Subsurface offset behaviour in velocity analysis with extended reflectivity images

Migration velocity analysis with the constant‐density acoustic wave equation can be accomplished by the focusing of extended migration images, obtained by introducing a subsurface shift in the imaging condition. A reflector in a wrong velocity model will show up as a curve in the extended image. In the correct model, it should collapse to a point. The usual approach to obtain a focused image involves a cost functional that penalizes energy in the extended image at non‐zero shift. Its minimization by a gradient‐based method should then produce the correct velocity model. Here, asymptotic analysis and numerical examples show that this method may be too sensitive to amplitude peaks at large shifts at the wrong depth and to artefacts. A more robust alternative is proposed that can be interpreted as a generalization of stack power and maximizes the energy at zero‐subsurface shift. A real‐data example is included.     

Improving the gradient of the image‐domain objective function using quantitative migration for a more robust migration velocity analysis

Migration velocity analysis aims at determining the background velocity model. Classical artefacts, such as migration smiles, are observed on subsurface offset common image gathers, due to spatial and frequency data limitations. We analyse their impact on the differential semblance functional and on its gradient with respect to the model. In particular, the differential semblance functional is not necessarily minimum at the expected value. Tapers are classically applied on common image gathers to partly reduce these artefacts. Here, we first observe that the migrated image can be defined as the first gradient of an objective function formulated in the data‐domain. For an automatic and more robust formulation, we introduce a weight in the original data‐domain objective function. The weight is determined such that the Hessian resembles a Dirac function. In that way, we extend quantitative migration to the subsurface‐offset domain. This is an automatic way to compensate for illumination. We analyse the modified scheme on a very simple 2D case and on a more complex velocity model to show how migration velocity analysis becomes more robust.     

GPR velocity determination by image-wave remigration

We have studied in detail the theoretical and numerical properties of a finite-difference algorithm for image-wave time-remigration. For a number of synthetic models, numerical experiments have been performed. For these examples, we obtained perfect agreement between the theoretical predictions and numerical results. The examples also prove the computational efficiency of the algorithm. An application to ground-penetrating-radar (GPR) data demonstrates that image-wave remigration can be used to estimate models with laterally varying velocities. The quality of the latter is confirmed by a final zero-offset time migration.     

高分辨率P-SV波速度分析

本文在常规转换波速度分析算法基础上,引入两个速度检测因子,提出了两种高分辨率的转换波速度分析方法,称之为转换波协方差速度分析法和转换波ECM（Enhanced Complex Matched Filter Method）速度分析法,同时对转换点计算公式、共转换点道集选排进行了研究,选取了最佳的转换点精确解计算公式,应用到基于时窗的CCP道集分选方法中,并与高分辨率速度分析同时进行,实现了转换波的高分辨率速度分析的算法,理论模型的计算结果表明,二者在时间及速度上都具有较高的分辨率.     

Refractor velocity analysis: a signal processing procedure

This paper presents a signal processing procedure to perform refractor velocity analysis. The procedure enables one to obtain the seismic velocity from the refracted wavefield without the picking of refracted arrival times. Two processing procedures are derived, one starting from a seismic interferometric approach and another, from the conventional reciprocal method and generalized reciprocal method approaches. The theoretical equivalence of the two approaches is also demonstrated. The proposed processing procedure is applied to synthetic data in order to test the influence of some procedural parameters and its capability to reconstruct a known velocity model starting from refracted signals, without and with perturbations, in arrival times and noise; finally, it is applied to a field data set.     

双参数展开CRP叠加和速度分析方法研究

椭圆展开共反射点(CRP)方法可以获得比常规倾角时差校正(DMO)方法更近似的零偏移距时间剖面和相应CRP速度场.大量研究和实践证实,在非均质性较弱的地区,该方法取得的成果显著.但由于该方法没有考虑速度的横向变化和转换波等情况,当地下介质存在较强非均质性时,该方法不再准确,需要引进反映速度横向变化的双参数(上行波与下行波的平均速度和速度比)进行改进.本文详细推导了引入双参数后的叠加和速度分析算法,并通过数值模型和地震资料处理证实,修正后的算法可以更好地解决地质复杂地区速度建模和叠加成像问题.     

