抛物Radon变换法近偏移距波场外推

本文给出了抛物Radon变换的基本原理,以及部分动校正后的CMP道集抛物线近似有效性的证明,基于带限正反最小平方抛物Radon变换的Levinson递推算法,对缺失的近偏移距地震波场进行叠前重建和外推.给出了抛物Radon变换法地震道重建外推的基本原理和叠前地震数据规则化的处理流程,另外对于Radon域均匀采样的情形,本文给出了均匀层状介质和Marmousi模型的近偏移距外推结果,计算结果验证了算法的稳定性和适用性.     

线性同相轴波场分离的高分辨率τ-p变换法

基于最小二乘τ-p变换和τ-p域模型稀疏分布的假设，本文给出高分辨率τ-p变换的推导及其模型空间域的离散采样公式，同时给出了保振幅线性同相轴波场分离的算法流程.在求解本文给出的高分辨率τ-p正变换时，由于待求解的矩阵不具备最小平方法所具有的Toeplitz结构，故采用Cholesky分解法进行计算.本文模拟了井间地震和阵列声波测井中的Stoneley上下行波的分离算法过程，高分辨率正反τ-p变换且滤波所得结果显示本文算法误差小和保振幅的特点.对于在τ-p域距离很近或时间域同相轴近于水平的线性波场，高分辨率算法的聚焦作用使得所分离波场畸变小，体现本文算法精度高的优点.理论模型试算表明本文给出的高分辨率τ-p变换线性波场分离算法具有稳定性、精度高和保振幅的特点.     

基于Hilbert变换的全波场分离逆时偏移成像

逆时偏移方法利用双程波算子模拟波场的正向和反向传播,通常采用互相关成像条件获得偏移剖面,是一种高精度的成像方法.但是传统的互相关成像条件会在偏移结果中产生低频噪声;此外,如果偏移速度中存在剧烈速度变化还可能进一步产生偏移假象.为了提高逆时偏移的成像质量,可在成像过程中先对震源波场和检波点波场分别进行波场分离,然后选择合适的波场成分进行互相关成像.本文基于Hilbert变换,推导了可在偏移过程中进行上下行和左右行波场分离的高效波场分离公式以及相应的成像条件,结合Sigsbee 2B合成数据,给出了不同波场成分的互相关成像结果.数值算例结果表明,采用本文提出的高效波场分离算法以及合理的波场成分互相关成像条件可以获得高信噪比的成像结果.     

多道瞬变电磁法(MTEM)虚拟波场提取技术

多道瞬变电磁法（MTEM）是近年来发展起来的一种新的地球物理勘查技术,其数据采集方式与地震法类似,因此,采用瞬变电磁拟地震解释方法对MTEM数据进行处理解释具有一定的优越性.研究MTEM虚拟波场有效提取方法具有重要意义.在以往奇异值分解法、预条件正则化共轭梯度法两种不同的常规提取方法研究基础上,本文提出采用相关叠加法提取MTEM虚拟波场信息.首先采用预条件正则化共轭梯度法对全时段MTEM数据进行虚拟波场提取,然后采用同种方法对划分的各时间段数据进行虚拟波场提取,最后对全时段提取结果与各个时间段提取结果进行相关性叠加,叠加结果作为最终的提取结果.实测MTEM数据虚拟波场提取结果表明,采用相关叠加法可以得到稳定、光滑的虚拟波场波形曲线,且抗干扰能力强.     

基于Bregman迭代的复杂地震波场稀疏域插值方法

在地震勘探中,野外施工条件等因素使观测系统很难记录到完整的地震波场,因此,资料处理中的地震数据插值是一个重要的问题。尤其在复杂构造条件下,缺失的叠前地震数据给后续高精度处理带来严重的影响。压缩感知理论源于解决图像采集问题,主要包含信号的稀疏表征以及数学组合优化问题的求解,它为地震数据插值问题的求解提供了有效的解决方案。在应用压缩感知求解复杂地震波场的插值问题中,如何最佳化表征复杂地震波场以及快速准确的迭代算法是该理论应用的关键问题。Seislet变换是一个特殊针对地震波场表征的稀疏多尺度变换,该方法能有效地压缩地震波同相轴。同时,Bregman迭代算法在以稀疏表征为核心的压缩感知理论中,是一种有效的求解算法,通过选取适当的阈值参数,能够开发地震波动力学预测理论、图像处理变换方法和压缩感知反演算法相结合的地震数据插值方法。本文将地震数据插值问题纳入约束最优化问题,选取能够有效压缩复杂地震波场的OC-seislet稀疏变换,应用Bregman迭代方法求解压缩感知理论框架下的混合范数反问题,提出了Bregman迭代方法中固定阈值选取的H曲线方法,实现地震波场的快速、准确重建。理论模型和实际数据的处理结果验证了基于H曲线准则的Bregman迭代稀疏域插值方法可以有效地恢复复杂波场的缺失信息。     

