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1.
Decoupled elastic prestack depth migration 总被引：1，自引：0，他引：1
This paper presents a new decoupled form of the formula for common-shot or common-receiver amplitude-preserving elastic prestack depth migration (PreSDM), which can be used for estimating angle-dependent elastic reflection coefficients in laterally inhomogeneous anisotropic media. The multi-shot or multi-receiver extension of this formula is suitable for automated prestack amplitude-versus-angle (AVA) elastic inversion of ocean-bottom cable (OBC), walkaway VSP (WVSP) or standard towed-cable data at any subsurface location. The essence of the theory is a systematic application of the stationary-phase principle and high-frequency approximations to the basic elastic Green's theorem. This leads to nonheuristic explicit wave mode decoupling and scalarization of vector PreSDM. Used in combination, ray-trace and finite-difference (FD) eikonal solvers create a useful tool to calculate accurate Green's function travel time and amplitude maps. Examples of synthetic OBC data and applications to field WVSP data show that the new imaging technique can produce a clear multi-mode elastic image. 相似文献
2.
We investigated the seismic attenuation of compressional (P‐) and converted shear (S‐) waves through stacked basalt flows using short‐offset vertical seismic profile (VSP) recordings from the Brugdan (6104/21–1) and William (6005/13–1A) wells in the Faroe‐Shetland Trough. The seismic quality factors (Q) were evaluated with the classical spectral ratio method and a root‐mean‐square time‐domain amplitude technique. We found the latter method showed more robust results when analysing signals within the basalt sequence. For the Brugdan well we calculated effective Q estimates of 22–26 and 13–17 for P‐ and S‐waves, respectively, and 25–33 for P‐waves in the William well. An effective QS/QP ratio of 0.50–0.77 was found from a depth interval in the basalt flow sequence where we expect fully saturated rocks. P‐wave quality factor estimates are consistent with results from other VSP experiments in the North Atlantic Margin, while the S‐wave quality factor is one of the first estimates from a stacked basalt formation using VSP data. Synthetic modelling demonstrates that seismic attenuation for P‐ and S‐waves in the stacked basalt flow sequence is mainly caused by one‐dimensional scattering, while intrinsic absorption is small. 相似文献
3.
本文对基于弹性波动理论的多波多分量高斯束偏移进行了完整且详细的分析和公式推导,实现了3D空间多分量(矢量)波场的直接成像.由于当前多数基于弹性波动方程的偏移方法只是假设应力边界条件为自由地表边界条件,这种假设不符合垂直地震剖面(VSP)和海底电缆(OBC)等地震数据.为此本文详细分析了实际应用中常见的三种弹性各向同性介质模型的应力边界条件:自由空间、海底和自由地表模型.在上行传播假设情况下,获得了应力边界条件与位移边界条件的关系式.在此基础上,准确推导了3D多波多分量高斯束波场延拓和偏移成像公式,并在偏移过程中实现了完整的多波型自动分离.由于常规的互相关成像条件不适用于矢量波场成像,本文引用了散度/旋度互相关成像条件.通过约定PS转换波的正向旋转方向解决了3D空间PS成像极性翻转问题.利用2D和3D模型数据偏移成像验证了我们所提出的多波多分量高斯束偏移方法的可行性. 相似文献
4.
纵横波波场分离是弹性波偏移方法的必要条件,通过纵横波成像的差异可以获取更多地下介质的信息.目前所用的纵横波波场分离方法多采用Helmholtz分解,这样得到的波场不仅物理意义发生了变化,振幅和相位也会发生改变.本文采用纵横波解耦的弹性波方程,将其应用于三维介质,对比分析了纵横波解耦方法相对传统Helmholtz分解方法在相位、振幅上的优势.将该解耦的波场分离方法应用于弹性波逆时偏移,能得到相位、振幅和物理意义不受改变的偏移结果.但是该解耦方法分离得到的纵横波波场均为矢量场,将该波场分离方法用于弹性波逆时偏移,还需要解决矢量场如何得到标量成像结果的问题.本文引入了Poynting矢量,通过Poynting矢量对矢量波场进行标量化,这样就能得到保振幅、相位,且无极性反转的标量PP和PS成像结果.同时针对S波Poynting矢量求取不准确的问题,采用拟S波应力场和S波速度场得到了更加准确的S波Poynting矢量.理论计算证明了本文采用的3D波场解耦的矢量波场分离方法的正确性和引入Poynting矢量对矢量波场进行标量成像的有效性. 相似文献
5.
