2-D Versus 3-D Magnetotelluric Data Interpretation

In recent years, the number of publications dealing with the mathematical and physical 3-D aspects of the magnetotelluric method has increased drastically. However, field experiments on a grid are often impractical and surveys are frequently restricted to single or widely separated profiles. So, in many cases we find ourselves with the following question: is the applicability of the 2-D hypothesis valid to extract geoelectric and geological information from real 3-D environments? The aim of this paper is to explore a few instructive but general situations to understand the basics of a 2-D interpretation of 3-D magnetotelluric data and to determine which data subset (TE-mode or TM-mode) is best for obtaining the electrical conductivity distribution of the subsurface using 2-D techniques. A review of the mathematical and physical fundamentals of the electromagnetic fields generated by a simple 3-D structure allows us to prioritise the choice of modes in a 2-D interpretation of responses influenced by 3-D structures. This analysis is corroborated by numerical results from synthetic models and by real data acquired by other authors. One important result of this analysis is that the mode most unaffected by 3-D effects depends on the position of the 3-D structure with respect to the regional 2-D strike direction. When the 3-D body is normal to the regional strike, the TE-mode is affected mainly by galvanic effects, while the TM-mode is affected by galvanic and inductive effects. In this case, a 2-D interpretation of the TM-mode is prone to error. When the 3-D body is parallel to the regional 2-D strike the TE-mode is affected by galvanic and inductive effects and the TM-mode is affected mainly by galvanic effects, making it more suitable for 2-D interpretation. In general, a wise 2-D interpretation of 3-D magnetotelluric data can be a guide to a reasonable geological interpretation.     

大地电磁局部畸变异常特征三维数值模拟研究

局部畸变问题曾经困扰大地电磁资料反演解释几十年,大地电磁三维数值模拟技术的发展为剖析局部畸变特点和得到可靠的反演成像结果提供了技术基础。本文采用三维数值模拟成像方法对典型三维局部畸变模型进行模拟分析。三维数值模拟结果显示:电场分量垂直电性分界面的极化模式视电阻率曲线(对应二维情况下TM模式)在穿越低阻异常体界面时,曲线会先上移后下移,而在穿越高阻异常体界面时,曲线会先下移后上移,这与电性分界面处积累面电荷产生的二次电场有关。三维模型中XY模式、YX模式视电阻率和相位在三维异常体附近的水平变化是呈现近似垂向对称的,该现象与电场垂直跨越电性界面时视电阻率的变化规律是吻合的,当测线分别沿X方向和Y方向展布时,三维情况下的XY和YX模式分别对应二维情况下的TM模式。低阻小异常体对区域构造响应的畸变影响比高阻小异常体要严重。低阻小异常体对二维区域响应的两种极化模式视电阻率和相位都有非常明显的畸变影响,相比较而言对TE模式的畸变要大于TM模式,因此我们在做二维反演解释时,可优先考虑拟合TM模式数据。位于小异常体中心上方测点的三维畸变响应虽然与对应真实二维区域响应的差异比较大,但可以等效于某种二维模型响应,这种由局部畸变造成的假二维响应在实际野外数据的解释中是需要注意的。     

A Robust Two-Step Inversion of Complex Magnetotelluric Apparent Resistivity Data

Though two-dimensional inversion is now a standard procedure, interpretation of magnetotelluric (MT) data under the assumption of isotropic and one-dimensional structures is a valuable procedure for a first step interpretation of exploratory and solid earth geophysics investigation data. Because its interpretation requires an efficient inverse modelling, we propose and evaluate an inversion procedure, which consists of two steps. Both steps employ jointly the modulus and the phase of the apparent resistivity function. The first one consists of the use of the asymptotic Bostick-Niblett approach. The second employs the result of the inversion obtained in the first step as a starting model to initialize the linearized inversion performed using a multiple re-weighted least-squares approach. We applied the analysis both to synthetic data and to field data from the Parana Basin in Brazil. The results show that the inversion procedure presents a faster convergence without loss of accuracy, increases the resolving power of the MT technique, and may improve its capability to delineate conductors up to a depth of one hundred kilometers. Therefore a reasonable interpretation of the data employing one-dimensional model can be achieved even in the presence of relatively noisy data, and under conditions that slightly violate the premise of lateral homogeneity.     

复杂二维/三维大地电磁的有限单元法正演模拟策略

复杂二维和三维大地电磁模型的正演数值模拟具有一定的挑战性。对于复杂的二维和三维大地电磁正演问题,我们采用有限单元法进行求解。有限单元法最后形成一个线性方程组,系数矩阵是大型稀疏的带状对称复系数矩阵,并且其条件数远大于1,为严重病态矩阵,求解其对应方程组会遇到很多困难。不完全LU分解处理的Bi-CGSTAB迭代方法可用于该线性方程组的求解,并且具有速度快、精度高和稳定性好等优点;为了模拟无穷远边界及满足计算机的内存需求,在保证计算精度的情况下设计了非均匀网格剖分;在程序编制中,只存储有限元系数矩阵的非零元素,大大减少了正演计算的时间。通过对二维和三维模型电磁响应的计算,验证了算法的正确性。     

