The role of large subsurface landforms produced during glaciations of the Pleistocene is still poorly understood with respect to groundwater flow. In particular, so-called tunnel valleys formed beneath ice sheets, acted as drainage systems of glacial meltwater. Their dimensions (up to 5 km width, 400 m depth, 100s of km length) reflect the massive amount of meltwater that incised into and flushed the subsurface beneath ice sheets.
To understand the potential of tunnel valleys as preferential flow pathways of offshore freshened groundwater (OFG) in the southeastern North Sea, we sailed 320 km of marine time-domain controlled-source electromagnetic surveys on 10 profiles using the surface-towed SWAN system on R/V ALKOR. In particular, we aim to answer the following questions: (1) Does the distribution of electrical resistivities indicate the presence of freshened groundwater in the subsurface of the North Sea? and (2) Can we delineate different resistivity distributions inside tunnel valleys?
Here we show our data acquisition and processing workflow as well as the subsurface electrical resistivity distribution from 2D inversions of the TD-CSEM data both for minimal constraints and for structural constraints inferred from a dense net of high-resolution 2D seismic reflection data. We use additional information from core data in similar geological setting in the interpretation to integrate geophysical and geological data.
The subsurface electrical resistivities show good correlation with the structures prevalent in the 2D seismic reflection data, where correlation is strongest for the upper and lower parts of the tunnel valleys. Plio-Pleistocene sediments show a widespread region of significantly increased electrical resistivities, which are interpreted to represent remnant offshore freshened groundwater from the flushing of meltwater below ice sheets during the Pleistocene.
How to cite: Lohrberg, A., Haroon, A., Moosdorf, N., and Krastel, S.: Resistivity anomalies in buried subglacial landforms and Plio-Pleistocene sediments of the southeastern North Sea revealed by surface-towed time-domain CSEM measurements, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10537, https://doi.org/10.5194/egusphere-egu25-10537, 2025.
Mantle tectonics, such as asthenospheric upwelling, usually controls subsequent fluid formation and migration from deep to shallow levels in the lower lithosphere and promotes rock failure and deformation, especially in southeastern Tibetan Plateau. Some models have proposed that lithospheric shearing and lower-crustal flow have controlled the crustal deformation in the Cenozoic. However, crustal rotation models show less possibility of a channel flow and relate the crustal deformation to the remote effect of India-Asia collision. Based on the concept whereby the fluid migration location defines the structural inheritance, we aim to relate crustal processes to mantle tectonics using fluids revealed by a new dense magnetotelluric (MT) array across the Ailao Shan-Red River belt with high quality, investigating above hypotheses. The 3D resistivity model of the study area reveals notable variations in the electrical property throughout the lithosphere. The lithosphere is interpreted to have been divided into two horizontal systems by a fluid diffusion layer at the bottom of upper crust, indicating a transition zone. In the lower lithosphere, two prominent near-vertical conductive regions are revealed and determined as consistent with the Dian-Qiong (DQ) suture and Song Da (SD) belt. These resistivity lows, which have spread at the bottom of upper crust, are inferred due to partial melting of deep lithosphere and reworking of the paleo sutures (DQ and SD) in the Late Cenozoic, because surface potassic magmatism in our study region was enabled by partial melting in the lower lithosphere. Therefore, fluid migration is considered to generate a transition zone featured by low-viscosity conductivity, which is inferred feeding by aqueous and melt fluids originating from the two channels and diffusing at depths from 15-20 km. Rather than the channel flow, this fluid migration process sensitively reflected in our model relates the mantle tectonics to crustal rotation by providing rheological conditions. We, hence, propose an inherited structure model featured by vertical mantle tectonic and upper-crustal translation-rotation that may occur before the major strike-slip event. The lithospheric transition zone could have provided convenience to induce the entirely upper crustal translation-rotation. The upper crust may move southwards and rotate clockwise, with respect to the lower part of the lithosphere, which is consistent with the surface geological and petrological observations to the north of our study region. Moreover, this transition-rotation process may have occurred within the time interval between potassic magmatism and strike-slip of the RRF (34-31 Ma), constrained by the geochronology results and duration estimation of shear zone by electrical model and energy equation. This translation-rotation process is inherited from the underlying mantle processes and may further be remotely affected by the upper crustal movement of the Tibetan Plateau, conforming with the mechanism interpretation of crustal rotation observed to the north of our study region. Furthermore, the crustal translation-rotation may also control the inherited strike-slip event in our study region.
How to cite: Di, Q., Zhang, K., and Xue, G.: A top-down control on upper crustal inheritance on the southeastern Tibetan Plateau, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1181, https://doi.org/10.5194/egusphere-egu25-1181, 2025.
The magnetotelluric (MT) method is widely used for reservoir exploration, including hydrocarbons, magma chambers, and other conductive fluids. However, this technique is sensitive to electrical anisotropy, which can complicate the determination of the target's dimensionality, potentially leading to inaccurate modelling and inversion. Identifying whether a region exhibits anisotropy is non-trivial, as the direction of anisotropy may be independent from the geoelectrical strike of the target structures.
In this study, we aim to evaluate the impact of anisotropy on MT modelling and inversion for reservoir identification of a hydrothermal system in a fractured zone. For this, we present a dataset of 45 broadband magnetotelluric sites, collected over a 100 km² area in the urban region of Annecy, France, within the Western Alpine Molasse Basin (WAMB). The recordings span from 1 to 12 days, with sampling rates from 256 Hz to 65 kHz. This area is of particular interest due to the potential presence of a low-enthalpy hydrothermal reservoir located within Jurassic marlstone-limestone units at depths of 1.5–2 km. The region also features brittle, seismogenic structures such as the Vuache Fault.
Fluid circulation in a fractured region might occur at preferred orientations, which signature can lead to electrical anisotropy that could be mistaken for a geoelectrical strike. To address this, we propose a methodology that incorporates magnetotelluric forward modelling and inversion under various dimensional scenarios (1D, 2D, and 3D) and geological settings. We begin with a dimensionality analysis, followed by isotropic and anisotropic forward modelling and inversion based on the area's geology to achieve the best fit to the observed data.
The dimensionality analysis yielded inconsistent results, suggesting that electrical anisotropy might be influencing the data. Consequently, we conducted 1D, 2D, and 3D modelling. Results from 1D forward and inverse models incorporating anisotropy show acceptable fits to the field data compared to isotropic models. These findings support the presence of conductive layers consistent with the hypothesis of a hydrothermal reservoir at those depths. In contrast, 2D models, whether isotropic or anisotropic, could not fit the data at the target frequencies related to the potential water boundary. Ongoing work includes 3D anisotropic forward modelling and inversions, which, in preliminary results, suggest indications of possible electrical anisotropy in the region.
How to cite: Riba Pereyra, C., Martí Castells, A., and Got, J.-L.: Assessment of Electrical Anisotropy Effects on Magnetotelluric Modelling and Inversion at Annecy, French Pre-Alps, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9421, https://doi.org/10.5194/egusphere-egu25-9421, 2025.
The Xiangshan volcanic basin hosts the largest volcanogenic uranium deposit in China. The host volcanic-intrusive complexes are the direct objects of the background study of metallogenic dynamics. However, the mechanism of its formation is still controversial. Given that the magma reservoir beneath the Xiangshan volcanic basin records the deep process of magma differentiation and evolution, the deep electrical structure and thermodynamic model were integrated to establish the dependence of melt fraction, temperature, and volatile phase content. Based on the high temperature and pressure petrophysical simulation and generalized three-phase Archie's law, the conductivity of each phase medium of the three-phase saturated magma reservoir and the bulk resistivity of the magma reservoir are estimated. The coupling relationship between the electrical parameters and the petrological data is finally established to find out the state of the Xiangshan magma reservoir. The results indicate that the highest volatile content of magma in the reservoir can reach 17 vol% and the highest melt fraction can reach 12 vol% based on the temperature pressure conditions and resistivity structure model of the crustal magma reservoirs here. This work provides new geophysical constraints and corroboration for the investigation of the formation mechanism of uranium-rich volcanic-intrusive complexes in the study area.
