人工震源地下介质变化动态监测

成像地球内部介质结构及其动态变化是地球物理学家的主要任务.相对于对地球介质静态结构的认识,对于地下介质性质随时间变化的了解明显不足.近年来随着观测资料的增加,已经开始利用重复地震和噪声等震源研究地下介质变化.受到源位置和时间等因素制约,基于地震和噪声等被动震源的观测结果的时间和空间分辨率有限.利用人工震源主动向地下发射...     

利用超声尾波干涉评价页岩应力敏感性

岩石应力敏感性是岩石内部微裂隙发育的一种反映,对于致密储层物性和成缝能力（可压性）评价具有重要作用。由于页岩油气储层基质模量高,对应力的敏感性相对较弱,需要寻求新的高灵敏度测试方法。尾波干涉是运用介质中的散射波来监测介质微弱变化的一种测量方法,对岩石应力变化具有较高的敏感性。借助GCTS RTR-1500高温、高压三轴测试试验系统,对重庆城口县下寒武统鲁家坪组露头页岩开展尾波干涉试验,研究了页岩尾波发育规律及其对应力的响应,分析了页岩在不同围压下的应力敏感性。结果表明：页岩尾波应力敏感性大于直达波,S波应力敏感性大于P波;尾波波速相对变化率随应力的变化能够较好地反映页岩内部裂隙的产生,与扩容点有很好的对应关系;围压使部分裂隙闭合,天然裂隙密度减小,页岩应力敏感性系数降低。尾波干涉可作为页岩的应力敏感性和成缝能力评价的方法。     

Ambient noise tomography with a large seismic array

The emergence of large-scale arrays of seismometers across several continents presents the opportunity to image the Earth's structure at unprecedented resolution, but methods must be developed to exploit the capabilities of these deployments. The capabilities and limitations of a method called “eikonal tomography” applied to ambient noise data are discussed here. In this method, surface wave wavefronts are tracked across an array and the gradient of the travel time field produces estimates of phase slowness and propagation direction. Application data from more than 1000 stations from EarthScope USArray in the central and western US and new Rayleigh wave isotropic and anisotropic phase velocity maps are presented together with an isotropic and azimuthally anisotropic 3D Vs model of the crust and uppermost mantle. As a ray theoretic method, eikonal tomography models bent rays but not other wavefield complexities. We present evidence, based on the systematics of an observed 1ψ component of anisotropy that we interpret as anisotropic bias caused by backscattering near an observing station, that finite frequency phenomena can be ignored in ambient noise tomography at periods shorter than ～ 40 to 50 s. At longer periods a higher order term based on wavefront amplitudes or finite frequency sensitivity kernels must be introduced if the amplitude of isotropic anomalies and the amplitude and fast-axis direction of azimuthal anisotropy are to be determined accurately.     

基于波动理论的近地表散射噪音衰减

基于波动理论,可将地震波场分为入射波场和散射波场。在一定的假设基础上,通过波阻抗差函数可把入射波场和散射波场联系起来。利用波阻抗差函数,可对地震数据中的散射波场进行预测,进而达到去除散射噪声的目的。经正演模型和对实际数据的应用,显示了该方法对衰减近地表散射噪声的优越性。     

Can unbiased source be retrieved from anisotropic waveforms by using an isotropic model of the medium?

Anisotropy is frequently present in geological structures, but usually neglected when source parameters are determined through waveform inversion. Due to the coupling of propagation and source effects in the seismic waveforms, such neglect of anisotropy will lead to an error in the retrieved source. The distortion of the mechanism of a double-couple point source located in an anisotropic medium is investigated when inverting waveforms using isotropic Green's functions. The anisotropic medium is considered to be transversely isotropic with six levels of anisotropy ranging from a fairly weak to rather strong anisotropy, up to about 24% in P waves and 11% in S waves. Inversions are based on either only direct P waves or both direct P and S waves. Two different algorithms are employed: the direct parametrization (DIRPAR, a nonlinear algorithm) and the indirect parametrization (INPAR, a hybrid scheme including linear and nonlinear steps) of the source. The orientation of the double-couple mechanism appears to be robustly retrieved. The inclination of the resulting nodal planes is very small, within 10° and 20° from the original solution, even for the highest degree of anisotropy. However, the neglect of anisotropy results in the presence of spurious isotropic and compensated linear-vector dipole (CLVD) components in the moment tensor (MT). This questions the reliability of non-double-couple components reported for numerous earthquakes.     