Reverse time migration of ground-penetrating radar with full wavefield decomposition based on the Hilbert transform

The conventional reverse time migration of ground-penetrating radar data is implemented with the two-way wave equation. The cross-correlation result contains low-frequency noise and false images caused by improper wave paths. To eliminate low-frequency noise and improve the quality of the migration image, we propose to separate the left-up-going, left-down-going, right-up-going and right-down-going wavefield components in the forward- and backward-propagated wavefields based on the Hilbert transform. By applying the reverse time migration of ground-penetrating radar data with full wavefield decomposition based on the Hilbert transform, we obtain the reverse time migration images of different wavefield components and combine correct imaging conditions to generate complete migration images. The proposed method is tested on the synthetic ground-penetrating radar data of a tilt-interface model and a complex model. The migration results show that the imaging condition of different wavefield components can highlight the desired structures. We further discuss the reasons for incomplete images by reverse time migration with partial wavefields. Compared with the conventional reverse time migration methods for ground-penetrating radar data, low-frequency noise can be eliminated in images generated by the reverse time migration method with full wavefield decomposition based on the Hilbert transform.     

On the downward continuation of electromagnetic fields1

A concise derivation is given for the downward continuation of the tangential fields on the surface to yield expressions for the fields at a specified depth. A homogeneous slab region is assumed for the analysis.     

虚拟波场降速条件下电性源瞬变电磁垂直磁场分量的波场反变换方法

通过波场反变换从瞬变电磁信号中提取虚拟波场是定位电性界面和刻画地下导电目标形态的有效手段.波场反变换过程中, 降低虚拟波场速度有利于改善虚拟波场对电性界面的分辨能力.然而, 波场变换核函数随时间、虚拟时间以及虚拟波场速度变化的动态范围极大, 且随虚拟波场降速而进一步扩大, 造成波场反变换问题高度不适定, 制约了虚拟波场提取的精度和效果.对此, 在虚拟波场降速的条件下, 本文以核函数在无穷区间上积分的解析解为准绳, 通过最优化与数据拟合得到了经验公式, 确定了波场反变换所需的最优积分区间; 在适应采集时间与虚拟波场速度的同时, 最大限度缩短了积分区间长度, 降低了核函数变化的动态范围, 压制了波场反变换方程的病态程度.此外, 通过引入精细积分方法求解波场反变换方程, 提高了波场反变换方程的求解精度.为验证方法的有效性与可靠性, 对正演模拟的电性源瞬变电磁垂直磁场分量及其时间导数分量进行了波场反变换; 结果回代后的拟合数据与正演数据的相对误差小于5%.在所提取的虚拟波场记录中观察到了合理的运动学特征, 如实反映了低阻目标层深度、厚度和二次场观测时间范围的变化.在计算条件相同的前提下, 通过降低虚拟波场速度, 改善了波场反变换的精度和效果.降速后提取的虚拟波场记录对低阻层上下界面的刻画更为清晰.在对实测数据的处理中, 本文的虚拟波场降速、积分区间最优化和精细积分求解方法得到了合理可信的虚拟波场提取结果, 为进一步利用虚拟波场开展低阻目标体成像提供了数据保障.

     

基于曲波变换的航空电磁数据调平方法研究

航空电磁法由于高效和高精度的特点广泛应用于地质填图、矿产资源、地下水、及环境与工程等勘查.然而,航空电磁系统处于动态环境,噪声影响严重,航空电磁数据处理至关重要.航空电磁数据噪声除随机成分外,还包括有各种效应引起的畸变,数据去噪需要依据噪声特征进行处理.航空电磁数据调平是航空电磁数据处理中至关重要的步骤,它能有效去除数...     

Interaction between the electromagnetic and acoustic fields in the marine environment

The possibility of generation of additional acoustic harmonics in the geomagnetic field after the propagation of an acoustic wave in the conducting medium through the region with a variable magnetic field has been considered. Two possible procedures of such a generation have been analyzed: the parametric procedure and the dynamic procedure related to ponderomotive forces. The expressions have been obtained for three acoustic harmonics generated by an oscillating magnetic field.     