This work was funded by the National Natural Science Foundation of China (grants 42130811, 42304090 and 42374097), the Jiangxi Provincial Natural Science Foundation (20242BAB20143) and by the Autonomous deployment project of National Key Laboratory of Uranium Resources Exploration-Mining and Nuclear Remote Sensing, East China University of Technology (2024QZ-TD-15).
How to cite: Chen, H., Wen, Y., Deng, J., Yu, H., and Wang, X.: Insights on the magma reservoirs beneath the Xiangshan volcanic basin: Revealed by integrated magnetotelluric and petrological data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14336, https://doi.org/10.5194/egusphere-egu25-14336, 2025.
The 1988 Lancang M7.6 and Gengma M7.2 earthquakes in southwestern Yunnan, China, provide a unique opportunity to study intraplate seismicity. A 3D magnetotelluric model reveals a prominent high-resistivity crustal body (R1), interpreted as a rigid asperity, located between the epicentres of the earthquake doublet. This resistive body is flanked by conductive fault zones (C1 and C2), associated with major NW-trending seismogenic faults. The resistivity transition zone correlates with aftershock distributions, with larger aftershocks concentrate near fault conductors, while the high-resistivity body hosts smaller aftershocks. These findings suggest that stress accumulation in R1 and its susequent release along the fluid-enriched fault conductors controlled the rupture process of the earthquake doublet, within the context of a prevailing NNE-oriented compressive stress regime. This study underscores the importance of resistivity structures in seismogenesis, offering new insights into intraplate earthquake mechanisms and providing valuable perspectives for improving earthquake hazard assessment in active tectonic regions.
How to cite: Ye, T., Chen, X., Huang, Q., Pei, S., Zhang, J., and Cai, J.: Magnetotelluric Insights into Resistivity Structure and Seismogenic Process of the 1988 Lancang-Gengma Earthquakes , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1858, https://doi.org/10.5194/egusphere-egu25-1858, 2025.
Orals: Thu, 1 May | Room -2.21
Electromagnetic induction (EMI) methods are commonly used to classify unexploded ordnance (UXO) in both terrestrial and marine settings. Modern time-domain systems used for classification are multicomponent which means they acquire many transmitter-receiver pairs at multiple time-channels. Traditionally, classification is performed using a physics-based inversion approach where polarizability curves are estimated from the EMI data. These curves are then compared with those in a library to look for a match based on some misfit measure. In this work, we developed a convolutional neural network (CNN) that classifies UXO directly from EMI data. Analogous to an image segmentation problem, our CNN outputs a classification map that preserves the spatial dimensions of the input. In this way, our CNN produces high-resolution results and can handle the multiple transmitter-receiver pairs and the acquisition of multicomponent systems. We train the CNN using synthetic data generated with a dipole forward model considering relevant UXO and clutter objects. A careful design of the clutter classes is needed to maximize clutter discrimination.
We use a two-step workflow. First, we train a CNN to detect metallic objects in field data. From this, we extract patches of data that contain only background signal and use these to generate a new training data set by adding this background noise to our synthetic data. A second CNN is trained with these data to perform the classification. We test our approach using field data acquired with the UltraTEMA-4 system in the Sequim Bay marine test site. Using this workflow, classification results for the field data show that our approach detects all of the UXOs and classifies more than 90% as the correct type while also discriminating ~70% of the clutter. A key advantage of our CNN is that, once trained, it may be used to provide real-time classification results on the field.
How to cite: Heagy, L., Lopez-Alvis, J., Oldenburg, D., Song, L.-P., and Billings, S.: Using convolutional neural networks to classify unexploded ordnance from multicomponent electromagnetic induction data , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13722, https://doi.org/10.5194/egusphere-egu25-13722, 2025.
The Magnetotelluric (MT) sounding is widely used in geophysical research and plays an important role in the exploration of oil and gas resources and the basic research of deep geological structures. Due to the use of diffusion fields in the MT method, its inversion exhibits significant non-uniqueness and lacks effective solution evaluation methods, making it difficult to achieve checkboard test similar to that in seismic tomography research. Currently, the most widely used inversion methods include the following: Occam’s inversion (Constable et al., 1987), which yields a model with the smallest roughness for a specified misfit, providing a stable and rapidly convergent solution; reduced basis Occam’s inversion (REBOCC), which transforms the linearized inverse problem from the model space to the data space (Siripunvaraporn & Egbert, 2000); and nonlinear conjugate gradients (NLCG), which avoid excessive evaluations of the full Jacobian matrix and the complete solution of a linearized inverse problem at each step of iteration (Rodi & Mackie, 2001). These inversion methods usually cast the mathematical constraint of the model with the minimum model, or the smoothest model into the objective function to relieve the non-uniqueness of the inverse problem and provide stable and smooth inversion results. However, these methods make it difficult to image the shape and sharp boundary of geological structures clearly. Although MT inversion based on Bayesian theory can effectively obtain sharp boundary information and estimate model uncertainties, its significant computing cost limits practical application in large-scale data.
Entropy can measure the average level of "uncertainty" in random variable systems and evaluate the stability of the model system. Zhdanov (2002) proposed introducing minimum entropy constraints into geophysical inversion as a means of regularization. This method aims to generate more focused and clear inversion results by limiting the entropy of the model. In geophysical diffusion field inversion, this method has been widely used and has shown remarkable results. Based on the constraint of minimum entropy, we further introduce statistical methods to calculate the uncertainty of the model. Assuming that each parameter of the model follows a Gaussian normal distribution, the probability density distribution of the model can be regarded as the superposition of the probability distributions of all parameters. On this basis, we introduce the variance of the model parameters as the inversion parameter into the objective function based on the minimum entropy constraint, thereby effectively quantifying the uncertainty of the inversion model. To verify the effectiveness of this method, we designed a checkboard model for synthetic test and applied it to the Gonghe geothermal basin on the northeastern edge of the Tibet Plateau. The results show that the algorithm can effectively characterize the spatial distribution characteristics of partial melt in geothermal basins. Our research not only obtained a focused resistivity inversion model, but also quantitatively evaluated the reliability of the results, providing a new strategy for accurate imaging of complex geological structures and quantitative analysis of the uncertainty of inversion results.
How to cite: huang, Y., Hao, J., Gao, J., zhang, H., and Moorkamp, M.: Magnetotelluric inversion based on minimum entropy and evaluation of solutions, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14473, https://doi.org/10.5194/egusphere-egu25-14473, 2025.
Geophysical electromagnetic survey techniques are indispensable for the exploration of mineral, groundwater, geothermal, or hydrocarbon resources. CustEM is currently one of few open-source modeling tools for related 3D electromagnetic simulations worldwide. It provides the community with a general tool for modeling of CSEM, semi-airborne EM, TEM, and MT data. CustEM support general anisotropic petrophysical parameters, including the conductivity, the magnetic permeability, electric permittivity, and Cole-Cole parameters to simulate induced-polarization effects. The code also supports inversion of CSEM field data in combination with the inversion library pyGIMLi.
The options to develop and provide sustainable codes for numerical solutions are very limited in the framework of geophysical project proposals. In the context of a new research projected dealing with a quality-assured and re-usable software development, we aim to upgrade the code base to ensure the long-term sustainability by consistently addressing the FAIR4RS standards while also changing some fundamental underlying libraries as they are no longer actively supported. The final goal is to supply the community with the new library custEMx, including tools for meshing, visualization, data I/O, partly suppling graphical user interfaces as well as an extended library of tutorials, examples and community benchmarks.
We share the state of progress of the transition from custEM to custEMx and show validation results as well as first improvements of the new underlying software to outline new possibilities. We want to closely interact with the user community to ease and support the transition to custEMx, using conferences and workshops not only to inform the community of the progress but also to gather feedback, which will influence the ongoing software development.
How to cite: Skibbe, N. and Rochlitz, R.: CustEMx - Sustainable software for computational electromagnetics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-16418, https://doi.org/10.5194/egusphere-egu25-16418, 2025.
