横向各向同性地层井孔伪瑞利波、弯曲波、螺旋波的低频极限速度

在多极源声测井中, 低频弯曲波或螺旋波被广泛应用于测量地层的横波速度. 前人的研究已证明在各向同性地层中井孔伪瑞利波、弯曲波及螺旋波的低频极限速度都等于地层横波速度. 大量的数值计算结果似乎表明此结论在横向各向同性（TI）地层情况下也能成立, 但缺乏理论证明. 本文在井孔平行于TI弹性地层对称轴的模型下, 考察了井内声源激发的波在流-固边界上的反射和透射情况, 阐述了非泄漏模式导波产生的必要条件并讨论了其速度上限值. 我们发现在各向异性参数满足一定条件的TI地层中,导波速度的低频极限值小于沿井孔方向上的横波速度. 通过对井孔导波的频散分析以及对时域全波列的数值模拟, 论证了在这类地层中进行多极源声测井时,不可能根据弯曲波或螺旋波的到达时间准确获取地层的横波速度值.     

利用随钻正交偶极子声波测井评价地层各向异性的数值研究

在随钻测井条件下,由于钻铤占据了井孔内的大部分空间,充液井孔中沿着井轴方向传播的模式波的特性与电缆测井非常不同.本文建立了随钻正交偶极子测井声学模型,采用三维有限差分方法模拟了偶极子声源在随钻条件下各向异性地层井孔内激发的声场,研究了地层的声学各向异性在随钻正交偶极子声波测井中的响应特征.数值模拟结果表明,在随钻测井条件下,对于井轴同TI地层对称轴垂直的情况,弯曲波分裂现象仍然存在,通过正交偶极子测量方式和合适的反演算法能够准确有效地确定地层的快横波方位角,可以考虑采用同正演理论相结合的反演算法来获得地层的快、慢横波速度及声学各向异性信息;对于井轴同介质对称轴呈一定夹角的TI地层井孔,情况变得非常复杂,不同井斜倾角下弯曲波的速度的变化趋势并非同对应的地层横波速度的变化趋势完全一致,不过在一定的频段内,地层横波速度仍然是弯曲波的最主要控制因素.对于本文研究的模型,当井轴同介质对称轴的夹角大于大于60°时,此时获得的弯曲波的各向异性值基本能够反映对应角度下地层横波速度的各向异性信息.     

山东临沂地区砂土剪切波速与标准贯入击数关系统计分析

剪切波速(V_S)与标贯击数(N)之间存在相关关系,受地区土壤条件影响很大。对临沂地区场地实测得到的砂土的剪切波速和标贯击数之间的关系进行了统计分析,得到了砾砂、粗砂、中砂和细砂相应的关系曲线。结果发现受沉积环境的影响,砂土层粒径与埋深呈正相关,砂土粒径越大其密实段样本点数量越多。为消除不同密实程度段之间的相互影响,以密实度为划分标准进一步进行分区段统计分析,得到了不同密实程度的四类砂土的相关关系方程,通过实际钻孔数据对比了分段与不分段的统计分析结果,分段模拟能更好地反映两者之间的相关关系。将砂土根据密实度进行划分再给出剪切波速和标贯击数的回归关系可以提高分析结果的准确性,同时可以考虑土体特性的影响,更为科学。此研究为临沂地区提供了一种简便预估剪切波速的方法,对相关地区的工程建设和科学研究也具有参考价值。     

Surface and downhole shear wave seismic methods for thick soil site investigations

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.     

