各向异性介质的大地电磁反演及其有关地震前兆场的讨论

论述了某些条件下地球介质呈现出的电导率各向异性行为及层状对称各向异性介质大地电磁资料的反演方法；实测大地电磁(MT)资料的各向异性模型反演解释有助于识别地震深部电导率各向异性变化前兆，并对研究地壳深部应力状态和形变带提供了若干有用的信息．      

各向异性介质的大地电磁反演及其有关地震前兆场的讨论

论述了某些条件下地球介质呈现出的电导率各向异性行为及层状对称各向异性介质大地电磁资料的反演方法；实测大地电磁（ＭＴ）资料的各向异性模型反演解释有助于识别地震深部电导率各向异性变化前兆，并对研究地壳深部应力状态和形变带提供了若干有用的信息     

利用大地电磁测深法识别EDA地震电性变化前兆

论述了起因于ＥＤＡ现象的地球介质电导率各向异性特征，分析研究了地球介质是忸率各向异性变化引起的大地电磁响应函数变化形态特征，简述了大地电磁测深法对ＥＤＡ电性变化前兆的可识别性及大地电磁资料解释中考虑电导率各向异性的必要性，研究了对称各向异性层状介质大地电磁资料的反演问题。     

基于交叉梯度结构约束的大地电磁主轴各向异性并行三维反演

各向异性介质对大地电磁观测数据的影响往往不可忽略,因此需要提高大地电磁各向异性三维反演的可靠性和有效性.为了满足大地电磁各向异性三维反演的需求,本文研究了一种基于交叉梯度结构约束的大地电磁主轴各向异性并行三维反演算法.根据大地电磁平面波理论假设,正演方程采用背景场与二次场分离的计算方式,二次场利用交错网格有限差分法求解.由于各向异性反演的多解性,本文将各向异性介质简化为主轴各向异性,并在此基础上进一步采用有限内存拟牛顿LBFGS法实现三维各向异性反演.为了提高各向异性反演的分辨率,反演目标函数中引入交叉梯度项,利用先验的结构信息,对三个方向的电阻率参数进行结构约束,最终的反演进一步利用MPI(Message Passing Interface,消息传递接口)技术实现分频并行计算,测试结果显示并行接近线性加速比.     

甘肃及邻近地区的各向异性介质MT反演及该区地壳深部应力场和形变带

分析了某些条件下地球介质的电导率各向异性行为，并应用一维层状对称各向异性介质大地电磁资料的反演方法，对甘肃及其邻近地区实测ＭＴ资料的各向异性模型进行反演解释，由此确定地壳深部应力场和形变带的方向。     

天祝-永登地区大地电磁资料的一维各向异性反演

对分布在甘肃天祝、永登一带的10个大地电磁测点资料进行了一维各向异性介质反演研究，并与二维对称各向异性介质反演结果进行了对比。结果表明在大多数情况下一维各向异性反演有可能揭示出地壳介质电导率各向异性结构的主要特征；同时利用某些平均阻抗和特征阻抗的一维各向同性介质反演亦可获得较为可信的电性分层界面。反演所获得之介质电导率可能为各主方向电导率主值的某种平均。     

大地电磁二维各向异性反演及其在青藏高原北部的应用

地球介质的电性特征在不同深度上都可能表现出各向异性,识别电各向异性有助于深入理解地球内部物质状态及变形环境,促进对动力学模型的理解.本研究在已实现的任意电各向异性大地电磁有限差分正演算法的基础之上,通过构建常规的各向异性反演目标函数,完成对目标函数及其梯度的计算.借助成熟的非线性共轭梯度反演技术,实现了对大地电磁各向异性介质的反演过程.在此基础上,构建了二维方位各向异性理论模型,来验证反演过程的稳定性.理论模型的反演结果直观地显示了大地电磁各向异性反演对模型参数中垂直电导率恢复上的局限性;在各向异性结构分析上需要将电导率张量作为整体考虑.对青藏高原北部东昆仑—柴达木盆地西段的实测大地电磁数据进行反演,并对比早期二维各向同性反演结果发现,各向同性结构中位于祁漫塔格山脉下方上地幔顶部存在低阻异常带,在相同的位置,各向异性结构中表现出明显的方位各向异性,其各向异性低阻主轴电阻率指示的剪切带走向与地表造山带走向不一致,这暗示该处的应力环境复杂,除了受印度—欧亚板块碰撞控制外,还可能受到邻近阿尔金走滑断裂带左行剪切运动的影响.     

