A physical model for shear-wave velocity prediction1

The clay-sand mixture model of Xu and White is shown to simulate observed relationships between S-wave velocity (or transit time), porosity and clay content. In general, neither S-wave velocity nor S-wave transit time is a linear function of porosity and clay content. For practical purposes, clay content is approximated by shale volume in well-log applications. In principle, the model can predict S-wave velocity from lithology and any pair of P-wave velocity, porosity and shale volume. Although the predictions should be the same if all measurements are error free, comparison of predictions with laboratory and logging measurements show that predictions using P-wave velocity are the most reliable. The robust relationship between S- and P-wave velocities is due to the fact that both are similarly affected by porosity, clay content and lithology. Moreover, errors in the measured P-wave velocity are normally smaller than those in porosity and shale volume, both of which are subject to errors introduced by imperfect models and imperfect parameters when estimated from logs. Because the model evaluates the bulk and shear moduli of the dry rock frame by a combination of Kuster and Toksöz’ theory and differential effective medium theory, using pore aspect ratios to characterize the compliances of the sand and clay components, the relationship between P- and S-wave velocities is explicit and consistent. Consequently the model sidesteps problems and assumptions that arise from the lack of knowledge of these moduli when applying Gassmann's theory to this relationship, making it a very flexible tool for investigating how the v_P-v_s relationship is affected by lithology, porosity, clay content and water saturation. Numerical results from the model are confirmed by laboratory and logging data and demonstrate, for example, how the presence of gas has a more pronounced effect on P-wave velocity in shaly sands than in less compliant cleaner sandstones.     

一种带误差校正的横波速度预测方法

本文提出一种基于纵横波速度关系的带误差校正的横波速度预测方法,它主要针对在进行横波速度预测时所采用纵波模量M近似变换结果与标准的Gassmann关系(K变换)之间存在的误差提出了校正的方法,并给出了完整的基于纵横波速度关系的带误差校正横波预测技术流程,该技术是一个简单实用的横波速度预测工具.     

On shear-wave triplication in transversely isotropic media

The exact solution to the problem of qSV triplication in homogenous transversely isotropic media has been long known, but the result is algebraically complex and is seldom applied in practice. We present an appropriate approximation (not assuming weak qSV-anisotropy) that simplifies the conditions for the onset of off-axis triplication as anisotropy is increased, identifying the anisotropy parameter σ as the controlling parameter. It follows that commonly reported surface-seismic P-wave move-out measurements imply that many formations in the earth's sedimentary crust support off-axis qSV triplications. For typical Vp/Vs velocity ratios and a horizontally stratified earth, however, off-axis qSV triplications appear to only occur for shear-wave incidence angles too far from the vertical to be sampled by surface-seismic converted-wave survey geometries.     

Error in shear-wave polarization and time splitting

Shear‐wave polarization and time delay are attributes commonly used for fracture detection and characterization. In time‐lapse analysis these parameters can be used as indicators of changes in the fracture orientation and density. Indeed, changes in fracture characteristics provide key information for increased reservoir characterization and exploitation. However, relative to the data uncertainty, is the comparison of these parameters over time statistically meaningful? We present the uncertainty in shear‐wave polarization and time delay as a function of acquisition uncertainties, such as receiver and source misorientation, miscoupling and band‐limited random noise. This study is applied to a time‐lapse borehole seismic survey, recorded in Vacuum Field, New Mexico. From the estimated uncertainties for each survey, the uncertainty in the difference between the two surveys is 31° for the shear‐wave polarization angle and 4 ms for the shear‐wave time delay. Any changes in these parameters greater than these error estimates can be interpreted with confidence. This analysis can be applied to any time‐lapse measurement to provide an interval of confidence in the interpretation of shear‐wave polarization angles and time splitting.     

用剪切波分裂研究台湾北部地壳各向异性

研究主要使用台湾北部13个地震台站记录到的1991年7月～2002年12月的波形数据,采用剪切波分裂SAM分析方法,对台湾北部地区的剪切波分裂特征进行了研究.发现位于宜兰盆地内的台站的快剪切波优势偏振方向为近E-W方向,而位于山脉(西部麓山带、雪山山脉和中央山脉)的台站的快剪切波优势偏振方向为NNE向或NE方向.位于海...     

