Three-dimensional forward modeling for magnetotelluric sounding by finite element method

A finite element algorithm combined with divergence condition was presented for computing three-dimensional(3D) magnetotelluric forward modeling. The finite element equation of three-dimensional magnetotelluric forward modeling was derived from Maxwell's equations using general variation principle. The divergence condition was added forcedly to the electric field boundary value problem, which made the solution correct. The system of equation of the finite element algorithm was a large sparse, banded, symmetric, ill-conditioned, non-Hermitian complex matrix equation, which can be solved using the Bi-CGSTAB method. In order to prove correctness of the three-dimensional magnetotelluric forward algorithm, the computed results and analytic results of one-dimensional geo-electrical model were compared. In addition, the three-dimensional magnetotelluric forward algorithm is given a further evaluation by computing COMMEMI model. The forward modeling results show that the algorithm is very efficient, and it has a lot of advantages, such as the high precision, the canonical process of solving problem, meeting the internal boundary condition automatically and adapting to all kinds of distribution of multi-substances.

Comparisons,processing and interpretation of the surveyed data by GMS-06 and other magnetotelluric sounding sys-tems

In recent years, the data collecting and processing of magnetotelluric sounding （MT） have progressed greatly. How to improve the quality of field surveyed data and obtain the high quality parameters such as apparent resistivity and phase is the most important link during the overall flow. It is closely related to the signal-to-noise ratio of magnetic sensors, the resolution of data collecting units and relevant processing methods as well. Produced by Germany Metronix Measurement Instruments and Electronics Ltd., the new type multi-channel geophysical measurement system GMS-06 was already adopted and used by domeslic units such as Institute of Geology and Geophysics of Chinese Academy of Sciences and Tongji University. Based on the brief introduction of GMS-06, this paper will give the comparisons and analyses of the synchronous surveyed MT data by GMS-06, MMS-03 MT system which was also produced by Metronix and V5-2000 MT system produced by Canada Phoenix Geophysics Ltd.. Then the preliminary processing and interpretation results of the long period MT data observed by GMS-06 in the area of Shanghai and Zhejiang are also discussed in the paper.

Geoelectrical structures of northern segment of Tan-Lu fault system in China and Russia areas based on magnetotelluric soundings

From China to Russia,Tan-Lu fault system stretches for thousands of kilometers,towards NE 30°.The authors have collected more than ten magnetotelluric profile that China and Russia have carried out,in the studied area of the Tan-Lu fault's northern section,and have analyzed electrical characteristics of the relevant profiles.Deriving the following conclusions:(1)Jiamusi-Bureya plots,etc.,demonstrate the high resistance of Kernel;(2)Fold belt attributed to the land showed high and low resistance stitching,associated with not only terrane accretion,but also upper mantle upwelling;below the sedimentary basin,the resistivity of the upper mantle is lower and closer to the surface comparing with the adjacent area(100-150);(3)In the system of Tan-Lu fracture,Yishu and Dunmi fracture,etc.are expressed as vertical low-resistivity zone;(4)There may exist subducted old-fashioned piece under Jiamusi and Bureya plots.

Magnetotelluric sounding study in the region of Zhongshan Station，East Antarctica

This paper deals with the results of the MT observations in the region of Zhongshan Station,Larsemann Hills area, East Antarctica and points out that the lithosphere thickness of the Larsemann Hills is 140 km and the crustal high conductivity layer is situated in 22 km.