隐伏地质构造的大地电磁有限单元法正演模拟

对常见的隐伏地质构造体进行大地电磁场正演模拟,并研究和总结了构造体对大地电磁场的响应特征.由麦克斯韦方程组导出大地电磁场的边界条件,应用有限单元法求解变分问题并得出刚度矩阵,根据线性方程组计算出相应的场值,再由改进的视电阻率定义式求出视电阻率.模拟结果显示:改进的视电阻率定义式相对于传统的Cagniard定义式能有效地提高视电阻率曲线的收敛速度和逼近程度;隐伏地质构造体对大地电磁场的响应具有一定的规律性,通过分析两种极化模式下的正演结果,可以有效地识别地质构造体的存在和分布情况.通过实例分析,得出隐伏地质构造正演模拟与实际观测资料的互相验证关系,为实践应用中的反演工作提供了重要的指导意义.     

利用积分方程法的大地电磁三维正演

利用积分方程法实现了均匀导电半空间三维大地电磁响应的数值模拟。求取张量格林函数积分时,采用二次剖分算法解决计算中奇异值问题,对于含有贝塞尔函数的积分项,利用结合连分式展开的高斯求积代替常规的快速汉克尔变换方法,确保了张量格林函数的正确计算并提高了计算精度。最后通过数值模拟结果的对比及模型试算验证了算法的正确性,所实现的三维大地电磁数值模拟算法为理论研究三维地电构造的大地电磁响应的分布规律提供了有效的工具,也为研究三维反演算法奠定了基础。     

Mechanical Stability Of Mt. Vesuvius Volcano: Effects Of Asymmetries On The Stress Field

The effect of nonaxisymmetric elasticheterogeneities on the mechanical stability of a nearly axisymmetricvolcanic structure, with particular reference to Mt. Vesuvius (Italy) is investigated. This is done solving numerical models using a finite element codein the framework of linear elasticity. The models include gravity,anisotropic depth-dependent regional stress, the edifice, thepossible presence of a pressurized feeding system and of heterogeneities inthe elastic behavior of country rocks. The criteria to assessinstability are the development of tensile stresses and, in compression, theNavier–Coulomb criterion. The presence of asymmetric heterogeneous structureswith lateral elastic contrast has been considered by solvingthree-dimensional (3D) models. To have computationally tractable models, axis-symmetric models were solved first, considering lateral symmetricheterogeneities of different shapes and sizes around the symmetry axis. This allowed us to assess the minimum size of smallermodels (submodels) to be developed in 3D. Thesubmodels were then solved including asymmetrical heterogeneities.In all the analyzed cases, the main characteristics of theinstabilities/stabilities found with the 3D asymmetric models are verysimilar to those found by the analogous symmetric models.Moreover, the presence of sharp lateral elastic contrast at shallow depthappear to produce a greater instability on the flanks of the volcanicedifice.     

嘉峪关地电场观测资料分析

研究了大地电场变化的基本要素,对嘉峪关地电场观测资料的质量进行了评价和分析,得到:嘉峪关台地电场观测基本反映了地电场的真实变化,特别对大地电场变化的记录准确.并研究了嘉峪关台附近及周围发生的4次中强地震前地电场、大地电场短临前兆现象,认为震前都显示了一定的短临异常.     

山西临汾中心地震台地电场观测数据质量分析评价

简要介绍了山西临汾中心地震台大地电场观测系统,并对2009年以来更换电极后,观测数据质量进行了分析评价,表明,临汾台大地电场观测数据能够反映出地电场强度的时空变化,数据质量符合规范要求,可以为地震前兆研究提供相应信息。     

"现代地球磁场监测与地磁基本数据积累"项目成果介绍

本文详细介绍了“现代地球磁场监测与地磁基本数据积累”项目的立项背景、目标、所开展的主要工作、取得的成果、研究成果的社会效益和对该领域研究未来工作的展望。     

大地电磁法监测地震活动能力的研究

我国大陆地震多发生在１０￣２５ｋｍ深处，在此部位的电性能前兆异常，按穿透深度规律传输至地表已相当微弱。当同时考虑场兆的作用时，由于电性异常区域的扩展，可大大增强大地电磁法监测地震活动的能力。     

云南弥渡地震台地电场观测数据分析评价

介绍了弥渡地震台地电场观测系统,对经过两次技术改造后的弥渡台2012年地电场观测数据进行了分析评价,结果表明:弥渡地电场观测数据能够反映出地电场时空变化特点,数据质量符合地电场观测的技术要求,数据可以为地震前兆研究和地震监测预报实践提供相应信息。     

磁异常场建模中的CHAMP卫星磁测数据通化技术的研究

根据2000～2008年CHAMP卫星磁测数据,研究磁异常场建模中的卫星磁测数据通化技术,建立了川滇地区磁异常场的泰勒多项式模型,并和最新的磁异常场模型MF6做了比较。     

弹性波场数值模拟的隐式差分多重网格算法

为了得到稳定的弹 性波数值模拟,而不得不选择隐式差分方程;为了提高解的精度,又不得不增加节点数目, 但同时也降低了隐式迭代求解的收敛速度. 为此,本文使用隐式差分的多重网格算法进行弹 性波数值模拟,多重网格算法通过粗网格收敛较快的迭代过程求出近似解,以近似解为初值 使用细网格进行精确的迭代求解,从而加速了隐式迭代求解的过程,能够以较高计算速度、 精度、稳定性完成弹性波传播过程的数值模拟.     

