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The Nari Caldera on Ulleung Island, an oceanic intraplate volcano, is a significant repository of information on the latest volcanic activity. To interpret the characteristics of the latest volcanic activity, it is essential to understand the caldera structures formed by the most explosive eruption of the Ulleung volcanic edifice. This study presents a three-dimensional (3-D) resistivity model based on audio-magnetotelluric (AMT) data and interprets the caldera structure of Ulleung Island. New land and ocean terrain models were used for the 3-D inversion of the AMT data, and a finer, nonuniform grid was generated for the caldera area. Subsequently, 3-D inversion and imaging were conducted on the AMT data at 25 stations. In the caldera area of Ulleung Island, the 3-D resistivity model is divided into two bodies: a high resistivity body located in the south and a low resistivity body located in the north. This structure is consistent with the location of the two calderas, Seongin Caldera and Nari Caldera, as inferred from geological studies. Furthermore, the high resistivity body located in the south exhibited a bowl shape in the 3-D space. Therefore, we suggest that the high resistivity body located south of the caldera on Ulleung Island is a structure of the Seongin Caldera. The Seongin Caldera has a diameter of approximately 1.5 km and a caldera fill height of approximately 0.8 km, as measured from the resistivity model. Based on the stratigraphy of Ulleung Island and the physical properties of the rock types, the interior of the Seongin Caldera was considered to have been filled with trachytic lavas of the Seonginbong Group. From the high geothermal gradient of Ulleung Island and the stratigraphy of the GH-4 borehole, the low resistivity body extending from the shallow depths of the Nari Caldera to the lower part of the Seongin Caldera could be interpreted as trachytic rocks that underwent hydrothermal alteration. In addition, a low resistivity body contains highly porous and/or weathered rock. This study presents information on the calderas of Ulleung Island that can aid in interpreting the characteristics of the latest volcanic activity. We expect this information to contribute to the preparation for potential volcanic hazards. [ABSTRACT FROM AUTHOR]

The Pannonian Basin is one of the best natural laboratories in the world to study the lithospheric response to continental extension and subsequent tectonic inversion. Here we address the topic of lithospheric structure by a combined geochemical and magnetotelluric analysis, which has been carried out in the framework of the Pannon LitH2Oscope project. The main objective was to detect the resistivity distribution over the entire lithosphere by magnetotelluric measurements, considering the lithological resistivity properties and relate the results to the structure and evolution of the Pannonian Basin. The Pannon LitH2Oscope MT array was used to estimate the depth of the Lithosphere-Asthenosphere Boundary (LAB), considering the legacy MT data and compared to previous estimates for the region. Using the MT and geomagnetic response functions, major structural zones of the Pannonian basin, such as the Mid-Hungarian Shear Zone or fault systems like the Makó Trough and the Békés Basin, were also imaged. In addition, we used the apparent resistivity soundings to compare 1D resistivity models computed from geochemistry and obtained from field MT measurements. This comparison provided new constrains for the composition, fluid and melt content variations at the local lithosphere-asthenosphere boundary. The Pannon LitH2Oscope MT dataset and the results presented in this paper provide input for more complex 3D inversions and further investigations of the lithospheric structure in the Carpathian-Pannonian region. [ABSTRACT FROM AUTHOR]

We provide a key magnetotelluric section, composed of archived magnetotelluric data along a NW-SE profile in Transdanubia, Hungary. For the interpretation of the key section, observations from raw magnetotelluric data and inversion results were used. In addition, other geophysical-geological information was also considered to confirm the conclusions based on the electrical resistivity sections. All this information was combined to identify the main structural lines and geologic units along the profile. Main structural lines observed on the resistivity sections are the Alpokalja line, Rába line, Balaton line, Kapos line, and Mecsekalja line. Geologic units that can be delineated due to their resistivity contrast include the Lower and Upper Austroalpine Units, the Transdanubian Range Unit, the Mid-Hungarian Megaunit, the Tisza Megaunit and sedimentary rocks filling the sub-basins of the Miocene Pannonian back-arc basin. The inversion results of the transverse magnetic (TM) polarization mode and the phase-depth sections of the raw data were found to be the most suitable for detecting the morphology and identifying the depth of the Pre-Cenozoic basement along the profile. [ABSTRACT FROM AUTHOR]