Intra-continental Orogeny:Insights from Magnetotelluric Data into the Mesozoic Uplift History of the Eastern Jiangnan Orogen in South China

Despite extensive efforts to understand the tectonic evolution of the Jiangnan Orogen in South China,the orogenic process and its mechanism remain a matter of dispute.Previous geodynamic studies have mostly focused on collisional orogeny,which is commonly invoked to explain the Jiangnan Orogen.However,it is difficult for such hypotheses to reconcile all the geological and geophysical data,especially the absence of ultrahigh-pressure metamorphic rocks.Based on the magnetotelluric data,we present a group of resistivity models produced through the combination of two-dimensional and three-dimensional inversions,revealing the geo-electrical structures of Jiangnan and a typical collisional orogen.In our models,the resistive crust is separated into three parts by a prominent conductive layer with opposite dipping directions on both sides.A special thrust-nappe system,which is different from that developed in a typical collisional process,is revealed in the Jiangnan Orogen.This structure suggests a process different from the simple collisional orogeny.To interpret our observations,an'intra-continental orogeny'is proposed to address the development of the Jiangnan Orogen in the Mesozoic.Furthermore,this'reworked'process may contain at least two stages caused by the decoupling of the lithosphere,which is revealed by an extra conductive layer beneath Jiangnan.

Electrical Structure of Wulingshan and Middle Jiangnan Orogen by Three-Dimensional Magnetotelluric Data Inversion

Funded by The National Key Research and Development Program of China,China Deep Exploration(Sinoprobe)and The China Geological Suvery Project on 2009–2019,a large scale magnetotelluric sounding(MT)survey grid(Fig.1)has covered whole south China.

Application of high-frequency magnetotelluric method in porphyry copper deposit exploration：a case study of Duobaoshan deposit area

The Duobaoshan mine area in Heilongjiang is located in the northeast section of Xingmeng orogenic belt and is in the west side of Hegang Mountain-Heihe fault zone. There exist many deposits in this area,and its metallogenic conditions are superior,which has been one of the hotspots in geological prospecting and metallogenic research in Northeast China. On the basis of previous studies,the authors used the EH-4 electromagnetic imaging system to carry out the data acquisition of three survey lines in Woduhe Village,Duobaoshan Town,Nenjiang County. Through the analysis of apparent resistivity section under TE and TM polarization modes,integrating regional geological data,it is concluded that:(1) the electrical characteristics of the metal ore in this area show a relatively low resistance,and according to its resistivity difference with surrounding rocks,the geometrical structures and apparent resistivity parameters of the low resistivity bodies in the lower section of the survey line are defined,and the electrical anomalies can be identified;(2)faults F1 and F2 may have a good metallogenic environment,so they are recommended for further exploration;(3) low resistance metal ore bodies have good correlation with local small structures or faults,which may play an iconic role for the delineation of key target areas;(4) in the process of using apparent resistivity to define the geometric structures of ore bodies underground,comprehensive analysis integrating the advantages of TE and TM models should be carry out to achieve more reliable inversion results.

Electrical Structure of Yarlung Zangbo Suture from Magnetotelluric Profiling of INDEPTH-MT

The INDEPTH MT results show that there are no electrical features of deep fractures along the Yarlung Zangbo River, but a large high conductivity body exists in the area between Gyangze and Rinbung. It dips northward, extends downward up to the depth of about 55 km and indicates the exposure of the possible real position of the Yarlung Zangbo suture. There are three sets of electrical gradient and distorted zones reflecting the structure of faults in the high conductivity region. These three fault belts, which dip northward and gradually converge downward to the main fault belt, and a series of south dipping faults in the north side form the exhaled structural characteristics of the Yarlung Zangbo suture. There is a close relationship between the large high conductivity body and the underground thermal state.

Crustal Electrical Conductivity Structure of Southern Tibet from Magnetotelluric Survey

Two superwide bands of frequency magnetotelluric (MT) profiles (Yadong-Xuegula, Jilong-Cuoqin) across the Yaluzangbu suture were deployed along the west-east direction, for the research into the electrical conductivity structure in the shallow and deep crust along the west-east and north-south directions in the southern part of Tibet plateau. The main characters of the electrical conductivity structure in this region are: (1) large-scale high resistive bodies exist near the Yaluzangbu suture surface, which extends to the maximum depth of more than 30 km. They are the reflection of the Gangdise granite; (2) small-scale conductive bodies exist in the southern part of the Yaluzangbu suture, and large-scale ones under the suture and in the northern part; (3) conductive bodies widely spread in the crust along the profiles. They are discontinuous, mainly decline to the north and become larger in scale, steeper near the suture, deeper gradually from south to north; (4) under the Yaluzangbu suture, the conductive bodies become larger in scale, more conductive gradually from west to east. These important electrical characters are caused possibly by the India plate subduction to the north. The variation in characters of the large-scale conductive bodies from west to east may be the proof that the plate collision might cause substantial movement along the west-east direction.