In marine controlled-source electromagnetic (CSEM) inversion, electromagnetic (EM)-field components, for example, the inline electric field components, are frequently utilized. For dependable data processing and inversion, precise orientations of the sources and receivers are essential, including their heading, tilt, and roll. Nevertheless, there could be potential uncertainties in the inversion and interpretation process. This is particularly true when the orientation information is lost during field data collection, for example, when the compass and tilt recordings are not obtainable. This research proposes a novel marine CSEM inversion algorithm. Instead of relying on EM fields, the orientation-independent rotational invariants are used. This approach allows for the inversion without having to take into account the orientations of the sources or receivers. The application of rotational invariants in the frequency-domain inversion of marine CSEM data is described. Numerical tests are conducted for both 1-D and 2-D scenarios using both the rotational invariants.
How to cite: Li, G. and Qin, L.: Frequency-domain marine controlled-source electromagnetic inversion using rotational invariants, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14992, https://doi.org/10.5194/egusphere-egu25-14992, 2025.
Over the last two decades, controlled source (CS) electromagnetic (EM) methods using an extended frequency range from 1-1000 kHz have been developed. Controlled sources complement the Radio-magnetotellurics (RMT) method by providing a wider frequency range, a greater depth of investigation and more stable responses. In addition, they remain operational even without radio antennas in the vicinity of the measurement area. While initially CS-/RMT was originally considered only in far-field conditions, it has recently been extended to include data from the intermediate zone, closer to the transmitter. This approach has several advantages over the solemn far-field consideration: (i) beneficial logistics, as there is no need to ensure sufficient source-receiver offsets; (ii) improved signal-to-noise ratio; (iii) combination of the unique resolution properties of CSEM and RMT, among others. Since EM fields in the intermediate zone depend on the source geometry, it is crucial to model the source geometry accurately.
We present a 3D inversion and resolution study for different inductively and galvanically coupled sources for synthetic data. We use (i) conventional CSEM single source configurations and (ii) CS/RMT source configurations with two orthogonal transmitters to provide two polarisations. Due to the lack of sufficiently accurate current measurements in the RMT frequency range, we invert: (i) impedance and tipper, (ii) admittance and (iii) interstationary transfer functions instead of univariate transfer functions. For modelling we use our newly in-house developed software package MR3DMod. The inversion package of MR3DMod follows the recipes of well established codes such as MODEM. The CS 3D forward modelling is based on a secondary field approach including quasi analytical modelling of the primary field. We have extensively tested the stability of the primary field solution for high frequency stability including displacement currents.
Based on our analysis, we are able to make further recommendations regarding high frequency CSEM and CS/RMT survey design. This includes the identification of favourable source configurations for specific shallow targets, such as waste deposits.
How to cite: Schöttle, S., Smirnov, M., Grayver, A., Smirnova, M., Yogeshwar, P., Mörbe, W., and Tezkan, B.: 3D Inversion and resolution study of different CS-/RMT transfer functions for different source configurations, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19541, https://doi.org/10.5194/egusphere-egu25-19541, 2025.
Efficient Transdimensional Bayesian Inversion of 2D Magnetotelluric Data with Deep Learning based Surrogate Modelling
Koustav Ghosal1, Arun Singh2 and Deepak K. Gupta3
1Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, India
2Indian Institute of Technology Roorkee, Uttarakhand, India
3Indian Institute of Technology (Indian School of Mines) Dhanbad, Dhanbad, India
Email id: koustav.20dr0066@agp.iitism.ac.in
Geophysical inverse problems are ill-posed and non-unique in nature, which makes the estimated model parameters prone to uncertainty. Uncertainty quantification is a vital step in geophysical inversion. Traditional gradient-based methods often struggle to provide reliable estimates of uncertainty with their single best fit model. Alternative approaches, such as the trans-dimensional Bayesian approach, have gained popularity for their ability to address this issue by generating an ensemble of models, which allows for uncertainty quantification. However, curating an ensemble for 2D magnetotelluric data inversion is computationally expensive and requires efficient sampling methods or rapid forward solvers to reduce the computation cost. The development of surrogate models has emerged as a promising solution to mitigate these computational challenges, enabling faster evaluations while maintaining accuracy in the inversion process. We proposed a 2D forward solver based on a convolutional neural network to accelerate the forward computation and integrate with trans-dimensional Bayesian framework. To enhance the generalization capabilities of the deep neural network, the subsurface resistivity models used for training were generated using Gaussian Random Fields (GRFs). This approach increased the network's robustness, even when dealing with unseen, out-of-distribution data.
The developed algorithm was tested on synthetic data, demonstrating that with surrogate modeling, the trans-dimensional inversion was ten times faster compared to the finite-difference-based forward solver, while producing similar results. Subsequently, the algorithm was applied to a subset of the CORPA dataset, covering a 200 km profile. The derived subsurface structure revealed a 5 km thick sedimentary layer sitting above a resistive basement. In the middle of the profile, the conductive North American conductor known as the Plain is clearly visible. The results not only align well with deterministic outcomes but also provide comprehensive uncertainty estimates.
How to cite: Ghosal, K., Singh, A., and Gupta, D.: Efficient Transdimensional Bayesian Inversion of 2D Magnetotelluric Data with Deep Learning based Surrogate Modelling, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-20290, https://doi.org/10.5194/egusphere-egu25-20290, 2025.
Posters on site: Wed, 30 Apr, 14:00–15:45 | Hall X3
The deep electrical conductivity structure (surface-to-100 km) beneath the Cappadocia Volcanic Province (CVP) in Central Anatolia, Türkiye was examined using three-dimensional magnetotelluric modeling of long-period data (>100 sec) that were collected as part of an U.S. NSF funded multi-disciplinary project titled the Continental Dynamics-Central Anatolian Tectonics (CD-CAT). The long-period dataset comprised seventy-eight observation points distributed on a wide 200 km x 200 km covering Cappadocia. Following the strike angle and dimensionality determination using electromagnetic impedance and phase tensor analyses, inverse models were developed to decipher the conductivity structure of the lower crustal/upper mantle depths beneath regional entities such as Mount Hasan, Mount Erciyes, and Niğde Massif. Some sensitivity tests were performed to control the validity of the key anomalies. The final model suggested that there are apparent high conductivity anomalies beneath these features with mantle roots. As a result, a clear and widespread highly conductive block is observed to be underlying a highly resistive anomaly near the Niğde Massif where the deep anomaly may represent fluid flow beneath the robust massif. As a second important outcome, the model suggested that there are deep conductive anomalies beneath the Hasan and Erciyes volcanoes, which are linked to the shallow potential magma chambers with horizontal dike-like structures.
How to cite: Tank, B.: Exploring the electrical conductivity structure of the Cappodacian upper mantle, Türkiye by long-period magnetotellurics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-792, https://doi.org/10.5194/egusphere-egu25-792, 2025.
The world’s largest rare earth element (REEs) deposit at Bayan Obo, northern North China craton (NCC), is geologically complex and its genesis is still debated due to the lack of geophysical constraints at depth. In this abstract, we newly discover two Mesozoic thrust faults and trace ore-bearing rocks by magnetotelluric (MT) method to constrain a carbonatite magmatic system. The Haoqin-North Jianshan thrust fault (F1) transported Boluotou-East-Main-West-Dongjielegele carbonatites (Ca1) and Paleoproterozoic metasediments over Paleozoic sediments, and it correlated the Shuiyuantou klippe, constraining a displacement of ~13 km. The Boluotou-Donjielegele-south West Pits thrust fault (F2) transported the carbonatite (Ca2), which was intruded by Permian granite, and gneiss in the hanging wall from Kulue, 30 km south of Bayan Obo, evidenced by similar four left-step en echelon high aeromagnetic anomalies (200-1000 nT). By restoring these faults, the Ca1 and Ca2 carbonatites situated above low resistivity zones (<500 Ω·m) at ~13 km south of the Bayan Obo deposit and Kulue, respectively. Based on low resistivity from 60-70 km in depth, high aeromagnetic anomalies and mantle-derived Mg, Fe and O isotopes of carbonatites, a carbonatite magmatic system consisting of a magma chamber and sill complex near the lithosphere-asthenosphere boundary (LAB) and two conduits, pluming magma upward to generate the Ca1 and Ca2 carbonatites is established. Our MT result is first a geophysical constraint on the igneous processes of the giant REE deposit from the LAB.
