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.

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.

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.

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.

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.

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.

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.

The Deep Geophysical Structure of the Middle Section of the Longmen Mountains Tectonic Belt and its Relation to the Wenchuan Earthquake

Investigation of the deep geophysical structure of the Longmen Mountains tectonic belt and its relation to the Wenchuan Earthquake is important for the study of earthquakes.By using magnetotelluric sounding profiles of the Luqu-Zhongjiang and Anxian-Suining; seismic sounding profiles of the Sichuan Maowen-Chongqing Gongtan,the Qinghai Huashi Gorge-Sichuan Jianyang,and the Batang-Zizhong; and magnetogravimetric data of the Longmen Mountains region,the deep geophysical structure of the Songpan-Ganzi block,the western Sichuan foreland basin,and the Longmen Mountains tectonic belt and their relation was discussed.The eastward extrusion of the Qinghai-Tibet Plateau thrusts the Songpan-Ganzi block upon the Yangtze block,which obstructs the eastward movement of the Qinghai-Tibet Plateau.The Maoxian-Wenchuan,Beichuan-Yingxiu,and Anxian-Guanxian faults of the Longmen Mountains fault belt dip to northwest with different dip angles and gradually converge in the deeper parts.Geophysical structure suggests that an intracrustal low-velocity,low-resistivity,and high-conductivity layer is common between the middle and upper crust west of the Longmen Mountains tectonic belt but not in the upper Yangtze block.The Sichuan Basin has a thick low-resistance sedimentary layer on a stable high-resistance basement; moreover,there are secondary paleohighs and depression structures at the lower part of the western Sichuan foreland basin with characteristic of high magnetic anomalies,whereas the Songpan-Ganzi block has a high resisitivity cover of upper crust and continues to a low-resistance layer.Considering the Longmen Mountains tectonic belt as the boundary,there are Bouguer gravity anomalies of ＂one belt between two zones.＂ Thus,we infer that there is a corresponding relation between the inferred crystalline basement of the Songpan block and the underlying basin basement of the Longmen Mountains fault belt.Furthermore,there may be an extensive ancient Yangtze block,which is west of the Ruoergai block.In addition,the crust-mantle ductile shear zone under the Longmen Mountains tectonic belt is the main fault,whereas the Beichuan-Yingxiu and Anxian-Guanxian faults at the surface are earthquake faults.The Wenchuan Ms 8.0 earthquake might be attributed to the collision of the Yangtze block and the Qinghai-Tibet Plateau.The eastward obduction of the eastern edge of the Qinghai-Tibet Plateau and eastward subduction of its deeper part under the influence of the collision of the Indian,Pacific,and Philippine Plates with the Eurasia Plate might have caused the Longmen Mountains tectonic belt to cut the Moho and extend to the middle and upper crust; thus,creating high stress concentration and rapid energy release zone.

Deep Seismogenic Environment in the Southern Section of the Longmenshan Fault Zone on the Eastern Margin of the Tibetan Plateau and Lushan M_s 7.0 Earthquake

The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20th, 2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional （3D） P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric （MT） sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover, at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya＇an, the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Low- velocity anomalies are distributed underneath the are of seismogenic origin, The inversion results of the MT retest data after the April 20th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake, Therefore, we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and pIayed a triggering role in the Lushan earthquake.

Research of the Conductive Structure of Crust and the Upper Mantle beneath the South-Central Tibetan Plateau

With the super-wide band magnetoteiluric sounding data of the JUong （吉隆）-Cuoqin （措勤） profile （named line 800） which was completed in 2001 and the Dingri （定日）-Cuomai （措迈） profile （named line 900） which was completed in 2004, we obtained the strike direction of each MT station by strike analysis, then traced profiles that were perpendicular to the main strike direction, and finally obtained the resistivity model of each profile by nonlinear conjugate gradients （NLCG） inversion. With these two models, we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture： the upper crust of the research area is a resistive layer with resistivity value range of 200-3 000 Ω.m. The depth of its bottom surface is about 15-20 km generally, but the bottom surface of resistive layer is deeper in the middle of these two profiles. At llne 900, it is about 30 km deep, and even at line 800, it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15-45 km, and a conductive layer is beneath it with resistivity even less than 5 Ω.m. This conductive layer is composed of individual conductive bodies, and at the south of the Yalung Tsangpo suture, the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However, at the north of the Yalung Tsangpo suture, the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively, the conductive bodies of line 900 are thinner than those of line 800, and the depth of the bottom surface of line 900 is also shallower. At last, after analyzing the effect factors to the resistivity of rocks, it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick, hot, flabby, and waxy.