Piecewise‐continuous velocity inversion

We suggest a new method to determine the piecewise‐continuous vertical distribution of instantaneous velocities within sediment layers, using different order time‐domain effective velocities on their top and bottom points. We demonstrate our method using a synthetic model that consists of different compacted sediment layers characterized by monotonously increasing velocity, combined with hard rock layers, such as salt or basalt, characterized by constant fast velocities, and low velocity layers, such as gas pockets. We first show that, by using only the root‐mean‐square velocities and the corresponding vertical travel times (computed from the original instantaneous velocity in depth) as input for a Dix‐type inversion, many different vertical distributions of the instantaneous velocities can be obtained (inverted). Some geological constraints, such as limiting the values of the inverted vertical velocity gradients, should be applied in order to obtain more geologically plausible velocity profiles. In order to limit the non‐uniqueness of the inverted velocities, additional information should be added. We have derived three different inversion solutions that yield the correct instantaneous velocity, avoiding any a priori geological constraints. The additional data at the interface points contain either the average velocities (or depths) or the fourth‐order average velocities, or both. Practically, average velocities can be obtained from nearby wells, whereas the fourth‐order average velocity can be estimated from the quartic moveout term during velocity analysis. Along with the three different types of input, we consider two types of vertical velocity models within each interval: distribution with a constant velocity gradient and an exponential asymptotically bounded velocity model, which is in particular important for modelling thick layers. It has been shown that, in the case of thin intervals, both models lead to similar results. The method allows us to establish the instantaneous velocities at the top and bottom interfaces, where the velocity profile inside the intervals is given by either the linear or the exponential asymptotically bounded velocity models. Since the velocity parameters of each interval are independently inverted, discontinuities of the instantaneous velocity at the interfaces occur naturally. The improved accuracy of the inverted instantaneous velocities is particularly important for accurate time‐to‐depth conversion.     

Borehole radar velocity inversion using cokriging and cosimulation

A new radar velocity tomography method is presented based on slowness covariance modeling and cokriging of the slowness field using only measured travel time data. The proposed approach is compared to the classical LSQR algorithm using various synthetic models and a real data set. In each case, the proposed method provides comparable to or better results than LSQR. One advantage of this approach is that it is self-regularized and requires less a priori information. The covariance model also allows stochastic imaging of slowness fields by geostatistical simulations. Stable characteristics and uncertain features of the inverted models can then be easily identified.     

基于稀疏反演理论的自动叠加速度反演方法

叠加速度分析技术是常规地震资料处理中的重要环节,也是经典的时间域速度建模方法.叠加速度分析技术主要包括速度谱计算和拾取两个步骤.至今为止,多数研究工作通过提高速度谱的分辨率以及抗噪声能力,获得高质量的速度谱从而有利于拾取.本文的目标是将叠加速度分析技术转为一个全自动化的处理流程.从参数估计的角度出发,将叠加速度估计转化为稀疏反演框架下的模型参数估计问题,并通过稀疏反演算法自动反演叠加速度,进而提高叠加速度建模的效率.为实现这一目标,首先给出了正问题的定义,即层状介质中CMP道集的预测模型,利用叠加速度、垂向双程走时(t_0)以及反射子波以及CMP道集时距关系(如双曲时距关系)可以预测CMP道集.接着,速度分析反问题可以描述为已知观测的CMP道集,估计模型参数(叠加速度及t_0时间等).利用模型参数的稀疏性作为约束条件并用L_0范数作为模型稀疏性的度量准则,叠加速度分析可以转化为L_0范数约束下的稀疏反演问题.本文提出了一种基于预测校正思想的匹配追踪算法求解上述反问题,实现了自动叠加速度建模并为后续的高精度速度反演方法提供较好的初始模型.理论和实际资料的测试结果证明了本文方法的有效性.     