The prerequisite of inversion is the forward modeling study. In recent years, structured electromagnetic method for forward modeling has become mature. However, it has low computing efficiency and difficulty in simulating complex geological bodies. The electromagnetic field equations are represented by the coupled equations of magnetic vector potential and electric scalar potential. The node-based finite element method can efficiently calculate the electromagnetic field. The linear equations obtained are solved using a symmetric successive overrelaxation preconditioned conjugate gradient solver. This iterative method provides a faster solution speed. Unstructured grids used in numerical simulation can effectively simulate complex geological bodies. The use of adaptive finite elements optimizes the grid and improves calculation accuracy. The accuracy of the forward modeling code is verified through the establishment of layered models and three-dimensional models; the significant advantage of unstructured grids in simulating complex geological bodies is confirmed by the forward modeling simulation of complex ellipsoidal anomaly bodies; the effectiveness of the adaptive finite element method is verified by the forward modeling simulation of the prism model. [ABSTRACT FROM AUTHOR]

Electrical resistivity imaging survey was carried out at the warm spring site in Kasane, northwest Botswana, to investigate the subsurface distribution of geological structures and how they influence the existence of the warm spring. This investigation was carried out along eight 480-m-long parallel profiles with an electrode spacing of about 10 m and 50 m line spacing. We used a Pole–dipole electrode array to acquire the data. The data inversion employed in this study was based on the smoothness constrained nonlinear least-square inversion algorithm. 2D data inversion for all the eight 2D datasets was performed, and the eight 2D datasets were then collated into a single 3D data file before performing a 3D data inversion. The resultant 2D inversion models display the variation of electrical resistivity beneath the respective profiles; whereas, the resultant 3D models show the distribution of electrical resistivity in both the lateral and vertical directions. The 2D inversion results revealed two distinct anomalies with low resistivity values, e.g., < 46.2 Ω m of the topmost layer indicating the presence of silty clays and relatively high resistivity material with resistivity values exceeding 249.3 Ω m, which was attributed to basalts. Also observed in the 2D inversion results are low-intermediate resistivity zones of about 46.2 Ω m extending to greater depths beyond the boundaries of the models, which are attributable to the existence of fluid-filled fractures/faults. The 3D inversion results resolved a laterally extensive high-resistivity basaltic unit, two NE–SW-trending vertical/near-vertical low-resistivity features which are attributable to fluid-filled fractures or faults, and a broad low-resistivity zone occurring beneath a depth of 40 m on the southeastern side of the study area, which is likely to be indicating part of the geothermal reservoir. The low-resistivity vertical/near-vertical features observed in this study are likely to be enabling the flow of geothermal fluids, and the high resistivity basaltic unit acts as a caprock overlying the reservoir. [ABSTRACT FROM AUTHOR]

In this study, a robust 1D inversion was performed on magnetotelluric (MT) data by utilizing the Huber loss function to avoid misleading interpretation of results, which is caused by the presence of outliers in the data. The MT method utilizes the ratio between the electric field (E) and the magnetic field (H), which are perpendicular to each other, to obtain impedance values (Z). These values are used to extract subsurface information in the form of electrical resistivity variations with depth. In the study, forward modeling of the 1D MT responses was conducted by recursively calculating Z values. Robust inversion was performed using the Huber loss function as an objective function to minimize the difference between the calculated and observed data. The Huber loss function combines the squared loss and the absolute loss to anticipate the presence of outliers in MT observation data. The robust inversion was performed on synthetic data with additional noise and field data. The inversion process utilized 50 layers with a thickness of 500 m, a hyperparameter δ = 5 × 10 - 2 , λ = 0.01 , and λ t = 100 . The number of iterations used was 500 for inversions that used synthetic data and 3000 for inversions that used field data. The robust inversion scheme using the Huber loss function successfully overcame the presence of outliers and accurately estimated the actual model parameters, both for synthetic data and field data that contained outliers. The misfit was relatively small and close to zero, indicating that the inversion code works well. [ABSTRACT FROM AUTHOR]

