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1.
The ability to analyse shear-wave anisotropy in a mine environment is greatly aided by using multiple source orientations of a reproducible, impulsive shear-wave source. The analysis of what is probably the first controlled source shear-wave experiment in a mine environment demonstrates clearly that shear-wave polarizations and time delays between split shear-wave arrivals are reliably measured because of the use of multiple source orientations rather than a single shear-wave source. Reliability is further aided by modelling the shear-wave source radiation pattern, which allows for the unequivocal discrimination between seismic raypaths where shear-wave splitting did and did not occur. The analysis also demonstrates the great importance of high reproducibility of the seismic source for the use of shear waves in time-lapse surveys to monitor changes in a rockmass. 相似文献
2.
There are two main sources of non-orthogonality in multicomponent shear-wave seismics: inherent non-orthogonal split shear waves arising from substantial ray deviation in off-symmetry planes due to strong anisotropy or complex overburden, and apparent non-orthogonal split shear waves in the horizontal plane due to variation of the angle of incidence even if the two shear waves along the raypath are orthogonal. Many techniques for processing shear-wave splitting in VSP data ignore these kinds of non-orthogonality of the split shear waves. Assuming inherent non-orthogonality in zero-offset VSPs, and apparent non-orthogonality in offset VSPs, we derive equations for the four-component data matrix. These can be solved by extending the linear-transform technique (LTT) to determine the shear-wave polarizations in zero-offset and offset VSPs. Both full-wave synthetic and field data are used to evaluate the technique and to examine the effects of non-orthogonal polarized split shear waves. If orthogonality is incorrectly assumed, errors in polarization measurements increase with the degree of non-orthogonality, which introduces a consistent decreasing trend in the polarization measurements. However, the effect of non-orthogonality on the estimation of geophone orientation and time delays of the two split shear waves is small and negligible in most realistic cases. Furthermore, for most cases of weak anisotropy (less than 5% shear-wave anisotropy) apparent non-orthogonality is more significant than inherent non-orthogonality. Nevertheless, for strong anisotropy (more than 10% shear-wave anisotropy) with complicated structure (tilted or inclined symmetry axis), inherent non-orthogonality may no longer be negligible. Applications to both synthetic and real data show that the extended linear-transform techniques permit accurate recovery of polarization measurements in the presence of both significant inherent and apparent non-orthogonality where orthogonal techniques often fail. 相似文献
3.
由三分量数字地震仪组成的小孔径流动台网记录了1982年10月19日河北卢龙Ms=6.1级地震的部分余震.用质点运动图的方法对横波的偏振进行了分析。研究结果表明,在横波窗内的各观测点都存在横波的分裂现象.不同离源角和方位角快波偏振的水平投影都具有近NE40°方向的优势取向,与根据卢龙地震两组断层错动在各向同性介质中所辐射的横波的偏振方向不一致.这可以由传播介质中应力所导致裂隙的定向排列来解释.这一观测结果提供了卢龙地区脆性上地壳大范围膨胀各向异性（EDA）的证据,并表明这一地区直立平行排列裂隙取向和水平主压应力的方向为NE40°. 相似文献
4.
Plomerová Jaroslava Granet Michel Judenherc Sebasten Achauer Ulrich Babuška Vladislav Jedlička Petr Kouba Daniel Vecsey Luděk 《Studia Geophysica et Geodaetica》2000,44(2):195-209
We present the first results of a comparison of deep lithosphere structure of three Variscan massifs - the Armorican Massif, French Massif Central and Bohemian Massif, as revealed by recent tomographic studies of seismic anisotropy. The data originate from several field measurements made in temporary arrays of stations equipped with both short-period and broadband seismometers with digital recording. The study is based on teleseismic body waves and a joint inversion of anisotropic data (P-residual spheres, the fast shear-wave polarizations and split times) and demonstrates that the three Variscan massifs appear to consist of at least two parts with different orientation of large-scale fabric derived from seismic anisotropy. The boundaries of anisotropic lithospheric domains are related to prominent tectonic features recognised on the surface as sutures, shear zones or transfer fault zones, as well as grabens, thus indicating that some of them extend deep through the entire lithosphere. 相似文献
5.
