旋转交错网格在横波分裂和再分裂模拟与分析中的应用

横波分裂可以用来探测地下介质的方位各向异性特征,如地幔流动方向、地壳内断层和应力的方向,以及近地表裂缝发育方位等,在地球深部结构探测和油气田勘探中具有重要作用。由于地下一般存在多层方位各向异性介质,记录的横波可能发生过多次分裂,因此,采用正演方法对横波分裂和再分裂进行数值模拟与分析,对于横波分裂反演和地下横向结构探测具有重要意义。旋转交错网格由于将速度和应力的不同分量置于同一网格点,可以避免各向异性参数的插值,正演更为精确。因此,采用旋转交错网格对方位倾斜横向各向同性介质的一阶速度-应力方程差分离散,并构造完全匹配层边界,实现了多层方位各向异性介质多分量逆VSP资料的正演;然后以此为基础,分析了横波发生分裂和再分裂时分裂参数随方位变化的规律;最后采用层剥离策略对多层方位各向异性进行反演和补偿,并探讨了层剥离方法的应用条件和技巧。     

横波在各向异性复合介质中的传播

横波在各向异性介质中传播时将发生横波的分裂。把玻璃和水泥组成的复合介质看做是对裂缝简单而合理的近似,对裂缝引起的各向异性介质中横波的分裂进行了较详细的观察,当横波沿着平行于层面的方向传播时,可以观察到明显的横波分裂。裂缝引起的各向异性与裂缝参数和声源之间有密切的关系。在一定范围内,介质的横波速度各向异性程度随裂缝密度及裂缝宽度的增加而变大,且较高频率的探头对介质的各向异性更敏感。     

地震波横波分裂现象的数值模拟特征

开展了横波分裂的机理、横波分裂的影响因素及横波分裂的可识别条件等3个方面的研究。采用交错网格高阶有限差分法,对垂直裂缝介质中地震波的传播特征进行了数值模拟。研究结果表明:由于入射横波在裂缝介质中存在两个正交极化分量导致横波分裂现象的发生。横波分裂的因素与震源、传播介质、分界面、观测系统等有关。横波分裂可识别的条件是:采用剪切力源或集中力源及VSP或井间地震观测系统,更容易识别横波分裂现象;对于非均匀各向异性介质,如果观测方位合适,会观测到横波分裂和再分裂现象,该项工作对实际中裂隙检测和地震波多波多分量勘探有一定的理论指导意义。     

腾冲地区中小震群横波分裂到时差变化研究

基于云南腾冲强震区8个台站观测得到的260条地方震横波分裂数据,分析快慢横波到时差在时间和空间上的分布特征。结果表明,横波到时差的变化与观测时窗内3个中小震群的震级有关联,多个台站的横波到时差随各震群中的较大地震同步变化,这些横波到时差数据携带了震群发生过程中应力场变化的信息,前提是各震群中地震发生的空间位置集中,排除了地震射线传播路径长度和空间方位的差异对横波到时差的影响。与以往大多数情况不同,3个震群的震级较小,最大震级在M3.2至M5.2之间,同时震群的震源距普遍大于30 km,最大达到70 km左右,实现通过较远中小震群横波到时差获取震群发生过程中应力变化信息的目标,对更广泛地利用远距离小震监测区域应力变化有积极意义。     

基于比值法叠后快慢横波分离方法的裂缝检测

定向地下裂缝是低渗透率油气藏油气的储集空间和运移通道,对油气的开采有着重要的意义.本文基于横波分裂理论,提出了比值法叠后快、慢横波分离方法,通过分离后的快、慢横波的振幅差异和时间延迟可求取地下介质的裂缝走向方位和密度以及各向异性系数。     

横波各向异性的分析处理（2）

地壳的各向异性分析计算中，非常重要的两个参数是：分裂的横波中高速和低速横波之间的时间延迟和高速横波的极化方向。在试算中的合成地震记录显示两种方法（ＡＳＲ和ＡＩＲ）都大大快于目前使用的数字方法（分别是１０倍和１００倍），而且结果与模型值一致。野外资料的分析计算显示ＡＳＲ相对于ＡＩＲ的抗干扰性要强且结果稳定。     

基于横波分裂方法的海南地幔柱研究

利用横波分裂方法,对北京大学、南方科技大学及桂林理工大学在广东和广西布设的宽频带流动台阵以及国家地震台网在广东、广西和海南的固定台站记录的XKS震相进行计算,得到海南及邻近地区的上地幔各向异性参数,分裂延迟时间在0.35～1.90s之间.综合研究区域内前人发表的和本文的横波分裂结果,认为自新生代以来,伴随东部太平洋板块...     