针对辽河坳陷西部凹陷曙光潜山高陡构造带顶界面断裂成像精度差、内幕地层识别难的问题,开展双井Walkaway VSP应用研究.利用大阵列检波器接收、炸药与震源联合采集方法,获得了高品质采集数据,是高陡构造成像的基础;子波一致性校正、基于射线追踪的矢量波场分离、VSP数据与地面地震数据联合各向异性速度建模、双井Walkaway VSP成像方法对比等针对性处理技术和波阻抗反演、地震属性提取等综合研究方法的应用,精细刻画了潜山顶界面,新识别出井旁小断层,落实了大倾角潜山构造形态,分析了内幕地层的接触关系,对板岩隔夹层及储层分布有进一步认识,展现了Walkaway VSP技术在高陡构造地区的应用前景.研究得到以下认识:(1)逆时偏移成像方法对高陡潜山成像更具适用性;(2)增加检波器进入潜山内幕的深度,避开潜山高速顶界面的能量屏蔽及滑行波、转换波等干扰,对潜山勘探具有重要意义;(3)双井Walkaway VSP联采时,两井距离要控制在一定范围内,防止两井间距离过大对深层偏移成像造成影响. 相似文献
6.
Seismic attenuation in Faroe Islands basalts 总被引：2，自引：1，他引：1
F. Shaw M.H. Worthington R.S. White M.S. Andersen U.K. Petersen the Seifaba Group 《Geophysical Prospecting》2008,56(1):5-20
We analysed vertical seismic profiling (VSP) data from two boreholes at Glyvursnes and Vestmanna on the island of Streymoy, Faroe Islands, to determine the magnitude and causes of seismic attenuation in sequences of basalt flows. The work is part of SeiFaBa, a major project integrating data from vertical and offset VSP, surface seismic surveys, core samples and wireline log data from the two boreholes. Values of effective seismic quality factor (Q) obtained at Glyvursnes and Vestmanna are sufficiently low to significantly degrade the quality of a surface reflection seismic image. This observation is consistent with results from other VSP experiments in the North Atlantic region. We demonstrate that the most likely cause of the low values of effective Q at Glyvursnes and Vestmanna is a combination of 1D scattering and intrinsic attenuation due to seismic wave‐induced fluid flow within pores and micro‐cracks. Tests involving 3D elastic wave numerical modelling with a hypothetical basalt model based on field observations, indicate that little scattering attenuation is caused by lateral variations in basalt structure. 相似文献
7.
相对保幅的角度域VSP逆时偏移(英文) 总被引：3，自引：1，他引：2
本文介绍了一种改进的角度域VSP逆时偏移方法。对VSP逆时偏移中的逆推公式进行了改进,为方便数值计算出相对保幅的角度域共成像点道集(ADCIGs)。此外VSP记录到的波场信息丰富,包括上行波场、下行波场和直达波场等,本文分析了这些波场的响应特征,发现直达波和下行波在角度域共成像点道集(ADCIGs)上都产生了成像噪音,直达波产生的噪音尤为严重。把该方法用于我国西部地区实际观测的VSP资料,不仅获得相对保幅角度域共成像点道集(ADCIGs),而且压制了成像噪音。通过数值模型试算,实际资料的应用验证了该方法的实用性与有效性,从而为VSP偏移速度分析、VSP AVA/AVO分析和反演等提供可靠的基础资料。 相似文献
8.
VSP资料上下行波场发育丰富.本文在分析VSP直达波、上行反射波、下行反射波传播路径及其照明范围的基础上,指出了常规VSP波动方程偏移方法缺陷,进而通过修改波场延拓方式,提出了上下行反射波联合成像方法,并在高频近似下分析了该方法的成像原理.该方法不需要进行VSP上下行反射波场分离,能够同时对VSP资料中的一次反射波、自由表面多次波、层间多次波进行成像,比常规成像剖面具有更宽的成像范围和更好的成像效果.该方法能够对下行一次反射波进行成像,从而可以实现常规偏移方法难以处理的高陡倾角构造成像.模拟资料和实际资料处理证明了本文方法的正确性. 相似文献
9.
