Three-dimensional electrical structure of the northwestern margin of the Karamay region,China, revealed by magnetotelluric data

In order to investigate the three-dimensional structures of intrusive granite and the deep structure of the Darbut fault in the northwestern margin of the Karamay region, western Junggar Basin, China, new magnetotelluric data were collected along six profiles across the Darbut fault. The magnetotelluric data were processed using a robust estimation technique to obtain the magnetotelluric impedance. Then the off-diagonal components of the impedance tensor were inverted using a three-dimensional nonlinear conjugate gradient inversion technique, which was performed using open-source three-dimensional electromagnetic inversion software. The final three-dimensional model includes two major low-resistivity anomalies and two major high-resistivity anomalies. The first low-resistivity anomaly corresponds to the location of the Darbut fault, which indicates that metallic elements are abnormally enriched there. The second one is located beneath the Darbut fault, and it is most likely a magma channel in the middle crust. The two major high-resistivity anomalies are distributed on either side of the Darbut fault. We interpret them as Karamay rocks and Akebasitao rocks and suggest that they were formed in an extensional setting. The cross sections of three-dimensional magnetotelluric inversion result reveal that the Darbut fault has been reformed by the later magmatism, leading to the variation of its downward extent along its strike. Moreover, our inversion result also indicates that a magma channel exists in the middle crust of the region.     

深圳市断裂新活动与地震研究进展

回顾了深圳地区断裂最新活动性和地震的研究历史,总结了前人所作的主要工作与认识,以此为基础进一步介绍了深圳市活断层探测与地震危险性评价项目的最新工程探测成果与研究进展。项目选择了横岗-罗湖断裂、莲塘断裂、温塘-观澜断裂为研究对象,采用了卫星遥感解译、微震精确定位、野外地震地质调查、钻孔和探槽探测、浅层地震勘探、深部结构大地电磁阵列探测、地下电磁波勘探等工程技术方法,对断裂的近地表及中深部结构特征、新活动性和地震危险性进行了探测与评价,获得了一系列新的认识。最后,结合深圳市断裂构造运动和地震研究现状与存在的问题,提出了今后开展工作的若干建议。     

老虎山断裂带大地电磁地震监测研究

本文分析研究了2004年10月至2006年9月在老虎山断裂带附近一测点的5次大地电磁重复观测资料。 观测期间在其断裂带的近距测点十数公里处,发生了最大震级为M_L4.6的一组3级以上地震。 揭示出了由大地电磁法观测到该地震活动过程所显示的深部电性变化的前兆特征,表明在震源区一相应深度范围在地震孕育过程中介质电阻率出现下降—上升—发震的变化过程。     

A geoelectric model for the area of the Tolbachik eruption (named after the 50 year anniverary of the Institute of Volcanology and Seismology)

We consider the methods and results of magnetotelluric sounding in the AMTS and MTS modifications. Audiomagnetotelluric sounding (AMTS) was carried out for the first time in the area of a recent Tolbachik eruption. The results from our analysis of the magnetotelluric parameters show that the geoelectric medium involving a regional fault can be fitted by a 2D inhomogeneous model. The longitudinal and transverse sounding curves were assumed as the leading elements for interpretation. A joint analysis of these curves and of pseudo-sections of impedance phases provides evidence of a geoelectric inhomogeneity in the area where the Naboko Vent is situated. A bimodal inversion of the AMTS curves yielded a geoelectric section that contains a conductive inhomogeneity that is possibly related to a fault that carried fluids up to the ground surface. Along with AMTS, we used MTS curves in a broader range to identify a crustal conductive anomaly at depths of 15–35 km. The data from AMTS, MTS, and other geological and geophysical information were used to develop a conceptual model for the area of study that characterizes a possible origin of the anomalous zones. We obtained approximate estimates of rock porosity in the fault zone that transported magma melts upward into the overlying rocks in the area of the Naboko Vent.     

