3D MODELLING OF DIFFRACTIONS OBSERVED ON DEEP REFLECTION LINE DEKORP 2-S1

DEKORP 2-S is the first profile carried out in the German continental reflection seismic programme. Besides numerous reflections in the lower crust, the seismic section is characterized by strongly curved events that are interpreted as diffractions. These diffractions occur as clusters, mainly in two areas of the profile: the Dinkelsbühl and the Spessart area. This paper deals with the Dinkelsbühl diffractions where three-dimensional control is available. The control is provided by two additional profiles P-1 and Q-40 which run parallel and perpendicular to the main line, DEKORP 2-S. The type and the location of the diffractors are determined by traveltime-modelling using crustal velocity functions derived from in-line wide-angle observations. A model with inclined line diffractors provides the best fit to the data for all three profiles. Projections of these line diffractors to the surface show that they are aligned parallel to the strike direction of the Variscides. This suggests that the diffractions are associated with the suture zone between the Saxothuringian and Moldanubian geological provinces.     

SEISMIC VIBRATOR MODELLING1

The wavefield in, and at the surface of, a homogeneous, isotropic, perfectly elastic half-space, excited by a traction distribution at the surface of the medium is investigated. The emitted wavefield is a spatial convolution of the surface tractions and the spatial impulse response. The properties of the wavefield in the far-field of the medium are derived and it is shown that the far-field particle velocity is essentially equal to a weighted sum of the time derivative of the integrated surface tractions, that is, of the components of the ‘ground force’. The theory is valid for an arbitrary geometry and orientation of the surface tractions, and is independent of the boundary conditions at the surface of the medium. The surface tractions are related to a source that consists of a mass distribution with an arbitrary force distribution imposed upon it. A boundary condition is introduced that accounts for the mass load and the forces applied to it but neglects vibrations within the mass. The boundary condition follows from the equation of motion of the surface mass load. The theory is applied to the Vibroseis configuration, using a P-wave vibrator model with a uniformly distributed force imposed on top of the baseplate, and assuming that horizontal surface traction components are absent. The distribution of displacement and stress directly underneath the baseplate of a single vibrator and an array of vibrators is investigated. Three different boundary conditions are used: (1) assuming uniform pressure, (2) assuming uniform displacement, (3) using the equation of motion of the baseplate as a boundary condition. The calculations of the distribution of stress and displacement over the plate for different elastic media and several frequencies of operation show that only the results obtained with the mixed boundary condition agree with measurements made in the field. The accuracy of three different phase-feedback signals is compared using synthetic data. Baseplate velocity phase-feedback leads to huge deviations in the determination of the far-field wavelet; reaction mass acceleration phase-feedback looks stable but neglects the differentiating earth filter; and phase-feedback to a weighted sum of baseplate and reaction mass accelerations becomes unstable with increasing frequency. The instability can be overcome using measurements over the whole baseplate. The model can be extended to a lossy layered earth.     

SEISMIC MODELLING: HIGH-FREQUENCY RANGE1

An increase in the grain/pore size can cause the received echo signal to be distorted due to the preferential attenuation of the high-frequency components of the transmitted acoustic signal. Therefore, a rock acts as a low-pass filter and the property of the filter has a linear relation to the grain/pore size. The results have shown that P- and S-wave attenuations can be used to characterize selected internal characteristics of a rock. For example, an increase in the differential stress can cause an increase in the attenuation, due to the sensitivity of the S-wave to microcracks.     

三分量地震波场的近似解析离散模拟技术

首先将二维各向异性介质中地震波动二阶偏微分方程组降为关于时间ｔ的一组一阶偏微分波动方程组，然后利用Ｔａｙｌｏｒ展式和插值逼近的方法构制了一种与有限元法、反射率法、射线追踪法、有限差分法等传统方法不同的近似解析离散方法（ＮＡＤＭ）．对双层各向同性介质以及横向各向同性介质中的三分量地震波场进行了模拟，并与传统的有限差分法作了比较，结果表明ＮＡＤＭ算法可行．     

复杂地震系统的数字模拟

借用细胞自动机的计算处理方法，对地震孕育过程中整个系统能量输入，积累，释放，传播等进行了数字模拟，并研究了复杂地震系统的整体特征，结果表明，整体调整系统，地震时能量的损耗系数，外部构造应力场的驱动力大小会影响到地震系统中能量积累和释放的状况，影响到Ｇ－Ｒ关系中ｂ值的大小。     

3D AND 2½ D MODELLING OF GRAVITY ANOMALIES WITH VARIABLE DENSITY CONTRAST1

The decrease of density contrast in sedimentary basins may be approximated by a quadratic function. A sedimentary basin may be viewed as a number of prisms placed in juxtaposition. Equations in closed form for the gravity anomalies of 3D and 2½ D prismatic models are derived. Approximate equations for these models are also derived for rapid calculations. Efficient methods are developed for anomaly calculation by an appropriate use of the exact and approximate equations, and hence, for 3D and 2½ D modelling. The depths to the basement are adjusted iteratively by comparing the calculated anomalies with the observed anomalies. These methods are applied for analysis of the residual anomaly map of the Los Angeles Basin, California.     

