data are valuable to the study of the inverse scattering problems in seismology. Some numerical results are presented and also compared, if possible, with experimental measurements.     

地震动多点差异输入对架空管道地震响应影响研究

架空管道由于地震波传递、地震动衰减以及场地不均匀产生各支撑点地震动差异,为了研究这种差异对架空管道地震响应的影响,通过有限元软件ADINA建立架空管道有限元模型,利用MATLAB软件编写具有相干效应的人工地震波,计算分析了多点地震动相干函数法输入、行波输入与一致输入下地震响应。结果表明:①随着视波速的增加管道轴向应变变小,有接近一致激励情况的趋势;②同一相干函数模型,考虑和忽略场地效应,管道轴向应变最大值存在差异;不同相干函数模型,管道轴向应变最大值也存在差异。结论认为,如果场地比较均匀且管段较短,可采用行波法进行地震输入;长柔管道应采用相干法进行地震响应分析,场地不均匀的长柔管道,应同时考虑场地效应。     

Simulation and analysis of GPR signal based on stochastic media model with an ellipsoidal autocorrelation function

Target detection using ground penetrating radar (GPR) is based on the contrast between the electrical parameters of the target and the background medium, such as dielectric permittivity, conductivity and permeability. The application mainly concentrates on the detection of the medium interface and the target shape. In any theoretical study, a simulation model is built with a homogeneous medium. However, real detection encounters heterogeneous media which might produce scattering and diffraction at electrical interfaces and distort the radar pulse shape and affect the detection resolution. In this paper, we build multi-scale random media model with an ellipsoidal autocorrelation function and use FDTD method to simulate the GPR signal response. We then estimate and analyze the arrival time, layer thickness, permittivity and the physics relation in different scale random models according to the S transform method and the transmission wave method. The results demonstrate that we can use GPR to obtain geophysical information of multi-scale heterogeneous media, and provide a foundation for real media detection and complex media inversion.     

基于加权一致性的蚁群算法在断层检测中的应用

突出地震数据振幅空间不连续性的断层增强属性体是断层自动解释的基础,而如何压制噪声、地层残余响应、角度不整合接触等伪断层的影响是断层自动检测的关键问题之一.在指纹图像边缘检测处理中,一致性是对指示局部方向场信息各向异性强度的一个度量,本文将其作为一种新的断层增强属性引入到地震数据处理中.为了更有效地压制噪声,本文通过对一致性进行加权处理,再结合蚁群算法,提出了新的断层自动检测方法——基于加权一致性的蚁群算法.理论地震模型和实际地震数据的处理结果表明,对于相同的断层检测过程,对比常规的基于方差属性的蚁群算法和基于C3相干属性的蚁群算法,本文所提方法不仅更有效地压制了噪声和地层残余响应产生的伪断层信息,而且更完整地展现了断层的延伸长度,同时具有更高的稳定性和计算效率.     

山西代县震群序列特征及其指示意义

2006年4月3日至9月19日在山西代县发生一次3.7级震群,着重研究该震群活动的时序列特征、韵律特征及震源机制解等相关参数。进一步探讨其对周围地区中强震的指示意义。分析结果表明,此次震群活动是该区域一次正常的地震起伏,并与周围350 km范围内华北地区中强震有较好的呼应关系。     

海洋和大气对Chandler摆动激发的贡献

用1985～1995年期间海洋角动量变化序列和大气角动量变化序列,分析它们对Chandler摆动的激发能量以及它们与天文观测激发的相干性. 结果表明，在Chandler摆动频带内，海洋角动量变化提供的激发能量大约占观测激发的64髎，其中洋底压力变化起主导作用；NCEP/NCAR(美国国家环境预测中心/国家大气研究中心)和JMA(日本气象厅)大气角动量变化提供的激发能量分别为观测激发的23髎和214髎. 海洋激发与观测激发的相干系数约为0.52，接近99髎置信度下限(0.54)，海洋激发的相位滞后观测激发约19°；NCEP/NCAR和JMA的大气激发与观测激发的相干系数分别为0.32和0.37，大气激发的相位超前观测激发分别约47°和19°. 用更短期间(9年、6年)的海洋和大气角动量序列与更长期间(16、39年)的大气角动量序列作分析表明，在Chandler摆动频带内，无论是它们的激发能量，还是它们与观测激发之间的相干系数都呈现很大的变化，而且这些变化具有某种程度的随机性.     