In summary, a REE-bearing carbonatite magmatic system consisting of a magma chamber, sill complex and two conduits has been established . This system can compare to the active carbonatite magmatic system in South Africa and volcanic system in Hawai’i . It also constrains the magma of the world’s largest Bayan Obo deposit originating from deeper level at the LAB, rather than the Moho.
How to cite: Xue, G., Zhang, J., Chen, W., Wu, X., Di, Q., and Li, X.: The newly constrained carbonatite magmatic system of the world’s largest Bayan Obo REE deposit, China, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1843, https://doi.org/10.5194/egusphere-egu25-1843, 2025.
The Yangtze block is a crucial component of the Rodinia and Columbia supercontinents, providing insights into their evolutionary history. The newly identified Caiziyuan-Tongan accretionary complex (CAC), situated on the southwestern margin of the Yangtze block, serves as an ideal window for understanding its Precambrian evolution. The exposure of the CAC suggests the presence of an ancient ocean basin that divides the Yangtze block into northern and southern micro-blocks. During the convergence of the Rodinia supercontinent, this ocean basin underwent subduction and eventual closure; however, its associated subduction polarity remains ambiguous. To address this issue, this study employed magnetotelluric in the Caiziyuan-Tongan area. Through three-dimensional inversion, three distinct features were delineated: an upper crustal conductor in the southern CAC, a middle-upper resistor trending stepwise north in the northern CAC, and a middle-lower crustal conductor encompassing the entire area. In conjunction with previously published geological and seismological observations, it is proposed that the first two features may be remnants associated with the northward subduction of the south Yangtze micro-block, while the third feature may indicate the existence of crustal flow related to the collision of the Indo-Asian plate. This finding enhances our understanding of the Precambrian evolution within the Yangtze block and contributes to the reconstruction of paleogeographic frameworks associated with the Rodinia supercontinent.
How to cite: Shibin, X., Qiao, W., Jian, Y., and Dewei, L.: A possible northward subduction in the southwestern Yangtze block during the Rodinia assembly： New constraints from three-dimensional magnetotelluric imaging, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-2974, https://doi.org/10.5194/egusphere-egu25-2974, 2025.
The water liberated from the breakdown of hydrous minerals while sinking in subduction zones may be transported into the overlying mantle wedge and may subsequently trigger magmatism and induce magnetotelluric (MT) anomalies above the subduction zone.Geophysical observations such as MT reveal that the electrical conductivity in subduction zone is up to 1 S/m. In contrast, the electrical conductivity of some hydrous mineral before dehydration remains very low which cannot explain the high electrical conductivity. Therefore, the dehydration effect on the electrical conductivity of hydrous minerals need to be determined.Electrical conductivity of several typical hydrous minerals before and after dehydration has been investigated. We observed significantly enhanced conductivities when most of hydrous minerals were heated to temperatures beyond their thermodynamic stability fields. The increase in conductivity are interpreted as several dehydration models. The increased electrical conductivity due to dehydration may responsible for the high-conductivity anomalies observed in subduction zones.
How to cite: Wang, D., Shen, K., and Zhang, Z.: Electrical conductivity of hydrous minerals during dehydration and geophysical implications, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-3942, https://doi.org/10.5194/egusphere-egu25-3942, 2025.
How to cite: Kangazian Kangazi, M. and Farquharson, C. G.: Constructing piecewise-constant conductivity models for the McArthur River, Canada, uranium mine audio-magnetotelluric data-set, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-13703, https://doi.org/10.5194/egusphere-egu25-13703, 2025.
Lithospheric foundering, through mechanical delamination or convective dripping, has been invoked to elucidate various perplexing geological phenomena, including near-surface deformation, sedimentation, and volcanism. Compared to delamination, direct evidence for the existence of modern lithospheric drips has been challenging to acquire because of their small-scale and transient nature. Here we present an image of the crustal and upper mantle electrical resistivity derived from high-quality, long-period magnetotelluric array data that extend from the southwest Songliao Basin (SLB), crossing the Great Khingan Range and onto the eastern Mongolia Plateau. The model reveals widespread layers of low resistivity in the deep crust and uppermost mantle beneath the mountainous regions surrounding the largely resistive SLB, where intense volcanism has occurred episodically during the Late Cenozoic. In the deep upper mantle, the model consistently reveals a set of elongated columns exhibiting high resistivity, interspersed with sub-vertical conductive anomalies. By incorporating additional petrologic, geochemical and experimental constraints, these drip-shaped resistive anomalies were interpreted as cold and dry lithospheric drips within a relatively hot and volatile-rich asthenospheric mantle. In light of the geological background, the dripping process may be linked to the edge-driven flow induced by the lithosphere thickness step between the rifted SLB and the adjacent mountainous areas. These results indicate that lithospheric dripping and induced asthenospheric upwelling may be significant factors in driving intraplate volcanism.
How to cite: Li, X., Deng, Y., Chen, Y., Moorkamp, M., Kuvshinov, A., and Huang, Z.: Magnetotelluric imaging of cold, dry lithospheric drips within a hot and volatile-rich asthenospheric mantle, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-9755, https://doi.org/10.5194/egusphere-egu25-9755, 2025.
Using an Magnetotelluric profile from the southern margin of the Sichuan Basin, we conducted an electrical structure study of the southern Sichuan and northern Guizhou region that the profile traverses, employing magnetotelluric sounding processing and inversion. A wealth of oil and gas exploration data within the Sichuan Basin indicates that the Pre-Sinian rift has a significant controlling effect on the Anyue-Deyang rift trough in the Sichuan Basin. Through comprehensive comparative analysis of the MT profile with the depth, seismic data, other MT data, and physical properties of the Pre-Sinian system within the Sichuan Basin, it is believed that the southern Sichuan and northern Guizhou region also exhibits a Pre-Sinian rift structure similar to the typical ones found within the Sichuan Basin. This finding has certain indicative significance for subsequent oil and gas resource exploration in the region and also deepens our understanding of the Neoproterozoic tectonic evolution in this area.
How to cite: Peng, Y., Fang, H., Liang, H., Zhang, X., Zhang, Y., He, D., Lü, Q., Wang, G., He, M., and Qiao, H.: The identification of Pre-Sinian graben-horst structures in the southern Sichuan and northern Guizhou region and their petroleum geological significance, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-5503, https://doi.org/10.5194/egusphere-egu25-5503, 2025.
The key to geothermal resource exploration lies in accurately determining the spatial distribution characteristics of geothermal reservoirs, including their depth, size, and geometric structure. However, due to the limited resolution of single geophysical datasets and the inherent non-uniqueness in geophysical inversion, inversion models independently derived from different datasets are difficult to accurately reveal the characteristics of subsurface structures. To reduce the uncertainty of inversion results and improve their reliability, We employed a 3-D approach to jointly invert gravity, magnetotelluric, and seismic surface wave dispersion data in the Huangshadong, Huizhou, based on the cross-gradient structural consistency constraints. The results indicated that the physical property structure obtained by the joint inversion was more comprehensive, with density, resistivity, and shear wave velocity models exhibiting good structural consistency, and provided a clearer characterization of the relief shape of the thermal reservoir interface, the thickness of the sedimentary, and the spatial distribution of hidden faults. Finally, we proposed a geothermal geological model for the region based on the geological information and joint inversion results, which will provide a scientific basis for effective geothermal exploration in the future.
How to cite: Liao, C. and Lin, J.: Joint inversion of gravity, magnetotelluric and surface wave dispersion data for geothermal exploration, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8066, https://doi.org/10.5194/egusphere-egu25-8066, 2025.