The article demonstrates the possibility of joint interpretation of seismic, density, magnetic, and geoelectric models of the deep structure of the lithosphere along the Spassk-Dalniy–Zerkalnaya Bay geotraverse. Seismic and density boundaries form the main structural framework of the integrated model of the lithosphere. Magnetic heterogeneities map the elements associated with magmatism. The electrical model reflects the most recent processes of matter redistribution. The resulting integrated geophysical model demonstrated that for the identified terranes on the surface and most of the faults separating them, the structural style is well expressed only within the upper and middle parts of the crust to a depth of 10–20 km. The lithosphere of the Khanka superterrane retains its structure both in the crust and mantle. Structures of the crust and upper mantle of the Sikhote-Alin orogenic belt are different and associated with each other through highly conductive areas and zones. [ABSTRACT FROM AUTHOR]

The Central Eastern Desert and Red Sea region have emerged as a significant area of interest for geothermal energy exploration, owing to their unique geological characteristics and active tectonic activity. This research aims to enhance our understanding of the region's geothermal potential through a comprehensive analysis of gravity and magnetic data. By utilizing a 3D gravity inversion model, a detailed examination of subsurface structures and density variations was conducted. Similarly, a 3D magnetic inversion model was employed to investigate subsurface magnetic properties. Integration result from the Pygimli library ensured robustness and accuracy in the inversion results. Furthermore, a temperature model was developed using the WINTERC-G model and inversion techniques, shedding light on the thermal structure and potential anomalies in the study area. The analysis of the Bouguer gravity map, 3D gravity inversion model, and magnetic data inversion yielded significant findings. The Red Sea exhibited higher gravity values compared to the onshore Eastern Desert, attributed to the presence of a thinner and denser oceanic crust as opposed to the less dense continental crust in the Eastern Desert. The 3D gravity inversion model revealed distinct variations in density, particularly high-density zones near the surface of the Red Sea, indicating underlying geological structures and processes. Conversely, density gradually decreased with depth along the onshore line, potentially influenced by a higher concentration of crustal fractures. The magnetic data inversion technique provided additional insights, highlighting areas with demagnetized materials, indicative of elevated temperatures. These findings were consistent with the correlation between high-density areas and low magnetic susceptibility values, reinforcing the proposition of increased heat transfer from the Red Sea. Comparative analysis of temperature profiles further confirmed the presence of elevated temperatures in promising zones, emphasizing the geothermal potential associated with heat transfer from the Red Sea. This research contributes to the understanding of the geothermal resources in the Central Eastern Desert and Red Sea region. The results from gravity and magnetic data inversions, combined with temperature profiles, provide valuable information for future geothermal exploration and utilization efforts. The findings underscore the importance of geothermal energy in achieving sustainability and contribute to the global discourse on renewable energy sources. [ABSTRACT FROM AUTHOR]

Three-dimensional inversion based on source growing uses systematic searches. Compared with the regularization inversion, this method has lower computational requirements and faster processing speed. The criteria determining the source growth is crucial for the quality of the results. This study proposes an inversion based on source growing with full-tensor gravity gradiometry data to improve vertical inversion effectiveness. First, a depth weighting function is introduced for the criteria to optimize the determination of source growing at different depths. Second, the weights of different data are adjusted based on the inversion results of single-component gradient data, and a joint inversion method is established. Finally, matrix compression reduces memory occupation and improves computational efficiency. By the tests of synthetic data and real data from Vinton Dome, it is demonstrated that the proposed method can effectively guide source growing, providing stronger ability for distinguishing deep targets and being suitable for the inversion of complex-shaped targets. Furthermore, the method has high computational efficiency and anti-noise ability. [ABSTRACT FROM AUTHOR]