Two-dimensional sensitivity kernels for cross-correlation functions of background surface waves

Ambient noise tomography has now been applied at scales ranging from local to global. To discuss the theoretical background of the technique, a simple form of a two-dimensional (2-D) Born sensitivity kernel was developed at a finite frequency for a cross-correlation function (CCF) of background surface waves. The use of far field representations of a Green’s function and a CCF in a spherically symmetric Earth model, assuming a homogeneous source distribution, is an efficient approach to the calculation of phase sensitivity kernels. The forms of a phase sensitivity kernel for major and minor arc propagations are the same as those for phase-velocity measurements of earthquake data. This result indicates the validity of ambient noise tomography under the given assumptions; however, the kernels are not equivalent in the case of an inhomogeneous source distribution.     

Towards improving ambient noise tomography using simultaneously curvelet denoising filters and SEM simulations of seismic ambient noise

The aim of this article is to investigate a possible way to improve ambient noise tomography by expanding the data base of useful cross-correlation measurements. We show that the curvelet transform makes it possible to compute synthetic noise correlations by simulating directly seismic ambient noise using the spectral element method. These synthetic correlations can in turn be used to identify surface waves and overtones even on correlations having a signal to noise ratio much lower than one. We hope that the possibility to compute SEM synthetic correlations and to extract more information from correlations will be useful to improve ambient noise tomography and monitoring.     

Azimuthal variation of Pg velocity in the Moldanubian, Czech Republic: observations based on a multi-azimuthal common-shot experiment

We study the azimuthal velocity variation of Pg waves in the Moldanubian, which is a crystalline segment within the Bohemian Massif in the Czech Republic. We use the data from a multi-azimuthal common-shot experiment performed as part of the ALP 2002 refraction experiment, complemented by profile refraction data from the CELEBRATION 2000 experiment. We analyze the travel times of waves recorded by 72 portable seismic stations deployed along two circles with radii of 35 and 45 km around a shot. The observed travel times display an azimuthal variation indicating anisotropy of 2%. The minimum and maximum velocity values are 5.83 and 5.95 km/s, respectively. The direction of the maximum velocity is N50°E. These values characterize horizontal anisotropy of the uppermost crust down to 3 km. The strength and orientation of uppermost crustal anisotropy in the Moldanubian is consistent with the overall upper crustal anisotropy in the entire Bohemian Massif. The high-velocity direction is roughly perpendicular to the present-day maximum compressive stress in the Bohemian Massif and Central Europe and coincides with the orientation of structures formed by the main Variscan tectonic events in the area. This indicates that the anisotropy is caused predominantly by alignment of textural elements and minerals in the rocks, which developed in early geological stages rather than by a preferred orientation of cracks or microcracks due to present-day stress. If the crack-induced anisotropy is present in the medium, then its strength should not exceed 1% and the cracks should be water saturated.     

半无限介质内异质体方位探测数值研究

用数值模拟方法研究了地下异质体对波散射,分析了散射波对表面波相速度影响,结果表明：由相速度曲线扰动难以有效确定异质体方位。在表面波场中,散射波能量相对较小,不易辨别,以振源为中心,在测线上对称布置一组测点,通过两侧测点质点响应曲线比较,得到表面散射波场。在表面散射波场中,散射瑞利波是主要的,由散射瑞利波走时随测点位置变化可判断地下异质体的方位。最后,给出地下异质体方位测试方案。     

Noise Reduction Technique Applied to the Multichannel Analysis of Surface Waves

This paper shows the presence of noises and technique to reduce these noises during the surface wave analysis. The frequency-dependent properties of Rayleigh-type surface waves can be used for imaging and characterizing the shallow subsurface. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the phase velocities of Rayleigh waves. Among these interferences by non-planar, non-fundamental mode Rayleigh waves (noise) are body waves, scattered and non-source-generated surface waves, and higher-mode surface waves. For the reduction of noise, the filtering technique is implemented in this paper for the multichannel analysis of surface wave method (MASW). With the de-noising technique during the MASW method, more robust and reliable outcome is achieved. The significance of this paper is to obtain pre-awareness about noises during surface wave analysis and take better outcomes with de-noising performance in near surface soil investigations.     