In recent years, there has been increasing interest in the phenomenon of induced polarization (IP) in airborne time-domain electromagnetic (ATEM) data. How to effectively extract the polarization information of the medium from the electromagnetic data containing the IP effect is a challenging problem. Most of the current studies can only recover relatively accurate resistivity information, but the recovery effect of other IP parameters is poor. In order to better extract the IP parameters of underground media, a new constrained inversion method must be sought. Using the process of z-score normalization, we define the data variance degree. Due to the correlation between IP parameters, the data difference degree is added to the multi-parameter inversion as a constraint, and a new inversion framework including the data difference degree is formed. The inversion results of synthetic data show that this method can improve the inversion results, obtain more accurate information of dielectric IP parameters, and reduce the difficulty of geological interpretation in the later stage. In order to test the stability of this constrained inversion method, we add different levels of noise for testing and obtain good inversion results. In addition, this data difference degree constraint can also be more widely used in the inversion of different geophysical methods.
How to cite: Lei, D.: Inversion of three-dimensional airborne transient electromagnetic excitation parameters under the restriction of data difference degree, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-1867, https://doi.org/10.5194/egusphere-egu25-1867, 2025.
Chargeable materials in the mining industry cause the induced polarization (IP) phenomenon when exposed to an electromagnetic field, which affects the electromagnetic response in transient electromagnetic (TEM) surveys. This distortion of TEM data is a challenge for traditional inversion methods, which typically focus on resistivity and may fail to provide reliable results when IP effects are significant. To address this limitation, this paper introduces a Bayesian inversion framework that incorporates full dispersive resistivity, using the Cole-Cole model to simulate both electromagnetic induction and IP phenomena. This approach allows for the effective recovery of Cole-Cole parameters from TEM data. A key advantage of Bayesian inversion is its ability to assess the confidence of inversion results, which is critical given the non-uniqueness of the inverse problem under these conditions.
Through numerical simulations and field examples, the proposed method demonstrates its ability to accurately recover both resistivity and Cole-Cole parameters, particularly in cases involving conductive and highly chargeable targets. However, the method struggles with resistive targets, where the inversion results exhibit lower accuracy and confidence. Despite this, the method is able to reliably resolve conductive regions, even when resistive regions are less accurately recovered, ensuring that model parameters are precisely estimated in conductive areas.
A field test conducted at Keyou Qianqi in Inner Mongolia confirmed the method’s effectiveness, successfully locating and validating a conductive, high-polarization silver-lead-zinc ore body. The study highlights the coupling of electromagnetic induction and IP effects, which generate time-decaying electromagnetic fields that are difficult to separate. By introducing dispersive resistivity, we can simulate these coupled responses and analyze the data’s ability to resolve dispersive resistivity model parameters. The analysis reveals that while considering IP increases the inversion’s complexity, it also enhances the resolution for resistivity and chargeability, although the resolution for time constant and frequency dependency is lower.
Our results show that multi-parameter inversion of IP-affected TEM data can effectively extract resistivity, chargeability, time constant, and frequency dependency parameters, particularly in conductive, high-polarization targets. For resistive, high-polarization targets, the inversion results are less accurate, highlighting the limitations of TEM data resolution for these structures. Nevertheless, the method still provides accurate parameter estimates for conductive zones, with high confidence in the target areas, making it a powerful tool for target identification and resource exploration.
How to cite: Li, H., Li, K., and Li, Z.: Inversion of IP-affected TEM data with full parametrization of dispersive resistivity, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-8197, https://doi.org/10.5194/egusphere-egu25-8197, 2025.
The Tibetan Plateau, the highest in the world, has been shaped by the ongoing collision of the Indian and Eurasian plates, which began ~55 Ma. This collision has caused widespread continental deformation, creating earthquake-prone faults along Tibet’s northern margin and in the surrounding intracontinental regions. Many studies suggest that the Alxa Tectonic Belt was primarily formed during the Paleozoic-Mesozoic through terrane accretion and closure, and later reactivated in the Cenozoic. Understanding crustal structures and identifying fossil suture zones are essential for interpreting these tectonic dynamics. However, the extensive sedimentary cover in the Alxa region, including the Tengri and Badain Jaran Deserts, complicates these efforts.
We conducted a 450 km MT survey from the Qilian Shan to the Alxa Tectonic Belt to map the subsurface crustal structure. The resistivity model delineates the Alxa basement boundaries, revealing the northern boundary of the Alxa Craton at Beidashan, near the Tebai-Quagan Qulu ophiolite belts, and the absence of the Shalazhashan-Zongnaishan Belt. This finding aligns with zircon U-Pb ages and Hf isotopes, indicating that the Yabulai-South Beidashan belts originate from ancient crustal sources, while the Shalazhashan-Zongnaishan and North Beidashan belts are juvenile.
Additionally, the electrical resistivity model reveals significant structural differences between the southern and northern Alxa basement, with distinct responses to Cenozoic deformation.
Deep structural analysis shows a conductive anomalous structure beneath the Qilian crust, possibly linked to fossil traces of northward subduction from the northern branch of the Paleo-Tethys Ocean. This structure is clearly delineated from the northern crustal resistive structure by the deep extension of the Haiyuan Fault. In the northern Hexi Corridor, southward-dipping thrust and blind thrust faults dominate, but their influence is mainly confined to the upper crust, where resistive bodies thin. Crustal decoupling at the middle-upper crust boundary may explain this. The middle and lower crust host Paleozoic mafic intrusions that facilitated vertical decoupling between the upper and lower crust. This interpretation is reinforced by seismic profiles, which reveal low-velocity anomalies and a thickened Moho consistent with such processes. The heat and magma from these intrusions thermally weakened the lower crust, causing partial melting, deformation partitioning, and the formation of a weak zone that enhanced decoupling.
In contrast, the northern boundary of the Alxa basement represents a coherent block structure, typically indicative of a strong, high-viscosity lithospheric composition. This robust structural framework favors vertical coupling between the upper and lower crust, contrasting with the decoupled zones observed in the south. This suggests that the outward expansion of the Tibetan Plateau stress is preferentially released along heterogenous boundaries between tectonic units, as evidenced by the extensive strike-slip deformation observed at Beidashan.
Acknowledgments: This research was supported by Phase I of the Northwest Region Magnetotelluric Subnetwork (2024ZD1000201), the China Geological Survey (DD20230254, DD20230255), and the Science and Technology Innovation Foundation of the Comprehensive Survey&Command Center for Natural Resources (KC20240024).
How to cite: Chen, C., Guo, C., Zhang, L., Yuan, W., Guo, P., Fan, J., and Wang, B.: Geophysical Imaging and Tectonic Features of the Alxa Basement and Its Role in the Tibetan Plateau's Evolution, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-10341, https://doi.org/10.5194/egusphere-egu25-10341, 2025.
The Iberian Pyrite Belt (IPB), which is located in the South Portuguese Zone (SPZ) of the Iberian Massif, is one of the most prominent sections of the Variscan orogenic belt in Western Europe. It extends over about 250 km in length and a width of 60 km forming an arc-shaped belt comprising several series of asymmetric basins that are tectonically controlled. These basins reflect the process of heterogeneous continental thinning triggered by left-lateral transpressive convergence with the Iberian Terrane.
Economically, it is an important European mining region with over 90 massive sulphides deposits shared between Portugal and Spain. It is home to world-famous and huge deposits such as Neves-Corvo, Aljustrel, Rio Tinto, Tharsis, Aznalcollar-Los Frailes, Las Cruces, among others. These mining activities show the economic dimension of the province based on the resources of Cu, Zn, Pb, Ag, Au, and Sn. The deposition of massive sulphides was related to the felsic volcanism and the black shales of the IPB volcano-sedimentary complex (VSC), which overlies the siliclastic sediments of the phyllite-quartzite group.
Nevertheless, many aspects are still poorly understood, especially those related to the deep lithospheric structure and the extent of the IPB to the southwest. The growing interest in the search for mineral deposits has led to many geophysical surveys being carried out over the years. However, most of them are limited to the first hundred meters in local areas or have a low spatial resolution, so a complete and global picture of the IPB extent isn't possible.