三维各向异性介质中弹性波方程交错网格高阶有限差分法数值模拟

三维数值模拟是研究各向异性介质中复杂弹性波波场特征和传播规律的重要手段。根据Taylor展开式，推导出了交错网格一阶空间导数的任意偶数阶精度有限差分近似式和相应的差分系数计算式，给出了三维各向异性介质中弹性波一阶双曲型应力-速度方程交错网格任意偶数阶精度差分格式和稳定性条件，并推导出了三维各向异性介质PML法吸收层边界条件公式和相应的交错网格差分格式。对方位各向异性介质模型和正交各向异性介质模型中弹性波的传播进行的三维数值模拟结果表明，弹性波在三维各向异性介质中传播时存在拟P波、拟SV波和拟SH波，并出现横波分裂、横波分裂盲区、波面三分叉等特殊现象，另外，弹性波场在空间是变化的，其拟P波、拟SV波和拟SH波的耦合关系比较复杂。     

基于DSI测井横波分裂的水平井压裂缝检测研究

利用偶极横波测井检测水平井压裂缝不同于直井压裂缝检测,目前还存在较多的理论和技术问题。针对直井、斜井和水平井等不同情况,给出了三维TTI介质中SH波和SV波的相速度和各向异性系数计算公式,从理论方面研究其横波各向异性系数与裂缝倾角、裂缝方位角、井斜方位角、井斜角之间的关系,开展水平井横向压裂缝的DSI实验模拟研究,并在实际水平井QP52井中进行压裂缝检测应用。研究发现,DSI可以检测水平井的纵向缝和斜交缝,但无法检测水平井的横向正交压裂缝。横波各向异性系数随裂缝倾角的增大而减小,随裂缝方位角与井斜方位角之差的增大而减小;而井斜角的改变不会影响横波分裂速度和各向异性系数的检测与计算。研究结果可为工区水平井实施压裂缝检测的DSI项目提供重要理论指导和依据,即通过压前、压后近井地带的横波各向异性对比来检测压裂缝与识别压开层段。     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

利用剪切波分裂研究地壳介质各向异性

地壳介质的各向异性主要是裂隙各向异性,剪切波穿过各向异性介质传播会发生分裂现象.剪切波分裂的偏振特性和时间延迟特性与裂隙介质的几何特性和物理性质有关,可以用来研究地壳应力场及其动态变化.     

华南块体各向异性分区及上地幔动力学含义

利用SKS波分裂方法, 对布设在华南地区的宽频带流动地震观测台阵数据进行分析。研究结果表明, 华南块体(SCB)的SKS波分裂自西向东存在明显的变化。在华南块体西部克拉通岩石圈保留完整的四川盆地区域, SKS波分裂不明显, 反映该区域岩石圈厚且缺乏大规模的一致性变形; 在四川盆地东缘褶皱带区域, SKS波分裂快波方向主要为NNE向, 分裂延迟时间可以达到1 s左右, 记录了该区域岩石圈地幔经历的一致的显著变形; 华南块体东部的SKS波分裂快波方向主要为ENE向, 分裂延迟时间为1 s左右, 该区域各向异性主要来自软流圈流动的贡献, 与大地幔楔的流动方向契合; 四川盆地东缘以东200 km范围内, 各向异性结果大部分显示为Null值, 显示该区域在地质历史上可能发生过复杂的地球动力学过程。     

The depth distribution of azimuthal anisotropy in the continental upper mantle

The most likely cause of seismic anisotropy in the Earth's upper mantle is the lattice preferred orientation of anisotropic minerals such as olivine. Its presence reflects dynamic processes related to formation of the lithosphere as well as to present-day tectonic motions. A powerful tool for detecting and characterizing upper-mantle anisotropy is the analysis of shear-wave splitting measurements. Because of the poor vertical resolution afforded by this type of data, however, it has remained controversial whether the splitting has a lithospheric origin that is 'frozen-in' at the time of formation of the craton, or whether the anisotropy originates primarily in the asthenosphere, and is induced by shear owing to present-day absolute plate motions. In addition, predictions from surface-wave-derived models are largely incompatible with shear-wave splitting observations. Here we show that this disagreement can be resolved by simultaneously inverting surface waveforms and shear-wave splitting data. We present evidence for the presence of two layers of anisotropy with different fast-axis orientations in the cratonic part of the North American upper mantle. At asthenospheric depths (200-400 km) the fast axis is sub-parallel to the absolute plate motion, confirming the presence of shear related to current tectonic processes, whereas in the lithosphere (80-200 km), the orientation is significantly more northerly. In the western, tectonically active, part of North America, the fast-axis direction is consistent with the absolute plate motion throughout the depth range considered, in agreement with a much thinner lithosphere.     