Surface waves in media having lateral inhomogeneities

Summary On the basis of Alsop's method (1966), approximated reflection and transmission coefficients are obtained for surface waves obliquely incident at a vertical discontinuity. Rayleigh and Love waves are arranged in such a way that they form a homogeneous eigenfunction system and their mutual conversion into each other is compatible with this theory. The structure of the eigenvalue equation is responsible for the mathematical complications in many surface wave problems. On the basis of a computer routine, first result concerning the angular dependence of the reflection and transmission coefficients including mode conversion are presented. The problem of phase jump is discussed. A comparison of theoretical and experimental results shows the applicability of Alsop's method for 90°-corners, too.     

基于局部直方图规定化的相干体增强

Coherence analysis is a powerful tool in seismic interpretation for imaging geological discontinuities such as faults and fractures. However, subtle faults or fractures of one stratum are difficult to be distinguished on coherence sections (time slices or profiles) due to interferences from adjacent strata, especially these with strong reflectivity. In this paper, we propose a coherence enhancement method which applies local histogram specification (LHS) techniques to enhance subtle faults or fractures in the coherence cubes. Unlike the traditional histogram specification (HS) algorithm, our method processes 3D coherence data without discretization. This method partitions a coherence cube into many sub-blocks and self-adaptively specifies the target distribution in each block based on the whole distribution of the coherence cube. Furthermore, the neighboring blocks are partially overlapped to reduce the edge effect. Applications to real datasets show that the new method enhances the details of subtle faults and fractures noticeably.     

Current measurements and spectral analyses in the upper 450 m and deep layers of the northeastern South China Sea

On the basis of the current measurements from the moored Long Ranger ADCP in the upper 450 m layer and the deep current measurements at 2000 and 2300 m from the moored cur-rent meters with the time series data of about 7 months at the mooring station in the northeastern South China Sea, the spectral analyses and calculation have been made. The major results are as follows: (ⅰ) From the progressive vector diagrams of the observed daily currents at the water lev-els from 50 m to 400 m, its temporal variation of velocity rotated counterclockwise in most of the observing time. This agrees basically with the result from the qualitative analysis of the sea surface height data, which was obtained from TOPEX/ERS-2 altimeter data by CCAR. The daily and monthly average velocities are both the largest in November, next in October and minimum in Au-gust. (ⅱ) At the 2000 and 2300 m levels, the daily and monthly average velocities are both the largest in January, next in September and minimum in August. From the seasonal change of cur-rents, the current velocity is the strongest in winter (January-March), next in autumn, and weak in summer. (ⅲ) There exists the variation of tidal current with the change of depth. In the upper layer, the height of diurnal peak is higher than that of semidiurnal peak. However, the semidiurnal peak is higher than the diurnal peak at the levels from 200 m to 400 m. In the layers above 450 m the clockwise component is dominant in their fluctuations. In the layers below 1500 m the diurnal peak is again higher than the semidiurnal peak. (ⅳ) There is the prominent periodic fluctuation of more than two months in the layer from 50 m to 2300 m. The period of this prominent peak is 75 d and its fluctuation is counterclockwise in the upper 450 m layers, and is 68 d and 69 d at the depths of 2000 and 2300 m, respectively, and the counterclockwise component is dominant in their fluctua-tions. (ⅴ) There are the variations of periods fluctuating with the change of depth in the upper 450 m layers. For example, when f＞0, there are the prominent fluctuations of about 22 d and 15 d pe-riods at the 50 and 100 m levels. However, there are no such periods at the layer from 200 m to 400 m, where only the fluctuation of about 13 d period occurs. (ⅵ) There are the fluctuations with periods of more than one month, 23 d and 15 d at the 2000 m and 2300 m levels. (ⅶ) In the layer from 50 m to 2300 m there are the following prominent peaks: ⅰ) the fluctuation in the period range of about 4-8 d, which occurs in the weather process; ⅱ) the fluctuation with inertial period, the fluctuation is clockwise; and ⅲ) the fluctuations with short periods of about 8 h and 6 h. (ⅷ) From the cross spectral estimates between two time series, it is shown that there are significant coherence peaks with the periods of more than two months (T = 68.3 d) and more than one month between the two time series of currents at 2000 m and 2300 m depths, and also those with periods of about half a month (15.5 d), 2 d and so on between two time series of currents at 100 m and 2300 m depths.     