A two-stage CO-PSO minimum structure inversion using CUDA for extracting IP information from MT data

The study of induced polarization （IP） information extraction from magnetotelluric （MT） sounding data is of great and practical significance to the exploitation of deep mineral, oil and gas resources. The linear inversion method, which has been given priority in previous research on the IP information extraction method, has three main problems as follows： 1） dependency on the initial model, 2） easily falling into the local minimum, and 3） serious non-uniqueness of solutions. Taking the nonlinearity and nonconvexity of IP information extraction into consideration, a two-stage CO-PSO minimum structure inversion method using compute unified distributed architecture （CUDA） is proposed. On one hand, a novel Cauchy oscillation particle swarm optimization （CO-PSO） algorithm is applied to extract nonlinear IP information from MT sounding data, which is implemented as a parallel algorithm within CUDA computing architecture; on the other hand, the impact of the polarizability on the observation data is strengthened by introducing a second stage inversion process, and the regularization parameter is applied in the fitness function of PSO algorithm to solve the problem of multi-solution in inversion. The inversion simulation results of polarization layers in different strata of various geoelectric models show that the smooth models of resistivity and IP parameters can be obtained by the proposed algorithm, the results of which are relatively stable and accurate. The experiment results added with noise indicate that this method is robust to Gaussian white noise. Compared with the traditional PSO and GA algorithm, the proposed algorithm has more efficiency and better inversion results.

Non-seismic geophysical analysis of potential geothermal resources in the Longgang Block,Northeast China

Although geothermal energy has many clear advantages,including its sustainability and environmentally friendly nature,research into potential geothermal resources across the Longgang Block,Northeast China,has been limited.Here we present the first analysis of the potential geothermal resources in this region that employs joint geological and non-seismic geophysical methods to identify target areas that may be economically viable.We acquire and analyze high-precision gravity,magnetic,and magnetotelluric sounding data,which are constrained using the petrophysical parameters of outcropping rocks across the Longgang Block,to conduct a comprehensive evaluation of the region’s deep geological structures and their geothermal resources potential,with a focus on identifying faults,rock masses,and thermal storage structures.We find that Archean granitic gneiss and Mesozoic rock masses in the deeper section of the Longgang Block possess weak gravity anomalies and high resistivities.We also identify thermal storage structures near these deeper geological units based on their extremely low resistivities.The data are used to infer the dip and depth of known or hidden faults,to constrain the spatial distribution of intrusive rock masses,and to determine the spatial distribution of subsurface thermal storage structures.The potential of the target areas for geothermal resources exploitation is divided into three grades based on contact depths between faults and thermal storage structures,and the scale of their thermal storage structures.Our results suggest that a joint non-seismic geophysical approach can be effective in locating and evaluating geothermal resources in complex geological settings.

Crustal electric structure of Haiyuan arcuate tectonic region in the northeastern margin of Qinghai-Xizang Plateau, China

Through the analysis and 2-D inversion for the 5 profiles in Haiyuan arcuate tectonic region (105°-107°E, 36°-37.5°N) in the northeastern margin of Qinghai-Xizang Plateau, we have obtained the electric structure within a range of 160 km in width (east-west) and 60 km in depth in the studied area. The results show that the crustal electric structure can be divided into 6 sections, corresponding respectively to Xiji basin (I), Xihuashan-Nanhuashan uplift (II), Xingrenbu-Haiyuan basin (III), Zhongwei-Qingshuihe basin (IV), Zhongning-Hongsibu basin (V) and west-margin zone of Ordos (VI) from the southwest to the northeast. The crustal electric structure is characterized by a broom-shaped pattern, which scatters to the northwest and shrinks to the southeast. The structures in the top part of Haiyuan arcuate tectonic region are complete and large, however, they diminish from the arc top to the northwest and southeast ends. In the depth from 0 km to 10 km, the resistivity is high in the sections II and VI, but relatively low in the other four sections, showing a similar pattern of basin depression. The electrical basement in the section III is the deepest, displaying a dustpan shape that is deep in the southwest and shallow in the northeast. A series of discontinuous zones with high conductivity exist in the middle-lower crust in Haiyuan arcuate tectonic region, which is possibly related to the moderate and strong earthquakes in the region. The resistivity distribution in the focal area of the 1920 Haiyuan earthquake is significantly heterogeneous with an obviously high conductivity zone near the hypocenter regime.

Deep electrical conductivity structure in the Great Wall Station area， Fildes Peninsula， West Antarctica

MT measured in Great Wall Station area shows that the electrical conductivity major axis of the Wind Valley Fault is 110°NE and the crustal thickness in the Fildes Peninsula is about 22. 3 km. The crust contains four main resistivity layers with their thicknesses being 1. 3 km, 6. 7 km, 1. 2 km and 13.1 km respectively. The upper crustal thickness is 9. 2 km and the lower crustal thickness is 13. 1 km.

Comparison Analysis of Longitudinal Electrical Conductivity Distribution and Seismic Tomography Velocity Models for the Central Tien Shan Region

The paper is dedicated to consider the approach of longitudinal electrical conductivity maps construction for Central Tien Shan region and to complete the comparison analysis of these maps with related seismic tomography data.The results of joint analysis of longitudinal electrical conductivity distribution and seismic tomography velocity models with respect to geological and geophysical senses allow to confirm the already known and to establish the new patterns of structure and geodynamics of the Tien Shan segment of Central Asia such as the manifestation of characteristic structural ensembles,reflection of the longitudinal and transverse segmentation of the Tien Shan,the existence within the framework of a“single”orogen of segments with different near-surface and deep structure and morphological expression.As well,the correlation between the velocity characteristics and geoelectric properties was found.This correlation allows to identify signs of reflection of fault structures in geophysical fields and to clarify the territorial and structural position of both visible and hidden faults.Additionally,in the paper,the geological interpretation of geophysical anomalies is considered.