We consider a general case of a magnetotelluric (MT) study to reveal three-dimensional (3D) distribution of the electrical conductivity within the Earth based on measurements of electromagnetic (EM) fields by a two-dimensional (2D) array. Such an MT array observation can be regarded as a spatially discrete sampling of the MT responses (impedances), and each observation site can be regarded as a sampling point. This means that MT array measurements must follow the Nyquist–Shannon sampling theorem. This paper discusses how the sampling theorem is applied to MT array studies and what kind of consideration is required in the application on the basis of synthetic model calculations, with special attention to spatial resolutions. With an aid of the EM scattering theory and the sampling theorem, we can show that an observation array resolves some features of the MT impedance but does not others. We call the resolvable and unresolvable features the MT signal and noise, respectively. This study introduces the spatial Fourier transform of array MT data (impedances) which helps us investigating sampling effects of lateral heterogeneity from a different angle (in the wavenumber domain). Shallow heterogeneities cause a sharp spatial change of impedance elements near structural boundaries. High wavenumber Fourier components are required to describe such a feature, which means the site spacing must be sufficiently short to be able to resolve such features. Otherwise, a set of array MT data will suffer from aliasing, which is one of the typical causes of MT distortion (MT geologic noise). Conversely, a signal due to a deep-seated conductivity anomaly will have more reduced amplitude at higher wavenumbers, which means focused imaging of such an anomaly is generally difficult. Finally, it is suggested to properly consider the sampling theorem in an observation array design, so as to have best performance in resolving MT signals. [ABSTRACT FROM AUTHOR]

In this research, an arbitrarily oriented electric dipole at subsurface is used to simulate Seismogenic Electromagnetic (SEM) radiation emanating from a seismic zone during its gestation phase. Analytical synthesis of responses at the Lijiang magnetotelluric (MT) station has revealed that SEM radiation could induce identifiable anomalies in the electromagnetic (EM) spectrum, apparent resistivity and phase within specific frequency bands. Background variations were extracted from long-term observation data of Dali and Lijiang MT stations, enabling the identification of SEM anomalies related to the Yunlong and Yangbi earthquakes. Multiple parameters of dipole sources at subsurface were obtained by applying the Differential Ant Colony Optimization (DACO) algorithm to anomalous data of two stations with multi-frequencies and various response functions. The spatial distribution of these predicted dipoles is predominantly clustered in or around the seismogenic area, with their azimuthal orientation aligning towards the seismogenic fault in general. This study has demonstrated the potential of using subsurface electric dipole simulations for SEM radiation analysis, offering a feasible approach for the prediction and understanding of seismogenic zones. [ABSTRACT FROM AUTHOR]

Abstract—During the magnetotelluric studies in North Vietnam, simultaneous Phase Roll Out from Quadrant effect was detected in the behavior of impedance phases Zxy and Zyx at one of the sounding sites, CTH, in the Cam Thuy district, Thanh Hoa province. The investigation of this phenomenon has shown that it is accompanied by negative (directed upwards) components of the Umov–Poynting vector of electromagnetic energy flux in the entire range of the recorded periods of magnetotelluric variations. This energy flux direction remains unchanged when the initial coordinate system is rotated by angles of 45° and 60°. Another characteristic feature of this phenomenon is an unusual behavior of the apparent resistivity curves: the apparent resistivity magnitudes increase starting from a period of 200 s up to a period of 104 s, and this increase occurs practically along the asymptote. The analysis of the magnetotelluric field polarization and phase tensor asymmetry parameter (skew angle) β has shown that the section is characterized by significant three-dimensional (3D) distortions which manifest themselves by β values ranging between 20° and 90°. [ABSTRACT FROM AUTHOR]