The most diagnostic effect of anisotropy on shear waves is shear-wave splitting. This phenomenon creates a distinctive signature in the 3D particle motion. Methods to extract the effects of anisotropy from shear-wave data attempt to measure details of this motion. Many techniques have been published recently which process the shear waves in the time or frequency domain. Here we examine the way in which information on the interference effects between the split shear waves is contained within the frequency domain, and suggest some criteria which may be used in future processing algorithms. The time-delay between the split shear waves, and the polarization direction of the leading shear wave can be converted into easily measured features from analysis of the Fourier spectrum of the shear-wave signal on each component of motion. These features arise in the spectral interference patterns which are formed by the interaction between the two closely-spaced and similar waveforms. The interference patterns are interpreted for synthetic and observed seismogram data. 相似文献
6.
Shear waves can provide valuable information about seismic anisotropy. On entering an anisotropic medium, a shear wave generally splits (shear-wave splitting) into a fast and a slow quasi-shear wave with polarizations fixed by the elastic properties of the medium and direction of travel. If the medium contains planar discontinuities with common normals, the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities. Measuring this polarization, using a VSP geometry with oriented three-component geophones in the borehole, offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth. Such a shear-wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. The clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured. In our shallow VSP experiment, shear-wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear-wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval. The polarization direction of the fast S-wave did not correspond exactly with the strike which was obtained from geological information on the fissures. The geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole. The discrepancy, however, could be explained in terms of dipping fissures, and such a dip was confirmed by the geological and geotechnical information. The orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned. This study thus illustrates a practical application of shear-wave splitting observed in shallow shear-wave VSP for geology and geotechnical engineering. 相似文献
7.
A marine VSP is designed to estimate the orientation and density of fracturing within a gas-producing dolomite layer in the southern North Sea. The overburden anisotropy is firstly estimated by analysing shear waves converted at or just below the sea-bed, from airgun sources at four fixed offset azimuths. Full-wave modelling helps confirm that the background has no more than 3% vertical birefringence, originating from TIH anisotropy with a symmetry axis orientated perpendicular to the maximum horizontal compressive stress of NW–SE. This finding concurs with current hypotheses regarding the background rock matrix in the upper crust. More detailed anisotropy estimates reveal two thin zones with possible polarization reversals and a stronger anisotropy. The seismic anisotropy of the dolomite is then determined from the behaviour of locally converted shear waves, providing a direct link with the physical properties of its fractures. It is possible to utilize this phenomenon due to the large seismic velocity contrast between the dolomite and the surrounding evaporites. Two walkaway VSPs at different azimuths, recorded on three-component receivers placed inside the target zone, provide the appropriate acquisition design to monitor this behaviour. Anisotropy in the dolomite generates a transverse component energy which scales in proportion to the degree of anisotropy. The relative amplitudes, for this component, between the different walkaway azimuths relate principally to the orientation of the anisotropy. Full-wave modelling confirms that a 50% vertical birefringence from TIH anisotropy with a similar orientation to the overburden is required to simulate the field observations. This amount of anisotropy is not entirely unexpected for a fine-grained brittle dolomite with a potentially high fracture intensity, particularly if the fractures contain fluid which renders them compliant to the shear-wave motion. 相似文献
8.
J. Plomerova L. Vecsey V. Babuska M. Granet U. Achauer 《Studia Geophysica et Geodaetica》2005,49(4):541-560
We present results of an array study of seismic anisotropy beneath the Bohemian Massif (BM) showing distinct lateral and directional
variations of the fast shear-wave polarization and split-delay time consistent with variations in the P-residual spheres,
in which directional means of relative residuals are subtracted. Our analysis of the S- and P-wave anisotropy suggests that
the mantle lithosphere of the BM consists of at least three large domains with different orientation of the large-scale fabric
separated by sutures cutting most likely the whole lithosphere. Boundaries of the units are characterised by a null or small
shear-wave splitting, as well as by smaller values in the P-residual spheres. We present self-consistent 3D anisotropic models
of the lithosphere domains of the BM resulting from joint inversion of anisotropic parameters searching for a 3D orientation
of mantle fabric. While in the Saxothuringian and Sudetes parts of the BM the (a, c) foliations dip prevailingly to the N-NW,
they dip to the S and SW in the southern and eastern parts of the Moldanubian. 相似文献
9.
