P and S reflection and P refraction: An integration for characterising shallow subsurface

Characterization of shallow structures was performed by using different approaches analysing both P- and S-wave seismic data with different resolution. The refraction tomography provided P and S velocity models of the first 80 m, while the reflection seismic processing gives a reasonable stacking velocity field until 300 m depth for both P- and S-wave data. So, we estimated the V_p/V_s ratio and an empirical relationship between the two velocities. We characterised the shallow layers using tomographic velocity models and the deeper layers using seismic images with different resolution. The seismic images were obtained by conventional CMP reflection seismic processing and by a novel multi-refractor imaging technique.     

Ultrasonic elastic characteristics of five kinds of metamorphic deformed coals under room temperature and pressure conditions

The calibration of the elastic characteristics of deformed coals is essential for seismic inversion of such units, because the prediction of coal deformation is essential for both mining safety and methane production. Therefore, many samples of broken and mylonitic deformed coal were tested with ultrasonic waves in the laboratory. These samples came from four mining areas: the Huainan, Pingdingshan, Hebi and Jiaozuo coal mines, which present five different metamorphic ranks shown as cylinders striking across circular limits of steel. Under normal pressures and temperatures, ultrasonic P- and S-wave tests show that the velocities, quality factors, and elastic moduli of the deformed coals were greatly reduced compared with undeformed coals. Also, some correlation was found between the P- and S-wave velocities in the deformed coals. However, there is no evidence of linear correlations between velocity and density, velocity and quality factor, or the quality factors of P- and S-waves. Compared with the elastic characteristics of undeformed coals, such as P- and S-wave velocity ratios or Poisson’s ratio, those of deformed coals generally decrease and the P-wave quality factors are less than those of S-waves. Moreover, the analysis of the relationship between pore structure and elastic modulus shows a better correlation between the P- and S-wave velocities and effective porosity, pore volume and specific surface area. Also, there are similar relationships between the pore structure and the Young’s and shear moduli. However, there are no such correlations with other moduli. Correlations between these elastic moduli, pore structure, coal rank and density were not found for the various samples of deformed coals, which is consistent with only structural destruction occurring in the deformed coals with other physical properties remaining unchanged. The experimental results show that it is possible to predict the deformation of coals with multi-component seismic elastic inversion.     

Shear wave velocity prediction during CO<Subscript>2</Subscript>-EOR and sequestration in the Gao89 well block of the Shengli Oilfield

Shear-wave velocity is a key parameter for calibrating monitoring time-lapse 4D seismic data during CO₂-EOR (Enhanced Oil Recovery) and CO₂ sequestration. However, actual S-wave velocity data are lacking, especially in 4D data for CO₂ sequestration because wells are closed after the CO₂ injection and seismic monitoring is continued but no well log data are acquired. When CO₂ is injected into a reservoir, the pressure and saturation of the reservoirs change as well as the elastic parameters of the reservoir rocks. We propose a method to predict the S-wave velocity in reservoirs at different pressures and porosities based on the Hertz–Mindlin and Gassmann equations. Because the coordination number is unknown in the Hertz–Mindlin equation, we propose a new method to predict it. Thus, we use data at different CO₂ injection stages in the Gao89 well block, Shengli Oilfield. First, the sand and mud beds are separated based on the structural characteristics of the thin sand beds and then the S-wave velocity as a function of reservoir pressure and porosity is calculated. Finally, synthetic seismic seismograms are generated based on the predicted P- and S-wave velocities at different stages of CO₂ injection.     

Generation of a pseudo-2D shear-wave velocity section by inversion of a series of 1D dispersion curves

