Imaging the Architecture of Mineral Systems and the Pathways of Ore-forming Fluids across Mongolia with Magnetotellurics

In the framework of a mineral system approach,a combination of components is required to develop a mineral system.This includes the whole-lithosphere architecture,which controls the transport of ore-forming fluids,and favorable tectonic and geodynamic processes,occurring at various spatial and temporal scales,that influence the genesis and evolution of ore-forming fluids(Huston et al.,2016;Groves et al.,2018;Davies et al.,2020).Knowledge of the deep structural framework can advance the understanding of the development of a mineral system and the emplacement of mineral deposits.Deep geophysical exploration carried out with this aim is increasingly important for targeting new ore deposits in unexplored and underexplored regions(Dentith et al.,2018;Dentith,2019).

Spatial structural characteristics of the Deda ancient landslide in the eastern Tibetan Plateau:Insights from Audio-frequency Magnetotellurics and the Microtremor Survey Method

It is of crucial importance to investigate the spatial structures of ancient landslides in the eastern Tibetan Plateau’s alpine canyons as they could provide valuable insights into the evolutionary history of the landslides and indicate the potential for future reactivation.This study examines the Deda ancient landslide,situated in the Chalong-ranbu fault zone,where creep deformation suggests a complex underground structure.By integrating remote sensing,field surveys,Audio-frequency Magnetotellurics(AMT),and Microtremor Survey Method(MSM)techniques,along with engineering geological drilling for validation,to uncover the landslide’s spatial feature s.The research indicates that a fault is developed in the upper part of the Deda ancient landslide,and the gully divides it into Deda landslide accumulation zoneⅠand Deda landslide accumulation zoneⅡin space.The distinctive geological characteristics detectable by MSM in the shallow subsurface and by AMT in deeper layers.The findings include the identification of two sliding zones in the Deda I landslide,the shallow sliding zone(DD-I-S1)depth is approximately 20 m,and the deep sliding zone(DD-I-S2)depth is 36.2-49.9 m.The sliding zone(DD-Ⅱ-S1)depth of the DedaⅡlandslide is 37.6-43.1 m.A novel MSM-based method for sliding zone identification is proposed,achieving less than 5%discrepancy in depth determination when compared with drilling data.These results provide a valuable reference for the spatial structural analysis of large-deepseated landslides in geologically complex regions like the eastern Tibetan Plateau.

Three-dimensional magnetotelluric modeling in the spherical and Cartesian coordinate systems:A comparative study

With the increase in the coverage area of magnetotelluric data,three-dimensional magnetotelluric modeling in spherical coordinates and its differences with respect to traditional Cartesian modeling have gradually attracted attention.To fully understand the influence of the Earth’s curvature and map projection deformations on Cartesian modeling,qualitative and quantitative analyses based on realistic three-dimensional models need to be examined.Combined with five representative map projections,a type of model conversion method that transforms the original spherical electrical conductivity model to Cartesian coordinates is described in this study.The apparent resistivity differences between the spherical western United States electrical conductivity model and the corresponding five Cartesian models are then compared.The results show that the cylindrical equal distance map projection has the smallest error.A meridian convergence correction resulting from the deformation of the map projection is introduced to rotate the Cartesian impedance tensor from grid north to geographic north,which reduces differences from the spherical results.On the basis of the magnetotelluric field data,the applicability of the Cartesian coordinate system to western and contiguous United States models is quantitatively evaluated.Precise interpretations of the contiguous United States model were found to require spherical coordinates.

Improved Responses with Multitaper Spectral Analysis for Magnetotelluric Time Series Data Processing:Examples from Field Data

In order to attain good quality transfer function estimates from magnetotelluric field data(i.e.,smooth behavior and small uncertainties across all frequencies),we compare time series data processing with and without a multitaper approach for spectral estimation.There are several common ways to increase the reliability of the Fourier spectral estimation from experimental(noisy)data;for example to subdivide the experimental time series into segments,taper these segments(using single taper),perform the Fourier transform of the individual segments,and average the resulting spectra.

Rheological Evidence of the Lithospheric Destruction of the Eastern Block of the North China Craton

The Eastern Block of the North China Craton(NCC)(Fig.1)has undergone severe lithospheric destruction,with crustal thinning down to 100 km depth(Chen et al.,2009),contrasting sharply with the stable Kalahari and Rae cratons.However,there remains controversy over the destruction pattern(e.g.,Zhu et al.,2017).During the Early Mesozoic,crustal thickening occurred in the Xuhuai and Qinling orogens,followed by lithospheric delamination leading to crustal thinning(Chen et al.,2023).The middle and upper crustal thinning in the Yanshan and Taihang uplifts was induced by mafic magma underplating(Ji et al.,2009).