The study compares the three deep learning approaches and assesses their relative performance solving the 1-D magnetotellurics (MT) inverse problem. MT data from a 1-D geothermal-type structure are used as an example to examine Variational Autoencoder (VAE), Residual Network (Res-Net), and U-Net architectures, adapted for 1-D MT inversion. Root Mean Square Error (RMSE) and Pearson correlation coefficient are applied as misfit measure and similarity criterion, and box plot tools are used to parameterize individual model parameters. The results show that the U-Net provides the most successful recovery of the 1-D resistivity models, even though all three approaches can produce accurate inversions of MT data. To investigate applicability of results to real data sets, the models performance are examined for the case of data containing noise. Three deep learning algorithms are robust with respect to data noise, although the U-Net is relatively superior. The study results provide a platform for more complex magnetotelluric inverse problems and ones involving real data sets. [ABSTRACT FROM AUTHOR]

Western Desert region between Cairo and Fayoum in Egypt is a geologically interesting area due to its active seismicity status and the continuous bidirectional urban expansions ongoing northward and southward growth. This work aims to develop a geological model of the region by combining magnetic intensity data (aeromagnetic) and potential field data (airborne gravity) with magnetotelluric (MT) data to offer decision-makers an additional evaluation tool for long-term future planning. Applying a number of filters to aerial gravity and magnetic data allowed us to calculate the subsurface density distribution and structure of the area between Cairo and Fayoum. In addition, the surface of the crust was imaged down to a depth of 6 km using a 3-D inversion of gravity data. Magnetotelluric data were gathered at eight locations over the NS-extended Cairo-Fayoum Road profile. MT data were inverted in 2-D to generate a resistivity model that defines the subsurface structure of the researched region. The subsurface geometry in the MT-derived resistivity model agrees with the results of airborne potential measurements, and no consideration was given to any faults that may have been undetected from the potential maps. Long-term seismological observations indicated that the likely active sources of earthquakes are limited to the recognized main faults and that the ongoing "1185 buildings" construction project did not and will not cause unanticipated seismicity. The paper concludes that urban growth is secure so long as the earthquake codes are rigorously considered while planning civil projects. The northern study region must be regularly monitored for induced seismicity. On the Cairo-Fayoum Road's western side, the middle sector of the study area is the safest place for future civil developments. [ABSTRACT FROM AUTHOR]

Non-trivial spin textures driven by strong exchange interaction, magneto-crystalline anisotropy, and electron correlation in a low-dimensional magnetic material often lead to unusual electronic transitions. Through a combination of transport experiments in exfoliated nanoflakes down to 16 layers and first principle calculations, we unravel emergent electronic phases in quasi-2D van der Waals ferromagnet, Fe4GeTe2, possessing ferromagnetic TC ~ 270 K, along with a spin-reorientation transition (TSR ~ 120 K) with the change of magnetic easy axis. Two electronic transitions are identified. The first transition near TSR exhibits a sharp fall in resistivity, followed by a sign change in the ordinary Hall coefficient (R0), together with, maximum negative magnetoresistance (MR) and anomalous Hall conductivity. Another unusual electronic transition, hitherto unknown, is observed near ~ 40–50 K (TQ), where R0 again changes sign and below which, the resistivity shows a quadratic temperature dependence, and MR becomes positive. An analysis of the experimental data further uncovers the role of competing inelastic scattering processes in anomalous magnetotransport behavior. The density-functional theory based first-principle calculations unveil two possible magnetic phases, followed by a low-energy model Hamiltonian which captures the essence of these phases as well as explains the observed magnetotransport behavior. Thus, we demonstrate an interplay between magnetism and band topology and its consequence on electron transport in Fe4GeTe2, important for spintronic applications. [ABSTRACT FROM AUTHOR]