Groundwater resources survey of tongchuan city using the audio magnetotelluric method

The development of Tongchuan City,Shaanxi Province,located in the northwestern region of China,is restricted by water resources.The direct current resistivity and induced polarization sounding methods are typically applied in finding urban groundwater.These methods,however,are not effective due to their complicated topography and geological conditions.The application practice shows that the audio magnetotelluric(AMT)method has a large depth of exploration,high work effi ciency,and high lateral resolution.To investigate the distribution of groundwater resources,we deployed three audio-frequency magnetotelluric profiles in the city area.The impedance tensor information of AMT data is obtained using SSMT2000.AMT data dimension analysis reveals that the two-dimensional structural features of the observation area are obvious.The main structure of the observation area is about 45°northeast,as indicated by structural trend analysis.A shallow two-dimensional electrical profile of 1 km in Tongchuan City is obtained by two-dimensional nonlinear conjugate gradient inversion.Finally,combined with regional geological information,the geological structure characteristics reflected by the electrical profile were obtained along with the detailed characteristics of water-rich structures in the area.The infl uence of the structure on the groundwater distribution was analyzed,and the water-rich areas were identifi ed.This work contributes to the prospective development of Tongchuan City.

准噶尔盆地乌伦古凹陷电性结构特征

弄清准噶尔盆地乌伦古凹陷地下深部电性结构,对于准噶尔盆地乌伦古凹陷的深部油气勘探具有重要意义。综合钻井、地震、测井、地质露头观测及分析化验等资料,总结了研究区的地球物理特征,包括地层岩石密度特征、磁性特征、电性特征。在此基础上,对乌伦古凹陷的一条大地电磁测深原始时间序列的电场数据进行视电阻率曲线平滑处理和静态偏移校正处理,并采用Occam法进行反演,得到乌伦古凹陷地下10km深度范围内的电阻率剖面。依据该电阻率剖面,结合地震及钻井资料,初步确定了准噶尔盆地乌伦古凹陷地层的构造形态,发现乌伦古凹陷西北端和东南端地下存在高阻异常,且基底中存在低阻地层。研究结论对准噶尔盆地乌伦古凹陷的油气勘探具有重要的价值。

陆域大地电磁基础网规划与展望

阵列式大地电磁测深是区域地球物理调查的重点发展方向之一,对标国外先进水平和国家社会的基础性需求,中国阵列式大地电磁测深工作程度依然较低,缺乏整体规划,开展全国性观测计划亟需提上日程。系统总结了当前区域地球物理学的发展趋势及国内外大地电磁网建设现状,提出了合作共建陆域多层级大地电磁基础网和大数据中心的顶层设计和规划布局,并对中国大地电磁基础网未来的重点发展方向进行了展望和思考。相关工作的顺利开展将为中国的区域地球物理调查拓展一项新属性参数,推动构建物性地球、结构地球和地球物理场四维模型,进而变革区域地球物理研究范式和工作模式,支撑中国深地探测走向世界前列。

基于深度学习的大地电磁二维反演研究

为了解决传统卷积神经网络反演由于层数过多而影响准确度的问题,开展了基于残差神经网络的大地电磁二维反演研究。通过大地电磁二维正演建立大量数据集,以TE和TM模式下的视电阻率和相位数据作为四通道网络输入,以对应的地电模型作为标签及输出进行有监督的学习,利用残差神经网络实现二维大地电磁反演。不同噪声水平的地电模型反演结果表明残差网络不仅可以很好地消除层数过多带来的准确度下降问题,还具有很强的抗噪作用。对冀中坳陷实测电磁资料进行反演,获得深部碳酸盐岩电阻率分布,据此分析了工区热储构造特征。理论模型和实测数据反演结果表明该方法具有良好的学习能力和抗噪性能,反演效果稳定可靠。