二维对称各向异性介质大地电磁反演

考虑一种二维构造走向与一水平各向异性主方向一致的对称各向异性介质. 在这种特殊情况下,若假设电导率沿垂向和倾向表现相同的各向异性, 即sigma;zz=sigma;yy,则得到形式完全等同于各向同性情形TE和TM极化方程的微分方程. 因此, 就可用各向同性完全相同的反演方法, 获得二维各向异性介质的电导率. 亦即各向同性介质的TE和TM极化反演结果, 可以解释为对称各向异性介质两主方向上的电阻率. 这为MT解释提供了新的解释途径及其理论依据. 最后用由此发展起来的各向异性介质反演方法,对甘肃天祝永登一带大地电磁资料进行了反演试验,获得了该区地壳介质各向异性电导率结构      

大地电磁相位张量和倾子的三维主轴各向异性反演

地壳和上地幔普遍存在的电性各向异性是大地电磁测深方法在实践应用中的关键和难点问题,而由近地表局部地电异常引起的电流型畸变可进一步加剧各向异性反演的多解性和不确定性.为压制电流型畸变影响,本文实现了一种利用大地电磁相位张量和倾子数据获取地下介质主轴电性各向异性结构的三维反演算法.该算法正演问题的求解采用交错采样有限差分法,反演问题的求解采用非线性共轭梯度法.理论模型测试表明,相位张量和倾子的三维各向异性反演不仅恢复了真实的各向异性异常分布特征,且压制了为拟合电流型畸变出现的假异常.由于相位张量和倾子数据不含幅值信息,先验模型选取对反演模型电阻率值的恢复效果影响较大,但初始模型的改变基本不影响反演结果.为进一步验证该算法的有效性,将其应用于USArray项目在美国西北部采集的109个长周期大地电磁实测数据.反演结果表明,相对于阻抗数据,相位张量和倾子数据的各向异性反演结果对浅层结构的刻画更为准确,与地表地质特征更为吻合.     

面向目标自适应有限元法的带地形三维大地电磁各向异性正演模拟

传统三维大地电磁各向异性模拟均是基于规则六面体网格,计算精度有限且较难拟合复杂地质条件.本文采用面向目标自适应非结构矢量有限元法,对三维大地电磁各向异性介质进行模拟.首先从电场双旋度方程出发,利用伽辽金方法建立变分方程;然后利用电流密度连续性条件构建适合大地电磁各向异性问题的加权后验误差估计方法,实现面向目标的网格自适应正演;最后通过典型算例分析各向异性对网格自适应和大地电磁响应的影响特征以及各向异性的识别方法.本文算法能够高精度地拟合起伏地表和任意各向异性介质,适用于分析复杂地电条件大地电磁响应特征,为提高大地电磁资料解释水平提供了理论基础.     

大地电磁三维反演方法综述

大地电磁测深（MT）资料的三维正、反演问题,已成为国际地球内部电磁感应领域研究的前沿课题.文中从算法思想方面简要地介绍了当前国内外MT三维反演的几种主要方法,探讨了今后MT三维反演研究的方向.     

Three-dimensional interpretation considering the static and the sea-effects of magnetotelluric data obtained in Jeju,Korea

Three-dimensional (3D) magnetotelluric (MT) surveys have been performed in Jeju, the largest volcanic island in Korea to figure out any possible structures or potential anomaly for remnant deep geothermal resources. Various approaches have been applied to interpret MT data observed in Jeju. MT dataset shows generally simple stratigraphy of four layers, though contains the severe static and the sea-effects. In our previous works, the induction vectors and 3D inversion results have commonly indicated the existence of a conductive anomaly in central parts of the island, beneath Mt. Halla. The 3D inversion dealt the static shifts as inversion parameters. The Jeju MT dataset, however, still contains the effect of conductive sea water surrounding the island.The sea-effect on MT impedance can be represented as a distortion tensor and excluded from the Jeju MT dataset by an iterative sea-effect correction. In this study, 3D inversion incorporating static shift parameterization was conducted using MT dataset corrected using 1D resistivity model obtained from the iterative scheme. Reasonably reconstructed images are obtained through the 3D inversion and using the MT dataset with sea-effect correction. The inversion result still shows the conductive anomaly in a similar depth. RMS misfits converged to a lower value than that of inversion using MT data before the sea-effect correction. From the fact, it is highly possible that the conductive anomaly is not an artifact but a real underground structure. Further investigation about the anomaly including exploration drilling is needed to see if it is from a fracture containing conductive sea water or related to the old volcanic activities.     