中国东北上地幔各向异性研究

利用中国数字地震台网记录的地震波形,观测其穿过地球外核和地幔的SKS波分裂特征,确定了快波S波的振动方向和快慢波时间延迟。这种经过地幔传播的SKS震相的横波分裂主要来源于上地幔的各向异性。可以用应变引起的上地幔中矿物质的结晶优势排列来解释。结果表明,对于所分析的10个观测台站均发现了明显的横波分裂现象。时间延迟在0.4～1.8 s。在东部地区,快波振动方向与中国东北西太平洋俯冲带的板块相对运动方向基本一致,说明俯冲的太平洋板块地幔流对各向异性有重要意义。在西部地区,快波振动方向和时间延迟都具有较大差异,说明其总体形变的起因是复杂的,各向异性受到地幔流和历史构造运动的双重影响。     

Mid-mantle deformation inferred from seismic anisotropy

With time, convective processes in the Earth's mantle will tend to align crystals, grains and inclusions. This mantle fabric is detectable seismologically, as it produces an anisotropy in material properties--in particular, a directional dependence in seismic-wave velocity. This alignment is enhanced at the boundaries of the mantle where there are rapid changes in the direction and magnitude of mantle flow, and therefore most observations of anisotropy are confined to the uppermost mantle or lithosphere and the lowermost-mantle analogue of the lithosphere, the D" region. Here we present evidence from shear-wave splitting measurements for mid-mantle anisotropy in the vicinity of the 660-km discontinuity, the boundary between the upper and lower mantle. Deep-focus earthquakes in the Tonga-Kermadec and New Hebrides subduction zones recorded at Australian seismograph stations record some of the largest values of shear-wave splitting hitherto reported. The results suggest that, at least locally, there may exist a mid-mantle boundary layer, which could indicate the impediment of flow between the upper and lower mantle in this region.     

Crust-mantle coupling in North China: Preliminary analysis from seismic anisotropy

Data from the CASN (Capital Area Seismograph Network), NSNC (National Seismograph Network of China), and IRIS (Incor-porated Research Institutions for Seismology) are compared with data from a temporary North China Seismic Array to obtain the background orientation of the horizontal crustal principal compressive stress at NE 95.1°±15.4° in North China. Data are corrected for disturbances of faults and irregular tectonics, and are used to constrain the fast SKS polarization at NE 110.2°±15.8° in North China. Individual station analyses suggests that there is consistently more than 10° difference between the polarizations of fast shear-wave in the crust and those of fast SKS phases. Azimuthally anisotropic phase velocities of Rayleigh waves at different periods also indicate an orientation change for fast velocity with depth. It suggests the crust-mantle coupling in North China follows neither the simple decoupling model nor the strong coupling model. Instead, it is possibly some inhomogeneous combination of two models or some gradual-change model of physical characteristics. This study shows that anisotropy in the crust and mantle could be multiply characterized more correctly and crust-mantle coupling could be analyzed further, if increasing near-field shear-wave splitting data that indicate crustal anisotropy, combined with the azimuthal anisotropy of Rayleigh waves, besides the result of SKS splitting travelling through lithosphere and surface GPS measurements.     

Ni／Cr多层膜的磁各向异性

利用振动样品磁强计和铁磁共振仪研究了Ｎｉ／Ｃｒ多摹 磁各向异性。     

Compressive stress field in the crust deduced from shear-wave anisotropy: an example in capital area of China

The rocks in the crust are pervaded by stress-aligned fluid-saturated microcracks, and the complex fault tectonics and stress control the configuration of the microcracks, however shear-wave splitting could indicate this kind of characteristics. In this paper, Capital Area Seismograph Network （CASN）, the widest scope and highest density of regional seismograph network presently in China, is adopted to deduce the principal compressive stress field distribution pattern from polarizations of fast shear- waves, based on shear-wave splitting analysis. The principal compressive stress in capital area of China is at NE85.7°±41.0° in this study. Compared with the results of principal compressive stress field in North China obtained from other methods, the results in this study are reliable in the principal com- pressive stress field distribution in capital area. The results show that it is an effective way, although it is the first time to directly obtain crustal stress field from seismic anisotropy. It is effectively applied to the zones with dense seismograph stations.