The phase relations between the first arrivals of compressional and shear waves, and their signification to full wave logging

This paper describes the three phenomena observed in full wave experiments： 1. the amplitudes （absolute value） of the first compressional and shear arrivals have the same variation rules; 2. the phases of the first compressional and shear arrivals are always opposite to each other; and 3. the amplitude variation periods of the first compressional and shear arrivals are 2π. A full analysis and interpretation points out that these phenomena should appear under the full wave logging condition. Hence,the basis of using phase diffrences to extract useful information from the full wave is found.     

Mapping crustal S—wave velocity structure with SV—component receiver function method

In this article,we analyze the characters of SV-component receiver function of teleseismic body waves and its advantages in mapping the S-wave velocity structure of crust in detail.Similar to radial receiver function,SV-component receiver function can be obtained by directly deconvolving the P-component from the SV-component of teleseismic recordings.Our analyses indicate that the change of amplitude of SV-component receiver function against the change of epicentral distance is less than that of radial receiver function.Moreover,the waveform of SV-component receiver function is simpler than the radial receiver function and gives prominence to the PS converted phases that are the most sensitive to the shear wave velocity structure in the inversion.The synthetic tests show that the convergence of SV-component receiver function inversion is faster than tnat of the radial receiver function inversion.As an example,we investigate the S-wave velocity structure beneath HIA sta-tion by using the SV-component receiver function inversion method.     

中国大陆下扬子及邻区地壳结构基本特征：深地震测深研究综述

The Deep Seismic Sounding( DSS) projects carried out from the 1970 s in the lower Yangtze region and its neighboring area were reviewed in this paper,then the basic wave group features of those wide angle reflection / refraction record sections,and of the crustal structure are summarized. It shows that there were in total five clear wave groups on the record sections,which include the first arrival Pg,the reflection P1 from the bottom interface of the upper crust,the reflection P3 from the bottom interface of the middle crust,the strong reflection Pm from the Moho boundary,and the refraction Pn from uppermost mantle. In general,these phases are easily consistently traced and compared,despite some first arrivals being delayed or arriving earlier than normal due to the shallow sedimentary cover or bedrocks. In particular,in the Dabie Mountain region the seismic events of a few gathered shots always have weak reflection energy,are twisted,or exhibit disorganized waveforms, which could be attributed to the disruption variations of reflection depth,the broken Moho,and the discontinuity of the reflection boundary within crust. The regional crustal structures are composed of the upper,middle and lower crust,of which the middle and lower layers can be divided into two weak reflection ones. The crustal thickness of the North China and Yangtze platform are 30km- 36 km,and the Moho exhibits a flat geometry despite some local uplifts. The average pressure velocity in lower crust beneath this two tectonic area is 6. 7 ± 0. 3km / s. Nevertheless,beneath the Dabieshan area the crustal thickness is 32km- 41 km,the Moho bends down sharply andtakes an abrupt 4km- 7km dislocation in the vertical direction. The average pressure velocity in the lower crust beneath the Dabieshan area is 6. 8 ± 0. 2km / s.     

Study on Shear Wave Velocity Structure and Velocity Ratio Beneath Ordos Block and Its Eastern and Southern Margins

Using pure S wave fitting method, we studied the shear wave velocity structures under the Ordos block and its eastern and southern marginal areas. The results show that the velocity structure beneath Yulin station in the interior of Ordos block is relatively stable, where no apparent change between high and low velocity layers exists and the shear wave velocity increases steadily with the depth. There is a 12km thick layer at the depth of 25km under this station, with an S wave velocity (Vs=3.90km/s) lower than that at the same depth in its eastern and southern areas (Vs≥4.00km/s). The crust under the eastern margin of Ordos block is thicker than that of the Yulin station, and the velocity structures alternate between the high and low velocity layers, with more low velocity layers. It has the same characteristic as having a 10km-thick low velocity layer (Vs=3.80km/s) in the lower crust but buried at a depth of aout 35km. Moreover, we studied the Vp/Vs ratio under each station in combination with the result of P wave velocity inversion. The results show that, the average velocity ratio of the Yulin station at the interior of Ordos block is only 1.68, with a very low ratio (about 1.60) in the upper crust and a stable ratio of about 1.73 in the mid and lower crust, which indicates the media under this station is homogenous and stable, being in a state of rigidity. But at the stations in the eastern and southern margins of the Ordos block, several layers of high velocity ratio (about 1.80) have been found, in which the average velocity ratio under Kelan and Lishi stations at the eastern margin is systemically higher than that of the general elastical body waves (1.732). This reflects that the crust under the marginal areas is more active relatively, and other materials may exist in these layers. Finally, we discussed the relationship among earthquakes, velocity structures beneath stations and faults.     