Electrical Characteristics of Tangjiawan Landslide in Lixian, Sichuan

A wide range of terrain features and landforms,which are exemplified by intricate geological formations and diverse rock compositions,are found in the western mountainous regions of China.These areas frequently encounter geological disasters.As one of the natural disasters,landslides lead to considerable loss of human life and property.Considering mitigation of the losses caused by landslide disasters,a necessary measure for disaster prevention and mitigation involves conducting detailed investigations and monitoring of landslides,which is also the cornerstone of landslide warning.This study compares and analyzes the feasibility of the magnetotelluric detection method for landslides using the results of engineering geological surveys and landslide monitoring.The study aims to address the scientific problem of the validity of using magnetotelluric methods to detect landslide development processes.The Tangjiawan landslide signal on the left side of the K94+000~K94+145 section of the Wenma Expressway is analyzed by employing engineering geological survey,magnetotelluric detection,landslide monitoring,landslide analysis,and other methods.Analysis results provide the static electrical characteristics of lithology,structure,and groundwater,as well as the dynamic electrical characteristics of landslide development.This study focuses on analyzing the relationship between the methods of magnetotelluric detection and engineering geological surveys and the results of landslide monitoring.The workflow and methods for data collection,processing,inversion,interpretation,and analysis using the magnetotelluric method to detect the dynamic development process of landslides are presented in the conclusion.Preliminary conclusions indicate a strong correlation between the dynamic changes in magnetotelluric wave impedance with the surface displacement of landslides and the dynamic changes in groundwater.The use of the magnetotelluric method for landslide detection and monitoring is a feasible example.The research results can offer certain technical references for the detection and monitoring of landslides using magnetotelluric methods and also provide references and guidance for the selection of diversified landslide monitoring methods in the future.

The Continental Scale USMTArray:Lessons Learned and Synergies with SinoProbe-Ⅱ

The USMTArray was completed on June 27,2024,comprising a network of 1779 transportable long-period magnetotelluric(MT)stations(Fig.1)with nominal 70-km grid spacing spanning the conterminous United States,an area of 8.1×10^(6)km^(2).Each station operated for weeksto-months,as required to meet data quality standards over the period band of 10–10000 s.The USMTArray shares similarities with the planned SinoProbe-II MT Array,with its 1-degree station spacing(~111 km in the latitudinal direction)spanning an area of 9.6×10^(6)km^(2).

Tectonic Evolution and Lithospheric Structure of the Beishan Orogen:Insights from Magnetotelluric Studies

The Beishan orogen,located in the central segment of the Tianshan–Solonker suture within the southern Central Asian Orogenic Belt(CAOB),is crucial for understanding the accretionary processes and continental growth in Central Asia.This orogen developed through the episodic amalgamation and accretion of continental margin arcs,island arcs,ophiolites,and accretionary wedges,undergoing a complex process of accretion and evolution.Since the Phanerozoic,the Beishan orogen has experienced multiple phases of magmatic and collision events.The intricate distribution of magmatic arc rocks has obscured the complete basement traces,and the spatial superposition of multiple magmatic arc phases has complicated the study of its evolutionary history.

Electrical Resistivity Structures of the Beishan Block,NW China,and Tectonic Implications

The Central Asian Orogenic Belt(CAOB)is a giant orogenic belt located between the Siberian Plate,the Tarim Plate,and the North China Plate,which records the longterm and complex geologic evolution of the Paleo-Asian Ocean from the Early Neoproterozoic(ca.1000 Ma)to the Late Paleoproterozoic(ca.250 Ma)process.The Beishan Block is located in the middle and southern edge of the Central Asian orogenic belt,at the intersection of the Tarim plate,the Siberian Plate and the Kazakhstan Plate.

Application of integrated geophysical techniques in geothermal exploration in Binhai County, Jiangsu Province

Integrated geophysical technology is a necessary and effective means for geothermal exploration.However,integration of geophysical technology for large‐scale surveys with those for geothermal reservoir localization is still in development.This study used the controlled source audio‐frequency magnetotelluric method technology for large‐scale exploration to obtain underground electrical structure information and micromotion detection technology to obtain underground wave velocity structure information.The combination of two detection technologies was used for local identification of geothermal reservoirs.Further,auxiliary correction and inversion constraint were implemented through the audio magnetotelluric sounding technology for maximum authenticity restoration of the near‐and transition‐field data.Through these technology improvements,a geothermal geological model was established for the Binhai County of Jiangsu Province in China and potential geothermal well locations were identified.On this basis,a geothermal well was drilled nearly 3000m deep,with a daily water volume of over 2000m3/day and a geothermal water temperature of 51°C at the well head.It is found that predictions using the above integrated geophysical exploration technology are in good agreement with the well geological formation data.This integrated geophysical technology can be effectively applied for geothermal exploration with high precision and reliability.