井间电磁场时域有限差分数值模拟

为进一步了解井间地下介质结构及其电性特征,本文用时域有限差分方法(FDTD),实现了井间电磁场响应的数值模拟.激发源是探测深度相对磁偶极子更大一些的电偶极子,首先选择一个位于井轴上的垂直电偶极子作为场源,并假设参与计算的介质相对于发射井井轴是轴向对称的,这样可将研究区域作为二维问题处理.推导了二维井间电磁波传播时域差分公式.该方法适用于任何方向入射的电偶极子源,尤其善于解决频域差分方法所难以描述的宽频脉冲.给出了井间金属圆柱和矿体圆柱数值模拟2个例子,结果表明,时域有限差分方法能有效的模拟井间地下介质中电磁波的传播,揭示电磁波传播规律.该方法速度快、精度高、结果稳定,适合用于井间电磁场反演成像的正演响应计算.     

电性源时域地空电磁数据小波去噪方法研究

基于飞艇的时间域地空电磁探测系统,具有勘探深度大、效率高、空间分辨率高、飞行控制容易等优势.但在低空飞行测量过程中,飞艇飞行高度、航迹、姿态等受风向、大气气流、地形、地面局部温度场变化等影响而发生变化,导致固定在艇囊前端的接收线圈发生运动,切割大地磁场,产生了电磁噪声、运动噪声、基线漂移等,从而影响电磁数据的电阻率成像质量.因此,研究地空电磁信号中多种噪声的去除方法,对数据的反演解释非常重要.由于地空电磁信号中有效信号频带与部分噪声频带相重叠,使用传统滤波或消噪方法具有一定局限性.因此,本文提出一种综合小波去噪法:根据地空电磁信号的特点,采用sym8小波基;基于小波多分辨率分析原理,利用小波高尺度近似分量估计基线漂移,以校正电磁数据中的基线;基于小波阈值收缩原理,采用5层小波分解、极小极大阈值配合硬收缩函数的消噪方法,来压制数据中的其余噪声.最后,通过异常环模型的理论响应和实测数据进行算法的验证,结果表明这种综合消噪法对多种噪声均有很好的抑制作用,是一种实用有效的时间域地空电磁数据消噪方法.     

Variations of the electromagnetic fields and ionospheric parameters in the Baikal Rift Zone

Variations in the geomagnetic and electric fields and variations of the total electron content (TEC) of the ionosphere recorded in the Baikal Rift Zone (BRZ) during the expeditions in 2009 and 2010 are analyzed. Synchronous bursts in the geomagnetic field on the ground and in the ionosphere, which are caused by propagation of electromagnetic disturbances (spherics) generated by the remote lightning discharges, are revealed. The analysis of the occurrence frequency of the electromagnetic disturbances at an altitude of ∼700 km shows that there is a preferred region of predominant propagation of these disturbances from the Earth-ionosphere waveguide to the upper ionosphere. When the ionospheric penetration point moves through this preferred region, the frequency spectrum of TEC variations changes, and the northern boundary of the region of spectral alteration is located at ∼54°N. The bursts in TEC that map on the zones of the main faults in the Tunka valley are identified. The results probably suggest a relation between the electromagnetic phenomena in the ionosphere and the structures in the lithosphere.     

On the stimulation of acoustic emission in rock samples by electromagnetic fields

The combined impacts of the electromagnetic fields generated by two independent sources on the process of crack formation in loaded heterogeneous materials (rocks) are studied. The interest in such experiments lies in the fact that in natural conditions, the combination of several fields—the potential triggers of seismic activity—is rather the rule than the exception. It is shown by examples that in the conditions of crossed electric and magnetic fields (CrEMFs) the enhancement in acoustic activity can be triggered with a lower strength electric field than in case of the electric impact alone.     

计算层状介质中电磁场的层矩阵法

现有人工源频率域电磁法的研究大多仅针对某种具体的方法,而较少将问题综合起来分析.本文综合多种方法的共同点提出了层矩阵法,它采取了源置于层间的模型进行公式的推导,理论上可以计算任意层状介质中任意位置的任意场源在空间中任意位置产生的场强,可适用于多种电磁法的正演模拟计算.层矩阵法的核心是对空间域的变量x, y, z中的x和y变量进行傅氏变换后转换到波数域kx和ky中,在波数域利用边界条件,用层矩阵建立起各层的关系后计算得到各层的波数域电磁场值,然后经过二维反傅氏变换最终得到空间域中任意位置的场值.因为文中定义的层矩阵是建立层关系的关键,所以称此方法为层矩阵法.本文以水平电偶源为例独立推导了层状介质中人工源频率域电磁场解的理论公式.为了验证方法的正确性,文中建立了多种模型,利用自行编排的程序将层矩阵法与现有文献的各种解析公式的解进行了对比,结果表明本文提出的层矩阵法是灵活的、可靠的.     