中国东部海域及邻区岩石层地幔的P波速度结构与构造分析

利用中国大陆东部及台湾地区、日本和琉球群岛的地震观测数据,通过体波地震层析技术反演了中国东部海域及其邻近地区的P波速度结构.以此为依据分析了不同地区的岩石层性质和深部动力学条件,探讨了中朝与扬子块体、扬子与华夏块体在海区的深部边界及其构造属性,揭示出菲律宾海板块与欧亚大陆的碰撞以及板片俯冲下沉、弧后扩张作用对中国东部海域岩石层结构的影响.结果表明,中国东部海域的岩石层地幔存在明显的横向非均匀性,它们与区域构造的形成演化有一定的联系.中国大陆东部的五莲－青岛断裂与朝鲜半岛西缘断裂、济州岛南缘断裂共同构成中朝和扬子块体的边界,江绍断裂向东延伸至朝鲜半岛南端成为分隔扬子和华夏块体的边界;东海陆架与冲绳海槽的岩石层结构差异明显,东海陆架具有中国东部地区的岩石层特征,属于欧亚大陆向海域的延伸;冲绳海槽的岩石层强烈减薄,为大陆向大洋过渡的区域;沿着日本－琉球－台湾俯冲带,菲律宾海板块俯冲下沉引起的地幔扰动对中国东部海域产生了较大的影响,欧亚大陆与菲律宾海板块之间的相互碰撞导致台湾地区岩石层明显增厚.     

Simultaneous inversion for velocity structure and hypocenters in Slovenia

We use nearly 2100 P-wave arrival times from 166 local earthquakes to investigate the 3-D compressional velocity structure of the upper crust of Slovenia using the simultaneous inversion algorithm developed by (Michelini and McEvilly, 1991). Remarkable and stable features of the resolved model are the relatively high velocities in western Slovenia and the low velocities in central Slovenia, SE of the Ljubljana basin. The boundary between these two anomalies follows approximately the NNW-SSE direction that coincides with the general strike of the External Dinarides. We interpret this feature as the upper crustal expression caused by the tectonic processes occurring along the active margin of the Adria promontory/microplate.     

基于平均速度分析的近地表纵波速度反演

面波频散曲线对于横波速度、纵波速度、层厚等近地表地球物理参数的敏感度相差较大, 现阶段通过频散曲线可以获得较为精确的近地表横波速度与厚度信息, 但无法直接对纵波速度进行反演.研究表明, 泊松比对于波长(W)-探测深度(D)关系较为敏感.基于这一发现, 本文根据频散曲线与反演获取的横波速度结构计算平均速度, 获取W-D关系曲线.但合成数据测试证明, 因近地表浅层对应的W-D曲线变化较小, 且浅层纵波速度反演不准确会使误差累积, 直接反演W-D曲线无法获取可靠的纵波速度剖面.本文改进了基于平均速度分析的近地表纵波速度反演方法, 在目标函数中加入了对浅层信息较为敏感的W/D-D信息, 同时对W-D曲线与W/D-D曲线进行联合反演.合成数据测试证明可以获取较为准确地浅层与深层纵波速度.将该方法应用于实际地震数据中, 联合反演得到的纵波速度剖面与微测井数据较为吻合, 证明本文提出的方法可以不借助其他信息, 仅通过面波频散信息, 获取更为准确地近地表纵波信息.