We develop the true‐amplitude prestack migration of multicomponent data based on the use of elastic Gaussian beams for walkaway vertical seismic profile (VSP) acquisition systems. It consists in a weighted summation of multishot data with specific weights, computed by tracing elastic Gaussian beams from each imaging point of the target area towards the sources and receivers. Each pair of beams may be connected with either a pair of P‐rays (PP‐image) or the P‐ray towards sources and the S‐ray to receivers (PS‐image) and is uniquely determined by dip (the angle of the bisector between the rays and the vertical direction) and opening (the angle between the rays) angles. Shooting from the bottom towards the acquisition system helps to avoid well‐known troubles, in particular multipathing for the imaging conditions in complex velocity models. The ability to fix the dip angle and implement summation over opening angles leads to the so‐called selective images that contain mostly interfaces with desired slopes. On the other hand, a set of images computed for a range of opening angles by summation over all available dip angles is used as input of an AVO‐like inversion procedure for the recovery of elastic parameters. The feasibility of this imaging procedure is verified by synthetic data for 2D realistic elastic models. 相似文献
10.
F. AMINZADEH 《Geophysical Prospecting》1989,37(8):893-906
Data from offshore Norway is used to study applications of elastic VSP modelling in detecting shear waves and observing the effects of successive mode conversion in field-recorded VSP data. The shear-wave velocities and densities from log data are used in conjunction with compressional wave velocities determined from surface seismic and log data in the VSP modelling. The time domain non-normal incidence elastic VSP modelling technique of Aminzadeh and Mendel is used as the modelling algorithm. Two surface seismograms are computed first. One is the vertical component and the other is the horizontal component for plane waves that have specified incident angles. A downward continuation method is then applied to generate seismograms at different depth points. The collection of these seismograms constitutes non-normal incidence VSPs. Both vertical and horizontal components of VSP data can be obtained by this procedure. In this paper non-normal incidence VSPs are generated for a 12.5° incident plane wave. The modelling results of layered earth systems of thin layers and thick layers are both compared with field data, and the effect of mode conversions in thin layers is observed. Several events in the field data can be explained by this elastic VSP modelling. Comparison of the model data and field data enabled a probable tube wave or out-of-plane event to be identified, the removal of which significantly improved the final VSP section. This study also shows how the VSP data helped the interpretation of the surface 3D data. 相似文献
11.
油气勘探需要重新开发玄武岩下目的层的成像技术。我们今天遇到的最重要的问题之一是玄武岩下地震成像。近年来,由于利用长炮检距这个问题部分已经得到解决。然而,由于地表以及玄武岩的内部非均质性引起的复杂波形,近炮检距的资料仍然不能充分地被利用。影响玄武岩下成像的近垂直入射资料对于了解玄武岩层内部结构是十分有用的。对比较均匀的玄武岩目标层可选用转换波。这里我们利用几个合成模型例子重点描述了更接近现实的非均质玄武岩流引起的实际困难。模拟计算了全波地震记录以有助于了解玄武岩内沉积物对地震资料的影响。本文介绍了印度德干圈闭的一个研究实例。首先探讨了夹层沉积物对整个地震成像的影响。其次利用该区声测井资料以反射系数法计算全波场响应与实际地震资料对比, 通过一系列速度-深度剖面探讨了利用模式转换波(顶底玄武岩界面上的P波转化为S波或相反)对印度库奇地区玄武岩下成像的可行性。通过与野外资料相比较证明玄武岩中多个薄层的效应严重恶化我们所要解释和利用的图象的质量。 相似文献
12.
We describe a method to invert a walkaway vertical seismic profile (VSP) and predict elastic properties (P‐wave velocity, S‐wave velocity and density) in a layered model looking ahead of the deepest receiver. Starting from Bayes's rule, we define a posterior distribution of layered models that combines prior information (on the overall variability of and correlations among the elastic properties observed in well logs) with information provided by the VSP data. This posterior distribution of layered models is sampled by a Monte‐Carlo method. The sampled layered models agree with prior information and fit the VSP data, and their overall variability defines the uncertainty in the predicted elastic properties. We apply this technique first to a zero‐offset VSP data set, and show that uncertainty in the long‐wavelength P‐wave velocity structure results in a sizable uncertainty in the predicted elastic properties. We then use walkaway VSP data, which contain information on the long‐wavelength P‐wave velocity (in the reflection moveout) and on S‐wave velocity and density contrasts (in the change of reflectivity with offset). The uncertainty of the look‐ahead prediction is considerably decreased compared with the zero‐offset VSP, and the predicted elastic properties are in good agreement with well‐log measurements. 相似文献
13.