新疆乌什地震带断裂研究的综合地球物理方法

本文介绍了重力、磁力、大地电磁测深、地震等地球物理资料与地震相关的活动断裂的识别标志,并以新疆乌什地震带为例分析了综合各种物探资料研究活动断裂的方法。结果显示根据不同的研究目的选择合适的针对性方法可以较好的获得断裂的展布、深部或者活动性等特征。     

The electrical resistivity signature of a fault controlling gold mineralization and the implications for Mesozoic mineralization: a case study from the Jiaojia Fault,eastern China

We use 3D audio magnetotelluric method to the south segment of Jiaojia fault belt, and obtain the 3D electrical model of this area. Regional geophysical data were combined in an analysis of strata and major structural distribution in the study area, and included the southern segment of the Jiaojia fault zone transformed into two fault assemblages. Together with the previous studies of the ore-controlling action of the Jiaojia fault belt and deposit characteristics, the two faults are considered to be favorable metallogenic provinces, because some important features coupled with them, such as the subordinate fault intersection zone and several fault assemblages in one fault zone. It was also suggested the control action of later fault with reversed downthrows to the ore distribution. These studies have enabled us to predict the presence of two likely target regions of mineralization, and are prospecting breakthrough in the southern section of Jiaojia in the Shandong Peninsula, China.     

Application of the 3D magnetotelluric inversion code in a geologically complex area

The WSINV3DMT code makes the implementation of 3D inversion of magnetotelluric data feasible using a single PC. Audio‐magnetotelluric data were collected along two profiles in a Cu‐Ni mining area in Xinjiang, China, where the apparent resistivity and phase curves, the phase tensors and the magnetic induction vectors indicate a complex 3D conductivity structure. 3D inversions were carried out to reveal the electrical structure of the area. The final 3D model is selected from the inversion results using different initial Lagrange values and steps. The relatively low root‐mean‐square (rms) misfit and model norm indicate a reliable electrical model. The final model includes four types of low resistivity areas, the first ones coincide with the known location of an orebody and further forward modelling indicates that they are not in full connectivity to form a low resistivity zone. The second ones are not controlled by magnetotelluric sites and embody little information of the observed data, they are considered as tedious structures. The third one is near to the regional Kangguer fault and should be treated carefully considering the effect of the fault. The last ones are isolated and existing at a limited level as the first ones, they should be paid more attention to.     

大地电磁资料精细处理和二维反演解释技术研究(五)——利用阻抗张量成像识别大地线性构造

大地线性构造往往与块体或物性的边界、断裂的几何延伸相关联,是地学研究的重要目标构造.在大地电磁法中,传统的方法是通过反演得到的电性结构的差异来解译线性构造,是一种间接分析方法.由于线性构造发育处走向和倾向明显,二维性较其他地方一般会更强,故而在阻抗张量成像分析中,相应的二维有效因子就会比较大,最佳主轴统计分布会更加明显.据此,本文利用大地电磁阻抗张量成像(包括最佳主轴统计成像、构造维性成像)来探测识别大地线性构造.本文首先通过三维理论模型的研究结果来验证这一技术在线性结构探测中的有效性,然后将其应用于郯庐断裂带南段实测剖面的具体分析中,展示了这一技术在线性构造探测中的实用性及其数据处理与分析过程.本文研究表明,大地电磁阻抗张量成像技术可独立于反演为地质解释尤其是线性构造的解释提供诸多参考信息.     

The deep structure of the southern kamchatka volcanic zone from geophysical data

We discuss the interpretation method and results of magnetotelluric soundings in combination with other geological and geophysical data. The interpretation method was developed by studying possible distortions in MTS curves using 3D-numerical modeling of the magnetotelluric field. Deep conductivity was studied by using longitudinal MTS curves below a period of 400 s, which are nearly unaffected by the induction effect due to marine electrical currents. Transverse curves were used to obtain more detail for the geoelectric model. Inversion of average longitudinal MTS curves resulted in a geoelectric section of the lithosphere down to a depth of 60 km. Anomalies of high conductivity in the lithosphere were detected and were found to produce certain effects in gravity and seismic velocities. MTS and seismic tomography data were used to determine the possible origin of the high conductivity anomaly and to estimate rock porosity and the concentration of magma melts.     

