In this paper, the new development of MT work in China, including the equipments, dataprocessing, inversion and application have been reviewed, since the middle of 1980s. home results and aspects of MT application on petroleum exploration and investigation of electrical structure in the crust and upper mantle in China are presented.

A controlled-source audio-frequency magnetotelluric (CSAMT) survey has been carried out to investigate potential iron (Fe) and polymetallic (Pb-Zn-Cu) deposits in Longmen region, which is one of the main metallogenic belts in southern China. Conducting geophysical surveys in this area is quite difficult due to mountainous terrain, dense forest, and thick vegetation cover. A total of 560 CSAMT soundings were recorded along twelve surveying lines. Two-dimensional Occam's inversion scheme was used to interpret these CSAMT data. The resulting electric resistivity models showed that three large-scale highly conductive bodies exist within the surveying area. By integrated interpretation combined with available geologic, geophysical, and geochemical data in this area, three prospective mineral deposits were demarcated. Based on the CSAMT results, a borehole penetrating approximately 250-m depth was drilled at the location of 470 m to the northwest end of line 06, defined with a massive pyrite from the depth of 52-235 m with 7%-16% Fe content, as well as locally high-grade Pb-Zn- and Ag-Ti-bearing ores.

Recent developments in the speed and quality of data acquisition using the radiomagnetotelluric (RMT) method, whereby large amounts of broadband RMT data can be collected along profiles, have prompted us to develop a strategy for routine inverse modeling using 2D models. We build a rather complicated numerical model containing both 2D and 3D elements believed to be representative for shallow conductors in crystalline basement overlain by a thin sedimentary cover. We then invert the corresponding synthetic data on selected profiles, using both traditional MT approaches, as well as the proposed approach, which is based on the determinant of the MT impedance tensor. We compare the estimated resistivity models with the true models along the selected profiles and find that the traditional approaches often lead to strongly biased models and bad data fit, in contrast to those using the determinant. In this case, much of the bias is removed and the data fit is improved. The determinant of the impedance tensor is independent of the chosen strike direction, and once the a priori model is set, the best fitting model is found to be practically independent of the starting model used. We conclude that the determinant of the impedance tensor is a useful tool for routine inverse modeling.

本文系统地研究了大地电磁测深(MT)资料中倾子多数的特征。研究结果表明:倾子异常主要反映了地电构造的水平非均匀性;倾子异常的幅值不仅与水平非均匀界面两边介质电导率的对比度有关。还与上覆层的电导率和厚度以及观测频率有关;利用倾于异常的频率特征。可以定量地解释出垂直断面的上覆层厚度。在倾子资料解释方法上,可采用图象处理中的模板相关匹配技术。以理想模型的响应为样板,对野外实测资料进行匹配。可以较准确地解释出垂直断面的位置和埋深以及电阻车的分布情况等。文中对一条测线的野外实测倾子资料进行了处理和解释,在没有参考任何其它地质信息的情况下,解释结果能很好地反映该测线的地质构造特征,分辨率和可信度均较常规解释方法要高。

本文在MT倾子定义基础上首次建立了视倾子的数学物理基础及其定义,它可以由视电阻率和相位资料直接计算得到,克服了实测倾子易受干扰、品质较差而难以利用的缺陷,并通过各类模型试验系统地分析了视倾子各项参数的特征,证明视倾子异常可以反映地电构造的水平非均匀性,其模、相位、实部和虚部以及本文提出的对视倾子所做的类Bostick反演均能对断裂的倾向、位置、规模、顶面埋深和电性差异有较好的揭示.文中还结合对实测MT倾子资料的处理与对比,说明了视倾子资料和实测倾子资料具有很好的可比性,对实际资料的处理解释结果也表明了该方法的效果,在不需要更多先验信息的条件下,利用视倾子可以直接判断电性横向的变化,尤其是断裂的展布,因此为研究地电构造和地质解释创造了新的条件.

Galvanic distortion of magnetotelluric (MT) data is a common problem in the study of the Earth’s electrical properties. These distortions are local, they affect independently each MT site, and where restricted to distortion of the electric field are manifest as vertical shifts in the apparent resistivity curves (static shift). The removal of the static shift is necessary to avoid misinterpreting MT data. We present a method that allows us to partially retrieve the regional response of the TE-mode data in a 2D case. The method determines relative changes between distortion parameters along a profile, and is based in the Faraday ’s law, and uses only magnetotelluric responses: measured impedance tensor and geomagnetic transfer function (tipper). The method is valid under the assumption that the variation of horizontal magnetic field can be neglected, and a test for checking this criterion has been developed. The mathematics involved in the procedure are straightforward, and can be stated as a linear regression. We present successful applications to both synthetic and real (COPROD data) datasets.