APPLICATION AND STATISTICAL ANALYSIS OF RELATIONSHIP BETWEEN SHEAR WAVE VELOCITY AND DEPTH OF SOIL-LAYERS

The shear wave velocity is one of the important parameters in seismic engineering.The common mathematical models of relationship between shear wave velocity and depth of soil-layers are linear function model,quadratic function model,power function model,cubic function model,and quartic function model.It is generally believed that the regression formulae based on aforementioned mathematical models are mainly used for preliminary estimation of the local shear wave velocity.In order to increase the value of test data of wave speed in boreholes,the calculation formulae for the thickness of ground cover layer are derived based on the aforementioned mathematical models and their fitting parameters.The calculation formulae for the mean shear wave velocity of soil-layers are derived by integral mean value theorem.Accordingly,the calculation formulae for the equivalent shear wave velocity of soil-layers are derived.The calculation formulae for the depth of reflective waves in time-depth conversion of the reflection seismic exploration are derived.Through the statistical analysis of test data of shear wave velocity of soil layers in Changyuan County,Henan Province,regression formulae and their fitting parameters of aforementioned mathematical models are obtained.The results show that in the determination of the quality of these regression formulae and their fitting parameters,the adjusted R-square,root mean square error and residual error,the matching on the statistical range between the geometry of function of mathematical models used and the scattergram of the measured data,the application purpose and the simplicity of the regression formulae should be considered.With the aforementioned new formulae,the results show that the calculated values of equivalent shear wave velocity of soil-layers and thickness of ground cover layer meet the engineering needs.The steps for statistics and applications of the relationship between shear wave velocity and depth of soil-layers for a new area are as follows:(1) Analyze the relevant data about the site such as the drilling and wave speed test data,etc.and divide the site into seismic engineering geological units;(2) In a single seismic engineering geological unit,make statistical analysis of the data of borehole wave speed test,comprehensively identify and select mathematical models and their fitting parameters of the relationship between shear wave velocity and depth of soil-layers;(3) Substitute the selected fitting parameters into the formulae,based on their mathematical models for the thickness of ground cover layer,or the equivalent shear wave velocity of soil-layers,or the depth of reflective wave,then the thickness of ground covering layer,equivalent shear wave velocity,and depth of reflective wave are obtained.     

Tube wave to shear wave conversion at borehole plugs

In hydraulic fracturing experiments, perforation shots excite body and tube waves that sample, and thus can be used to characterize, the surrounding medium. While these waves are routinely employed in borehole operations, their resolving power is limited by the experiment geometry, the signal‐to‐noise ratio, and their frequency content. It is therefore useful to look for additional, complementary signals that could increase this resolving power. Tube‐to‐body‐wave conversions (scattering of tube to compressional or shear waves at borehole discontinuities) are one such signal. These waves are not frequently considered in hydraulic fracture settings, yet they possess geometrical and spectral attributes that greatly complement the resolution afforded by body and tube waves alone. Here, we analyze data from the Jonah gas field (Wyoming, USA) to demonstrate that tube‐to‐shear‐wave conversions can be clearly observed in the context of hydraulic fracturing experiments. These waves are identified primarily on the vertical and radial components of geophones installed in monitoring wells surrounding a treatment well. They exhibit a significantly lower frequency content (10–100 Hz) than the primary compressional waves (100–1000 Hz). Tapping into such lower frequencies could help to better constrain velocity in the formation, thus allowing better estimates of fracture density, porosity and permeability. Moreover, the signals of tube‐to‐shear‐wave conversion observed in this particular study provide independent estimates of the shear wave velocity in the formation and of the tube wave velocity in the treatment well.     

裸眼井中弹性波传播的非对称模式的数值研究

本文从数值上研究了裸眼井中弹性波传播的非对称模式,给出了合成微地震图和导波（弯曲波）的频散曲线,发现在“硬”地层和“软”地层的井中,导波都是高度频散的,其最大相速度等于地层的横波速度,其截止频率低于对称模式的伪瑞利波的截止频率;在低频（2-3kHz）和长源距（3-4m）的条件下,由非对称的声源（如声偶极子）所产生的微地震图中,初至信号是以横波速度传播的,而以纵波速度传播的信号被抑制。本文的结果对研制横波速度测井仪是有意义的。     

Retrieval of local surface wave velocities from traffic noise – an example from the La Barge basin (Wyoming)

In regions where active source seismic exploration is constrained by limitations of energy penetration and recovery, cost and logistical concerns, or regulatory restrictions, analysis of natural source seismic data may provide an alternative. In this study, we investigate the feasibility of using locally‐generated seismic noise in the 2–6 Hz band to obtain a subsurface model via interferometric analysis. We apply this technique to three‐component data recorded during the La Barge Passive Seismic Experiment, a local deployment in south‐western Wyoming that recorded continuous seismic data between November 2008 and June 2009. We find traffic noise from a nearby state road to be the dominant source of surface waves recorded on the array and observe surface wave arrivals associated with this source up to distances of 5 kms. The orientation of the road with respect to the deployment ensures a large number of stationary points, leading to clear observations on both in‐line and cross‐line virtual source‐receiver pairs. This results in a large number of usable interferograms, which in turn enables the application of standard active source processing methods like signal processing, common offset stacking and traveltime inversion. We investigate the dependency of the interferograms on the amount of data, on a range of processing parameters and on the choice of the interferometry algorithm. The obtained interferograms exhibit a high signal‐to‐noise ratio on all three components. Rotation of the horizontal components to the radial/transverse direction facilitates the separation of Rayleigh and Love waves. Though the narrow frequency spectrum of the surface waves prevents the inversion for depth‐dependent shear‐wave velocities, we are able to map the arrival times of the surface waves to laterally varying group and phase velocities for both Rayleigh and Love waves. Our results correlate well with the known geological structure. We outline a scheme for obtaining localized surface wave velocities from local noise sources and show how the processing of passive data benefits from a combination with well‐established exploration seismology methods. We highlight the differences with interferometry applied to crustal scale data and conclude with recommendations for similar deployments.     