SEISMIC MODELLING OF SEAM WAVES EXCITED BY ENERGY TRANSMISSION INTO A SEAM1

The traditional method of exciting channel waves in coal deposits underground consists of firing explosive sources in a mid-seam position generating seam waves of the Rayleigh and Love type. We investigate various source positions and excitation mechanisms within the bedrock structure surrounding the seam to evaluate the effects of source positions adjacent to the seam. The investigation is based on analogue and numerical modelling of half- and full-space cases, for which the excitation and the nature of Rayleigh channel waves are examined. In the analogue modelling, sources, located from mid-seam out into the bedrock, along the edge of a 2D plate model, excited channel waves through a conversion of the free surface Rayleigh wave at the edge of the plate. The excited channel wave belongs to the normal mode range. Frequency-wavenumber analysis shows that the symmetric 2nd mode of the channel wave is excited with frequencies comparable to the forcing frequency of the source signal. The polarization changes from retrograde to prograde, as the wave develops from the front to the rear of the seam, respectively. The amplitude-depth distribution resembles that of an ordinarily excited seam wave, for the symmetric component. However, the antisymmetric component does not show the characteristic change of sign in amplitudes across the mid-seam axis. Numerical modelling with sources located in the bedrock (full-space case) shows that relocating the source away from the seam lowers the frequency content of the excited channel wave. Based on these investigations, the influence of a lower-frequency source signal on the excitation of the channel wave is examined in an analogue experiment. Sources are sited in the bedrock adjacent to the seam at three locations. A lower-frequency wavelet is calculated for each source location from the results obtained in the numerical analysis. For comparison, a higher-frequency wavelet is also used which is known to be optimal for this model geometry when excited by a mid-seam source location. It is found that in two cases the use of the lower-frequency wavelet improves the channel wave excitation, while no amplification is achieved in one case.     

中朝地台东北缘3km深度地震层析成像的地质解释

利用本区丰富的地质,人工地震探测资料,对３ｋｍ深度层析成象成果进行了分析对比,结果表明：（１）３Ｋｍ深度速度值与人工地震探测所获地震波速值吻合较好;（２）３ｋｍ深度速度分布与该深度处的岩石介质分布有较好的对应关系,高速区对应地层主要为太古界－下元古界变质岩和地槽区的古生界浅变质岩,低速区对应主要为中生界火山,碎屑岩,燕山期花岗岩体和盖层性质的中元古－古代界沉积岩。（３）层析成象结果可划分深度不同的     

A STABLE AND FLEXIBLE PROCEDURE FOR THE INVERSE MODELLING OF SEISMIC FIRST ARRIVALS1

Multiple coverage reflection seismic data provide an important source of information concerning the subsurface. However, due to the stacking and migration techniques used in the processing, the first arrivals are muted and details about the upper part of the sections are generally lost. This paper describes a computerized method for the inverse modelling of laterally varying velocities and shallow depths which are not sufficiently resolved in the reflection seismic processing. The method minimizes, in a least-squares manner, the difference between the observed first arrivals, picked from the reflection traces, and a set of synthetic traveltimes, calculated by ray tracing in a cell model. An initial model, e.g. from a priori knowledge or the application of a conventional interpretation method, is refined iteratively until no further essential improvement can be achieved. Traditional first-arrival inversion methods cannot, in general, provide such flexible modelling. The technique is successfully tested on synthetic data as well as on first arrivals picked automatically from the records of a reflection seismic survey in North Jutland, Denmark.     

MODELLING 1D WAVE PROPAGATION IN A SYSTEM OF ABSORBING LAYERS1

The purpose of this paper is to derive relationships useful for the analysis of shallow transmission and reflection seismic data. Since the effects of absorption cause such data to be non-stationary, the derived results do not rely on frequency-domain representations being independent of the time origin. Rather, results are expressed in the z-domain and are suitable for implementation in the time domain. In order to give an unambiguous system of notation a classic approach to the problem of 1D wave propagation in a layered system is fully described. Absorption is included in a general but realistic way and the implications of representing it digitally are considered in detail. Computational efficiency is improved by grouping the layers in blocks of similar absorptive effect. The transmissivity and reflectivity of systems composed of such blocks are obtained and a dereverberation filter identified. The filter is described succinctly in terms of a non-linear recursion relationship. The sequences obtained by convolving the filter with the transmissivity and reflectivity are discussed in detail. The results are illustrated by nominally realistic synthetic examples computed in the time domain.     

VISCOELASTIC SEISMIC RESPONSES OF 2D RESERVOIR MODELS1

The use of relaxation mechanisms has recently made it possible to simulate viscoelastic (Q) effects accurately in time-domain numerical computations of seismic responses. As a result, seismograms may now be synthesized for models with arbitrary spatial variations in compressional- and shear-wave quality factors (Q₉, and Q_s, as well as in density (ρ) and compressional- and shear-wave velocities (V_p, and V_s). Reflections produced by Q contrasts alone may have amplitudes as large as those produced by velocity contrasts. Q effects, including their interaction with V_p, V_s and p, contribute significantly to the seismic response of reservoirs. For band-limited data at typical seismic frequencies, the effects of Q on reflectivity and attenuation are more visible than those on dispersion. Synthetic examples include practical applications to reservoir exploration, evaluation and monitoring. Q effects are clearly visible in both surface and offset vertical seismic profile data. Thus, AVO analyses that neglect Q may produce erroneous conclusions.     