Geophysical data inversion typically involves the numerical solution of high-dimensional ill-posed problems. To reduce the non-uniqueness, prior information in the form of appropriate regularization schemes, together with a proper representation of the quantities of interest, is helpful. To this end, methods are recommended that take into account a low-dimensional formulation of the inverse problem, a suitable representation for the quantities of interest, and a helpful numerical procedure. Structural aspects of the objects of interest, such as the dimensions of structures, are often available as prior knowledge in archaeological magnetic surveys. However, they are not easily exploited by classical geophysical data inversion applications; for example, a typical representation together with Tikhonov-like regularization schemes or total variation, which promote smoothness in the solutions, occlude underlying structural information or retrieve fuzzy boundaries. In this work, a three-dimensional inversion of magnetic data is developed based on an evolution strategy. It is adapted to a numerical representation that easily incorporates aspects achievable at archaeological sites. Synthetic test cases and a magnetic dataset corresponding to an archaeological site are used to report the results. [ABSTRACT FROM AUTHOR]

Fe−6.5%Si has higher electrical resistivity, lower magnetocrystalline anisotropy, and lower magnetostriction than traditional Fe−3.2%Si silicon steel. The reduced iron losses of Fe−6.5%Si render it a highly favorable candidate for high-speed motors and transformers. However, large-scale production of wide Fe−6.5%Si tape by rapid solidification can be challenging mainly because of its high melting point. In this work, boron is alloyed to Fe−6.5%Si to reduce its melting temperature and interfacial energy to improve the alloy's processability. Boron additions from 0.01 wt.% to 2.24 wt.% into Fe−6.5%Si and its effect on ribbon thickness, grain size, magnetic, and mechanical properties were studied. Minor boron alloying significantly changed the melt pool stability and wetting on the quench wheel and in turn increased the quench rate with minimum impact on the magnetic saturation and ductility. Boron addition of < 0.06 wt.% was also found beneficial to the magnetic property of the alloy by lowering both its hysteresis and eddy current losses. [ABSTRACT FROM AUTHOR]

Induced polarization datasets, which assess the overvoltage characteristics of the subsurface, are becoming increasingly prevalent. These datasets are not limited to mineral resource exploration but are also gaining traction in various engineering applications. We present the result of joint inversion of direct current resistivity and induced polarization data. We have obtained two subsurface properties: electrical resistivity and chargeability, by minimizing the single objective function. For this purpose, we have computed the distorted apparent resistivity using the secondary voltage measure at each time gate. We compared our results with the conventional inversion of induced polarization on synthetic data with both high and low chargeable mineralized ore bodies. We noted that our approach provides a more reasonable model for high chargeable ore bodies. However, both joint inversion and nonlinear approaches give similar results for low-chargeable ore bodies. Nonlinear inversion of induced polarization was accomplished by solving two objective functions: One for resistivity and another one for induced polarization. Thus, error in the resistivity inversion propagates in the chargeability computation due to consequences. Further, the developed joint inversion algorithm is also applied to a field data set measured at the Beldih mine located in the West Bengal, India. We compared our final chargeability models with the local geology and geologic models obtained via other geophysical methods on the same profiles and found an excellent correlation in both models. [ABSTRACT FROM AUTHOR]

The spatiotemporal evolution of earthquakes induced by fluid injections into the subsurface can be erratic owing to the complexity of the physical process. To effectively mitigate the associated hazard and to draft appropriate regulatory strategies, a detailed understanding of how induced seismicity may evolve is needed. In this work, we build on the well-established continuous-time random walk (CTRW) theory to develop a purely stochastic framework that can delineate the essential characteristics of this process. We use data from the 2003 and 2012 hydraulic stimulations in the Cooper Basin geothermal field that induced thousands of microearthquakes to test and demonstrate the applicability of the model. Induced seismicity in the Cooper Basin shows all the characteristics of subdiffusion, as indicated by the fractional order power-law growth of the mean square displacement with time and broad waiting-time distributions with algebraic tails. We further use an appropriate master equation and the time-fractional diffusion equation to map the spatiotemporal evolution of seismicity. The results show good agreement between the model and the data regarding the peak earthquake concentration close to the two injection wells and the stretched exponential relaxation of seismicity with distance, suggesting that the CTRW model can be efficiently incorporated into induced seismicity forecasting. [ABSTRACT FROM AUTHOR]