On shear-wave splitting in the Los Angeles basin

Three-component seismograms at the three USC stations, PVP, GFP and DHB, have been examined. Most earthquakes, with magnitudes ranging from 1.4 to 5.0, within a period from 1985 to 1988, show evidence of shear-wave splitting. The preferred polarization of the first split-shear wave arrivals at PVP is nearly in N-S which is consistent with both regional maximum horizontal compressive stress direction and local subsurface fault strike, showing that shear-wave splitting is caused by liquid-filled cracks or fractures associated with the N-S faulting. The polarizations of first shear wave arrivals at GFP are roughly divided into two almost perpendicular directions, ENE-WSW and NNW-SSE, which are parallel or perpendicular to the strike of the geology or topography near the station. Because GFP is near the foothills of Santa Monica Mountains, the shear-wave arrivals may be disturbed by topographic irregularities and by subsurface dipping interfaces. Two examples at DHB clearly display shear-wave splitting. Their polarizations of shear wave are in the direction of N-S, which agree with the fragmentary surface and fracturing direction there. From some relatively reliable delay times, the crack densities at three stations are given, that is, 0.025 at PVP, 0.020 at GFP and 0.045 at DGB. No systematic change of shear-wave polarization is discovered in this study.     

地壳介质剪切波分裂研究的部分进展

剪切波穿过各向异性介质传播时会发生分裂现象,而在地壳中产生各向异性的主要因素是大量充满液体的定向排列的微裂隙。通过剪切波分裂参数可以研究地壳介质的特性、地壳应力状态及应力场的变化,并可用于地震预测。利用地壳介质各向异性特征研究断层性质也是一个新的动向。     

各向异性介质中频率相关横波分裂参数的提取算法(英文)

基于曾新吾和MacBeth提出的时域内双源累积旋转方法,建立了频率相关介质中分析多分量VSP数据的横波分裂参数提取算法(DCTF)。该算法可以在频域中针对单个频率提取横波各向异性参数(快横波的极化方向以及快、慢横波间的时间差),从而避免了目前常用方法中使用窄带通滤波可能带来的误差。通过对地震合成记录的数值分析,确定了该算法的可行性和正确性,并与目前常用方法的应用结果进行了比较。结果显示,频率相关横波分裂参数可以利用DCTF从地震四分量数据中直接提取。在地震频率范围内,含较大尺度裂缝时各向异性参数将表现出频率相关性,这意味着在地震频率范围会出现频散。随着频率的增加,各向异性有降低的趋势。     

Estimation of shear-wave attenuation characteristics for the Klyuchevskoi volcanic edifice

This paper reports a study of intrinsic attenuation and scattering in the Klyuchevskoi volcanic edifice. The data set consisted of small volcano-tectonic earthquakes occurring as deep as 30 km beneath Klyuchevskoi Volcano and recorded by radio telemetry seismograph stations installed on the edifice and near it. The digital seismograms of small volcano-tectonic earthquakes were processed by the multiple lapse time window analysis (MLTWA) method currently in use in seismology. The method uses experimental normalized integrals of 3D seismic energy density determined from several time windows applied to earthquake records that have been put through bandpass filters. The parameters that characterize the intrinsic attenuation and scattering of seismic energy of small volcano-tectonic earthquakes in Klyuchevskoi Volcano were estimated by adjusting the coefficients to ensure the best fit between experimental and theoretical integrals. An analytical solution to the seismic energy transfer equation was used to calculate the theoretical integrals of 3D seismic energy. Reliable estimates of the parameters that characterize the intrinsic attenuation and scattering in the material of the Klyuchevskoi volcanic edifice have been obtained. Our estimates of the Q for the edifice are below those derived by other workers for the Kamchatka lithosphere.     

山西地区地壳剪切波分裂特征

通过对山西数字地震台网2000年6月—2012年12月的波形记录资料的分析, 使用剪切波分裂系统分析方法, 即SAM综合分析方法, 获得了山西地区18个数字地震台站的快剪切波偏振结果. 结果表明： 位于活动断裂上的台站的快剪切波偏振优势方向与活动断裂的走向基本一致; 个别距离断裂较远的台站的快剪切波偏振优势方向与震源机制解及GPS主压应变方向完全一致; 少数位于几条断裂交汇处的台站的快剪切波偏振优势方向则较为复杂, 与活动断裂的走向和GPS主压应变方向均不一致, 反映了该地区断裂背景和应力分布特征的复杂性.     

辽宁盖州震群剪切波分裂参数的时间演化特征

自2012年2月2日起,辽宁盖州地区发生多次4级以上地震,地震活动呈震群态势,且2020年依然持续.利用辽宁区域数字地震台网2008年1月至2018年12月近场小地震波形资料,采用剪切波分裂( SAM)方法,计算盖州震群中75次中小地震的剪切波分裂参数,并利用盖县( GAX)地震台(下文简称盖县台)记录分析剪切波分裂特...     

