叠前逆时偏移假象去除方法

叠前逆时偏移(RTM)求解全波动方程进行波场延拓,可以精确描述地震波的传播,在陡倾界面和复杂构造成像方面具有显著优势.但在实际应用中,互相关成像条件会带来低波数高振幅的假象.本文分析了假象产生的原因,实现并比较了Laplace滤波、坡印廷矢量法和上下、左右行波分解这三种假象压制方法,针对不同的计算平台分析其优劣.数值试验表明,坡印廷矢量法当波场简单时较为有效;而上下、左右行波分解法和Laplace滤波可以应对较复杂的波场.就去假象结果而言,使用波场分解成像条件并施加Laplace滤波效果最佳;然而就计算效率而言,CPU平台应用波场分解成像条件只增加少量的计算量,但GPU平台的实现则需要对数据的读写付出较大的代价.因此,综合考虑,对于CPU平台上述两种方案的组合为最佳策略;而基于GPU平台的逆时偏移采用Laplace滤波去噪仍然是目前最经济的选择.Marmousi模型和SEG/HESS VTI模型试验验证了上述结论.     

基于虚拟偏移距方法的各向异性转换波保幅叠前时间偏移

In this paper, we use the method of pseudo-offset migration （POM） to complete converted wave pre-stack time migration with amplitude-preservation in an anisotropic medium. The method maps the original traces into common conversion scatter point （CCSP） gathers directly by POM, which simplifies the conventional processing procedure for converted waves. The POM gather fold and SNR are high, which is favorable for velocity analysis and especially suitable for seismic data with low SNR. We used equivalent anisotropic theory to compute anisotropic parameters. Based on the scattering wave traveltime equation in a VTI medium, the POM pseudo-offset migration in anisotropic media was deduced. By amplitude-preserving POM gather mapping, velocity analysis, stack processing, and so on, the anisotropic migration results were acquired. The forward modeling computation and actual data processing demonstrate the validity of converted wave pre-stack time migration with amplitude-preservation using the anisotropic POM method.     

Analysis and application of pseudo-offset method in the converted-wave prestack time migration

Pseudo-offset migration （POM） is a new method for prestack time migration of converted waves that improves on equivalent-offset migration （EOM）. The mapping of POM is different than EOM but the purpose of the two methods is to map the input samples to the common conversion scatter point （CCSP） gathers. This paper introduces the principles of the two migration methods and model parameter sensitivity tests for both POM and EOM. At large offset-to-depth ratios the hyperbolic approximation, the three-term approximation and the double square root （DSR） equation are used to NMO-correct the mapped POM gathers to obtain more accurate migration velocities. These equations were derived and calculated by small pseudo offset. POM is then used to image complex structure and prestack time migration.     

虚拟偏移距方法在转换波叠前时间偏移中的分析与应用

虚拟偏移距偏移(POM)是一种新的转换波叠前时间偏移方法,是对等效偏移距偏移(EOM) 的一种改进,它与EOM方法映射方式上虽然不同, 但本质都是相同的,都是将原始输入道集映射成为共转换散射点道集。本文主要介绍了两种偏移方法的原理,通过理论模型来验证两种方法对模型参数的敏感性。在远偏移距情况下,为了能在速度分析中得到更精确的偏移速度,分别用小排列拟合的双曲线公式、三阶近似式、双平方根旅行时公式对映射后的POM道集做动校正,从而验证了三个公式的精度。最后用POM方法对复杂构造进行了叠前时间偏移。     

用于AVO分析的振幅保真平面波叠前时间偏移

To support amplitude variation with offset （AVO） analysis in complex structure areas, we introduce an amplitude-preserving plane-wave prestack time migration approach based on the double-square-root wave equation in media with little lateral velocity variation. In its implementation, a data mapping algorithm is used to obtain offset-plane-wave data sets from the common-midpoint gathers followed by a non-recursive phase-shift solution with amplitude correction to generate common-image gathers in offset-ray-parameter domain and a structural image. Theoretical model tests and a real data example show that our prestack time migration approach is helpful for AVO analysis in complex geological environments.     