Multichannel Analysis of Surface Waves utilizes a multichannel recording system to estimate near-surface shear (S)-wave velocities from high-frequency Rayleigh waves. A pseudo-2D S-wave velocity (v_S) section is constructed by aligning 1D models at the midpoint of each receiver spread and using a spatial interpolation scheme. The horizontal resolution of the section is therefore most influenced by the receiver spread length and the source interval. The receiver spread length sets the theoretical lower limit and any v_S structure with its lateral dimension smaller than this length will not be properly resolved in the final v_S section. A source interval smaller than the spread length will not improve the horizontal resolution because spatial smearing has already been introduced by the receiver spread.In this paper, we first analyze the horizontal resolution of a pair of synthetic traces. Resolution analysis shows that (1) a pair of traces with a smaller receiver spacing achieves higher horizontal resolution of inverted S-wave velocities but results in a larger relative error; (2) the relative error of the phase velocity at a high frequency is smaller than at a low frequency; and (3) a relative error of the inverted S-wave velocity is affected by the signal-to-noise ratio of data. These results provide us with a guideline to balance the trade-off between receiver spacing (horizontal resolution) and accuracy of the inverted S-wave velocity. We then present a scheme to generate a pseudo-2D S-wave velocity section with high horizontal resolution using multichannel records by inverting high-frequency surface-wave dispersion curves calculated through cross-correlation combined with a phase-shift scanning method. This method chooses only a pair of consecutive traces within a shot gather to calculate a dispersion curve. We finally invert surface-wave dispersion curves of synthetic and real-world data. Inversion results of both synthetic and real-world data demonstrate that inverting high-frequency surface-wave dispersion curves – by a pair of traces through cross-correlation with phase-shift scanning method and with the damped least-square method and the singular-value decomposition technique – can feasibly achieve a reliable pseudo-2D S-wave velocity section with relatively high horizontal resolution.     

The influence of dry and water saturated cracks on seismic velocities of crustal rocks - A comparison of experimental data with theoretical model

The pressure dependence of P- and S-wave velocities, velocity anisotropy, shear wave splitting and crack-porosity has been investigated in a number of samples from different crustal rock types for dry and wet (water saturated) conditions. At atmospheric pressure, P-wave velocities of the saturated, low-porosity rocks (< 1%) are significantly higher than in dry rocks, whereas the differences for S-wave velocities are less pronounced. The effect of intercrystalline fluids on seismic properties at increased pressure conditions is particularly reflected by the variation of the Poisson's ratio because P-wave velocities are more sensitive to fluids than S-wave velocities in the low-porosity rocks. Based on the experimental data, the respective crack-density parameter (), which is a measure of the number of flat cracks per volume unit contained within the background medium (crack-free matrix), has been calculated for dry and saturated conditions. There is a good correlation between the calculated crack-densities and crack-porosities derived from the experimentally determined volumetric strain curves. The shear wave velocity data, along with the shear wave polarisation referred to a orthogonal reference system, have been used to derive the spatial orientation of effective oriented cracks within a foliated biotite gneiss. The experimental data are in reasonable agreement with the self consistent model of O'Connell and Budiansky (1974). Taking the various lithologies into account, it is clear from the present study, that combined seismic measurements ofV _p andV _s , using theV _p V _s -ratio, may give evidence for fluids on grain boundaries and, in addition, may provide an estimate on the in-situ crack-densities.     

测井资料Xu-White模型预测横波速度的一些新观点

目前横波预测的方法大致可以分为两种:经验公式预测和理论岩石物理模型。由于经验公式预测一般具有区域性,研究者更重视岩石物理模型预测。目前大多数岩石物理模型预测横波的方法假定地下流体的物性参数(速度和密度)不受地层深度的影响,且孔隙扁率是恒定的,实际上这并不科学。因为矿物的体积模量和剪切模量随所处地层深度发生改变,而对于孔隙扁率则随颗粒形状、孔隙度等的变化有较大变化。针对这些情况,提出一种新的改进的Xu-White横波预测方法,并可取得较好的效果。     

利用SPAC法估算地壳S波速度结构

S波速度结构能够反映地球介质的物性差异,是地壳内低速区结构特征判别的重要依据.本文尝试利用空间自相关法(SPAC法)从地震台站微动信号的垂直分量中提取瑞利波相速度频散曲线,通过对频散曲线的反演获得地下介质的S波速度结构.以国家数字测震台网8个宽频带地震台站的实测微动数据为例,采用SPAC方法获得了首都圈地区北京附近约30 km 深度范围内的一维S波速度结构.结果表明,该区结晶基底埋深较浅约2 km;分别在5~8 km 和12~16 km 深处发育S波低速层;8 km 和 20 km 处是S波速度差异较大的速度分界面.这一结果与以往地震学及人工地震探测结果较为吻合,表明SPAC法估算地壳S波速度结构是可行、有效的.     