Preliminary analysis of the shear-wave splitting observations from the Qiaojia seismic array* 
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下载免费PDF全文 A total of 351 shear-wave splitting results at 25 stations were obtained from the seismic data recorded in period of January, 2013 to December, 2016, by a broadband seismic array deployed in the northern segment of Xiaojiang Fault Zone (n-XJFZ). Meanwhile, the stress field of the n-XJFZ was determined by inverting 140 focal mechanism solutions of the small earthquakes within the study area which were recorded in the same period. This determination confirmed a compressive stress in NW-SE orientation and an extensional stress in the NE-SW orientation, with little difference from those released by previous studies. The shear-wave splitting results show a spatial complexity in polarization orientation, different from one site to another. However, the polarization orientations integrated for the subareas suggest that the fault trends seemingly played important roles. All the subareas bear two dominant orientations, N10°E and N90°E, both of which are different from the azimuths of the principal compressive stress, due to the fault distribution. The time delay averaged over the entire region is 4.56 ms/km, close to that of the upper boundary of the generally accepted interval worldwide but larger than those in most of the investigated regions in the Chinese mainland, which probably implies an alignment of more micro-cracks in the n-XJFZ. Interestingly, the 2014 MS6.5 Ludian earthquake was found to have caused a variation in the time delays of the slow shear waves within the study area though its epicenter was outside. This earthquake resulted in an evident drop of the time delays remaining for 4 months, however, lifted a bit the time-delay level with respect to that prior to the earthquake. 相似文献
10.
The 3D shear experiment over the Natih field in Oman: the effect of fracture-filling fluids on shear propagation 总被引：1，自引：0，他引：1
This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three-component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear-wave velocities. One is that the shear-wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear-wave velocity which senses both the fractures and the fracture-filling fluid. Using a new effective-medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear-wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear-wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P-wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective-medium modelling and the geology.
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator. 相似文献
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator. 相似文献
11.
The scattering of shear-waves in the crust 总被引：2，自引：0，他引：2
Stuart Crampin 《Pure and Applied Geophysics》1990,132(1-2):67-91
The two major sources of scattering for shear-waves in the crust, interactions with the topography at the surface and the effective anisotropy of aligned cracks throughout the rockmass, introduce first-order changes to the shear-wave particle-motion. At the surface, shear-waves are scattered by the topography within a wavelength or two of the recording site so that, unless the effective incidence angle is less than the critical angle sin–1
V
S/V
P, the recorded waveforms may bear little relationship to the waveforms of the incident wave. Within the rockmass, shear-waves are scattered by extensive-dilatancy anisotropy (EDA), the distribution of stress-aligned fluid-filled cracks, microcracks, and preferentially oriented pore-space pervading most rocks in the crust. Analysis of this shear-wave splitting yields new information about the internal structure of thein situ rockmass which is not otherwise available. 相似文献
12.
利用蒙古中南部地区布设的69套宽频带数字地震仪2011年8月—2013年7月记录的远震事件,使用时间域反褶积方法提取接收函数,并挑选高质量Pms震相,通过改进的剪切波分裂方法对研究区地壳各向异性参数进行了研究,最终获取了1473对各向异性参数.经过统计分析,有48个台站可以归纳出两个方向的各向异性,11台站得到单个方向的各向异性,而剩余10个台站各向异性方向比较发散.结果显示,各向异性在蒙古中南部地壳中呈不均匀分布,有54个台站得到了NE-SW向各向异性,快波偏振方向平均值为N58°E±16°,与最大水平主应力σHmax方向和区域内主要断层走向一致,说明这部分地壳各向异性的主要成因存在于上地壳,可能与流体填充的微裂隙有关.而NW-SE向各向异性在53个台站被观测到,各向异性方向变化范围平均N132°E±16°,与研究区大部分SKS分裂快波方向具有较好的一致性,说明下地壳成岩矿物晶体定向排列是各向异性的主要成因.研究区地壳各向异性的分层特征总体上支持岩石圈受到NE-SW向挤压的动力学模型. 相似文献
13.
Quantifying Damage, Saturation and Anisotropy in Cracked Rocks by Inverting Elastic Wave Velocities 总被引：1，自引：0，他引：1
Alexandre Schubnel Philip M. Benson Ben D. Thompson Jim F. Hazzard R. Paul Young 《Pure and Applied Geophysics》2006,163(5-6):947-973
Crack damage results in a decrease of elastic wave velocities and in the development of anisotropy. Using non-interactive
crack effective medium theory as a fundamental tool, we calculate dry and wet elastic properties of cracked rocks in terms
of a crack density tensor, average crack aspect ratio and mean crack fabric orientation from the solid grains and fluid elastic
properties. Using this same tool, we show that both the anisotropy and shear-wave splitting of elastic waves can be derived.