Multiwave velocity analysis based on Gaussian beam prestack depth migration

Prestack depth migration of multicomponent seismic data improves the imaging accuracy of subsurface complex geological structures. An accurate velocity field is critical to accurate imaging. Gaussian beam migration was used to perform multicomponent migration velocity analysis of PP- and PS-waves. First, PP- and PS-wave Gaussian beam prestack depth migration algorithms that operate on common-offset gathers are presented to extract offset-domain common-image gathers of PP- and PS-waves. Second, based on the residual moveout equation, the migration velocity fields of P- and S-waves are updated. Depth matching is used to ensure that the depth of the target layers in the PP- and PS-wave migration profiles are consistent, and high-precision P- and S-wave velocities are obtained. Finally, synthetic and field seismic data suggest that the method can be used effectively in multiwave migration velocity analysis.     

逆时偏移成像技术研究进展

近年来,基于双程波动方程的逆时偏移方法在地震成像领域吸引了越来越多地球物理学家的关注.逆时偏移方法相对于实际油气勘探过程中被广泛应用的Kirchhoff偏移方法和单程波波动方程偏移方法,具有原理简单、成像精确且无倾角限制、适应任意复杂速度模型等诸多优点.本文首先概述了逆时偏移近三十年的发展历程,然后重点介绍了逆时偏移方...     

逆时偏移速度误差影响分析

逆时偏移作为一种先进的地震偏移成像方法,其成像结果的好坏取决于很多因素,其中最关键的是速度.速度越准确,成像效果就越好.但在实践当中,速度往往是未知的,只能通过速度反演等各种手段来估计近似速度.由此导致逆时偏移成像结果存在误差,从而降低后续地震解释的精度.那么速度误差对逆时偏移成像的影响到底有多大呢?实际当中一般估计出来的近似速度可能在模型每一个地方都不同,很难直接进行逆时偏移速度误差系统分析.为了简化,本文采用等效速度误差——速度模型整体平均误差,进行逆时偏移成像分析.逆时偏移成像采用互相关成像条件,低频噪声压制采用振幅补偿拉普拉斯滤波方法,源波场正推和接收波场逆推采用波动方程一阶应力.速度形式及交错网格有限差分方法.首先对比分析不同速度误差对逆时偏移成像结果的影响,并和单程波波动方程偏移进行对比分析;然后分析速度误差对逆时偏移和单程波波动方程偏移成像位置的影响;最后基于偏移速度分析方法估计得到的近似速度模型,进行逆时偏移和单程波波动方程偏移成像试验.结果 表明:在不同速度误差情况下,逆时偏移成像在同相轴连续性和能量聚焦等方面要好于单程波波动方程偏移;速度误差对逆时偏移和单程波波动方程偏移成像位置的影响程度相当.本文研究成果对逆时偏移在实践中的应用具有一定的参考价值.     

逆时偏移震源子波影响分析

逆时偏移作为一种先进的地震偏移成像方法,其成像结果的好坏取决于很多因素,其中成像奈件是关键的一个因素.成像条件一般采用互相关成像条件,它是将源波场沿时间进行正推,接收波场沿时间进行逆推,然后将源波场和接收波场进行互相关,从而得到偏移成像结果.源波场正推和接收波场逆推都是采用同一个波动方程,这里采用波动方程一阶应力-速度形式,数值计算方法采用交错网格有限差分方法.采用振幅补偿拉普拉斯滤波方法压制逆时偏移成像中的低频噪声.采用三次样条插值方法解决逆时偏移成像中的波形不光滑问题.在接收波场逆推过程中,是将地震记录作为边界条件.而在源波场正推过程中,需要给定震源子波作为初值条件.理论上来讲,震源子波为脉冲时是最佳的,但在实际当中难以实现.在源波场正推时,一般采用子波函数.子波函数形式非常多,最常用的是雷克子波.不同主频震源子波产生的源波场频率也不同,从而导致逆时偏移互相关成像效果也不同.那么震源子波主频对逆时偏移成像影响到底有多大呢?为了简便,本文基于不同主频雷克子波,初步分析了相应的逆时偏移成像效果,同时对速度模型存在不同误差时的情形也进行了试验分析.结果 表明:在不同速度误差情况下,震源子波主频越低,成像同相轴连续性越好,散射点聚焦能量越强;震源子波主频越高,虽然分辨率提高,但成像同相轴连续性变差,散射点聚焦能量变弱,同时背景噪声增强.本文研究结果对逆时偏移成像技术在实践中的应用具有一定的参考价值.     