基于变形P-L模型的矩阵方程迭代精细横波预测

横波速度预测问题的关键有两个,一是如何建立合理的岩石物理模型,二是针对建立的横波预测目标函数,如何准确高效地求解.针对第一个问题,对Pride模型和Lee模型(P-L模型)进行变形,提出拟固结指数的概念,将干岩石模量和岩石基质模量相联系,变形后的P-L模型在没有降低P-L模型准确度的情况下简化了问题的复杂度,建立起了饱和流体岩石弹性模量与干岩石模量、岩石基质模量、混合流体模量之间的关系,进而计算理论上的纵波速度,并通过比较实测纵波速度与计算的理论纵波速度大小,最终建立了横波预测的目标函数.针对第二个问题,借鉴了地震反演的思路,将该目标函数的最优化问题转化为线性矩阵方程组迭代求解问题,通过几步迭代就可以求解出合适的拟固结指数,进而得到预测横波速度.实际验证和应用表明,该横波预测方法具有很好的稳定性和准确性,并且岩石物理模型的构建和目标函数的求解思路可用于其他储集类型地层的横波预测.     

基于散度和旋度纵横波分离方法的改进

纵、横波的分离是多波多分量地震资料处理中很重要的一步,其分离结果直接影响到后续数据处理的质量.各向同性介质中纵波为无旋场,横波为无散场,因此可以在频率-波数域利用散度和旋度算子对地震记录进行纵、横波分离,但是此处理过程必须知道地表处的纵、横波速度.本文给出了一种估算地表纵、横波速度的方法,可以在纵、横波速度值未知的情况下,将其估算出来.针对弹性波场进行散度和旋度运算时,纵、横波的相位和振幅比发生改变的问题,本文给出了相位和纵、横波振幅比的校正方法.     

玛沁-靖边剖面S波资料研究与探讨

对玛沁——靖边剖面深地震测深S波资料处理解释，获得本区S波二维地壳速度结构和波速比结构．结果表明，该区S波二维速度结构和波速比结构，沿剖面存在着明显的差异．剖面西段和海原地区下方呈现S波速度偏低，而波速比偏高的结构特征；剖面中段和东段的S波速度和波速比正常．根据S波速度结构和波速比结构的横向变化特征，讨论了两大异常区岩性的变化．推测海原大地震孕育发生不仅与构造活动有关，而且与该区的岩石性质有关．      

T-phase Observations in Northern California: Acoustic to Seismic Coupling at a Weakly Elastic Boundary

—?Plans for a hydroacoustic network intended to monitor compliance with the CTBT call for the inclusion of five T-phase stations situated at optimal locations for the detection of seismic phases converted from ocean-borne T phases. We examine factors affecting the sensitivity of land-based stations to the seismic T phase. The acoustic to seismic coupling phenomenon is described by upslope propagation of an acoustic ray impinging at a sloping elastic wedge. We examine acoustic to seismic coupling characteristics for two cases; the first in which the shear velocity of the bottom is greater than the compressional velocity of the fluid (i.e., v _p > v _s > v _w ), the second is a weakly elastic solid in which v _s << v _w < v _p . The former is representative of velocities in solid rock, which might be encountered at volcanic islands; the latter is representative of marine sediments. For the case where v _s > v _w , we show that acoustic energy couples primarily to shear wave energy, except at very high slope angles. We show that the weakly elastic solid (i.e., v _s << v _w ) behaves nearly like a fluid bottom, with acoustic energy coupling to both P and S waves even at low slope angles.¶We examine converted T-wave arrivals at northern California seismic stations for two event clusters; one a series of earthquakes near the Hawaiian Islands, the other a series of nuclear tests conducted near the Tuamoto archipelago. Each cluster yielded characteristic arrivals at each station which were consistent from event to event within a cluster, but differed between clusters. The seismic T-phases consisted of both P- and S-wave arrivals, consistent with the conversion of acoustic to seismic energy at a gently sloping sediment-covered seafloor. In general, the amplitudes of the seismic T phases were highest for stations nearest the continental slope, where seafloor slopes are greatest, however noise levels decrease rapidly with increasing distance from the coastline, so that T-wave arrivals were observable at distances reaching several hundred kilometers from the coast. Signal-to-noise levels at the seismic stations are lower over the entire frequency spectrum than at the Pt. Sur hydrophone nearby, and decrease more rapidly with increasing frequency, particularly for stations furthest from the continental slope.     

三维地震与OBS联合勘探揭示的神狐海域含水合物地层声波速度特征

以三维高分辨地震与海底高频地震仪（OBS）联合勘探数据为基础,获得海底之下沉积层的地震反射成像剖面及多波信息,并以此确定研究区含天然气水合物沉积层的纵、横波速度的变化特征.根据走时反演获得的横波速度与纵波速度对比分析发现,研究区海底之下500 m深度范围内的某些沉积层具有较高的纵横波速度,这一纵波速度升高区域与水合物稳定带对应,而纵波速度下降并且横波速度变化较小的区域,可能与游离气的存在相关.游离气的可能存在与基于这一区域2007年钻探测井结果的普遍认识不完全相符.     