Two simple crack distributions are considered for which the predicted anisotropy depends strongly on the saturation, reaching
up to 60% in the dry case. Comparison with experimental data on two granites, a basalt and a marble, shows that the range
of validity of the non-interactive effective medium theory model extends to a total crack density of approximately 0.5, considering
symmetries up to orthorhombic. In the isotropic case, Kachanov's (1994) non-interactive effective medium model was used in
order to invert elastic wave velocities and infer both crack density and aspect ratio evolutions. Inversions are stable and
give coherent results in terms of crack density and aperture evolution. Crack density variations can be interpreted in terms
of crack growth and/or changes of the crack surface contact areas as cracks are being closed or opened respectively. More
importantly, the recovered evolution of aspect ratio shows an exponentially decreasing aspect ratio (and therefore aperture)
with pressure, which has broader geophysical implications, in particular on fluid flow. The recovered evolution of aspect
ratio is also consistent with current mechanical theories of crack closure. In the anisotropic cases—both transverse isotropic
and orthorhombic symmetries were considered—anisotropy and saturation patterns were well reproduced by the modelling, and
mean crack fabric orientations we recovered are consistent with in situ geophysical imaging.
Our results point out that: (1) It is possible to predict damage, anisotropy and saturation in terms of a crack density tensor
and mean crack aspect ratio and orientation; (2) using well constrained wave velocity data, it is possible to extrapolate
the contemporaneous evolution of crack density, anisotropy and saturation using wave velocity inversion as a tool; 3) using
such an inversion tool opens the door in linking elastic properties, variations to permeability. 相似文献
14.
本文首先制作了TI地层模型井,对模型井的速度、密度等物理参数进行了测量,通过快速模拟退火算法得到了该介质的五个TI刚性系数,然后采用缩尺的偶极子探头在HTI地层井孔内进行超声测量实验模拟了实际的偶极子声波测井,并结合数值模拟的结果,研究了不同偏振方向的声源在HTI地层井孔中进行偶极子声波测井的响应特征.模拟结果表明,弯曲波的幅度和速度随声源的偏振方向不同而不同.正交分量的波形幅度在声源偏振方向平行或垂直于快横波面方位时最小,在与快横波面方位呈一定夹角时较大;同向分量波形幅度随声源的偏振方向的变化规律同正交分量相反.弯曲波速度在声源的偏振方向与快横波面方位的夹角小于30°或者大于60°时,分别接近于快弯曲波和慢弯曲波的速度,对于夹角的变化不敏感;当夹角在30°和60°之间时,弯曲波的速度对该夹角变化非常敏感,由接近快弯曲波速度快速变化到接近慢弯曲波速度. 相似文献
15.
We present the results of a shear-wave reflection experiment and in situ measurements in opencast lignite exploration. Near-surface coal seams have lower shear-wave velocities (～ 200 m/s) and lower densities than sand and clay layers. Due to strong reflection coefficients, a shear-wave reflection survey provides a powerful tool in lignite prospecting. Due to shorter seismic wavelengths shear waves will yield a higher resolution of shallow subsurface structure than compressional waves. Low shear-wave velocities and strong lateral velocity variations, however, require a dense data acquisition in the field. The variation of stacking velocities can exceed ± 15% within a profile length of 300 m. The different steps in processing and interpretation of results are described with actual records. The final CMP-stack shows steep-angle fault zones with maximum dislocations of 20 m within a coal seam. 相似文献
16.
W.H. Owens 《Physics of the Earth and Planetary Interiors》1981,27(2):106-113
The model, applicable to fine-grained haematite, assumes that the magnetization of a grain is constrained to lie in the basal plane, within which there is a single easy axis. Following Stoner and Wohlfarth (1948), torque curves and rotational hysteresis are calculated, for various field strengths, as a function of the orientations of the basal plane and easy axis. The results show that, for certain orientations, rotational hysteresis may be expected to persist to very high field strengths. Further, the dependence of rotational hysteresis on orientation implies that rotational hysteresis is anisotropic and thus, that, for a distribution of grains, the anisotropy of resultant rotational hysteresis will reflect the degree of preferred orientation. It may therefore serve as a rock fabric parameter. Calculated models of resultant rotational hysteresis for assumed preferred orientation distributions compare favourably with measurements on a sample of Cambrian purple slate from North Wales. 相似文献
17.