在二维地电模型中,考虑到大地电磁测深(MT)倾子响应依赖于横电(TE)模式的数值模拟问题,笔者采用矩形网格单元和双二次插值对MT的倾子响应进行求解。首先,给出了二维地电条件下的边值问题和变分问题,并通过有限单元法对模型进行单元剖分、插值、积分和整体合成;其次,通过求解复系数方程组得到了每个节点的电场值;最后,采用差分方法求得电场沿横向和垂向的偏导数值,从而计算出二维MT的倾子响应。通过对2个二维地电模型的倾子响应进行数值模拟,获得了倾子响应的实部、虚部和振幅,结果很好地反映了不均匀体的横向分布情况。

在二维地电模型中,倾子资料作为大地电磁测深实测资料之一,能够很好地反映地下介质在水平方向的不均匀性,可以作为复杂地质构造的表征。这里采用矩形网格单元的双二次插值有限元法对MT的倾子响应进行正演模拟,通过对异常体在不同条件下倾子数值的分析,结果表明,倾子异常的幅值不仅与上覆层的电导率、厚度以及观测频率有关,还与水平非均匀界面两边介质电导率的对比度,以及地下介质尺寸的大小有关。因为倾子响应的异常幅值很小,在实际观测中由于受到多方面的干扰,且地下异常体的几何不规则性,于是倾子响应的异常也是诸多情况的综合反映。通过对倾子影响因素的分析,可以更加准确地利用倾子资料进行地质解释。

磁场的垂直分量表征介质的水平不均匀性。描述H_z与H_x、H_y之间关系的感应矢量,是研究电性水平不均匀性的有利参数,它具有受局部畸变影响小,对电性噪声干扰不敏感等优点。由实感应矢量的方向和长度,可以确定地下电性不均匀体的位置、规模以及与周围介质的电性差异。实际应用表明,长白山天池火山区从表层到深部都存在低阻的电性不均匀体,浅部不均匀体与天池水和热资源有关,而深部的不均匀体可能与地下岩浆囊有关。

感应矢量的本质就是倾子,感应矢量表示了磁场的垂向分量与水平分量的复数比值。在均匀平面波假设下,一次磁场中不存在垂直分量,在横向均匀一维介质中,二次场中也不会产生垂直磁场,只有在二维或者三维的情况下才会出现垂直磁场分量。感应矢量主要反映大地电性结构的横向变化。在直角坐标系中,磁场的垂直分量和水平分量之间满足关系式:

鄣源构造带现今呈北东―南西向展布的混杂新元古代早期基性岩块以韧性变形构造为显著特点，并强烈叠加印支―燕山期逆冲-走滑脆性断裂的区域性超壳断裂带。构造带历经多期构造变形，主期构造变形以晋宁期弧-陆（扬子陆块）碰撞在地壳内部层圈结构相对薄弱部位形成的向南逆冲式韧性变形为特征。鄣源构造带中的蚀变枕状－块状基性熔岩、辉长岩、辉绿岩等基性岩块在空间上紧密相伴，形成于低速扩张的陆缘小洋盆扩张脊环境，具初始洋壳基性岩特征，属皖南伏川蛇绿岩西延组成部分。鄣源构造带及其两侧广布的新元古代早期火山-沉积建造体形成于大陆边缘与板块构造体制相关的构造-沉积环境，鄣源构造带南侧为裂解海盆沉积体系，北侧属于与大陆边缘火山弧有关的盆地沉积体系，鄣源构造带内显示深海―半深海沉积特征。鄣源构造带北东、南西向延伸分别与皖南伏川蛇绿混杂岩带及景德镇-宜丰深断裂带相接，是江南造山带北缘以皖南伏川蛇绿岩为代表的新元古代早期陆缘小洋盆俯冲-碰撞拼合带的重要组成部分。

华南中部中新元古代造山带可以划分为5个地质构造单元:乐平-歙县构造混杂岩亚带、万年海相 -滨海相沉积-火山沉积建造、赣东北蛇绿混杂岩亚带、怀玉火山-火山碎屑岩系、东乡-龙游混杂岩亚带.通过对不同构造单元形成大地构造环境分析,认为它们 分别形成于火山弧-弧后盆地、弧间盆地、大洋岛弧、洋中脊、火山岛弧、弧前盆地等大地构造环境;华南中部中新元古代造山带属陆-弧-弧-陆碰撞造山带,发 育在汇聚型板块边缘地带,古洋盆为一个多岛洋体系.中元古代末期(约1 024 Ma)古华南多岛洋开始关闭,大约在850 Ma左右,整个古华南多岛洋最终关闭.

江西北部地处欧亚大陆板块东南沿海前陆推覆冲断带,与扬子、华夏古板块结合带叠加部位.有色金属矿床的形成、演化,具有"多次成矿叠加、燕山期最强";" 块、带"成矿分区,块体边缘深断裂带聚矿,沿前陆推覆冲断带"向洋"分带;走滑剪切、扭动与推覆构造复合控岩控矿;岩浆多层次造浆、多层次就位成矿;大型 矿集区产布环境的偏在性等成矿特征.具有较大的矿产资源潜力.

Jiangshan-Shaoxing fault zone is a famous deep fault zone in China. There are various view-points on whether and how it stretches to the Hangzhou Bay. According to the field investigation by the authors in the main islands of Hangzhou Bay and the mountain lands around it, and with reference to some new data of the seismic exploration and the oil-drilling, we obtained some new knowledge, i.e. Jiangshan-Shaoxing fault zone strike turns from NE to NEE direction over past Pingshui and Fusheng of Shaoxing City, it continues along Changhe and Yandong in north Cixi county, and finally stretches to East China Sea through Dajushan island of Daishan county.