Feasibility Study on Determining Focal Mechanism Solutions of Small Earthquakes Using the Velocity Amplitude Ratio of -and -Waves

The focal mechanism parameters of small earthquakes are determined by the maximum velocity and displacement amplitude ratio of the direct ^-P- and ^-S-waves recorded by digital stations. The displacement is obtained from the velocity by emulation, and the two results are compared and analyzed. Results of theoretical analysis and practical measurement indicate that the two results of velocity and displacement are consistent, and it is feasible that the maximum displacement amplitude ratio be replaced by the maximum velocity amplitude ratio of the direct ^-P-and ^-S- waves recorded by regional seismic networks when determining focal mechanism solutions of small earthquakes.     

新疆巴楚和塔什库尔干地震台下方地壳结构研究

Using the teleseismic waveform data recorded by the seismic station Bachu( hereafter referred to as station BCH) i n the Tarim Basin and the seismic station Taxkorgan(h ereafter referred to as station TAG) i n the west Kunlun Mountains for years,we applied the receiver function H-κ stacking method to study the crustal structure beneath stations BCH and TAG. The results showed that there are obvious differences in the crustal thickness beneath stations BCH and TAG,and the regional crustal thickness and terrain have a very good corresponding relationship. There are high crustal average V P/ V S values beneath the two stations. The crustal thickness is 44 km,and the crustal average wave velocity ratio is 1. 849 beneath station BCH. There is a sharp discontinuity in the middle of the crust beneath station BCH at a depth of 21 km. There is a low average P wave velocity and low V P/ V S from the surface to the discontinuity beneath station BCH.The depth of the discontinuity is consistent with the lower interface of the focal depth from accurate location in the Jiashi earthquake source area adjacent to station BCH; and may be the crustal brittle-ductile conversion boundary. The crustal thickness is 69 km,and the crustal average wave velocity ratio is 1. 847 beneath station TAG,a thicker crust and high V P/ V S may indicate that materials in the lower crustal are prone to plastic flow,which is responsible for the thickening of the crust.     

地球物理技术在天然气水合物预测中的应用

Based on the sensitivity of geophysical response to gas hydrates contained in sediments, we studied the prediction of gas hydrates with seismic techniques, including seismic attributes analysis, AVO, inverted velocity field construction for dipping formations, and pseudo-well constrained impedance inversion. We used an optimal integration of geophysical techniques results in a set of reliable and effective workflows to predict gas hydrates. The results show that the integrated analysis of the combination of reflectivity amplitude, instantaneous phase, interval velocity, relative impedance, absolute impedance, and AVO intercept is a valid combination of techniques for identifying the BSR （Bottom Simulated Reflector） from the lower boundary of the gas hydrates. Integration of seismic sections, relative and absolute impedance sections, and interval velocity sections can improve the validity of gas hydrates determination. The combination of instantaneous frequency, energy half attenuation time, interval velocity, AVO intercept, AVO product, and AVO fluid factor accurately locates the escaped gas beneath the BSR. With these conclusions, the combined techniques have been used to successfully predict the gas hydrates in the Dongsha Sea area.     