Three-dimensional magnetotellurics modeling using edgebased finite-element unstructured meshes

Three-dimensional forward modeling magnetotellurics （MT） problems. We present a is a challenge for geometrically complex new edge-based finite-element algorithm using an unstructured mesh for accurately and efficiently simulating 3D MT responses. The electric field curl-curl equation in the frequency domain was used to deduce the H （curl） variation weak form of the MT forward problem, the Galerkin rule was used to derive a linear finite-element equation on the linear-edge tetrahedroid space, and, finally, a BI-CGSTAB solver was used to estimate the unknown electric fields. A local mesh refinement technique in the neighbor of the measuring MT stations was used to greatly improve the accuracies of the numerical solutions. Four synthetic models validated the powerful performance of our algorithms. We believe that our method will effectively contribute to processing more complex MT studies.

3D Step-by-step inversion strategy for audio magnetotellurics data based on unstructured mesh

A three-dimensional(3D)step-by-step inversion strategy for audio magnetotellurics(AMT)is investigated in this study.The objective function is minimized by iteratively solving the Gauss-Newton normal equation,and the inversion region is discretized with unstructured tetrahedral elements.The inversion proceeds step-by-step from a coarse mesh to a fine mesh.In the inversion iteration process,a mesh is adaptively optimized according to the spatial gradient information about the model resistivity to fine delineate the boundaries of abnormal bodies.In the early stage of inversion execution,a coarse mesh is used for inversion,and the inversion stability is improved by reducing the number of inversion elements.In addition,mesh refinement is performed in the iterative inversion process.The inversion results obtained from the previous mesh are used as the reference and initial models for the next mesh iterative inversion.The step-by-step inversion strategy can ensure that the inversion is performed in the correct direction,improving the inversion stability and results gradually.Synthetic results show that the step-by-step inversion strategy with a Gauss-Newton method for 3D AMT inversion is stable and reliable,which lays a foundation for further practical 3D AMT data inversion.

Proterozoic tectonothermal processes imaged with magnetotellurics and seismic reflection in southern Australia

Over the last two decades,co-located seismic and magnetotelluric(MT) profiles provided fundamental geophysical data sets to image the Australian crust.Despite their complimentary nature,the data are processed and often interpreted separately without common processes in mind.We here qualitatively compare 2 D resistivity inversion models derived from MT and seismic reflection profiles across a region of Archean-Proterozoic Australia to address the causes of variations in seismic response and anomalous conductivity in the crust.We find that there exists a spatial association between regions of low reflectivity in seismic sections and low resistivity in co-located2 D MT modelled sections.These relationships elucidate possible signatures of past magmatic and fluid-related events.Depending on their diffuse or discrete character,we hypothesize these signatures signify fossil melting of the crust due to mafic underplating,magma movement or hydrothermal fluid flow through the crust.The approach discussed herein is a process-oriented approach to interpretation of geophysical images and a significant extension to traditional geophysical methods which are primarily sensitive to a singular bulk rock property or state.

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.

3D model of Svecofennian Accretionary Orogen and Karelia Craton based on geology,reflection seismics,magnetotellurics and density modelling:Geodynamic speculations