叠前时间偏移技术在深层断陷区地震成像中的应用

Kirchhoff积分叠前时间偏移应用波动方程的Kirchhoff积分解实现地下反射层的偏移问题,该技术应用所有偏移距的地震资料,能适应纵横向速度变化较大的情况,是复杂地区地震资料成像的较理想的方法.叠前时间偏移是对偏移处理和偏移速度修改的一个迭代过程,偏移速度的精度直接影响到偏移效果.对大庆油田古龙断陷某工区的三维地震资料,应用叠前时间偏移技术进行成像处理,对于埋深较大的目的层,断陷结构复杂,介质的各向异性较为严重,所以应用考虑各向异性参数的速度修改公式,得到较为精确的偏移速度体,进而保证偏移CRP道集拉平,得到成像良好的偏移数据体.实际地震数据偏移处理结果表明叠前时间偏移技术可对复杂构造的地震数据准确成像,可在古龙地区断陷地层勘探中推广应用.     

衰减补偿的黏弹性最小二乘逆时偏移

地震勘探是发现和认识油气藏的重要方法,随着国内油气勘探领域、勘探对象的日趋复杂化,油气勘探迫切需求地震成像的精准度、分辨率和保真度的大幅度提升.相比于常规逆时偏移方法,最小二乘逆时偏移成像有较少的伪像,成像振幅均衡,成像分辨率高.目前常用弹性最小二乘逆时偏移方法来对地下介质进行成像.但由于地下介质通常表现出黏弹性性质,...     

基于叠前反演的岩溶古河道充填特征描述方法

在塔河油田碳酸盐岩油藏中岩溶古河道作为一种重要的储集类型,易受沉积物充填,储层非均质性强,不同的充填程度和充填物类型导致古河道储集体产量存在差异,而古河道内部充填特征量化表征一直是勘探开发中难以解决的技术难题.本文以塔河工区某一典型河道为研究对象,从叠前反演的角度出发,介绍了一种岩溶古河道充填特征的定量描述方法,首先采...     

Seismic wavefield modeling based on time-domain symplectic and Fourier finite-difference method

Seismic wavefield modeling is important for improving seismic data processing and interpretation. Calculations of wavefield propagation are sometimes not stable when forward modeling of seismic wave uses large time steps for long times. Based on the Hamiltonian expression of the acoustic wave equation, we propose a structure-preserving method for seismic wavefield modeling by applying the symplectic finite-difference method on time grids and the Fourier finite-difference method on space grids to solve the acoustic wave equation. The proposed method is called the symplectic Fourier finite-difference (symplectic FFD) method, and offers high computational accuracy and improves the computational stability. Using acoustic approximation, we extend the method to anisotropic media. We discuss the calculations in the symplectic FFD method for seismic wavefield modeling of isotropic and anisotropic media, and use the BP salt model and BP TTI model to test the proposed method. The numerical examples suggest that the proposed method can be used in seismic modeling of strongly variable velocities, offering high computational accuracy and low numerical dispersion. The symplectic FFD method overcomes the residual qSV wave of seismic modeling in anisotropic media and maintains the stability of the wavefield propagation for large time steps.     

相对保幅的角度域VSP逆时偏移(英文)

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。     

Seismic modeling with an optimal staggered-grid finite-difference scheme based on combining Taylor-series expansion and minimax approximation

Staggered-grid finite-difference (SGFD) schemes have been used widely in seismic modeling. The spatial difference coefficients of the SGFD scheme are generally determined by a Taylor-series expansion (TE) method or optimization methods. However, high accuracy is hardly guaranteed both at small and large wavenumbers by using these conventional methods. We propose a new optimal SGFD scheme based on combining TE and minimax approximation (MA) for high accuracy modeling. The optimal spatial SGFD coefficients are calculated by applying a combination of TE and MA to the dispersion relation, where the implementation of the MA method is based on a Remez algorithm. We adopt the optimal SGFD coefficients to solve first-order spatial derivatives of the elastic wave equations and then perform numerical modeling. Dispersion analyses and seismic modeling show the advantage of the proposed optimal method. The optimal SGFD scheme has greater accuracy than the TE-based SGFD scheme for the same spatial difference operator length. In addition, the optimal SGFD scheme can also adopt a shorter operator length to achieve the high accuracy reducing the computational cost.     