     

基于Hinge损失函数的垂向全变差约束全波形反演

全波形反演可以为叠前深度偏移成像提供更高精度的速度模型,但该方法具有较强的非线性,对初始速度模型的依赖性较强,尤其是在实际应用中,地质条件复杂多变,速度变化不连续,增加了反演非线性程度,常常使反演陷入局部极小值,影响反演的精度.全变差约束在图像去噪领域应用广泛,属于非光滑约束,在去噪过程中能有效的保留图像的不连续界面和边缘信息.本文提出基于Hinge损失函数的垂向全变差约束全波形反演方法,在全变差约束的基础上,利用Hinge损失函数控制模型的更新方向,并使用原-对偶混合梯度算法进行求解,给出这一优化问题的迭代格式,有效提高了对地下不连续界面的重构精度,同时也降低反演对初始速度模型的依赖程度.数值算例证明：与常规全波形反演方法相比,基于全变差约束的全波形反演方法可以有效的重构速度模型中的不连续界面,尤其对高速体边缘的重构效果更明显,但该方法对初始速度模型的依赖性仍然较强;基于Hinge损失函数的垂向全变差约束全波形反演方法降低了对初始速度模型的依赖程度,可以从一个较差的初始模型通过循环迭代的方式最终得到同样精确的速度模型,较好的重构了高速体边缘和不连续界面.

     

地震勘探中速度信号转化为加速度信号的分析及应用

目前,广泛应用在地震勘探中的检波器类型主要是速度型检波器和加速度型检波器,分别接收振动的速度信息和加速度信息,不同类型的检波器接收的地震资料具有不同的响应特征.在滩浅海地震勘探中,不同的地表区域使用不同类型的检波器接收,滩上使用速度检波器接收、海中使用加速度检波器和速度检波器组成的"双检"检波器接收,所以存在着两类检波器资料的拼接、融合处理问题.本文在理论上通过速度检波器和加速度检波器机电转化原理不同,跟踪不同物理量说明了两种信号之间的关系,并通过理论分析、实际的地震数据资料分析和证明了速度信号通过微分计算转化为加速度信号的可行性和正确性,并进行了相应的对比和验证.在滩浅海以及浅海地震勘探中应用加速度检波器和速度检波器联合采集后,应用这种转换方法进行资料的拼接处理和融合处理,得到的地震剖面质量得到了很好的提升.     

地层衰减对地震波速度逆散射反演的影响研究

在利用地震波数据进行地球物理反演时,地层对地震波的吸收衰减效应会对地层物性参数的准确反演产生较大的影响,因此利用黏弹性声波方程进行反演更符合实际情形.本文在考虑地层衰减效应进行频率空间域正演模拟的基础上,提出基于黏弹性声波方程的频率域逆散射反演算法并对地震波传播速度进行反演重建,在反演过程中分别用地震波传播复速度和实速度来表征是否考虑地层吸收衰减效应.基于反演参数总变差的正则化处理使反演更加稳定,在反演中将低频反演速度模型作为高频反演的背景模型进行逐频反演,由于单频反演过程中背景模型保持不变,故该方法不需要在每次迭代中重新构造正演算子,具有较高的反演效率;此外本文在反演过程中采用了基于MPI的并行计算策略,进一步提高了反演计算的效率.在二维算例中分别对是否考虑地层吸收衰减效应进行了地震波速度反演,反演结果表明考虑衰减效应可以得到与真实模型更加接近的速度分布结果,相反则无法得到正确的地震波速度重建结果.本文算法对复杂地质模型中浅层可以反演得到分辨率较高的速度模型,为其他地震数据处理提供比较准确的速度信息,在地层深部由于地震波能量衰减导致反演分辨率不太理想.     

频率多尺度全波形速度反演

以二维声波方程为模型,在时间域深入研究了全波形速度反演.全波形反演要解一个非线性的最小二乘问题,是一个极小化模拟数据与已知数据之间残量的过程.针对全波形反演易陷入局部极值的困难,本文提出了基于不同尺度的频率数据的"逐级反演"策略,即先基于低频尺度的波场信息进行反演,得出一个合理的初始模型,然后再利用其他不同尺度频率的波场进行反演,并且用前一尺度的迭代反演结果作为下一尺度反演的初始模型,这样逐级进行反演.文中详细阐述和推导了理论方法及公式,包括有限差分正演模拟、速度模型修正、梯度计算和算法描述,并以Marmousi复杂构造模型为例,进行了MPI并行全波形反演数值计算,得到了较好的反演结果,验证了方法的有效性和稳健性.     