A 3D model of deep crustal structure of the Archaean Karelia Craton and late Palaeoproterozoic Svecofennian Accretionary Orogen including the boundary zone is presented.The model is based on the combination of data from geological mapping and reflection seismic studies,along profiles 1-EU,4 B,FIRE-1-2 a-2 and FIRE-3-3 a,and uses results of magnetotelluric soundings in southern Finland and northern Karelia.A seismogeological model of the crust and crust-mantle boundary is compared with a model of subhorizontal velocity-density layering of the crust.The TTG-type crust of the Palaeoarchaean and Mesoarchaean microcontinents within the Karelia Craton and the Belomorian Province are separated by gently dipping greenstone belts,at least some of which are palaeosutures.The structure of the crust was determined mainly by Palaeoproterozoic tectonism in the intracontinental settings modified by a strong collisional compression at the end of the Palaeoproterozoic.New insights into structure,origin and evolution of the Svecofennian Orogen are provided.The accretionary complex is characterized by inclined tectonic layering:the tectonic sheets,~15 km thick,are composed of volcanic-sedimentary rocks,including electro-conductive graphite-bearing sedimentary rocks,and electro-resistive granitoids,which plunge monotonously and consecutively eastward.Upon reaching the level of the lower crust,the tectonic sheets of the accretionary complex lose their distinct outlines.In the seismic reflection pattern they are replaced by a uniform acoustically translucent medium,where separate sheets can only be traced fragmentarily.The crust-mantle boundary bears a diffuse character:the transition from crust to mantle is recorded by the disappearance of the vaguely drawn boundaries of the tectonic sheets and in the gradual transition of acoustically homogeneous and translucent lower crust into transparent mantle.Under the effect of endogenic heat flow,the accretionary complex underwent high-temperature metamorphism and partial melting.Blurring of the rock contacts,which in the initial state created contrasts of acoustic impedance,was caused by partial melting and mixing of melts.The 3 D model is used as a starting point for the evolutionary model of the Svecofennian Accretionary Orogen and for determination of its place in the history of the Palaeoproterozoic Lauro-Russian intracontinental orogeny,which encompassed a predominant part of the territory of Lauroscandia,a palaeocontinent combining North American and East European cratons.The model includes three stages in the evolution of the Lauro-Russian Orogen(-2.5,2.2-2.1 and 1.95-1.87 Ga).The main feature of the Palaeoproterozoic evolution of the accretionary Svecofennian Orogen and Lauroscandia as a whole lay in the causal link with evolution of a superplume,which initiated plate-tectonic events.The Svecofennian-Pre-Labradorian palaeo-ocean originated in the superplume axial zone;the accretionary orogens were formed along both continental margins due to closure of the palaeo-ocean.

Investigating the Whole-lithosphere Structure of a Mineral System——Pathways and Source of Ore-forming Fluids Imaged with Magnetotelluric Modeling

Whole-lithosphere structure has direct implications for both the genesis of minerals and the locations of mineral emplacement;thus knowledge of the deep structural framework of the lithosphere can advance understanding of the development and evolution of mineral systems.

Evolutionary Programming for Systematic Evaluation of Aquifers: A Case Study from Dholera, Cambay Basin, Gujarat, India

Joint inversion of different potentials improves subsurface model resolution. In this paper seismic refraction and magnetotelluric data are used to understand near subsurface features of Dholera, Gujarat, India. An extensive seismic and magnetotelluric survey was carried out in Dholera in order to delineate subsurface presence of aquifers. Ray Inversion for Near Surface Estimation (RINSE) is used for inversion of Dholera seismic data. The inversion output of seismic data is used as seed points for resistivity inversion of anomalies. Inversion of resistivity data is done using evolutionary programing method which is also a type of genetic algorithm. Here the optimization is done using four major steps, of evolutionary programing namely population generation, fitness function, crossover and mutation. This paper also compares the similarities between the natural and geophysical optimization. A Low Velocity Layer is identified up to a depth of 11 m from seismic refraction method. Three layers are identified after the interpretation of seismic and resistivity data. The average thicknesses of Layers one and two are calculated as 3.558 and 6.533 respectively.

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.

Parallel rapid relaxation inversion of 3D magnetotelluric data

We implement a parallel algorithm with the advantage of MPI （Message Passing Interface） to speed up the rapid relaxation inversion for 3D magnetotelluric data. We test the parallel rapid relaxation algorithm with synthetic and real data. The execution efficiency of the algorithm for several different situations is also compared. The results indicate that the parallel rapid relaxation algorithm for 3D magnetotelluric inversion is effective. This parallel algorithm implemented on a common PC promotes the practical application of 3D magnetotelluric inversion and can be suitable for the other geophysical 3D modeling and inversion.

Three-dimensional conjugate gradient inversion of magnetotelluric sounding data

Based on the analysis of the conjugate gradient algorithm, we implement a threedimensional （3D） conjugate gradient inversion algorithm with magnetotelluric impedance data. During the inversion process, the 3D conjugate gradient inversion algorithm doesn＇ t need to compute and store the Jacobian matrix but directly updates the model from the computation of the Jacobian matrix. Requiring only one forward and four pseudo-forward modeling applications per frequency to produce the model update at each iteration, this algorithm efficiently reduces the computation of the inversion. From a trial inversion with synthetic magnetotelluric data, the validity and stability of the 3D conjugate gradient inversion algorithm is verified.