A deep geoelectric model of the Bol’she-Bannyi hydrothermal system,Kamchatka

This paper reports the results of magnetotelluric sounding (MTS) in the area where the Bol’she-Bannyi hydrothermal springs are discharged. The MTS curves were inverted on the assumption of a twodimensional inhomogeneous model using longitudinal and transverse curves of apparent resistivity. It was found that the geoelectric section contains a nearly vertical anomaly of high electrical conductivity at depths of 5.5–8 km, which is the signature of a deep-seated fault. The resulting geoelectric section for the upper crust and data from regional magnetotelluric soundings were used as a basis for developing a conceptual deep model of the Bol’she-Bannyi hydrothermal system. We quote an approximate estimate of rock porosity. According to the model, deep fluids come from a crustal layer into the subvertical deep-seated fault then penetrate via fissures into the sedimentary–volcanogenic cover, and finally arrive at the ground surface in zones of high rock permeability. We provide a recommendation for drilling a deep well in order to determine the potential of the Bol’she-Bannyi hydrothermal field.     

Deep structure of the northeastern margin of the Parnaiba Basin, Brazil, from magnetotelluric imaging

The magnetotelluric (MT) method has been applied to the determination of the deep resistivity structure of the northeastern margin of the Parnaiba Basin. Transient electromagnetic (TEM) and MT data were collected in early 1999 along a 95 km long N–S line, extending from the coast across the projected subcrop position of a discontinuous fault found to the west of the study area that is believed to be a possible basin‐bounding fault. The MT data were processed to yield the TE‐ and TM‐mode responses and then corrected for static shift using central‐loop and single‐loop TEM data, respectively. Regularized 2D MT inversion was subsequently undertaken using a structured initial model with the near‐surface constrained by TEM inversion results. As a consistency check, we performed another set of 2D inversions using different smooth initial models. The various optimal 2D inversion models show clearly the presence of a major basement trough, over 2 km deep, located about 70 km from the coast. We interpret it as possibly marking the main basin margin and suggest that it may have implications for groundwater resource development in the area.     

安徽霍山地震区深部电性结构和发震构造特征

霍山地震区位于大别造山带北缘华北板块与扬子板块接触带上,是大别造山带及周边地震活动最频繁、最集中的地区.83个大地电磁测点组成的大地电磁三维阵列覆盖了整个霍山地震区.用多重网格法、印模迭代重构法和非线性共轭梯度法对阵列数据进行三维带地形反演,获得了地震区深部三维电性结构.电性结构显示,北大别、北淮阳区的中上地壳为电阻率1000Ωm以上的高阻区,中下地壳为电阻率数十欧姆米的相对低阻区;六安盆地电阻率整体较低,中地壳存在显著的电阻率为几欧姆米的壳内高导层.北西向的晓天—磨子潭断裂分隔了北大别高阻层和北淮阳高阻层,在浅部向NE倾,深部向SW倾;北东向的落儿岭—土地岭断裂切穿北大别上地壳高阻层.小震双差定位结果表明,地震主要发生在NE向延伸的落儿岭—土地岭断裂附近的北大别、北淮阳中上地壳的高阻区,并集中于NW向的晓天—磨子潭断裂运动所造成的构造薄弱带中;2014年M S4.3霍山地震震源深度较深,位于北大别高阻区内部的电性梯度较大的区域.综合上述结果我们认为,霍山地震区的主要发震断裂为落儿岭—土地岭断裂,断裂的运动变形充分利用了晓天—磨子潭断裂早先活动所形成的构造薄弱带,断裂下方壳源高导体中的流体沿断层传播使断层强度弱化,使得这些薄弱带区易于发生小地震.由于北大别、北淮阳构造区显著高阻层的存在,我们认为霍山地震区存在发生6级以上中强震的深部孕震环境.     

大兴安岭西坡德尔布干断裂地球物理特征与构造属性

对1:250万重力异常向上延拓不同高度并分别计算135°方向水平导数,德尔布干断裂表现为北东向延伸的重力场分界线,采用基于DCT法欧拉反褶积对1:10万高精度重磁数据向上延拓0～20 km追踪断裂轨迹,并进行1:100万大地电磁测深反演和海拉尔盆地地震剖面解释,识别出德尔布干断裂具有向南东倾斜,切割深度至下地壳的特征....     

海底大地电磁数据采集器

进行海底大地电磁数据采集，需借助于专用的水下探测设备 . 虽然大地电磁测深法无论是仪器或是测量手段在陆地上的应用已较为成熟，但把该方法移 植到海洋中，还需解决一些与海洋探测有关的工程技术问题. 主要包括：水下电场信号的提 取，海底环境的监测，仪器在非实时监控运行中的纠错对策以及测量部件的密封承压等等 . 在研制海底大地电磁数据采集器的过程中，采用先进的材料工艺，高精度的电子线路，可 靠的软件编程以及牢固的器件安装结构，有效地解决了上述技术问题. 用所研制的仪器首次 在我国海域进行了探测试验.     