A Coastal Aquifer Study Using Magnetotelluric and Gravity Methods in Abo Zenema, Egypt

Magnetotelluric (MT) soundings and gravity methods were employed to study the deep freshwater aquifer in the area north of Abo Zenema city on the eastern side of the Gulf of Suez, Egypt. Seven MT sites and 48 gravity stations were surveyed along northeast–southwest profiles as close as possible to a line perpendicular to the coast of the Gulf of Suez. The MT survey was conducted using high and low frequencies to investigate shallow and deep areas, respectively. One-dimensional inversion was conducted using a heuristic inversion scheme of the Bostick algorithm. The MT data were also inverted with a 2-D smooth model inversion routine using the nonlinear conjugate gradient method to infer variation in vertical and lateral resistivity inside the Earth. A 100-Ohm-m homogeneous half-space initial model was used to invert the TE mode data only. Then, the inverted model obtained from the TE mode data was used as an initial model for inversion of the TM mode data. The inverted model thus obtained from the TM mode data inversion was used as an initial model for the inversion of the joint TE and TM responses. Two-dimensional (2-D) forward modeling of the gravity data was conducted using the 2-D polygon method of Talwani’s algorithm for an arbitrarily shaped body and was based on the subsurface information from the MT survey and the available information about the geological structure of the study area. This method enabled us to obtain the basement structure of the coastal aquifer in the study area. The results from the analysis and the interpretation of MT and gravity data were used to detect and delineate the groundwater coastal aquifer in the study area.     

Large-scale three-dimensional inversion of EarthScope MT data using the integral equation method

In this paper we apply 3D inversion to MT data collected in the Northwestern United States as a part of the EarthScope project. By the end of 2009 MT data had been collected from 262 stations located throughout Oregon, Washington, Idaho, and most of Montana and Wyoming. We used data from 139 MT stations in this analysis. We developed fully parallelized rigorous 3D MT inversion software based on the integral equation method with variable background conductivity. We also implemented a receiver footprint approach which considerably reduced the computational resources needed to invert the large volumes of data covering vast areas. The data set used in the inversion was obtained through the Incorporated Research Institutions for Seismology (IRIS). The inversion domain was divided into 2.7 M cells. The inverted electrical conductivity distribution agrees reasonably well with geological features of the region.     

大地电磁非线性共轭梯度拟三维反演

提出了非线性共轭梯度法大地电磁拟三维反演.该方法选取共轭梯度反演算法为拟三维反演的核心.在计算灵敏度（Jacobian）矩阵时,吸取近似灵敏度矩阵思想,采用一维灵敏度矩阵来代替三维灵敏度矩阵,并对非测点的灵敏度元素提出一种近似方法.在第一次反演之后,采用拟牛顿法更新灵敏度矩阵.拟三维反演法在很大程度上节省了计算时间,并且理论模型和实际资料的反演试算结果表明大地电磁拟三维反演法具有一定的实用价值.     

大地电磁资料精细处理和二维反演解释技术研究(三)——构建二维反演初始模型的印模法

在主流的线性最优化大地电磁二维反演中,如何合理构建初始模型是一个亟待解决的问题.常用的是采用均匀半空间或一维反演结果构建初始模型,不易获得稳定可靠的反演效果.实践表明,尽管基于不同初始模型的大地电磁二维反演结果差别较大,但均较初始模型更为接近真实模型.基于这样一种认识,经过反复的理论和实践探索,我们提出构建大地电磁二维反演初始模型的印模法.印模法的基本思想是依据已有反演结果和均匀半空间模型之间的加权来确定下一步二维反演的初始模型,它一方面保留了已有反演结果中关于真实模型的宏观轮廓信息,另一方面,保证了深部电性结构的均匀性,从而满足大地电磁二维正演所要求的底边界条件.基于印模法,本文进一步提出了迭代重构的反演思想.通过多个理论模型和实测数据的反演计算,验证了上述方法可在很大程度上压制初始模型对反演结果的影响.     