Mutation Anomalies of the Wave Velocity Ratio at a Single Station Before the Lancang-Gengma Earthquake

In this paper,the process of dynamic variation of the wave velocity ratio that occurred at a single station,such as Lancang,Tengchong,and Eryuan stations,8 years before the Lancang-Gengma earthquakes(Ms=7.6 and 7.2),is studied by applying the synthetic method as a whole.It is found that the moderately strong and strong earthquakes 250 km away that occurred since 1983 may lead to mutation anomalies of the wave velocity ratio in the Lancang region.The mutation amplitude is increased with the approaching time and distance of a large event.The reliability of mutation data for the wave velocity ratio has been demonstrated in many ways.The application of mutation of the wave velocity ratio to earthquake prediction and its physical explanation are discussed.     

General velocity formula of boundary layer above mobile sediment bed induced by asymmetric waves

Sediment movement in the wave boundary layer above a mobile sediment bed is complex.A velocity formula for the boundary layer is proposed for sheet flow induced by asymmetric waves above a mobile sediment bed.The formula consists of a free stream velocity and a defect function which contains a phase-lead,boundary layer thickness and mobile sediment bed.Phase-lag of sediment movement is considered in the formula for the mobile sediment bed.The formula needs six dependent variables about asymmetric wave and sediment characteristics.Asymmetry effects on parameters(orbital amplitude,roughness height,bed shear stress,and boundary layer thickness)are properly considered such that the formula can yield velocity differences among onshore,offshore,acceleration,and deceleration stages.The formula estimates the net boundary layer velocity resulting from the mobile sediment bed and asymmetric boundary layer thickness.In addition,a non-constant phase-lead also contributes to the net boundary layer velocity in asymmetric oscillatory sheet flow.Results of the formula are as good as that of a two-phase numerical model.Sheet flow transport induced by asymmetric waves,and the offshore net sediment transport rate with a large phase-lag under velocity-skewed waves,can be adequately estimated by the formula with a power sediment concentration function.     

A physical model study of effect of fracture aperture on seismic wave

Based on Hudson’s theoretical hypothesis of equivalent fracture model,inserting aligned round chips in solid model can simulate fractured media. The effect of fractures on the propagation of P and S waves can be observed by changing the fracture thickness. The base model is made of epoxy resin,and the material of fractures is a kind of low-velocity mixture containing silicon rubber. With constant diameter and number of fractures in each model,one group of models can be formed through changing the thickness of fracture. These models have the same fracture density. By using the ultrasonic pulse transmission method,the experiment records time and waveform of P and S waves in the direction parallel and perpendicular to the fracture orientation. The result shows that,with the same fracture density,changing fracture aperture will affect both velocity and amplitude of P and S waves,and the effect on P-wave amplitude is much greater than that on the velocity. Moreover,the variation in velocity of S wave is more obvious in the slow shear wave (S2),while the variation in amplitude is more obvious in the fast shear wave (S1). These properties of wave propagation are useful for seismic data processing and interpretation.     

地震转换波对叠前同时反演的影响研究

叠前同时反演是油气探测的一种有效工具.其理论基础是平面P波Zoeppritz方程计算的反射系数的近似,是入射角的函数.叠前同时反演可以利用三项或两项Fatti方程进行反演分析.本文针对实际油田的测井数据,利用反射率法模拟了仅包含P波一次反射记录,包含P波一次反射和P波层间多次波记录以及全波场地震记录,再利用叠前同时反演对合成地震记录进行反演研究.研究结果表明,在大偏移距处P波主要反射受到其它模式波的污染,从而影响了叠前同时反演结果的精度.对于薄互层介质当转换波影响严重时,使用小角度数据的两项AVO反演比使用大角度数据的三项AVO反演更合理可靠.     

P－SV转换波的一种处理方法─叠前抽道集法

本文利用纵波和横波的迭加速度资料来计算Ｐ－ＳＶ波勘探中转换点的近似位置。再按转换点对Ｐ－ＳＶ波重新抽道集，得到可与反射资料ＣＭＰ道集相兼容的时间剖面图。利用理论模型对上述方法进行了可靠性检验，并用该方法对实际资料作了处理。同时还得到了实际资料的Ｐ波和Ｐ－ＳＶ波的彩色振幅比剖面图。结果表明：采用改进的叠前抽道集法所得到的剖面图具有很好的空间分辨率。     