玉溪盆地浅层土层颗粒特征与剪切波速的关系研究

以玉溪盆地16个钻孔的柱状图和波速数据为基础,统计分析了具有不同颗粒特征的土层埋深与剪切波速之间的关系,给出了玉溪盆地内角砾、砾石、圆砾、砾砂、细砂和淤泥质黏土等土层在80 m深度范围内深度与剪切波速的经验关系.通过分析不同颗粒大小、不同磨圆程度土层剪切波速特征,认为土层的波速特征与其形成时所处的沉积环境有一定的关系,并以玉溪盆地浅层土层为例,给出了一种根据具有明显颗粒特征土层估算未测波速钻孔的等效剪切波速的方法.      

Analytical expressions for the borehole seismic signatures excited by an explosive source buried in an elastic half‐space

Borehole guided waves that are excited by explosive sources outside of the borehole are important for interpreting borehole seismic surveys and for rock property inversion workflows. Borehole seismograms are typically modelled using numerical methods of wave propagation. In order to benchmark such numerical algorithms and partially to interpret the results of modelling, an analytical methodology is presented here to compute synthetic seismograms. The specific setup is a wavefield emanating from a monopole point source embedded within a homogeneous elastic medium that interacts with a fluid‐filled borehole and a free surface. The methodology assumes that the wavelength of the seismic signal is much larger than the borehole radius. In this paper, it is supposed that there is no poroelastic coupling between the formation and the borehole. The total wavefield solution consists of P, PP, and PS body waves; the surface Rayleigh wave; and the low‐frequency guided Stoneley wave (often referred as the tube wave) within the borehole. In its turn, the tube wave consists of the partial responses generated by the incident P‐wave and the reflected PP and PS body waves at the borehole mouth and by the Rayleigh wave, as well as the Stoneley wave eigenmode. The Mach tube wave, which is a conic tube wave, additionally appears in the Mach cone in a slow formation with the tube‐wave velocity greater than the shear one. The conditions of appearance of the Mach wave in a slow formation are formulated. It is shown that the amplitude of the Mach tube wave strongly depends on Poisson's ratio of the slow surrounding formation. The amplitude of the Mach tube wave exponentially decreases when the source depth grows for weakly compressible elastic media with Poisson's ratio close to 0.5 (i.e., saturated clays and saturated clay soils). Asymptotic expressions are also provided to compute the wavefield amplitudes for different combinations of source depth and source‐well offset. These expressions allow an approximate solution of the wavefield to be computed much faster (within several seconds) than directly computing the implicit integrals arising from the analytical formulation.     

软地层横波速度的偶极随钻测井反演

在电缆测井中,可以利用偶极声场中的弯曲波反演软地层的横波速度.然而,在随钻声波测井(LWD)中,钢制钻铤的存在使得井孔结构变得复杂,同时改变了井孔声场,弯曲波也变得难以测定.此外,弯曲波与钻铤波耦合在一起,使得地层横波速度的反演变得困难.本文计算分析了随钻声场的频散曲线和激发曲线,注意到了偶极舒尔特波在较宽频带内速度频散很弱,特别是在本文研究的软地层情况下,偶极舒尔特波速度在3至25kHz的频率范围内几乎为一个常值,并且该值与地层横波速度存在一一对应关系.舒尔特波速度远小于其他模式波速度,与其他模式波在时域上易分离.相对于其他地层参数,舒尔特波对地层横波速度十分敏感.因此,它可以用来反演软地层的横波速度.     