We present the preliminary results of a 3D resistivity model focusing on the lithospheric structure of the IPB down to a depth of 40 km. The model was calculated using data from 60 broadband Magnetotelluric (BBMT) stations, combining previous and newly acquired data, arranged in a 10x10 km grid along the IPB in Portuguese Terrain. The BBMT method provides a comprehensive resistivity image of the lithosphere, which is essential to decipher the geometry of the tectonic structures at depth. These structures play a key role in controlling the spatial distribution of many massive sulphide ore systems and offer potential insights into identifying new areas with deposits suitable for exploitation.
Acknowledgment
This work is supported by the Portuguese Fundação para a Ciência e Tecnologia, FCT, I.P./MCTES through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 https://doi.org/10.54499/LA/P/0068/2020).
References
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Kelbert, A., Meqbel, N., Egbert, G. D., & Tandon, K. (2014). ModEM: A modular system for inversion of electromagnetic geophysical data. Computers & Geosciences, 66, 40–53. https://doi.org/10.1016/j.cageo.2014.01.010.
Miensopust, M. P. (2017). Application of 3-D electromagnetic inversion in practice: Challenges, pitfalls and solution approaches. Surveys in Geophysics, 38(5), 869–933. https://doi.org/10.1007/s10712-017-9435-1.
Matos, J.X. et.al - Geophysical surveys in the Portuguese sector of the Iberian Pyrite Belt: a global overview focused on the massive sulphide exploration and geologic interpretation. In: Comunicações Geológica (2020), vol.107, Fasc. Especial III, p. 41-78.
de Oliveira, Daniel et.al - Mineral sustainability of the Portuguese sector of the Iberian Pyrite Belt. In: Comunicações Geológica (2020) vol.107, Fasc. Especial III, p. 11-20.
How to cite: Baltazar-Soares, P., Martinéz-Moreno, F. J., Gonzaléz-Castillo, L., Galindo-Zaldívar, J., Monteiro-Santos, F. A., Alves Ribeiro, J., Mateus, A., and Matias, L.: Preliminary results of the Iberian Pyrite Belt 3D resistivity model through Magnetotelluric data., EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-12919, https://doi.org/10.5194/egusphere-egu25-12919, 2025.
Abstract:
The Heyuanbei area, located in the western part of the Xiangshan region of China,is a promising area for deep exploration of polymetallic deposits.However, there is still a lack of sufficient understanding and reliable constraints regarding the spatial distribution of deep ore-controlling factors and the favorable zones for deep polymetallic mineralization.Therefore, identifying the spatial distribution of these deep ore-controlling factors is essential for advancing breakthroughs in deep exploration efforts.This study employs an 8-km-long audio-magnetotelluric (AMT) profile.After segmenting the data and applying impedance tensor rotations using the phase tensor analysis, a nonlinear conjugate gradient method was used for 2D inversion considering topographic effects. As a result, a refined electrical structure model of the study area was developed.Based on regional geology, drilling, and gravity and magnetic data, the following conclusions were drawn:(1)The red basin in the study area shows a two-layer electrical structure, with a low-resistivity red layer overlying a high-resistivity metamorphic basement. The volcanic basin, on the other hand, shows a three-layer electrical structure, consisting of a high-resistivity Ehuling Formation, a medium-low resistivity Daguding Formation volcanic layer, and a high-resistivity metamorphic basement.(2)The main ore-controlling faults in the Heyuanbei area are the He-Xiao faults and Ku-Xiao faults. The composite variation zones between these faults and the interfaces of the volcanic rock formation and the upper boundary of the metamorphic basement provide favorable spaces for the mineralizing hydrothermal fluids, controlling the deposition of polymetallic ores. The thickening of volcanic rocks at depth represents a promising segment for future breakthroughs in polymetallic exploration.(3)In the early stages of the western Xiangshan region, polymetallic-rich hydrothermal fluids mixed with downward-seeping atmospheric precipitation at depth, resulting in metasomatic exchange and precipitation, which formed polymetallic mineralization.Under continued extensional and tectonic stretching, later mineralizing hydrothermal fluids ascended, migrated along deep faults, and underwent differentiation and evolution, ultimately resulting in a vertically distributed polymetallic mineralization pattern.
Key words:Heyuanbei area；Audio frequency magnetotelluric sounding；Electrical structure；Ore-controlling factors；
Acknowledgments: This research was supported by Phase I of the Northwest Region Magnetotelluric Subnetwork(2024ZD1000201),the China Geological Survey for Regional Geophysical Survey in Beishan and Adjacent Areas (Grant No. DD20230254),the National Natural Science Foundation of China(42130811,41864004,41674077)
How to cite: Su, P., Guo, C., Meng, Y., and Deng, J.: Electrical Resistivity Structure and Ore-Controlling Factors in the Heyuanbei Area, Xiangshan Region, China: Implications for Polymetallic Mineralization, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14229, https://doi.org/10.5194/egusphere-egu25-14229, 2025.
The Midcontinent Rift (MCR) system formed ~1.1 Ga is a failed continental rift within the Superior Province of the Archean Laurentian continent. It is one of the important Precambrian geological features in the North American midcontinent. The abundance of igneous rocks exposed in the vicinity of Lake Superior contemporaneous to MCR is thought to be related to the upwelling of a Keweenaw mantle plume or anomalously hot/enriched mantle. However, in contrast to the classic three-arm model of continental rifting above a mantle plume, the lack of a northward-trending third rift branch or aulacogen in the MCR and the ~300 km deviation of the main rift arms from the inferred center of the mantle plume have not yet been well explained. To investigate this unique mantle plume-rift relationship and better constrain the influence range of the Keweenaw mantle plume, this study builds a three-dimensional electrical resistivity crust-upper mantle model that extends northward from the MCR to the Archean Superior Province using magnetotelluric (MT) data from the United States EarthScope and Canadian Lithoprobe project. The model reveals a prominent high conductivity anomaly near the base of the Western Superior Craton's lithospheric mantle, which is northwest-southeast trending, crosses the western branch of the MCR, and extends more than 300 km to both sides. It is inferred that the anomaly reflects an ancient mantle plume trail and is caused by the metasomatism and/or partial melting of the sulfide-rich basal lithospheric mantle during the Keweenaw mantle plume impingement.
How to cite: Lin, W., Schultz, A., Yang, B., Harris, L., and Hu, X.: Ancient mantle plume trail beneath the North American Midcontinent Rift revealed from Magnetotelluric data, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14390, https://doi.org/10.5194/egusphere-egu25-14390, 2025.
Abstract:
Due to the subduction of the Pacific plate and the wedging of the Indian plate, the area from Alxa Block to Ordos Block in the northeast margin of the Qinghai-Tibet Plateau has been experiencing intense tectonic activities, forming a north-south contact zone with complex geological structure and developed faults. In this study, a long-distance profile of magnetotelluric sounding distributed across Alxa Block, Hetao Basin-Helan Mountain-Yinchuan Basin-Ordos Block was conducted, and the following conclusions were drawn by dimensional analysis, and three-dimensional inversion research of the measured data, combined with other geological and geophysical data: (1) The Ordos Block shows a relatively stable layered resistivity structure. The shallow structure is mainly characterized by low resistance at 3-5km, the overall high resistance at 5-25km, and the low resistance structure at a large range at 25km. Alxa Block is characterized by high resistance, indicating that it still retains a rigid base, and two high-conductivity anomalies are upwelling to a depth of 20km in the middle of the block. The Hetao Basin-Yinchuan Basin is characterized by a typical basin-mountain structure with high resistance of mountain range and low resistance of basin. (2) F6 is Yellow River Fault, which cut into the upper mantle at a large angle. It is the boundary between Yinchuan Basin and Ordos Block. The eastern foot fault of Helan Mountain meets the Yellow River fault at a depth of about 20km. The F3, 4, 5, and 6 faults form a typical "negative flower structure". The low-resistivity melt from the upper mantle migrated upward in the weak zone and intruded into the high-resistivity Helan Mountain along the fault at the eastern foot of Helan Mountain at a depth of 20km. (3) The lithosphere on the western margin of the Ordos Block has undergone destruction and reconstruction, showing that the high-resistivity basement has been greatly reduced, while Alxa Block still retains a 40km thick basement. There are large-scale low-resistance bodies representing the asthenosphere of the northeastern margin of the Qinghai-Tibet Plateau, indicating that the surface area of the plateau's northeast-extending deformation has reached the Helan Shan-Yinchuan Basin. Alxa and Ordos Blocks were stretched and the low-resistivity fluid rose along the fault, and the low-resistivity fluid in Yinchuan basin invaded the middle part of Helan Mountain.