非零偏VSP地震资料是一种多分量资料,处理非零偏VSP资料,弹性波叠前逆时深度偏移技术无疑是最适合的处理技术.本文从二维各向同性介质的弹性波波动方程出发,研究了对非零偏VSP资料进行叠前逆时深度偏移的偏移算法,讨论了逆时传播过程中的边值问题和数值频散问题及其相应的解决方案;采用求解程函方程计算得到地下各点的地震波初至时间作为成像时间,实现了非零偏VSP资料的叠前逆时深度偏移.最后进行了模型试算和非零偏VSP地震资料的试处理,结果表明该方法不受地层倾角限制,较适用于高陡构造地区或介质横向速度变化较大地区的非零偏VSP地震资料处理. 相似文献
14.
与地面地震资料相比,VSP资料具有分辨率高、环境噪声小及能更好地反映井旁信息等优点.常规VSP偏移主要对上行反射波进行成像,存在照明度低、成像范围受限等问题.为了增加照明度、拓宽成像范围、提高成像精度,本文采用直达波除外的所有声波波场数据(全波),包括一次反射波、多次反射波等进行叠前逆时偏移成像.针对逆时偏移中的四个关键问题,即波场延拓、吸收边界条件、成像条件及低频噪声的压制,本文分别采用自适应变空间差分算子长度的优化有限差分方法(自适应优化有限差分方法)求解二维声波波动方程以实现高精度、高效率的波场延拓,采用混合吸收边界条件压制因计算区域有限所引起的人工边界反射,采用震源归一化零延迟互相关成像条件进行成像,采用拉普拉斯滤波方法压制逆时偏移中产生的低频噪声.本文对VSP模型数据的逆时偏移成像进行了分析,结果表明:自适应优化有限差分方法比传统有限差分方法具有更高的模拟精度与计算效率,适用于VSP逆时偏移成像;全波场VSP逆时偏移成像比上行波VSP逆时偏移的成像范围大、成像效果好;相对于反褶积成像条件,震源归一化零延迟互相关成像条件具有稳定性好、计算效率高等优点.将本文方法应用于某实际VSP资料的逆时偏移成像,进一步验证了本文方法的正确性和有效性. 相似文献
15.
COMPUTATION OF ZERO-OFFSET VERTICAL SEISMIC PROFILES INCLUDING GEOMETRICAL SPREADING AND ABSORPTION*
Synthetic vertical seismic profiles (VSP) provide a useful tool in the interpretation of VSP data, allowing the interpreter to analyze the propagation of seismic waves in the different layers. A zero-offset VSP modeling program can also be used as part of an inversion program for estimating the parameters in a layered model of the subsurface. Proposed methods for computing synthetic VSP are mostly based on plane waves in a horizontally layered elastic or anelastic medium. In order to compare these synthetic VSP with real data a common method is to scale the data with the spherical spreading factor of the primary reflections. This will in most cases lead to artificial enhancement of multiple reflections. We apply the ray series method to the equations of motion for a linear viscoelastic medium after having done a Fourier transformation with respect to the time variable. This results in a complex eikonal equation which, in general, appears to be difficult to solve. For vertically traveling waves in a horizontally layered viscoelastic medium the solution is easily found to be the integral along the ray of the inverse of the complex propagation velocity. The spherical spreading due to a point source is also complex, and it is equal to the integral along the ray of the complex propagation velocity. Synthetic data examples illustrate the differences between spherical, cylindrical, and plane waves in elastic and viscoelastic layered media. 相似文献
16.