目前人们利用4种基本的地震波现象研究地震各向异性,如横波双折射、面波散射、与传播方向有关的走时异常和PS转换波震相.本文利用面波散射产生的Quasi-Love（QL）波研究青藏高原上地幔顶部的各向异性结构特征.首先利用中国地震台网昌都（CAD）台记录的地震波形资料识别出产生QL波的路径,并利用合成地震记录和垂直偏振极性分析证实所观测到的为QL波,而不是高阶振型的Rayleigh波或其他体波震相;然后由Rayleigh波、Love波和QL波的群速度估算了各向异性结构横向变化的转换点;不同周期时,转换点的位置不同,这种频率依赖性还需要进一步的模拟研究.Love波向Rayleigh波耦合（产生QL波）的转换点位置揭示了青藏高原面波方位各向异性变化特征,并以南北向构造带的东西分段性、上地幔流引起的地球内力诱导岩石形变解释了青藏高原各向异性的东西向差异性. 相似文献
18.
—Reflectivity synthetic seismograms demonstrate that the type, layering and orientation of 1-D anisotropy influences strongly the coda of teleseismic P waves at periods T > 1 sec, particularly P-SH converted waves. We assume the simplest form of anisotropy described by an elastic tensor with a symmetry axis ? of arbitrary orientation. The resulting phase velocities vary as cos 2ξ with respect to that axis. Using three families of simple crustal models, we compare the effects of an anisotropic surface layer with reverberations caused by both "thick" and "thin" layers of anisotropy at depth. If anisotropy in the surface layer is significant, the polarization of direct P can be distorted to generate a transverse component, followed by Ps and a prominent shear reverberation converted from direct P at the free surface. If the anisotropic layer is buried, the first, and often the most prominent, arrival on the transverse component is the P-to-SH conversion at its upper surface. If the anisotropic layer is sufficiently thin, P-to-SH conversions from its boundaries interfere to form a derivative pulse shape on the transverse component, which could be mistaken as the signature of shear-wave splitting. If ? is horizontal, compressional (P) and shear (S) anisotropy both produce similar waveform perturbations with four-lobed azimuthal patterns, suggesting that a weighted stack of P coda from different back-azimuths would improve signal-to-noise. For ? tilted between the horizontal and vertical, however, the effects of P- and S-anisotropy differ greatly. The influence of P-anisotropy on P-to-S conversion is greatest for a symmetry axis tilted at 45° to the vertical, where its azimuthal pattern has two lobes, rather than four. Combinations of P- and S-anisotropy typically lead to a composite azimuthal dependence in the P-coda reverberations. 相似文献
19.
This paper describes a large-scale reservoir characterization experiment carried out in Oman in 1991 which comprised the acquisition, processing and interpretation of a 28.4 km2 3D multicomponent seismic experiment over the Natih field. The objective of the survey was to obtain information on the fracture network present in the Natih carbonates from shear-wave anisotropy. Shear-wave anisotropy in excess of 20% time splitting was encountered over a large part of the survey. The seismic results are confirmed by geological and well data but provide additional qualitative information on fracturing where this was not available before. Regions of stronger and weaker shear-wave anisotropy appear to be fault-bounded. The average well flow rates (which are fracture-dominated) within such blocks correlate with the average anisotropy of the blocks. The further observation that the anisotropy is largest in the fracture gas cap of the reservoir suggests that shear waves can provide a direct hydrocarbon indicator for fractured rock. 相似文献
20.
云贵高原东南部地壳各向异性初步研究 总被引：2，自引：0，他引：2
云贵高原东南部地区位于贵州西南部及广西西北部。 我们选用中国地震科学台阵一期在广西、 贵州区域内的50个流动台站记录的2011年6月至2013年3月的地震波形资料, 运用剪切波分裂系统分析方法(SAM), 得到了可用于分析的10个台站地震资料, 获得剪切波分裂参数即快剪切波偏振方向和慢剪切波时间延迟。 结果表明, 云贵高原东南部地区的地壳各向异性呈现南北分区的特点。 广西西北部地区的快剪切波优势偏振方向为NW—SE, 与华南地块主压应力方向一致。 贵州西南地区位于扬子板块和华南褶皱系右江褶皱带的过渡带, 其快剪切波优势偏振方向近NE—SW, 与华南地块主压应力方向近似垂直, 反映出区域内复杂的构造背景及应力环境。 将云贵高原东南部慢剪切波时间延迟与川滇地块以及华夏地块东南部对比可知, 云贵高原东南部地壳介质各向异性程度更接近于华南地块。 相似文献