AVO investigations of shallow marine sediments

Amplitude‐variation‐with‐offset (AVO) analysis is based on the Zoeppritz equations, which enable the computation of reflection and transmission coefficients as a function of offset or angle of incidence. High‐frequency (up to 700 Hz) AVO studies, presented here, have been used to determine the physical properties of sediments in a shallow marine environment (20 m water depth). The properties that can be constrained are P‐ and S‐wave velocities, bulk density and acoustic attenuation. The use of higher frequencies requires special analysis including careful geometry and source and receiver directivity corrections. In the past, marine sediments have been modelled as elastic materials. However, viscoelastic models which include absorption are more realistic. At angles of incidence greater than 40°, AVO functions derived from viscoelastic models differ from those with purely elastic properties in the absence of a critical angle of incidence. The influence of S‐wave velocity on the reflection coefficient is small (especially for low S‐wave velocities encountered at the sea‐floor). Thus, it is difficult to extract the S‐wave parameter from AVO trends. On the other hand, P‐wave velocity and density show a considerably stronger effect. Attenuation (described by the quality factor Q) influences the reflection coefficient but could not be determined uniquely from the AVO functions. In order to measure the reflection coefficient in a seismogram, the amplitudes of the direct wave and the sea‐floor reflection in a common‐midpoint (CMP) gather are determined and corrected for spherical divergence as well as source and streamer directivity. At CMP locations showing the different AVO characteristics of a mud and a boulder clay, the sediment physical properties are determined by using a sequential‐quadratic‐programming (SQP) inversion technique. The inverted sediment physical properties for the mud are: P‐wave velocity α=1450±25 m/s, S‐wave velocity β=90±35 m/s, density ρ=1220±45 kg/m³, quality factor for P‐wave Q_P=15±200, quality factor for S‐wave Q_S=10±30. The inverted sediment physical properties for the boulder clay are: α=1620±45 m/s,β=360±200 m/s,ρ=1380±85 kg/m³,Q_P=790±660,Q_S=25±10.     

两种不同势函数下半空间饱和多孔介质中Rayleigh波求解比较

分别对"考虑两种压缩波和幅值比例系数"和"考虑一种压缩波(P1或P2波)但不考虑幅值比例系数"两种不同势函数下的半空间饱和多孔介质中Rayleigh波求解进行详细推导,理论分析表明"考虑两种压缩波和幅值比例系数"下Rayleigh波求解推导更为严密,与饱和多孔介质中存在两种压缩波的事实相一致。在研究半空间饱和多孔介质中Rayleigh波时应采用"考虑两种压缩波和幅值比例系数"的势函数。     

Analytical wavefront curvature correction to plane‐wave reflection coefficients for a weak‐contrast interface

Most amplitude versus offset (AVO) analysis and inversion techniques are based on the Zoeppritz equations for plane‐wave reflection coefficients or their approximations. Real seismic surveys use localized sources that produce spherical waves, rather than plane waves. In the far‐field, the AVO response for a spherical wave reflected from a plane interface can be well approximated by a plane‐wave response. However this approximation breaks down in the vicinity of the critical angle. Conventional AVO analysis ignores this problem and always utilizes the plane‐wave response. This approach is sufficiently accurate as long as the angles of incidence are much smaller than the critical angle. Such moderate angles are more than sufficient for the standard estimation of the AVO intercept and gradient. However, when independent estimation of the formation density is required, it may be important to use large incidence angles close to the critical angle, where spherical wave effects become important. For the amplitude of a spherical wave reflected from a plane fluid‐fluid interface, an analytical approximation is known, which provides a correction to the plane‐wave reflection coefficients for all angles. For the amplitude of a spherical wave reflected from a solid/solid interface, we propose a formula that combines this analytical approximation with the linearized plane‐wave AVO equation. The proposed approximation shows reasonable agreement with numerical simulations for a range of frequencies. Using this solution, we constructed a two‐layer three‐parameter least‐squares inversion algorithm. Application of this algorithm to synthetic data for a single plane interface shows an improvement compared to the use of plane‐wave reflection coefficients.     