Study of the Ability of the Magnetotelluric Method to Monitor Earthquakes

Most earthquakes on the Chinese mainland have occurred within the depth range of 10-25 km,which is usually called the seismogenic layer.According to the rule of penetration depth,the precursor of the electric anomaly,which is transferred from the seismogenic layer to the Earth's surface,should be very weak,but the ability to monitor earthquakes can be greatly improved if the field precursor effect is taken into consideration because of expansion of the electric anomaly area.The results of one-and two-dimensional modeling computations are consistent with the observed fact that the apparent resistivity decreases before earthquakes.This provides good evidence for our inference.The corresponding physical mechanism is that the conductivity of the anomaly area may be increased by the rock dilatation and fluid permeation due to the stress effect during the developing process of earthquakes.     

长周期大地电磁测深数据采集及处理技术

研究岩石圈底部的上地幔盖层甚至软流圈的电性结构,需要采集周期长达几万秒的长周期大地电磁场信号,这对仪器设备、采集以及处理技术都提出了更高的要求.本文论述了用于采集长周期信号的仪器设备及其特殊性,讨论了长周期大地电磁测深测站的布置要求,并采用长周期大地电磁测深仪器试采了长周期大地电磁场信号.对试采信号的处理结果表明,采用长周期仪器设备和稳定性好的不极化电极,合理布置测站,能够采集到最长周期达几万秒的长周期大地电磁场信号.     

甘肃嘉峪关地震台大地电场观测数据干扰分析

文章对甘肃嘉峪关地震台2001年至2009年以来大地电场分钟值资料做了全面的分析和整理。对造成观测资料相关性低的电暴（磁暴）、人类活动、雷电、降水、观测系统本身变化（包括更换电极）、电阻率测量、公用接地网等干扰因素进行分析,并做出相关的解释。这将对分析和运用地电场资料进行地震预报有一定的意义。     

Hydrodynamical Modeling Of Oceanic Vortices

Mesoscale coherent vortices are numerous in the ocean.Though they possess various structures in temperature and salinity,they are all long-lived, fairly intense and mostly circular. Thephysical variable which best describes the rotation and the density anomaly associated with coherent vortices is potential vorticity. It is diagnostically related to velocity and pressure, when the vortex is stationary. Stationary vortices can be monopolar (circular or elliptical) or multipolar; their stability analysis shows thattransitions between the various stationary shapes are possible when they become unstable. But stable vortices can also undergo unsteady evolutions when perturbed by environmental effects, likelarge-scale shear or strain fields, -effect or topography. Changes in vortex shapes can also result from vortex interactions. such as the pairing, merger or vertical alignment of two vortices, which depend on their relative polarities and depths. Such interactions transfer energy and enstrophy between scales, and are essential in two-dimensional and in geostrophic turbulence. Finally, in relation with the observations, we describe a few mechanisms of vortex generation.     

The Selection of Field Acquisition Parameters for Dispersion Images from Multichannel Surface Wave Data

The accuracy and resolution of surface wave dispersion results depend on the parameters used for acquiring data in the field. The optimized field parameters for acquiring multichannel analysis of surface wave (MASW) dispersion images can be determined if preliminary information on the phase velocity range and interface depth is available. In a case study on a fill slope in Hong Kong, the optimal acquisition parameters were first determined from a preliminary seismic survey prior to a MASW survey. Field tests using different sets of receiver distances and array lengths showed that the most consistent and useful dispersion images were obtained from the optimal acquisition parameters predicted. The inverted S-wave velocities from the dispersion curve obtained at the optimal offset distance range also agreed with those obtained by using direct refraction survey.     

Processing And Interpretation Of Electromagnetic Induction Array Data

The simultaneous nature of array data can be exploitedin electromagnetic induction studiesfor three general purposes.First, one or more reference sites can be usedto reduce bias, improve signal-to-noise ratios, and provide bettercontrol over source complications and coherent noisein estimates of MT impedances and otherEM transfer functions (TFs).Although a single good reference site can dramatically improveTF estimates, improvements due to multiple sites are often rather modest, because local noise is the limiting factor.Secondly, arrays allow for estimation of inter-station transfer functions, and maps of anomalous horizontal field variations. Relatively straightforward modifications to inversion codes wouldallow quantitative interpretation of these additionalconstraints on resistivity variations.Finally, with arrays it is possible to estimate the response of theEarth to a richer spectrum of external source excitations. In particular,the natural extension to the usual uniform source assumption implicit inthe MT method allows for three curl-free magnetic gradient sources.Quantitative interpretation of the response of a three-dimensionalEarth to these sources could provide additionalconstraints on large scale variations in crustal and uppermantle resistivity, and might help to overcome problemsdue to aliasing of near surface distortion in widely spaced MT data.