THE INSENSITIVITY OF REFLECTED SH WAVES TO ANISOTROPY IN AN UNDERLYING LAYERED MEDIUM1

Propagation in the plane of mirror symmetry of a monoclinic medium, with displacement normal to the plane, is the most general circumstance in anisotropic media for which pure shear-wave propagation can occur at all angles. Because the pure shear mode is uncoupled from the other two modes, its slowness surface in the plane is an ellipse. When the mirror symmetry plane is vertical the pure shear waves in this plane are SH waves and the elliptical SH sheet of the slowness surface is, in general, tilted with respect to the vertical axis. Consider a half-space of such a monoclinic medium, called medium M, overlain by a half-space of isotropic medium I with plane SH waves incident on medium M propagating in the vertical symmetry plane of M. Contrary to the appearance of a lack of symmetry about the vertical axis due to the tilt of the SH-wave slowness ellipse, the reflection and transmission coefficients are symmetrical functions of the angle of incidence, and further, there exists an isotropic medium E with uniquely determined density and shear speed which gives exactly the same reflection and transmission coefficients underlying medium J as does monoclinic medium M. This means that the underlying monoclinic medium M can be replaced by isotropic medium E without changing the reflection and transmission coefficients for all values of the angle of incidence. Thus no set of SH seismic experiments performed in the isotropic medium in the symmetry plane of the underlying half-space can reveal anything about the monoclinic anisotropy of that underlying half-space. Moreover, even when the underlying monoclinic half-space is stratified, there exists a stratified isotropic half-space that gives the identical reflection coefficient as the stratified monoclinic half-space for all angles of incidence and all frequencies.     

Spatial coherence of seismic wave fields across an alluvial valley (weak motion)

The loss of coherence of seismic waves, even over small distances, can become significant and consequently important for engineering applications. This coherence loss depends on several parameters (frequency, inter-station distance), but their dependencies are not well understood yet. We show the results of a detailed analysis of the spatial variability of the coherence between two signals. This analysis is based on a simple model which is an exponential function of inter-station distance and frequency. Data from a temporary network of 36 weak-motion instruments installed at the EURO-SEISTEST site, north-east of Thessaloniki (Greece), is used in this work. The data offers the unique possibility to study the loss of coherence over a wide range of distances, as the inter-station distances are between 8 m and 5488 m. We test the influence on the coherence of the length of the time-window of the signals, of the type of the waves present in the time-window and of the common energy of the signals. We also show that, at least at this particular site, the loss of coherence with distance is probably marked by a "cross over" distance, distinguishing two different ranges: one for inter-station distances up to 100 m and the other above 100 m. Finally, we find that the coherence determined from noise recordings behaves in a rather similar way to the one determined from coda-waves and more stationary (longer) signals. Therefore, noise can be useful for a rough, but quick estimation of the loss of coherence, at least for inter-station distances larger than 100 m.