Seismic interferometry and ambient noise tomography in the British Isles

Traditional methods of imaging the Earth's subsurface using seismic waves require an identifiable, impulsive source of seismic energy, for example an earthquake or explosive source. Naturally occurring, ambient seismic waves form an ever-present source of energy that is conventionally regarded as unusable since it is not impulsive. As such it is generally removed from seismic data and subsequent analysis. A new method known as seismic interferometry can be used to extract useful information about the Earth's subsurface from the ambient noise wavefield. Consequently, seismic interferometry is an important new tool for exploring areas which are otherwise seismically quiescent, such as the British Isles in which there are relatively few strong earthquakes. One of the possible applications of seismic interferometry is ambient noise tomography (ANT). ANT is a way of using interferometry to image subsurface seismic velocity variations using seismic (surface) waves extracted from the background ambient vibrations of the Earth. To date, ANT has been used successfully to image the Earth's crust and upper-mantle on regional and continental scales in many locations and has the power to resolve major geological features such as sedimentary basins and igneous and metamorphic cores. Here we provide a review of seismic interferometry and ANT, and show that the seismic interferometry method works well within the British Isles. We illustrate the usefulness of the method in seismically quiescent areas by presenting the first surface wave group velocity maps of the Scottish Highlands using only ambient seismic noise. These maps show low velocity anomalies in sedimentary basins such as the Moray Firth, and high velocity anomalies in igneous and metamorphic centres such as the Lewisian complex. They also suggest that the Moho shallows from south to north across Scotland which agrees with previous geophysical studies in the region.     

Continental mantle seismic anisotropy: a new look at the Twin Sisters massif

Recent interpretations of upper continental mantle seismic anisotropy observations have often relied on fabric measurements and calculated anisotropies of upper mantle xenoliths. Seismic ray paths of P and S waves, which provide information on azimuthal compressional wave anisotropy and shear wave splitting, are tens to hundreds of kilometers, whereas, xenoliths are usually only a few centimeters in diameter. To place better constraints on field-based anisotropy observations and to evaluate anisotropy information provided by xenoliths, it is important to examine anisotropy in large ultramafic massifs which have originated in the upper mantle. One such massif is the Twin Sisters Range located in the western portion of the North Cascades of Washington State, USA. The Twin Sisters massif, a slab of unaltered dunite, is 16 km in length, 6 km in width and 3 km thick. Exposed along its south and west sides are mafic granulite facies rocks, which likely represent lower continental crustal fragments. The ultramafic rocks are porphyroclastic in texture, consisting of strained, flattened porphyroclasts of olivine and enstatite and strain-free olivine mosaics. Olivine fabrics are typical of those formed at high temperatures and low strain rates. Petrofabrics and calculated anisotropies of individual samples vary throughout the massif, however, overall anisotropy of the body is significant, with maximum P and S waves anisotropies of 5.4% and 3.9%, respectively. The maximum delay time for split shear waves traveling through a 100-km-thick slab is 0.8 s and two directions of shear wave singularity are observed. The directions of maximum shear wave splitting and shear wave singularities do not coincide with the directions of maximum and minimum compressional wave velocity. In general, individual hand samples show significantly higher anisotropy than the overall anisotropy of the massif. It is concluded that simple averages of xenolith anisotropies are unreliable for use in the interpretation of field anisotropy observations.     

水平井煤岩界面方位电磁波测井仪器探测性能

煤岩界面的预先、精准识别是实现智能化开采,提高开采效率和降低成本的关键技术之一,方位电磁波测井已在石油测井岩性界面识别中获得较好效果,但对煤岩界面识别情况的研究较少。为了研究方位电磁波在煤田测井环境中的适用性和探测性能,使用一维广义反射系数法计算磁场分量,并使用快速Hankel变换加快积分计算速度,模拟线圈系组合方式、频率、源距、线圈系半径、电阻率对比度、发射电流和电阻率各向异性对煤田方位电磁波测井响应的影响。结果表明:方位电磁波测井仪器能够分辨煤岩界面,对煤岩界面识别具有较大的应用潜力。线圈系半径和发射电流主要影响仪器响应信号的大小;在一定电阻率对比度范围内,幅度比和相位差在界面附近的幅值随电阻率对比度增大而增大,高阻地层电阻率各向异性对幅度比和相位差影响较小;仪器发射频率、源距、电阻率对比度同时影响方位信号最大探边深度。      

Fifty years of seismic anisotropy studies in Russia

This is a historic overview of seismic anisotropy studies in Russia run as part of seismic exploration work in the 1940s through the 1980s, with a focus on main research lines. At the early stage in the 1940s through 1950s, most important contributions belonged to A.G. Tarkhov, Yu.V. Risnichenko, and S.M. Rytov (averaging the parameters of stratified media), I.I. Gurvich (processing reflection and refraction traveltime curves in media with elliptical anisotropy), and N.I. Berdennikova (shear-wave velocity anisotropy). In the 1960s–1980s, there were two basic schools of thought: one of G.I. Petrashen’, with a more theoretical approach, and the other of N.N. Puzyrev dealing more with experimental work. Most of experiments addressed a newly discovered phenomenon of azimuthal anisotropy. This anisotropy appearing as “anomalous” polarization of shear and converted waves was found out to result from vertical fractures in rocks. The unusual polarization became understood owing to Klem-Musatov’s model of a subsurface with a system of aligned cracks. The problem was fully resolved after field data had been processed with an algorithm by I.R. Obolentseva and S.B. Gorshkalev, for separating the total field of interfering shear waves of two types into fast and slow phases polarized in crack-parallel and crack-orthogonal directions, respectively.     