APPLYING 3D INVERSION OF SINGLE-PROFILE MAGNETOTELLURIC DATA TO IDENTIFY THE SHADE AND YUNONGXI FAULTS

Many synthetic model studies suggested that the best way to obtain good 3D interpretation results is to distribute the MT sites at a 2D grid array with regular site spacing over the target area. However, MT 3D inversion was very difficult about 10 years ago. A lot of MT data were collected along one profile and then interpreted with 2D inversion. How to apply the state-of-the-art 3D inversion technique to interpret the accumulated mass MT profiles data is an important topic. Some studies on 3D inversion of measured MT profile data suggested that 2D inversions usually had higher resolution for the subsurface than 3D inversions. Meanwhile, they often made their interpretation based on 2D inversion results, and 3D inversion results were only used to evaluate whether the overall resistivity structures were correct. Some researchers thought that 3D inversions could not resolute the local structure well, while 2D inversion results could agree with the surface geologic features much well and interpret the geologic structures easily. But in the present paper, we find that the result of 3D inversion is better than that of 2D inversion in identifying the location of the two local faults, the Shade Fault(SDF)and the Yunongxi Fault(YNXF), and the deep structures. In this paper, we first studied the electrical structure of SDF and YNXF based on a measured magnetotelluric(MT) profile data. Besides, from the point of identifying active faults, we compared the capacity of identifying deep existing faults between 2D inversion models and 3D models with different inversion parameters. The results show that both 2D and 3D inversion of the single-profile data could obtain reasonable and reliable electrical structures on a regional scale. Combining 2D and 3D models, and according to our present data, we find that both SDF and YNXF probably have cut completely the high resistivity layer in the upper crust and extended to the high conductivity layer in the middle crust. In terms of the deep geometry of the faults, at the profile's location, the SDF dips nearly vertically or dips southeast with high dip angle, and the YNXF dips southeast at depth. In addition, according to the results from our measured MT profile, we find that the 3D inversion of single-profile MT data has the capacity of identifying the location and deep geometry of local faults under present computing ability. Finally, this research suggests that appropriate cell size and reasonable smoothing parameters are important factors for the 3D inversion of single-profile MT data, more specifically, too coarse meshes or too large smoothing parameters on horizontal direction of 3D inversion may result in low resolution of 3D inversions that cannot identify the structure of faults. While, for vertical mesh size and data error thresholds, they have limited effect on identifying shallow tectonics as long as their changes are within a reasonable range. 3D inversion results also indicate that, to some extent, adding tippers to the 3D inversion of a MT profile can improve the model's constraint on the deep geometry of the outcropped faults.     

大地电磁的人工鱼群最优化约束反演

大地电磁的反演问题是非线性,如果采用线性反演方法容易陷入局部极小,使得反演结果非唯一性严重.本文将人工鱼群算法引入到地球物理反演之中,提出了非线性的大地电磁人工鱼群最优化反演.该方法不需要进行偏导数的求取,可以对反演的范围进行约束,以减小反演结果的非唯一性.同时我们对搜索步长进行了改进,给出适用于大地电磁反演的人工鱼群参数.大量的理论数据试算表明,人工鱼群反演算法能够较好地寻找到全局最优解.实测数据的处理结果表明,该方法可以用来处理实际资料,并且能够取得很好的应用效果.     

稀疏测线大地电磁资料三维反演研究:合成算例(英文)

受勘探成本和工区环境等因素的影响,当前大多数大地电磁实际工作采取布置稀疏测线采集数据和使用二维反演方法解释这些稀疏测线数据的方式。然而,二维反演方法在解释三维地电构造数据时存在局限性,有时甚至做出错误的地质解释。本文尝试了使用三维反演方法对大地电磁稀疏测线数据进行反演解释。使用大地电磁全信息资料三维共轭梯度反演程序对理论模型合成稀疏测线数据进行了三维反演。结果表明:这种反演方案是可行与有效的。同时,我们发现在不同数据的三维反演结果中,四个张量阻抗元素和两个倾子数据同时反演的结果相对更为准确,更接近理论模型。     

Delaunay三角剖分插值算法在MT成图中的应用

研究并实现了一种基于Delaunay三角剖分的二维快速插值算法,并将其应用于大地电磁(MT)二维反演实时成像网格化处理中。实际资料试算结果表明,该算法具有稳定、插值效果好以及易于模拟地形数据等优点,能满足MT反演结果实时成图的要求。