Paul Sava 《Geophysical Prospecting》2012,60(1):29-48
Seismic waves propagate through the earth as a superposition of different wave modes. Seismic imaging in areas characterized by complex geology requires techniques based on accurate reconstruction of the seismic wavefields. A crucial component of the methods in this category, collectively known as wave‐equation migration, is the imaging condition that extracts information about the discontinuities of physical properties from the reconstructed wavefields at every location in space. Conventional acoustic migration techniques image a scalar wavefield representing the P‐wave mode, in contrast to elastic migration techniques, which image a vector wavefield representing both the P‐ and S‐waves. For elastic imaging, it is desirable that the reconstructed vector fields are decomposed into pure wave modes, such that the imaging condition produces interpretable images, characterizing, for example, PP or PS reflectivity. In anisotropic media, wave mode separation can be achieved by projection of the reconstructed vector fields on the polarization vectors characterizing various wave modes. For heterogeneous media, because polarization directions change with position, wave mode separation needs to be implemented using space‐domain filters. For transversely isotropic media with a tilted symmetry axis, the polarization vectors depend on the elastic material parameters, including the tilt angles. Using these parameters, we separate the wave modes by constructing nine filters corresponding to the nine Cartesian components of the three polarization directions at every grid point. Since the S polarization vectors in transverse isotropic media are not defined in the singular directions, e.g., along the symmetry axes, we construct these vectors by exploiting the orthogonality between the SV and SH polarization vectors, as well as their orthogonality with the P polarization vector. This procedure allows one to separate all three modes, with better preserved P‐wave amplitudes than S‐wave amplitudes. Realistic synthetic examples show that this wave mode separation is effective for both 2D and 3D models with strong heterogeneity and anisotropy. 相似文献
17.
偏移成像是VSP数据处理中的一个重要环节,常规的VSP成像方法通常利用VSP-CDP转换或Kirchhoff偏移,均存在保幅性差及成像精度低等问题,而波动方程叠前深度偏移被认为是对地下复杂构造进行成像的精确偏移方法.任意广角波动方程作为一种高精度的空间域单程波波动方程,同时由于只含有二阶偏导数项,易于数值实现,与其他单程波波动方程相比,具有更大的成像倾角,因此是偏移成像的有力工具之一.本文将AWWE推广应用到VSP数据成像中,实现了VSP时空域高角度单程波方程偏移.首先从三维标量任意广角波动方程出发,推导了完全匹配层吸收边界条件,在基本不增加计算量的前提下有效地压制了边界反射成像噪音,同时利用非线性反演算法优选参考速度来提高平方根算子的近似程度,从而提高高角度地层的成像精度.模型数值模拟实验验证了该方法的有效性,同时表明该方法在陡倾角构造情况下能取得很好的成像效果.最后对某地区实际观测的VSP资料进行了偏移成像,并与地面地震偏移结果进行了对比,显示出VSP波动方程偏移在成像分辨率上的优势. 相似文献
18.
We have developed a straightforward and ray based methodology to estimate both the maximum offset and reflection imaging radius for multi‐layered velocity models, which can be used for a 2D/3D VSP survey design. Through numerical examples, we demonstrate that the presence of a high‐velocity layer above a target zone significantly reduces the maximum offset and reflection imaging radius. Our numerical examples also show that including in a migration VSP data acquired beyond a recommended maximum offset, radically degrades the quality of the final VSP image. In addition, unlike the conventional straight‐line based approximation that often produces an incorrect large reflection imaging radius, our methodology predicts the VSP imaging radius with more accuracy than does the conventional approximation. 相似文献
19.
20.
Dhananjai Pandey Satish Singh Martin Sinha Lucy MacGregor 《应用地球物理》2007,4(3):183-193
Hydrocarbon exploration interests have renewed the need for developing new sub basalt imaging techniques. One of the most important problems encountered today is seismic imaging below basalt. In recent years, this problem appears to have been overcome partly by using long offset seismic data. However near offset data are yet to be fully utilised due to the complex waveform caused by the surface as well as internal heterogeneity of the basalts. The near normal incidence data, which influence the sub-basalt imaging, are highly useful to understand the internal structure within a basalt layer. The use of converted waves for such targets has been proposed as an alternative in a rather homogeneous basalt layer. With a few synthetic modelling exercises here we highlight the practical difficulties in dealing with more realistic and heterogeneous basalt flow. Full waveform seismograms are computed to understand the effects of intra-trappean sediments on the seismic data. A case study from the Deccan Traps of India is presented in this paper. First, we discuss the effects of intercalated sediments on the overall seismic image. Later, the sonic log data from the field are used to compute the full wave-field response using the reflectivity method and compared with the field data. The feasibility of using mode converted waves （P to S and vice-versa at the top and bottom basalt interfaces） for sub-basalt imaging in Kutch region is discussed through a series of velocity-depth profiles. By comparing with the field data we demonstrate that the effects of multiple thin layering within the basalt can strongly deteriorate the image we seek to interpret and exploit. 相似文献