综合物探方法在碎屑岩地区找水中的应用——以云南宣威大路边村为例

为探索地球物理方法在碎屑地区找水应用的有效性,文章以云南宣威大路边村为研究对象,选取高密度电阻率法、联合剖面法、音频大地电磁法对碎屑岩层进行找水研究。结果表明:三叠系上统飞仙关组(T1f)碎屑岩富水性中等,含水性较均匀;受地面场地狭窄限制,高密度电法及联合剖面法测线长度有限,探测深度较浅,受碎屑岩电阻率小的影响,其找水应用效果不佳,在可探测的深度范围内较难识别基岩裂隙等找水有效异常,需结合其他物探方法甄别异常;音频大地电磁法找水应用效果相对较好,具有较高的横向分辨率,可有效识别碎屑岩中的断裂破碎带及构造裂隙带,表现为相对低阻异常特征;视电阻率单支测深曲线能有效分辨不同深度的岩土层结构,具有较高的纵向分辨率,对裂隙破碎带有较好指示意义。在地面场地较为狭窄的碎屑岩地区开展找水工作,音频大地电磁法结合视电阻单支测深曲线能取得较好效果,以音频大地电磁法确定碎屑岩地区中的裂隙发育带位置,以视电阻单支测深曲线确定裂隙发育带的深度。

非均匀网格有限差分法大地电磁测深法正演模拟

为解决有限差分法大地电磁正演模拟中模拟精度与计算效率之间的矛盾,采用非均匀网格有限差分进行了大地电磁响应计算。基于二维大地电磁方程,推导出非均匀网格剖分下的TM极化模式的大地电磁方程的差分格式。通过均匀半空间模型和低阻异常体模型的正演模拟,验证了非均匀网格差分法的正确性,非均匀网格差分法可以准确反映出复杂模型的大地电磁响应特征,识别异常体分布范围。

基于双向循环插补的大地电磁脉冲类噪声处理

【目的】大地电磁测深是一种通过观测天然电磁场获取地下电性结构的勘探方法,较易受到噪声干扰。脉冲类噪声是大地电磁工作中的常见噪声,其幅值高、频带宽,会对数据质量产生较大影响。【方法】为了压制脉冲类噪声,以插补思想为基础,提出了基于时间序列双向循环插补模型(Bidirectional recurrent imputation for time series,BRITS)的大地电磁脉冲类噪声处理方法。首先,将噪声干扰段删除,此时大地电磁时间序列可视为待插补的缺失序列,而后利用该缺失序列构建训练集,对BRITS模型进行插补训练,训练完成后对缺失序列进行插补,即可得到去噪结果。通过仿真及实测含噪声数据处理,并与经验模态分解(Empirical mode decomposition,EMD)阈值方法进行了对比。【结果和结论】结果表明:BRITS方法对仿真噪声数据处理后与原始数据的归一化互相关系数可达0.999以上,信噪比可达29 dB以上,EMD阈值方法处理前后相关系数为0.778,信噪比为3.09 dB;在实测数据处理中,BRITS方法有效恢复了噪声干扰数据,相比EMD阈值方法,其阻抗奈奎斯特图更接近天然大地电磁信号特征。通过不同训练样本试验得出:对4分量大地电磁数据而言,数据中至少需包含两道正常分量,单个含噪分量中噪声占比不大于20%,且噪声连续干扰长度不超过10个采样点,此时,BRITS方法去噪后数据的相关系数在0.96以上,可以保证一定的去噪精度。