扩展成像条件下的最小二乘逆时偏移

逆时偏移(RTM)是复杂介质条件下地震成像的重要手段.因受观测系统限制、上覆地层影响以及波场带宽有限等因素的影响,现行的常规RTM所采用的互相关成像条件通常对地下构造进行模糊成像.最小二乘逆时偏移(LSRTM)通过最小化线性Born近似正演数据和采集数据之间的波形差异,采用梯度类反演算法优化反射系数模型,获得的成像结果具有更高的分辨率和更可靠的振幅保真度.然而,基于波形拟合的LSRTM对背景速度模型的依赖性很强.误差太大的速度模型容易产生周波跳跃现象,导致LSRTM难以获得全局最优解.为了克服这一问题,本文基于扩展模型的思想,在线性Born近似下,推导得到RTM扩展成像条件.并基于最小二乘反演理论,提出扩展成像条件下的LSRTM方法.理论模型试算表明,本文方法不仅可以提供分辨率更高、振幅属性更为可靠的成像结果,而且能够在一定程度上消除速度误差对反演成像的影响.     

川滇地区中小震重新定位与速度结构的联合反演研究

通过震源与速度结构联合反演, 利用2000年4月至2006年3月云南和四川区域地震台网给出的P波初至走时资料, 确定了川滇地区的三维速度结构, 同时获得了川滇地区6642次中小地震的重新定位结果。 结果表明: ① 川滇地区地震震源平均深度随震级增大而加深的特征明显, 地震震级越大, 震源深度越深, 但震源下界不超过25 km; ② 在瑞丽-龙陵、 丽江-小金河以及龙门山等断裂带以西地区, 震源深度偏浅, 大多在15 km以上, 15 km深度以下地震稀少; ③ 川滇地区中小地震分布具有与强震相同的地壳深部介质背景, 震源大多分布于正、 负异常过渡区的速度相对较高一侧, 而其下方主要为低速异常分布。     

Tomographic inversion for the three-dimensional seismic velocity structure of the Aswan region,Egypt

The Thurber iterative simultaneous inversion program is used to determine the three-dimensionalP-wave velocity structure in the Aswan seismic region of Egypt. The tomographic inversion presented in this study is based on 1131P-phase observations at 13 stations from 89 local earthquakes, all of which occurred within the Kalabsha fault zone. The assumed initial velocity model is that deduced from local explosion experiments. The results indicate that the Aswan region is characterized by a heterogeneous crust, consisting of a shallow, low-velocity zone and a deeper high-velocity anomaly. Seismic velocity structure within the shallow part demonstrates that the inferred change in velocity exists primarily across the east-west trending Kalabsha fault scarp, whereas the high-velocity zone is located south of this fault. Two well-resolved, low-velocity zones appear within the upper 6 km of the crust. The first coincides with a graben structure located between the Kalabsha and Seiyal faults and the second exists between the N-S Kurkur fault and the main axis of Lake Aswan. Both low-velocity zones occupy an area of approximately 30×40 km, located along the western bank of the lake. The most significant result of this study is that the location of the deeper, high-velocity anomaly coincides with the concentration of seismic activity in the lower crustal layer.     

云南通海盆地的浅层速度结构

为了研究通海盆地的地震动场地效应,在强地面运动的数值模拟计算中需要提供该地区的浅层速度结构模型.为此,在通海盆地开展了10 km高分辨率反射地震勘探和30个测点的面波联合勘探工作.综合本次探测结果与褐煤普查中的电测深和钻孔资料,得到了通海盆地的基岩埋深分布图.结果表明,在通海盆地中部,新生界厚度为100~300 m,整体表现出西南厚东北薄的趋势,盆地沉积层厚度变化明显受到小江断裂南段分支断裂的控制.浅层速度随深度分布表现出在深度相同的情况下,盆地边缘的速度高于盆地中部.