Ground motion modeling for site effects at L’Aquila and middle Aterno river valley (central Italy) for the MW 6.3, 2009 earthquake

Ground motion for the 6 April 2009 (M_w 6.3) earthquake is computed along 2-D cross-sections at L’Aquila by a hybrid method (modal summation plus finite differences) and validated with recordings at AQU, AQK, AQG, AQA and AQV stations. Parametric studies of S-wave velocities of the shallowest lithotypes allow to get a general agreement between synthetic and observed response spectra, despite the scaled point-source approach and the lack of detailed geological and seismic studies. It results that the megabreccia covering on lacustrine soils, characterizing the historical center of L’Aquila, is responsible of spectral amplifications along the vertical (2–7) and horizontal components (2–3) at a wide frequency range (0.6–7 Hz). The covering of alluvial soils in the middle Aterno river valley is responsible of amplifications at 2–7 Hz both in the horizontal and vertical planes of the motion. Such amplifications evidence that site effects might have been responsible of structural damages.     

Structural heterogeneity of an ultra-low-velocity zone beneath the Philippine Islands: Implications for core–mantle chemical interactions induced by massive partial melting at the bottom of the mantle

An ScP phase reflected and converted at the core–mantle boundary (CMB) beneath the region east of the Philippine Islands shows clear pre- and postcursors, recorded on short-period seismic networks in Japan. These waveform variations can be explained by interaction of the ScP wavefield with thin layers at the CMB. The results of forward modeling of double-array stacks reveal two different structural heterogeneities in the lowermost mantle beneath the region east of the Philippine Islands. One of the structures represents a decreased velocity, and increased density across the reflector at the lowermost ～10 km of the mantle, with P- and S-wave velocity reductions of 5–10% and ～30%, respectively, and an increase in density of 5–10%. Another structure consists of a pair of reflectors at ～10 km and ～5 km above the CMB, both of which are characterized by reduced P- and S-wave velocities. The upper reflector is the interface of a low-velocity zone in which P- and S-wave velocities decrease of 10% and 30%, respectively, accompanied by an extremely large increase in density (20–25%). The lower reflector is characterized by a 25% reduction in S-wave velocity relative to the above low-velocity layer, as well as a 5% decrease in P-wave velocity and no change in density. The nature of the low-velocity zone detected locally at the CMB is comparable with that of ultra-low-velocity zones (ULVZs) observed by various seismic probes in the South Pacific and Central America. Extensive observations of the ULVZ beneath the region east of the Philippine Islands indicate massive partial melting at the bottom of the mantle. Low-S-velocity basal layer partly detected within the ULVZ may be resulting from core–mantle chemical interactions, driven by massive partial melting.     

Inverting full waveforms into 1D seismic velocity model of the upper crust by neighborhood algorithm - Corinth Gulf,Greece

We investigated inversion of full waveforms into formal 1D velocity models. ‘Formal’ means that the models are primarily intended to simulate complete seismograms close to real records, rather than to reflect the true crustal structure from the geological point of view. The method is demonstrated for a magnitude M_w 5.3 earthquake (centroid depth of 4.5 km), recorded at 8 three-component stations in the Corinth Gulf region, Greece, spanning the epicentral distance range from 15 to 102 km, and frequency range from 0.05 to 0.2 Hz. The forward problem was solved by the discrete wavenumber method, while the inversion was performed with the neighborhood algorithm. As such, not only the best-fit models, but also suites of the models almost equally well satisfying data were obtained. The best resolution was found in the topmost ～10 km. Extensive testing of the model parametrization enabled identification of the most robust features of the solution. The P- and S-wave velocities are characterized by a strong increase with depth in the topmost ～4–5 km. This part of the model can be approximated by a layer with constant velocity gradient. Compared to a previously existing model of the region, the satisfactory waveform match was extended from the maximum frequency of 0.1 Hz up to 0.2 Hz. This extension will improve calculation of the seismic source parameters in the region, e.g. determination of source time functions and slip distributions of potential future M_w > 6 events.     