Keywords: Geophysics; Northeast margin of Qinghai-Tibet Plateau; Magnetotelluric profile; Deep electrical structure;
Acknowledgments:This research was supported by Phase I of the Northwest Region Magnetotelluric Subnetwork(2024ZD1000201),the China Geological Survey project – Regional Geophysical Survey of Wuwei to Yinchuan Region(DD20230255).
How to cite: Peng, W., Qi, C., Wang, J., Chang, C., Chang, J., and Zhu, D.: Deep Electrical Structure and Geological Significance of Alxa Block-Ordos Block in the Northeast Margin of Qinghai-Tibet Plateau, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14417, https://doi.org/10.5194/egusphere-egu25-14417, 2025.
Abstract:
The Beishan Orogenic Belt is a collage of multiple active continental margins, island arcs, and ophiolite belts during the closure of the ancient Asian Ocean since the Paleozoic era. In terms of geotectonic setting, the study area experienced polycyclic lateral orogeny and vertical accretion as well as complex splicing and strike-slip processes of blocks. Although a lot of studies have improved the overall research level of the Beishan ophiolite belts, there are still controversies about how the ophiolite belts are spread and buttressed on a regional scale, and about the subduction polarity of the ocean basins. Therefore, to better document the subsurface crustal structure of the Beishan Block, including the down‐dip orientation of the ophiolite belts, we carried out a 280‐km‐long Magnetotelluric (MT) survey from the Hexi Corridor to the northern Beishan with 16 stations along the overall survey length. The acquired data underwent rigorous analysis using ModEM software, The results of the profile analysis demonstrate that the southern and northern regions of the Beishan area exhibit divergent electrical structural characteristics, demarcated by low resistivity bodies indicative of ophiolite mélange zones. The northern part of the area is distinguished by a low-high-low resistivity pattern, which may suggest the presence of a colluvial accretionary crustal structure. Conversely, the southern part of the northern hill exhibits a more structurally intact high-resistivity body, which is hypothesised to potentially represent a more rigid and ancient landmass.
Key words:Electrical structure；Magnetotelluric;Beishan Orogenic Belt;
Acknowledgments: This research was supported by Phase I of the Northwest Region Magnetotelluric Subnetwork(Grant No.2024ZD1000201), the China Geological Survey for Regional Geophysical Survey in Beishan and Adjacent Areas (Grant No. DD20230254).
How to cite: Guo, C., Zhang, L., Chen, C., and Yuan, W.: Deep electrical structure and geological significance of Beishan Orogenic Belt, Southern CAOB, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14471, https://doi.org/10.5194/egusphere-egu25-14471, 2025.
Salt diapirs are key structures for understanding tectonic and sedimentary processes and are relevant for applications in resource exploration, carbon storage, and geological risk assessment. However, their study has been limited by challenges in acquiring detailed subsurface data, especially in offshore environments where access is restricted and costs are high. Onshore studies, therefore, play an important role in understanding salt tectonics by offering more accessible settings for investigation.
The Atlas Mountains in Morocco serve as an exceptional natural laboratory for studying salt tectonics. Formed through the tectonic inversion of an extensional basin, the Central High Atlas region hosts numerous diapiric structures related to evaporites deposited during the Triassic rifting phase. The Tazoult diapir stands out for its well-preserved surface exposure and accessibility. Magmatic activity associated with rifting further increased the complexity of Tazoult, with mafic intrusions emplaced both within and along its salt walls.
Seismic methods have traditionally been used to study salt diapirs, but they face limitations due to salt's high acoustic impedance and complex geometry, resulting in low-resolution images and interpretative challenges. Magnetotellurics (MT), on the other hand, offers a powerful alternative by leveraging electrical resistivity contrasts between salt and host rocks. This approach delineates internal geometries and formation processes while identifying salt extrusion features (e.g., salt glaciers), enabling the reconstruction of their geological and tectonic evolution.
To study the Tazoult diapir, we completed the acquisition of 102 wide-band MT soundings both within and around the diapiric structure, covering a frequency range of 10 kHz Hz to 0.001 Hz, over an area of approximately 40 × 40 km². The aim is to retrieve a high-resolution resistivity model to image the internal structure of the diapir, analyze salt extrusion geometries, and identify the salt source unit to better understand the geological and tectonic processes at a semi-regional scale. Preliminary transfer function analysis reveals significant resistivity contrasts at high frequencies within the diapir, identifying zones of high conductivity potentially linked to salt extrusion structures, as well as highly resistive zones associated with volcanic intrusions hosted within Tazoult. Mid-to-long periods exhibit (>1-10 s) a large split in transfer functions, indicating increased structure complexity. For longer periods (>100 s), very high apparent conductivities (10 S/m) have been identified, likely related to the salt source unit. These findings suggest a promising outlook for detailed imaging of the salt extrusion system.
How to cite: Castro, C., Hill, G., Závada, P., Kováčiková, S., Martín-Martín, J. D., Elgettafi, M., Himi, M., Kiyan, D., Mrlina, J., Beránek, R., and Amagar, F.: Diapirism and magmatic intrusions in the Tazoult salt wall revealed by Magnetotellurics, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14579, https://doi.org/10.5194/egusphere-egu25-14579, 2025.
The Oldoinyo Lengai stratovolcano, located in northern Tanzania, is the only active volcano currently erupting natrocarbonatite lavas. It is situated within the East African Rift, the longest continental rift in the world. While extensive geological research has been conducted on Oldoinyo Lengai, geophysical studies, particularly those using electromagnetic methods, remain limited. A recent seismological investigation in the region revealed a complex, highly interconnected lateral and vertical plumbing system. Additionally, recent Global Navigation Satellite System (GNSS) and Interferometric Synthetic Aperture Radar (InSAR) studies have shown subsidence around the volcano, suggesting the presence of a shallow, deflating magma source connected to deeper reservoirs. This has emphasized the urgent need for a comprehensive study of the inner structure of the volcano to assess potential hazards. Magnetotellurics (MT) has proven effective in imaging magmatic systems by mapping electrical resistivity distributions at depth, which are particularly sensitive to the presence of fluids and melt. Therefore, applying MT to infer the subsurface structure of Oldoinyo Lengai is highly promising, as it could provide valuable insights into the magmatic reservoirs driving volcanic-tectonic events, including their depth, geometry, and distribution.
This study evaluates the feasibility of using MT to detect the magmatic system beneath Oldoinyo Lengai by analyzing MT response distributions (e.g., Phase Tensor). We incorporate previous seismological findings to model the electrical conductivity structures at both shallow and deep levels. Our analysis focuses on: a) evaluating the resolving capacity of MT to identify conductive features that may correspond to magmatic reservoirs, and b) examining the spatial distribution and frequency behavior of MT responses based on equivalent conductivity models. Legacy data from the region has been reprocessed, providing a more accurate reference frame. This allows for a clearer contrast between the modeled structures and background resistivities, suggesting that MT is an ideal method for imaging the internal architecture of the volcano.
The station locations have been selected based on accessibility and logistical considerations. The proposed MT array will have higher resolution around Oldoinyo Lengai, with increasing interstation distances toward the outer array, extending up to ~40 km. This strategy aims to investigate the magmatic system beneath Oldoinyo Lengai and explore potential connections with surrounding volcanoes, opening doors for future regional-scale studies.
How to cite: Díaz, M. K., Junge, A., Castro, C., and Reiss, M. C.: Sensitivity of magnetotelluric responses in the exploration of the Oldoinyo Lengai magmatic plumbing system, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-14759, https://doi.org/10.5194/egusphere-egu25-14759, 2025.