Solution of the one-dimensional inverse magnetotelluric problem

Summary Methods of solving the inverse magnetotelluric problem are compared. Basic relations for Newton's method, the least-squares method and Marquardt's method are presented and the convergence properties of these methods are studied. The high effectiveness of Marquardt's method is demonstrated and its application to practical magnetotelluric data is discussed.     

Modelling the 2D terrain effect on MT by the boundary-element method

The boundary-element method is used to model the 2D terrain effect on the magnetotelluric (MT) field. Firstly, the boundary-value problem of a 2D magnetotelluric field is transformed into an integral equation problem by using Green's theorem. Then the boundary-element method is used to solve the integral equation and to obtain the MT field and its normal derivative on the terrain. From these values, the apparent resistivity can be calculated. Compared with the finite-element method, the boundary-element method is simpler in element division and the initial data preparation. The configuration of a terrain divided by the boundary-element method is more consistent with the practical terrain. The method proposed in this paper can be run on a microcomputer, so that it can be used in the field.     

MAGNETOTELLURIC MODELLING BY THE FINITE ELEMENT METHOD*

The finite element method, here viewed as a special case of the Galerkin projective method, is applied to the modelling of magnetotelluric problems, and its adaptation to geological profiles is outlined. A novel method for obtaining surface field values, involving matrix representation of the normal derivative operator, is presented in detail. Results obtained by this method are compared with well-known infinite series solutions for the vertical fault and the outcropping dyke. Two profiles containing sulphide zones are also modelled, the results being compared with field data; satisfactory agreement is obtained.     

上海奉城—浙江湖州长周期MT剖面揭示的深部电性结构

利用GMS-06电磁观测系统对上海奉城—浙江湖州剖面进行了大地电磁数据采集,获得了超过4000 s的高质量长周期观测数据,为沪浙地区深部电性结构研究提供了有利条件.通过反演处理,综合倾子解释结果和重磁资料以及最新的地震解释结果,在电性和断裂结构上获得了新的认识,对湖州—苏州、乌镇—马金、枫泾—川沙以及太仓—奉贤这几条深大断裂带的展布以及对区域构造格架的控制作用进行了新的评价,其中枫泾—川沙断裂是上海断隆和湖州—南通台拱之间推覆构造过渡带的控制断裂,太仓—奉贤断裂带的主体位置推断在亭林附近.同时,对高阻基底以上电性层进行了解释,揭示了岩石圈底界面的结构和构造形态,推断湖州—苏州断裂西侧存在壳内高导层.     

Geoelectric section of the Central Tien Shan: Sequential inversion of the magnetovariational and magnetotelluric data along the Naryn Line

The paper presents the results of 2D inversion of deep magnetotelluric (MT) and magnetovariational (MV) soundings along the Naryn Line. The method of partial (sequential) inversions is used. According to this method, at the first stage, magnetovariation responses are used for the localization of deep anomalies of electrical conductivity, and then the magnetotelluric sounding data are invoked to refine the structure of the host medium and the structural details in the upper part of the section. It is shown that this approach enables one to estimate the informativeness of separate components of the electromagnetic field, to reduce the distorting influence of the near-surface geoelectric inhomogeneities, and to increase the stability of the final solution of the inverse problem.     

冀中坳陷东北部河西务断裂深浅构造特征

浅层高分辨率地震探测揭示,河西务断裂在近地表为倾角70°—75°的高角度正断层,断裂由1条主干断裂及1条次级断裂构成。主干断裂上断点埋深170 m左右,活动时代为中更新世晚期。石油深地震探测结果和大地电磁测深资料分析结果显示,断层上陡下缓,呈铲形,两侧地层角度不整合,扩展向斜、褶皱和伴生断裂发育,断裂切割较深,在38 km左右深度与大兴断裂交汇向下延伸,进入上地幔,断裂的下盘存在壳内高导体(层)。结合精定位小震的分布以及华北地区地震孕育发生的特点认为,河西务断裂存在中强地震发生的构造背景。