Site-dependent shear-wave velocity equations versus depth in California and Japan

Shear-wave velocity of the near-surface ground is an important soil property in earthquake and civil engineering. Using the data from 643 boreholes from the KiK-net in Japan and 135 boreholes from California where the shear-wave velocity profile reaches at least 30 m, firstly, we classify sites by building code, then build site-dependent relationships between travel time and depth by regression utilizing the logarithmic model and power-law model, lastly, we get shear-wave velocity equations versus depth based on the mathematical relationship between travel time and velocity. The results show that: (1) the travel time is strongly correlated with depth, and the Pearson correlation coefficients range between 0.867 and 0.978. (2) there is a certain difference between linear velocity equations and power-law velocity equations, and the power-lower equations are generally more close to measured data than linear equations except for class E in Japan and class D in California. (3) the velocities are similar at the sites of each class for different regions but that the gradient of velocity with depth vary between different regions.     

四川宜宾地区S波分裂特征

本文采用纵横比与偏振分析相结合的方法测定了2013年4月25日~2015年12月31日四川宜宾地区10个台站S波分裂参数,即快波偏振方向和慢波延迟时间。结果表明,华蓥山断裂两侧台站呈现不同的快波偏振优势方向,断裂带以西的台站偏振优势方向为NW向,与区域应力场方向一致;位于断裂带以东的台站优势偏振方向为NE向,与断裂走向一致。在地震密集分布区域内的CNI台的优势偏振方向为NE向,与台站附近的断裂带走向基本一致。研究区域南段的3个台站(JLI、YAJ、XWE)优势偏振方向近NS向。各个台站平均慢波延迟时间在3.07~11.95ms/km范围内,慢波延迟时间最大的台站是CNI台,距离2013年4月25日06时10分M_L5.2地震震中位置最近,这反映出震源区地震波各向异性程度较强。CNI台站的慢波延迟时间显示,在2015年2月7日M_L4.8地震前观测到慢波延迟时间有明显的上升趋势。     

四川锦屏水电站地区的剪切波分裂参数变化

通过分析四川锦屏水电站地区2012年1月至2013年12月库区的地震活动性和木里台的剪切波分裂参数变化,研究了水电站3次大规模蓄水对该地区地震活动性和地壳应力的影响. 结果显示:经3次蓄水,该地区的地震活动呈持续增强趋势,最终保持在较高水平,尤其是第二次蓄水之后,地震活动增强最为明显;木里台的快剪切波偏振方向在锦屏水电站3次蓄水前后表现出明显的变化. 锦屏水电站在蓄水之前,木里台快剪切波偏振方向的优势取向为北东向,与锦屏山—小金河断裂走向一致;第一次蓄水之后,快剪切波偏振方向则变得较为分散,无明显的优势取向;第二次蓄水之后,快剪切波偏振方向偏转为南东向;第三次蓄水之后,快剪切波偏振方向表现为一致的南东向;慢剪切波延迟时间在第二次蓄水前后明显变短. 这些显著的变化说明了锦屏水电站第二次蓄水之后原地小范围内的地壳应力方向迅速改变,地壳应力也随着地震活动的增强而释放.      

Fluid-dependent shear-wave splitting in a poroelastic medium with conjugate fracture sets

The dependence of shear‐wave splitting in fractured reservoirs on the properties of the filling fluid may provide a useful attribute for identifying reservoir fluids. If the direction of wave propagation is not perpendicular or parallel to the plane of fracturing, the wave polarized in the plane perpendicular to the fractures is a quasi‐shear mode, and therefore the shear‐wave splitting will be sensitive to the fluid bulk modulus. The magnitude of this sensitivity depends upon the extent to which fluid pressure can equilibrate between pores and fractures during the period of the deformation. In this paper, we use the anisotropic Gassmann equations and existing formulations for the excess compliance due to fracturing to estimate the splitting of vertically propagating shear waves as a function of the fluid modulus for a porous medium with a single set of dipping fractures and with two conjugate fracture sets, dipping with opposite dips to the vertical. This is achieved using two alternative approaches. In the first approach, it is assumed that the deformation taking place is quasi‐static: that is, the frequency of the elastic disturbance is low enough to allow enough time for fluid to flow between both the fractures and the pore space throughout the medium. In the second approach, we assume that the frequency is low enough to allow fluid flow between a fracture set and the surrounding pore space, but high enough so that there is not enough time during the period of the elastic disturbance for fluid flow between different fracture sets to occur. It is found that the second approach yields a much stronger dependence of shear‐wave splitting on the fluid modulus than the first approach. This is a consequence of the fact that at higher wave frequencies there is not enough time for fluid pressure to equilibrate and therefore the elastic properties of the fluid have a greater effect on the magnitude of the shear‐wave splitting.     