高干扰矿集区大地电磁噪声抑制技术探索

大地电磁测深(MT)在矿产资源勘探方面应用广泛,但强人文电磁干扰严重制约了高质量的原始MT数据的获取。本文详细总结了国内常见电磁噪声源的类型,对其产生的电磁噪声特征进行分类分析;对国内外MT电磁噪声去噪方法进行对比,结合实际生产需求,总结了一套适用于高干扰矿集区快速有效的MT数据去噪施工与处理工艺。研究表明:Robust处理、远参考道技术和人工挑选是提高MT数据采集质量的有效和必要的手段,而通过理论计算,远参考站设置距离不小于3.56倍趋肤深度即可;在江苏洪泽盐盆矿集区开展的MT试验工作验证了该设定。

青藏高原东构造结林芝地热田浅部典型电性结构及热储关系

约40 Ma以来,受控于印度板块的俯冲及后期演化,青藏高原喜马拉雅山系东构造结成为了板块活动最强烈的地区之一;由于深部动力学过程中的浅表响应,该地区地热资源极为丰富。笔者等通过对两条音频大地电磁(AMT)测线进行数据处理与分析,查明了测点覆盖区域范围内二维电性结构及主要存在的深大断裂。依据电性结构推测研究区地下1 km深度范围内可分为4层,浅部低阻层为松散砂泥卵石层,下伏的中阻为砾卵石层,其下的低阻为砂岩、板岩、页岩强风化层,最底部的中高阻层推测为古元古界林芝岩群真巴岩组以片岩、花岗岩为主的地层。结合以往大地电磁测深及地震研究发现的地下10~20 km存在大规模近东西向展布且向上延伸熔融流变导致的低速高导体,推测可能是该地区深部热源所在。进一步通过对深部及浅部电阻率模型的综合对比研究,基于地热地质背景、电性结构特征,探讨了该地区的深部热源及热储关系。

超高密度电阻率法与音频大地电磁测深法对贵州普安某萤石矿区断层的探测效果

本文以贵州省普安县莲花山某调查区萤石成矿断层为研究重点,采用超高密度电阻率法和音频大地电磁测深法对同一断层进行了测量,查明了断层的平面分布情况和深部延伸情况,揭露出剖面峨眉山玄武岩组至茅口组“低阻-中阻-高阻”的三元分层电性结构。以峨眉山玄武岩组与茅口组接触带(蚀变体)的中阻特征为识别标志,结合断层延伸情况,推断出萤石矿体埋深范围,得到了钻孔验证。两种方法均对断层探测有较好的效果,也存在优劣势,超高密度电阻率法探测深度浅,测量精度高,但容易出现多种非目标体异常;音频大地电磁测深法探测深度深,但浅部分辨率较差,测量精度较低。实际工作中,应根据勘探目标的埋深情况选择合适的工作方法或综合两种方法,以达到更加可靠的物探解译成果。

大地电磁测深法在冀中坳陷深部碳酸盐岩热储调查评价中的应用

地热资源作为一种清洁能源,在“双碳”背景下,其开发和利用越来越受到重视。为了探查渤海湾盆地的冀中坳陷深部碳酸盐岩的热储条件,笔者等利用3条大地电磁测深剖面,进行了数据处理、分析和反演,获得可靠的二维电阻率模型和电阻率等深度平面图。分析研究区深部电性结构特征及主要断裂构造特征,同时根据基底隆起形成的高阻异常对深部碳酸盐岩分布和埋深进行了推断,并对冀中坳陷深部碳酸盐岩热储进行评价,以圈定地热异常远景区。分析认为:①研究区电性结构可以被划分为5层,其中第5电性层为高阻标志层,对应深部碳酸盐岩基底;②据此圈定了3类深部地热远景区:基岩埋深小于4000 m,如高阳低凸起地热远景区、黑龙口低凸起地热远景区(徐水凹陷)和河间潜山地热远景区(饶阳凹陷),基岩埋深4000~5000 m,如雁翎潜山地热远景区(霸县凹陷),基岩埋深5000~5500 m,如肃宁潜山地热远景区和留路潜山地热远景区;③大地电磁测深方法适用于冀中坳陷深部碳酸盐岩热储远景区的探查。深部热储远景区的圈定,可为后期的地热资源评价和开发利用建立基础。