Research and application of spectral inversion technique in frequency domain to improve resolution of converted PS-wave

Multi-wave exploration is an effective means for improving precision in the exploration and development of complex oil and gas reservoirs that are dense and have low permeability. However, converted wave data is characterized by a low signal-to-noise ratio and low resolution, because the conventional deconvolution technology is easily affected by the frequency range limits, and there is limited scope for improving its resolution. The spectral inversion techniques is used to identify λ/8 thin layers and its breakthrough regarding band range limits has greatly improved the seismic resolution. The difficulty associated with this technology is how to use the stable inversion algorithm to obtain a high-precision reflection coefficient, and then to use this reflection coefficient to reconstruct broadband data for processing. In this paper, we focus on how to improve the vertical resolution of the converted PS-wave for multi-wave data processing. Based on previous research, we propose a least squares inversion algorithm with a total variation constraint, in which we uses the total variance as a priori information to solve under-determined problems, thereby improving the accuracy and stability of the inversion. Here, we simulate the Gaussian fitting amplitude spectrum to obtain broadband wavelet data, which we then process to obtain a higher resolution converted wave. We successfully apply the proposed inversion technology in the processing of high-resolution data from the Penglai region to obtain higher resolution converted wave data, which we then verify in a theoretical test. Improving the resolution of converted PS-wave data will provide more accurate data for subsequent velocity inversion and the extraction of reservoir reflection information.     

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.     

岩石的纵、横波速度与密度的规律研究

选择了济阳坳陷第三系地层四类不同岩性、237块典型岩心样品,在变温变压条件下进行超声波纵、横波速度及密度的测定,样品的孔隙度范围从2％到27％.根据变温变压测试数据,得出纵横波速度的压力、温度校正公式,将高温高压条件下测量的弹性参数校正到60 MPa条件,再进行密度、Vp和Vs数据分析.分析结果表明：Vp、Vs具有很好的线性相关性,得到新的密度与Vp关系式.同时将Vp、Vs关系融入到Gardner公式中,将Gardner函数推广为〖JP2〗ρ=CVApVBs的函数形式（称为广义Gardner公式）,拟合出砂岩的密度、Vp和Vs间的新关系式：ρ=1.575 8V0.287 9pV0.019 7s.〖JP〗文中讨论了该公式在横波速度预测、岩石物理模拟和AVO正反演等的作用,指出这些规律性认识可服务于油气勘探和开发.     

共炮检距道集波动方程保幅叠前深度偏移方法

本文提出了一种基于双平方根算子的共炮检距道集波动方程保幅叠前深度偏移方法,将振幅误差补偿作为偏移的一部分与“运动学偏移”一起在偏移过程中实现.其基本内容包括：(1)从保幅的单平方根算子方程出发,推导出由双平方根算子定义的保幅单程波方程;(2)根据地震波摄动理论把速度场分裂为层内常速背景和变速扰动,分别在频率－波数域和频率－空间域求得波场深度延拓的偏移时移量及振幅校正系数,从而得到最终的DSR保幅波场延拓算子;(3)在高频假设条件下,把DSR保幅波场延拓公式中的积分运算进行稳相近似,得到保幅波场延拓的相移公式.理论分析和模型数值试验表明,该方法不但可以使散射能量聚焦、归位,提高成像精度;而且可以输出正确反映地下反射系数的振幅信息,为后续的地震属性分析（如AVO/AVA）提供更真实的地震信息.     

Multi-offset phase analysis of surface wave data (MOPA)

Inaccuracy in the shear wave velocity profile inverted from surface wave data manifests from both modelling error and data uncertainty. An alternative method for dispersion curve evaluation by weighted linear regression of phase-offset data can be applied to both equispaced and non-equispaced data for objective identification of these often overlooked error sources.From field data, near-field effects are noted to at most half a wavelength and lateral discontinuities identified by marked changes in wavenumber with offset. Transition frequencies to dominant higher modes appear lower than when identified from standard plane-wave transform methods. Effects can be discriminated by their frequency, position or offset dependence.When a non-corrupt dispersion curve is extracted, the errors are up to 5% at low frequency. Through theoretical Gaussian error propagation analysis, the resulting shear wave velocity profile shows up to 18% uncertainty at depth.