Comparison of waveform inversion, part 1: conventional wavefield vs logarithmic wavefield

This is the first in a series of three papers focused on using variants of a logarithmic objective function approach to full waveform inversion. In this article, we investigate waveform inversion using full logarithmic principles and compare the results with the conventional least squares approach. We demonstrate theoretically that logarithmic inversion is computational similar to the conventional method in the sense that it uses exactly the same back‐propagation technology as used in least‐squares inversion. In the sense that it produces better results for each of three numerical examples, we conclude that logarithmic inversion is also more robust. We argue that a major reason for the inherent robustness is the fact that the logarithmic approach produces a natural scaling of the amplitude of the residual wavefield by the amplitude of the modelled wavefield that tends to stabilize the computations and consequently improve the final result. We claim that any superiority of the logarithmic inversion is based on the fact that it tends to be tomographic in the early stage of the inversion and more dependent on amplitude differences in the latter stages.     

Recent studies of time variations of natural electromagnetic fields in Scotland

A series of geomagnetic induction studies has been undertaken in Scotland since 1973. It includes the operation of two geomagnetic arrays, one over northern Scotland and the other over southern Scotland, subsequent individual station and small-scale geomagnetic array studies, and three sets of magnetotelluric soundings which traverse Scotland from the Lewisian Foreland to the English border. The problems associated with the interpretation of induction data from an island located in the subauroral region are discussed qualitatively and the manner in which both coast and source field effects can be accounted for, is described. The geomagnetic deep sounding data (GDS) from all the observation sites have been collated, and examples of hypothetical event contour maps and traverses across the Great Glen and of individual events from the northern array are presented. They indicate that significant lateral variations in electrical conductivity structure within the crust and upper mantle are associated with the major geological faults in the region. Examples of the results of the magnetotelluric soundings are also presented, together with an outline of the procedure used for one- and two-dimensional modelling. Models of the geo-electric structure in both northern and southern Scotland have been obtained. These show distinctive features which are compatible with the qualitative interpretation of the magnetovariational data. For example, the major granitic blocks are highly resistive while regions of relatively low resistivity exist at upper crustal depths near the Great Glen, Highland boundary and Southern Uplands faults. A zone of low resistivity exists at lower crust-uppermost mantle depths throughout much of the region, the lowest value occurring under the Southern Uplands.     

Modelling and analysis of electromagnetic fields in 3D inhomogeneous media

The main progress in the 3D modelling of electromagnetic fields, achieved during the last few years, is summarized. Various 3D numerical modelling techniques are described and compared as to their possibilities and efficiency. Numerical studies are complemented by laboratory scale modelling.Conventional methods of the analysis of the magnetotelluric and magnetovariational surface characteristics are confronted with new trends in MT tensor analysis and decomposition. Various approaches to the analysis of simplified 3D structures are also presented.     

Contribution of the relative velocity between source and receivers to electromagnetic fields in the sea

The analysis of the primary electromagnetic fields caused by steady state or transient electric current flowing along a current loop moving with a constant velocity below the sea surface has several applications. It supports the analysis of submarine physical data and it is useful for protecting ships from the threat of sea mines. The usual approach to the solution for the primary field starts from a hertz vector potential in the frequency domain due to a magnetic dipole. Subsequently it employs Fourier, Laplace, and Hankel transforms to describe the time variation of the primary electromagnetic induced field due to a loop. The result is applicable to both shallow and deep sea water environments. Because of the difference in velocity between source and receiver, a careful application of the convolution integral is necessary in order to adapt the source pulse solution to any type of transmitting current waveform. Furthermore, since the scattered field represents a fraction of the primary field, even minor differences in it caused by the differential velocity renders inadequate interpretation of EMI data.     

Cauchy integral analogues for the separation and continuation of electromagnetic fields within conducting matter

The main results in the theory of the interpretation of geopotential fields are generalized to the case of arbitrary variable electromagnetic fields by means of elaborating electrodynamic analogues for the integral of the Cauchy type.The generalized Kertz method for separating a variable electromagnetic field into parts related to the sources located in different regions of space is elaborated on the basis of this technique. The generalized Kertz method allows the selection of external and internal, normal and anomalous parts of the geomagnetic field, as well as the separation of geomagnetic anomalies into the surface and deep components caused by conductivity inhomogeneities in the Earth's crust and upper mantle.The theory of analytical continuation of variable electromagnetic fields in a conducting medium is also developed in the present work using the technique of analogues for the integral of the Cauchy type. It is shown that analytical continuation of a field downwards permits the determination of the location and form of deep geoelectric inhomogeneities according to the configuration of the isolines of flux functions for magnetic and electric fields.