P- and S-wavefield simulations using both the first- and second-order separated wave equations through a high-order staggered grid finite-difference method

In seismic exploration, it is common practice to separate the P-wavefield from the S-wavefield by the elastic wavefield decomposition technique, for imaging purposes. However, it is sometimes difficult to achieve this, especially when the velocity field is complex. A useful approach in multi-component analysis and modeling is to directly solve the elastic wave equations for the pure P- or S-wavefields, referred as the separate elastic wave equations. In this study, we compare two kinds of such wave equations: the first-order (velocity–stress) and the second-order (displacement–stress) separate elastic wave equations, with the first-order (velocity–stress) and the second-order (displacement–stress) full (or mixed) elastic wave equations using a high-order staggered grid finite-difference method. Comparisons are given of wavefield snapshots, common-source gather seismic sections, and individual synthetic seismogram. The simulation tests show that equivalent results can be obtained, regardless of whether the first-order or second-order separate elastic wave equations are used for obtaining the pure P- or S-wavefield. The stacked pure P- and S-wavefields are equal to the mixed wave fields calculated using the corresponding first-order or second-order full elastic wave equations. These mixed equations are computationally slightly less expensive than solving the separate equations. The attraction of the separate equations is that they achieve separated P- and S-wavefields which can be used to test the efficacy of wave decomposition procedures in multi-component processing. The second-order separate elastic wave equations are a good choice because they offer information on the pure P-wave or S-wave displacements.     

Dynamic interaction numerical models in the time domain based on the high performance scaled boundary finite element method

Consideration of structure-foundation-soil dynamic interaction is a basic requirement in the evaluation of the seismic safety of nuclear power facilities. An efficient and accurate dynamic interaction numerical model in the time domain has become an important topic of current research. In this study, the scaled boundary finite element method （SBFEM） is improved for use as an effective numerical approach with good application prospects. This method has several advantages, including dimensionality reduction, accuracy of the radial analytical solution, and unlike other boundary element methods, it does not require a fundamental solution. This study focuses on establishing a high performance scaled boundary finite element interaction analysis model in the time domain based on the acceleration unit-impulse response matrix, in which several new solution techniques, such as a dimensionless method to solve the interaction force, are applied to improve the numerical stability of the actual soil parameters and reduce the amount of calculation. Finally, the feasibility of the time domain methods are illustrated by the response of the nuclear power structure and the accuracy of the algorithms are dynamically verified by comparison with the refinement of a large-scale viscoelastic soil model.     

基于时域相关分析的结构损伤指数

阐述了一种基于时域相关分析的结构损伤指数,并进行了该指数的实验研究,目的是探讨一种对损伤敏感的损伤识别方法。试验模型为具有4根柱子的2层钢框架,模型通过底部4根桩埋在土壤里。在底层柱顶端设置缺口代表损伤,设计了2种损伤,有限元数值模拟分析得到的基频变化率分别为0.42%和0.94%。采用激振器对底板进行正弦激励,激励频率为25Hz。利用顶板和中板响应计算该损伤指数,绘制了损伤指数及其变化率与结构损伤程度的关系曲线。结果显示,该指数对损伤敏感,具有进行损伤判定与标定的潜力。     

叶绿素a浓度的动态峰反演方法

叶绿素a是影响内陆水体光学特性的主要水色要素之一.作为反映水体富营养化程度的重要参数指标,国内外学者发展了大量的算法和模型进行水体中叶绿素a浓度的遥感反演.这些算法均使用固定的波段位置.叶绿素a的特征谱段随着其浓度的变化存在一定的位移,尤其是701nm反射峰,其存在着明显的红移现象,因此使用固定波段势必带来一定的误差.通过对不同叶绿素a浓度水样的光谱曲线分析,提出动态峰反演叶绿素a浓度的方法.使用反射峰范围最大值和吸收谷范围最小值,而不是固定波长位置进行建模,以求获得更高的反演精度.