A new method for near-topographic correction in gravity surveys

The paper is concerned with the problems of topographic reduction in gravity prospecting. Particular emphasis is placed on topographic conditions frequently encountered in mountainous areas such as in the Alps. New master curves are developed for several cases in which steep walls followed by inclined planes are located near the gravity sites. The computation methods are outlined and the precision as well as the rapidity are tested for various cases.Contribution No. 301. Institute of Geophysics, Swiss Federal Institute of Technology, CH-8093 Zurich, Switzerland.     

Temporal variations of shear-wave splitting in field and laboratory studies in China

Observations of shear-wave splitting at seismic stations above a swarm of small earthquakes on Hainan Island, China, and other examples world-wide, suggest that the time-delays of split shear-waves monitor the build up of stress before earthquakes and the stress release as earthquakes occur. Rock physics experiments on marble specimens also show variations of shear-wave time-delays with uniaxial pressure analogous to the field observations. The rock experiments show an abrupt decrease in time-delays immediately before fracturing occurs. Similar precursory behaviour has been observed before earthquakes elsewhere, and is believed to be important for two reasons. Precursory changes in shear-wave splitting could be used for short-term forecasting, but of greater importance may be the information such behaviour provides about the source processes in earthquake preparation zones.     

偶极横波远探测在裸眼井及套管井中接收波场的方位响应特征

偶极横波远探测技术的快速发展及其在裸眼和套管井中应用的需求增加使得研究井中远探测波场的方位特征和接收响应具有重要意义.将远探测声场看作是声源在反射体一侧的镜像点的辐射声场并对其做柱面波展开,得到了充液裸眼和套管井中偶极横波方位远探测接收声场的渐近解.据此对声场在井内流体中产生的径向位移和声压进行了模拟分析,深入研究了接收器偏心距、声源频率和地层类型对井中方位接收波场的影响,并对裸眼和套管井中的响应特征进行了对比,预示了快速和慢速地层条件下声场方位响应中的180°相位反转现象.渐近解析解与三维有限差分数值解的结果对比证实了这一现象,同时也验证了解析解的正确性和普适性.结果表明：接收器偏心距、声源频率和地层类型是井中接收声场方位响应特征的主要影响因素;SH横波入射引起的井内流体的径向位移和声压方位响应特征基本一致,而SV横波入射时二者特征却有较大差异;因此,接收信号中的SV分量有助于识别反射体方位;相对于裸眼井,套管井中接收声场的方位灵敏度更高,更有利于识别井外反射体的方位.本文的分析结果为偶极横波方位远探测技术的进一步发展和应用提供了理论支持.

     

防步兵地雷地球物理探测技术国内外研究动态

战后遗留的数百万颗地雷在许多国家带来了严重的人道主义问题.其中防布兵地雷因为其体积小金属含量低,一旦被埋在地下将很难被发现,所以目前迫切需要研究和发展新颖的探测技术来消除目前国际社会十分关注的探雷问题.而高敏感度、高可靠度和低花费是对探测技术的基本要求.当前正在使用和发展的探测技术包括地球物理探测技术和非地球物理探测技术两大类.地球物理探测技术中,正在使用和有发展前景的探测技术主要包括:低频电磁感应、红外成像、核四极矩共振和探地雷达等技术.本文将主要介绍以上几种探测技术的原理、现状,并总结它们各自的优缺点.研究表明将几种探测技术有机地结合于一体的复合探雷器是解决地雷探测问题的一个不错的选择和当前发展的一个趋势.     

用滑移界面理论模拟多裂缝体系的偶极横波远探测声场

偶极横波远探测已成为探测储层裂缝的重要测井技术,裂缝及其充填物质的弹性性质决定了其对远探测声场的响应特征.本文运用滑移界面理论来描述井外地层中裂缝对声场的响应特征,考察了SH横波的反射和透射系数随充填物的变化规律.建立了井外双裂缝远探测声场的几何模型,以双裂缝为例给出了偶极横波远探测声场的快速模拟方法,并将所得结论应用于现场远探测裂缝的识别和解释.本文的研究结果为偶极横波远探测数据的处理和解释提供了理论依据.