In easternmost Papua New Guinea, some of the most metal-enriched continental crust has formed, hosting some of the world’s largest copper and gold deposits that formed over the last 2-3 million years. The area is marked by a dynamic and complex geologic history at a convergent margin with arc-continent collision, subduction reversals producing a complex microplate mosaic, and continuous metasomatism of the mantle wedge. In 2023, the SO299 DYNAMET expedition set sail to target the New Ireland Basin with special focus on the South Lihir Volcanic Field and investigate the source of the youngest volcanic activity in this area. To detect potential ascending melts and aqueous fluids focusing in the suspected trans-lithospheric faults, our team deployed 16 ocean-bottom magnetotelluric stations (OBMT) with a bottom-time of up to 3 weeks in the New Ireland Basin south and west of Lihir Island. Given the assumed geological complexity, the stations were positioned in a 3D array with approximately 15 km spacing, including a dense cluster around the volcanic seamounts south of Lihir. After the deployment we found 12 stations recorded valid data which were segmented, filtered and robustly processed to electromagnetic impedances for periods between 60 and 20,000 s. Preliminary results based on analyzing the 1D Berdichevsky average response function, the 2D response functions and 1D inversions show anomalies for several stations close to Lihir island on an NW-SE pointing profile in approximately 20-30 kilometers depth. Dimensionality analysis points towards an at least 2D structured anomaly with a possible northern strike direction. However, the area's complex geometry and proximity to nearby islands introduce significant uncertainties and favors 3D inversion, which is currently work in progress.
How to cite: Reeck, K., Brandl, P. A., Zimmer, H., Beier, C., Klein, J., Panachi, E., Moorkamp, M., and Jegen, M.: A marine magnetotelluric survey in the New Ireland Basin, Papua New Guinea – First results from SO299 DYNAMET. , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-15160, https://doi.org/10.5194/egusphere-egu25-15160, 2025.
Understanding geological structures, particularly tectonic dislocations, is crucial for addressing various issues such as seismic hazards, slope instability, radon emission, and more. It also offers valuable insights into the tectonic evolution of the studied region. As part of the DISLOCAT project and bilateral project between Czech and Slovak Academy of Sciences, the magnetotelluric measurements with a focus on the Western Carpathians has been carried out. We focus on tectonic structures and their development in the area of the Tatry mountain range. Collected data were distributed in three groups, where the central group investigating structures through whole Tatry Mts. from north to south crossing structures of the Inner Carpathian Paleogene, crystalline complexes, and Tatric envelope units. The eastern group is imaging south - eastern boundary between Mesozoic units of Belianske Tatry Mts. and the Palaeogene. The last group is mapping contact zone or dislocation in the Western Tatry Mts. between crystalline basement and Paleogene sediments and its relationship with the surrounding tectonic units.
Thanks to the contrasting electrical conductivity of tectonic structures such as faults, fracture structures, or deeper suture zones, magnetotellurics is a suitable method for mapping them to depth. Under favorable conditions, it is possible to identify conductive fluids accumulated in the fracture zone in the brittle part of the crust or detect the presence of conductive minerals, such as graphite, in these zones. According to preliminary results, the deepening of the main Tatry fault is steep to the South.
Keywords: magnetotellurics, fault zones and dislocations, Western Carpathians, Tatry mountains
How to cite: Ondrasova, L., Vozar, J., Bezak, V., Klanica, R., Kovacikova, S., and Hill, G. J.: Magnetotelluric studies of tectonics in the Tatry Mts. (Western Carpathians) , EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17077, https://doi.org/10.5194/egusphere-egu25-17077, 2025.
The geoelectric field depends on the electrical structure of the ground and the time variations of the geomagnetic field, which become more intense during geomagnetic storms. We present a procedure for identifying the areas of higher amplitude of the geoelectric field and determining the orientation of the geomagnetic field needed to produce these higher amplitudes. The technique involves decomposing the tensorial magnetotelluric responses in the frequency domain (e.g., impedance Z) and using the representation of complex vectors. We apply this method to identify these areas in the Iberian Peninsula, using the latest version of the Electrical Resistivity Model of the Iberian Lithosphere (ERMIL). This approach can help assess the potential effects of geomagnetically induced currents (GIC).
How to cite: Marcuello, A., Hafizi, R., Martí, A., Ledo, J., Queralt, P., and Piña-Varas, P.: Estimation of the high amplitudes of the geoelectric field, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-17635, https://doi.org/10.5194/egusphere-egu25-17635, 2025.
The Karasjok Greenstone Belt (KGB), located in northern Norway, is a key Archean-Proterozoic geological feature that shows the tectonic and magmatic processes of the Fennoscandian Shield. This elongated, northeast-southwest trending belt consists predominantly of metavolcanic and metasedimentary rocks, including mafic and ultramafic volcanics, banded iron formations, and sedimentary schists. The KGB records complex geological histories, including episodes of subduction, rifting, and metamorphism, primarily during the Paleoproterozoic era. Geochemical studies reveal that the belt's volcanic rocks are of both arc and rift-related origin, reflecting dynamic lithospheric processes during its formation.
The belt hosts significant mineral deposits, including gold, base metals, and rare earth elements, making it an important target for mineral exploration. The integration of geological and geophysical data continues to refine our understanding of the Karasjok Greenstone Belt’s evolution, its role in the assembly of the Fennoscandian Shield, and its broader implications for Precambrian tectonics. This study presents geophysical findings derived from four-frequency (0.9 kHz, 3 kHz, 12 kHz, and 25 kHz) airborne electromagnetic data (AEM05) acquired by the Geological Survey of Finland in 2009. One-dimensional (1D) inversion was carried out by using P223, a publicly available code. The inverted 1D resistivity models along a few selected flight lines in the KGB region reveal conductive anomalies sandwiched between the surface layer and the basement. These conductive anomalies correspond closely with those identified in apparent resistivity images derived from individual frequencies which corresponds to different depths. Moving forward, detailed lithological models will be integrated to refine and enhance the accuracy of the 1D resistivity inversions and all 1D inverse models will be visualized in three-dimensional (3D), providing deeper insights into the region's subsurface structure. We will also make attempts to invert the data in 3D.
How to cite: Wang, S., Slagstad, T., and Baranwal, V. C.: Using Airborne Electromagnetic Data to Map Conductive Anomalies in the Karasjok Greenstone Belt Region, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-18755, https://doi.org/10.5194/egusphere-egu25-18755, 2025.
Geoelectric fields induced on the Earth's surface by geomagnetic storms represent a significant hazard to the operation of power grids and critical infrastructures. These disturbances, generate Geomagnetically Induced Currents (GICs), which can disrupt electrical systems, causing potential damage and operational downtime. The ability to estimate these geoelectric fields in near real-time and to develop accurate local forecasts is crucial for enabling timely mitigation strategies, thereby enhancing the stability and functionality of power grids.
With solar activity expected to peak in 2025, understanding the full scope of its impact on geomagnetic storms and associated space weather phenomena has become a priority for both scientific research and risk management in critical sectors.
In this context, the MARGE project, an abbreviation of " GEoelectromagnetic Risk MAp for Central Italy” plays a key role. Led by the National Institute of Geophysics and Volcanology (INGV) in collaboration with the University of Bari and the Institute of Environmental Analysis Methodologies at CNR, the project focuses on two primary objectives.
First, it employs broad-band, long-term magnetotelluric data to delineate large-scale lithospheric structures in the Central Apennines.
Second, it develops detailed geoelectric field maps for Central Italy, supporting Space Weather modeling and vulnerability analyses of critical infrastructure.
This study analyzes the characteristics of natural electric and magnetic fields generated by geomagnetic storms during the MARGE project’s magnetotelluric campaign.
The results will contribute to improving Space Weather models, advancing our ability to predict and mitigate the impacts of geomagnetic disturbances on critical systems.
How to cite: Pignatiello, G., Siniscalchi, A., De Michelis, P., and De Girolamo, M. and the MARGE TEAM: Space Weather and GIC Risks: Analyzing Geomagnetic Storms within the MARGE Project, EGU General Assembly 2025, Vienna, Austria, 27 Apr–2 May 2025, EGU25-19457, https://doi.org/10.5194/egusphere-egu25-19457, 2025.