二维层状介质中衍射波的计算

本文讨论了衍射波的计算,它是文献[5]的继续和发展,是为了应用射线法解决二维地震人机联作工作的一部分。根据波的物理性质,引入了等效炮点、等效检波点和等效界面等概念,将Berryhill导出的衍射波计算公式推广到倾斜界面和楔形结构等情形。这一工作与文献[1—3]中的工作相结合,在实用上解决了二维地震人机联作解释中的正演计算方法问题。     

全三维地震解释

三维地震为油气工业的增储上产作出了重大贡献，随着采集处理成本的不断降低，周期的不断缩短，全三维处理和叠前深度偏移等技术的推广应用，以及计算机技术的迅速发展，全三维地震解释已成为国内外致力发展的技术。     

复杂地层构造的地震模拟

复杂地层构造的地震模拟,是地震勘探理论的重要问题。本文提出了一个模拟系统:用有限元素表示复杂地层构造,在此基础上应用射线轨迹法计算二维合成记录,从而得到复杂地层构造的理论剖面。这个系统可以用来模拟构造和岩性,为解释人员提供理论依据。 作者研制了适用于任意复杂地层构造计算的通用程序,并用以计算了某测线的一批模型,所得合成剖面由电视加以显示,最后对结果作了简单的分析讨论。     

区域分解法地震正演模拟方法研究

区域分解法是近年开辟的计算偏微分方程数值解的一种新方法，该方法把研究区域分解为形态规则的小区域，在各子区域内采用最有效的方法求解，能充分发挥各种正演模拟方法的优点，大大地提高求解正演问题的有效性．本文首次把区域分解法引入地震正演模拟，构造出了适应性强、计算速度快、所需计算机资源量小，具有高度并行性的区域分解地震正演模拟方法，编制了区域分解地震正演模拟软件，对大量地质模型进行了计算，获得了良好的结果．     

区域分解法地震正演模拟方法研究

区域分解法是近年开辟的计算偏微分方程数值解的一种新方法,该方法把研究区域分解为形态规则的小区域,在各子区域内采用最有效的方法求解,能充分发挥各种正演模拟方法的优点,大大地提高求解正演问题的有效性．本文首次把区域分解法引入地震正演模拟,构造出了适应性强、计算速度快、所需计算机资源量小,具有高度并行性的区域分解地震正演模拟方法,编制了区域分解地震正演模拟软件,对大量地质模型进行了计算,获得了良好的结果．     

复杂地层构造的地震模拟

复杂地层构造的地震模拟,是地震勘探理论的重要问题。本文提出了一个模拟系统:用有限元素表示复杂地层构造,在此基础上应用射线轨迹法计算二维合成记录,从而得到复杂地层构造的理论剖面。这个系统可以用来模拟构造和岩性,为解释人员提供理论依据。 作者研制了适用于任意复杂地层构造计算的通用程序,并用以计算了某测线的一批模型,所得合成剖面由电视加以显示,最后对结果作了简单的分析讨论。     

τ-p SEISMIC DATA FOR VISCOELASTIC MEDIA – PART 1: MODELLING1

Viscoelastic modelling reveals that the interaction of compressional-wave velocity C_p, compressional-wave quality factor Q_p, shear-wave velocity C_s, shear-wave quality factor Q_s and Poisson's ratio as a function of time intercept τ and ray parameter p, is complicated; however, distinct, potentially diagnostic behaviours are seen for different combinations of viscoelastic parameters. Synthetic seismograms for three viscoelastic reservoir models show that variations in the Poisson's ratio produce visible differences when compared to the corresponding elastic synthetic seismograms; these differences are attributable to interaction of the elastic parameters with Q_p and Q_s. When the P-wave acoustic impedance contrast is small, viscoelastic effects become more apparent and more useful for interpretation purposes. The corresponding amplitude and net phase spectra reveal significant differences between the elastic and the viscoelastic responses. When P-wave reflectivities are large, they tend to dominate the total response and to mask the Q reflectivity effects. The attenuation effects are manifested as an amplitude decay that increases with both time and ray parameter. The sensitivity of the computed seismic responses for various combinations of viscoelastic parameters suggests the opportunity for diagnostic interpretation of τ-p seismic data. The interpretation of the viscoelastic parameters can permit a better understanding of the rock types and pore fluid distribution existing in the subsurface.     

不同地震烈度下边坡稳定性计算实例

按照工程场地地震安全性评价的有关规定，在充分研究场地工程地质，地震地质背景等工作的基础上，采用ＢＩＳＨＯＰ法以某拟建工程场地在不同地震烈度下其边坡的稳定性进行了计算。