Explicit use of the Biot coefficient in predicting shear-wave velocity of water-saturated sediments

Predicting the shear‐wave (S‐wave) velocity is important in seismic modelling, amplitude analysis with offset, and other exploration and engineering applications. Under the low‐frequency approximation, the classical Biot–Gassmann theory relates the Biot coefficient to the bulk modulus of water‐saturated sediments. If the Biot coefficient under in situ conditions can be estimated, the shear modulus or the S‐wave velocity can be calculated. The Biot coefficient derived from the compressional‐wave (P‐wave) velocity of water‐saturated sediments often differs from and is less than that estimated from the S‐wave velocity, owing to the interactions between the pore fluid and the grain contacts. By correcting the Biot coefficients derived from P‐wave velocities of water‐saturated sediments measured at various differential pressures, an accurate method of predicting S‐wave velocities is proposed. Numerical results indicate that the predicted S‐wave velocities for consolidated and unconsolidated sediments agree well with measured velocities.     

The P and S wave velocity structures of the crust and upper mantle beneath Tibetan Plateau

Using the P-and S-wave arrivals from the 150 earthquakes distributed in Tibetan Plateau and its neighboring areas, recorded by Tibetan seismic network, Sichuan seismic network, WWSSN and the mobile network situated in Tibetan Plateau, we have obtained the average P-and S-wave velocity models of the crust and upper mantle for this region:

Study of crustal structure with S—wave data from Maqen—Jingbian profile

2-D crustal velocity structure and vp/vs are obtained by processing and interpretation of S-wave data from Maqen-Jingbian deep seismic sounding(DSS)profile.The result shows that there exist obvious differences in 2-D S-wave velocity structure and vp/vs ratio structure along the profile.The S-wave velocities are low and vp/vs ration is high for the westem section of the profile and Haiyuan region,while they are normal for the middle and eastern sections.The changes in lithologic characters of two major anomalous zones are discussed according to lateral variation of S-wave velocity structure and vp/vs ratio structure.It is concluded that the development and occurrence of the Haiyuan strong earthquake is not only related to tectonic activities,but also to lithologic characters of the region.     

WEIGHTED STACKING FOR ROCK PROPERTY ESTIMATION AND DETECTION OF GAS*

Amplitude versus offset concepts can be used to generate weighted stacking schemes (here called geo-stack) which can be used in an otherwise standard seismic data processing sequence to display information about rock properties. The Zoeppritz equations can be simplified and several different approximations appear in the literature. They describe the variation of P-wave reflection coefficients with the angle of incidence of a P-wave as a function of the P-wave velocities, the S-wave velocities and the densities above and below an interface. Using a smooth, representative interval velocity model (from boreholes or velocity analyses) and assuming no dip, the angle of incidence can be found as a function of time and offset by iterative ray tracing. In particular, the angle of incidence can be computed for each sample in a normal moveout corrected CMP gather. The approximated Zoeppritz equation can then be fitted to the amplitudes of all the traces at each time sample of the gather, and certain rock properties can be estimated. The estimation of the rock properties is achieved by the application of time- and offset-variant weights to the data samples before stacking. The properties which can be displayed by geo-stack are: P-wave reflectivity (or true zero-offset reflectivity), S-wave reflectivity, and the reflectivity of P-wave velocity divided by S-wave velocity (or ‘pseudo-Poisson's ratio reflectivity’). If assumptions are made about the relation between P-wave velocity and S-wave velocity for water-bearing clastic silicate rocks, then it is possible to create a display which highlights the presence of gas.     

基于Love波相速度反演南北地震带地壳上地幔结构

收集了南北地震带区域地震台网中292个地震台站2008年1月至2011年3月期间的地震波形数据,由频时分析方法提取了Love波相速度频散曲线,经过反演得到了研究区内的Love波相速度分布.根据Love波纯路径频散,采用线性反演方法对0.25°×0.25°的网格点进行了一维S波速度结构反演,利用线性插值获取了南北地震带地区的三维S波速度结构.结果显示了松潘—甘孜地体和川滇菱形块体地区的下地壳具有明显的S波低速层分布,该异常分布特征支持解释青藏高原隆升及其地壳物质运移的下地壳流模型.在100至120km深度上,川滇菱形块体西北部呈现较强的S波高速异常,这可能是印度岩石圈板块沿喜马拉雅东构造结下插至该区域所致,该区域下地壳的低速软弱物质与上地幔的高速强硬物质形成了鲜明对比,暗示了地壳和上地幔可能具有不同的构造运动和变形方式,这为该区域的壳幔动力学解耦提供了条件.