水平界面上P-SV转换波转换点的精确解

转换波共转换点的叠加和道集选取都需要准确地计算转换点的位置.Tessmer和Behle、Taylor分别给出了水平单层介质中P SV转换波转换点坐标的解析解,由于其表达式的复杂性,在应用中几乎不被采用.在本文中,运用Snell定律重新建立了在水平反射界面上反射的P SV转换波的转换点坐标的四次方程,并严格地推导出与纵波速度、横波速度、炮检距和反射深度有关的转换点坐标的解析解,确定了惟一的解析表达式.将这一结果应用于P SV转换波的速度分析和叠加处理中.简化的公式有较好的应用价值.     

Crustal S-wave velocity structure of the Yellowstone region using a seismic ambient noise method

The Yellowstone volcano is one of the largest active volcanoes in the world, and its potential hazards demand detailed seismological and geodetic studies. Previous studies with travel time tomography and receiver functions have revealed a low-velocity layer in the crust beneath the Yellowstone volcano, suggesting the presence of a magma chamber at depth. We use ambient seismic noise from regional seismic stations to retrieve short-period surface waves and then study the shallow shear velocity structure of the Yellowstone region by surface wave dispersion analysis. We first obtained a crustal model of the area outside of the Yellowstone volcano and then constructed an absolute shear wave velocity structure in combination with receiver function results for the crust beneath the Yellowstone volcano. The velocity model shows a low-velocity layer with shear velocity at around 1.3 km/s, suggesting that a large-scale magma chamber exists at shallow levels within the crust of the Yellowstone volcanic region.     

CROSS-BOREHOLE OBSERVATION OF MODE CONVERSION FROM BOREHOLE STONELEY WAVES TO CHANNEL WAVES AT A COAL LAYER1

Conversion of borehole Stoneley waves to channel waves was observed in data from a seismic cross-borehole experiment conducted between wellbores penetrating a thin coal layer at 2022 m depth, near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves underwent partial conversion to channel waves at the coal layer. The channel waves were detected directly in an adjacent borehole 35 m away at receiver positions within the coal. Stoneley waves, subsequently produced by partial conversion of channel waves, were also detected at receiver positions located up to 50 m above and below the coal layer in the adjacent borehole. We infer the channel wave to be the first-higher Rayleigh mode by comparing the observed group velocity with theoretically derived dispersion curves. Identifying the conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing a transmitter or receiver in the coal itself.     

非对称圆弧片状声源在井旁地层中产生的声场

提出了一种可用于随钻三维反射声波成像测井的圆弧片状声源,推导了该声源声学性质在波数-频率域内的数学描述,并利用实轴积分的方法对该声源在井旁地层中产生的声场进行求解,考察了声源的线度、频率等因素对该声源向地层中辐射的声场的影响.研究结果表明,特定尺寸和特定频率下的圆弧片状声源产生的纵波场的水平和垂直指向性图中仅具有一个明显的主瓣,且主瓣三分贝角宽较窄,方位分辨率较高,指向性良好,适用于随钻三维反射成像测井;SV波场和SH波场的指向性图中存在两个或者多个角瓣,SV波场在垂直于井轴方向上辐射的能量为0,另外在井中发射和接收的反射SH波场和SV波场会相互干扰,所以难以利用SV波场和SH波场进行三维反射声波成像;声源的线度和频率等因素对其辐射声场影响较大,考虑到激励效率和方位分辨率等因素,对于本文描述的井孔模型,选择圆周角在75°和90°之间, 主频为12 kHz左右的声源是合适的.     

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated V_s and borehole measured V_s in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived V_s values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis.     

随钻声波测井FDTD模拟及钻铤波传播特性研究

幅度大的钻铤波掩盖地层信号是困扰随钻声波测井技术的难题,认识钻铤波特性对于消除或有效降低钻铤波至关重要.记录岩石动电效应引起电磁场的随钻动电测井,被认为有望彻底解决钻铤波干扰问题.本文采用有限差分法,模拟不同钻铤、不同井孔结构的随钻声场,对比分析了单极源钻铤波的传播特性,阐明存在伴随钻铤波动电信号的原因.结果表明:钻铤声波在沿钻铤传播时向外部介质辐射能量,透过井壁进入地层的具有视钻铤波速度的声波,与地层纵横波一样可发生动电转化,因而可导致随钻动电测井时产生钻铤波速度的电磁信号.计算还表明,高频情况下,随钻声波测井钻铤波呈现两阶模式:具有低频截止频率的高阶钻铤波幅度较小,其速度略低于钻铤纵波速度;无截止频率的低阶钻铤波幅度较大,其波速在高频时甚至低于钻铤横波速度.这种频散特性和多阶模式特性,是径向多分层开波导结构的导波属性.     