Source distribution of ocean microseisms and implications for time-dependent noise tomography

A qualitative analysis of ocean microseism source distribution observed in North America during fall and winter months was carried out. I review the theory of the origin of ocean microseisms and show that it can be used in conjunction with wave-wave interaction maps to quantify the source distribution anisotropy. It is demonstrated that microseisms generation in the North Atlantic and in the North Pacific Oceans are inherently different. North Atlantic microseisms are generated predominantly in the deep ocean, while North Pacific microseisms are dominated by coastal reflections. In spite of these differences both result from repeated ocean wave patterns that give rise to an anisotropic noise pattern, which cannot be randomized by time averaging. Considering time-varying ambient noise imaging, which aims to resolve a fraction of a percent changes in the crust over short distances, the source anisotropy would introduce a relatively significant error that needs to be accounted for.     

针对储层裂缝检测的三维P波地震采集设计参数分析

理论和实践表明,垂直定向排列的裂缝能够诱导方位各向异性,三维P波地震是一种经济有效的裂缝勘探方法。合理的三维勘探有利于从三维地震数据中提取和增强有效的属性参数,来表征方位各向异性,达到检测和预测储层裂缝的目的。这里通过综述目前常用的纵波裂缝检测技术,结合部分实际数据,总结了三维P波采集方案设计的一些基本原则,为储层裂缝检测提供了野外设计方面的参考。     

Lithospheric studies based on array analysis of P-coda and microseisms

A simple coherence measure—the semblance coefficient—has been used to sample the lithosphere beneath the NORSAR array in terms of its ability to produce coherent seismic energy. The data set is comprised of two samples of teleseismic P-coda waves (south of Honshu and Hindu-Kush events) and two samples of microseismic noise data with duration of about one hour each.Concerning the constitution of the earth structure there are two possibilities for coherent energy to be present in noise-like wavefields. The first is tied to scattering effects and the second may be brought about by seismic emission effects. The emissive component represents the seismic response of the medium to variations in the background stress field which may arise from various sources: passing seismic waves (body or surface), free oscillations of the Earth, tides, an abnormal level of heat flow, or geodynamic processes in general.The presence of a well known source of seismic signal—Hunderfossen dam—in the vicinity of NORSAR area provided an opportunity to test the data processing algorithm. Results obtained for two independent microseismic data samples point to the same location in close vicinity to the dam.For P-coda data the signal energy from the upper mantle areas, as indicated by the maximum of semblance measure values at the depth around 116 km, reaches about 3.8% and 5% of the total P-coda energy for Honshu and Hindu-Kush events, respectively. For microseismic noise data this value is about 1% at a depth around 100 km. Comparison with the results of holographic studies supports the assumption that, in the case of microseismic data, the detected signal energy is of emissive nature, while for P-coda data we need more coda samples to make a sound judgement about the nature (scattering or emission) of the detected signals.     

Intrinsic versus extrinsic seismic anisotropy: Surface wave phase velocity inversion

The precise determination and interpretation of anisotropy are relatively difficult because the apparent anisotropy is usually a mixture of intrinsic and extrinsic anisotropy, which might partly hide the true properties of the medium investigated. The artificial anisotropy can be due to the fact that seismic waves do not ‘see’ the real details of a medium, but a ‘filtered’ (or ‘upscaled’) version of the Earth model. This can be due to a bad quality of the data coverage, to limited frequency band effects, or to errors in the approximate theory. With the limitation to layered Earth models, through comparisons of the results of the homogenization method with those of the periodic isotropic two-layered model as an analytical solution, we illustrate that the Backus theory for the long wavelength equivalent effect can be extended to calculate the extrinsic anisotropy, due to upscaling effects at discontinuities for the general isotropic layered model, when its spatial scale is much less than or equal to the seismic wavelength. We find that the extrinsic radial S-wave anisotropy produced by the vertical heterogeneities in the upper mantle of the Earth can be as large as 3% (about 30% extrinsic anisotropy of the 10% radial anisotropy). To better recover information from seismic data, we propose a surface wave phase velocity inversion method based on the first-order perturbation theory. We show that resolution at discontinuities can be improved by adding overtone modes of surface wave data. For more general layered models, the homogenization method could be considered, which can flexibly adapt the scale of the model to seismic wavelengths. However, the periodic isotropic two-layered model can also help to analytically quantify the amount of extrinsic radial, and possibly azimuthal anisotropy produced by the tilted fine layering.