P‐wave stacking‐velocity tomography for VTI media

A major complication caused by anisotropy in velocity analysis and imaging is the uncertainty in estimating the vertical velocity and depth scale of the model from surface data. For laterally homogeneous VTI (transversely isotropic with a vertical symmetry axis) media above the target reflector, P‐wave moveout has to be combined with other information (e.g. borehole data or converted waves) to build velocity models for depth imaging. The presence of lateral heterogeneity in the overburden creates the dependence of P‐wave reflection data on all three relevant parameters (the vertical velocity V_P0 and the Thomsen coefficients ε and δ) and, therefore, may help to determine the depth scale of the velocity field. Here, we propose a tomographic algorithm designed to invert NMO ellipses (obtained from azimuthally varying stacking velocities) and zero‐offset traveltimes of P‐waves for the parameters of homogeneous VTI layers separated by either plane dipping or curved interfaces. For plane non‐intersecting layer boundaries, the interval parameters cannot be recovered from P‐wave moveout in a unique way. Nonetheless, if the reflectors have sufficiently different azimuths, a priori knowledge of any single interval parameter makes it possible to reconstruct the whole model in depth. For example, the parameter estimation becomes unique if the subsurface layer is known to be isotropic. In the case of 2D inversion on the dip line of co‐orientated reflectors, it is necessary to specify one parameter (e.g. the vertical velocity) per layer. Despite the higher complexity of models with curved interfaces, the increased angle coverage of reflected rays helps to resolve the trade‐offs between the medium parameters. Singular value decomposition (SVD) shows that in the presence of sufficient interface curvature all parameters needed for anisotropic depth processing can be obtained solely from conventional‐spread P‐wave moveout. By performing tests on noise‐contaminated data we demonstrate that the tomographic inversion procedure reconstructs both the interfaces and the VTI parameters with high accuracy. Both SVD analysis and moveout inversion are implemented using an efficient modelling technique based on the theory of NMO‐velocity surfaces generalized for wave propagation through curved interfaces.     

Signal reconstruction of surface waves on SASW measurement using Gaussian Derivative wavelet transform

Surface wave method consists of measurement and processing of the dispersive Rayleigh waves recorded from two or more vertical transducers. The dispersive phase data are inverted and the shear wave velocity versus depth is obtained. However, in case of residual soil, the reliable phase spectrum curve is difficult to be produced. Noises from nature and other human-made sources disturb the generated surface wave data. In this paper, a continuous wavelet transform based on mother wavelet of Gaussian Derivative was used to analyze seismic waves in different frequency and time. Time-frequency wavelet spectrum was employed to localize the interested seismic response spectrum of generated surface waves. It can also distinguish the fundamental mode of the surface wave from the higher modes of reflected body waves. The results presented in this paper showed that the wavelet analysis is able to determine reliable surface wave spectrum of sandy clayey residual soil.     

基于反卷积干涉法的土层地震反应特性研究

美国阿拉斯加安克雷奇德兰尼公园的土层地震反应观测台阵建有6个井下观测点和1个地表观测点, 每个观测点设置一个三分量加速度传感器, 该台阵建成后记录了多次地震事件. 本文通过反卷积地震干涉法对这些地震记录进行土层反应分析, 根据该分析结果提取土层结构的等效剪切波速和阻尼比. 所有层位的地震记录对地表记录的反卷积波形均反映出在土层中传播的上、 下行波, 其上行与下行至每一层的时间差别明显. 根据每一层的到时差, 确定土层中的等效剪切波速和分层等效剪切波速, 该结果与现场土层等效剪切波速测试结果吻合较好; 根据等效剪切波速计算出的土层卓越频率, 其一致性亦较好; 根据上、 下行波的峰值分析确定该场地土层的等效阻尼比, 也与软土的阻尼比经验值相当. 这些参数为后续的土层反应模拟和土结相互作用研究奠定了良好的数据基础. 结果表明, 反卷积干涉法能够用于土层反应分析和土层地震反应特性的提取.