The tectonic position of the southwest section of the Qinzhou Bay-Hangzhou Bay Tectonic Junction Zone (QHTJZ) can be determined by examining the Qinzhou-Fangcheng Junction Zone (QFJZ) in Guangxi. This zone is significant because it was the location of the largest earthquake ever recorded in the inland region of South China, specifically the 1936 LingshanM6¾ earthquake in Guangxi. Therefore, this region serves as an optimal location for researching the origins of intraplate earthquakes in South China. This study presents a display of a broadband magnetotelluric (MT) prospecting profile that traverses the Guangxi QFJZ and the Lingshan earthquake zone, extending from the northwest (NW) to the southeast (SE). A resistivity structure model was generated using three-dimensional (3D) inversion technology along the profile. The main faults in QFJZ were analyzed in terms of their deep extension forms and tectonic attributes. This analysis was performed by integrating the results obtained from geology, gravity, wave velocity ratio, Global Position System (GPS), and geothermal flow. The results showed that (1) the Dongzhong-Xiaodong fault (DXf), the eastern Fangcheng-Lingshan fault (FLf2), and the eastern Hepu-Beiliu fault (HBf2) were all trans-crustal deep faults, and crust-mantle ductile shear zones developed in the deep part. Two electrical boundary zones, DXf and HBf2, were identified. DXf inclined towards the northwest, while HBf2 inclined towards the southeast. The Fangcheng-Lingshan fault (FLf) exhibits a tectonic style resembling a "flower" shape in the upper crust. In the deeper section, it is characterized by an electrical boundary zone that gradually slopes towards the southeast direction. (2) The Hunan-Guangxi Passive Continental Margin (HGPCM) on the NW side of DXf had a stratified resistivity structure and relatively stable Bouguer gravity anomalies, which conformed to the quasi-craton tectonic attribute of the local failure at the southeastern margin of the Yangtze Block (YB). The southeastern side of this block is marked by the presence of the QFJZ and Yunkai Magmatic Arc (YKMA). These areas exhibit varying Bouguer gravity anomalies, indicating a combination of high and low resistivity in their electrical structures. This suggests that this zone has undergone multiple stages of structural evolution and transformation. The giant high-resistivity body under the Qinzhou-Fangcheng Remnant Ocean Basin (QFROB) might be the trace left by the extinction of the South China Ocean and the collision orogeny between YB and the Cathaysian Block (CB). The presence of sub-low-resistivity layers in the middle-lower crust between the Liuwandashan Magmatic Arc (LMA) and YKMA indicates that this particular zone is being influenced from a distance by magmatic activities originating from the Leiqiong mantle. (3) The focal area of the 1936 Lingshan earthquake was located in the brittle high-resistivity body with a low strain rate. Under the coupling action of NWW-SEE regional tectonic stress and deep thermodynamic force, the brittle high-resistivity body in the upper crust became the main body for accumulating the tectonic stress. The Lingshan earthquake occurred due to the dextral strike-slip fracture instability of FLf2, a rock layer with slightly lower strength in the sub-high-resistivity zone. This instability was triggered when the accumulated stress reached the ultimate rock strength. The unveiling of the seismogenic model of the Lingshan earthquake, as presented in this study, holds significant scientific importance in comprehending the factors contributing to intraplate earthquakes in the South China region. [ABSTRACT FROM AUTHOR]

A couple of studies show that Saudi Arabia has promising geothermal resources. The most prominent sites are located around Al-Lith, with four hot springs, where Ain Al-Harrah has the highest temperature. Previous studies suggest that this hot spring's geothermal reservoir is promising and may provide the Al-Lith region with long-term electricity. Our key objective is to investigate and explore more comprehensively the prospect of geothermal resources around the Ain Al-Harrah hot spring. To achieve this goal, we acquired broadband magnetotellurics (MT) data in the period range 0.001–60 s for three different sampling frequencies at 15 soundings along one profile. The 2D inversion results show three main units. A conductive anomaly (< 30 Ω.m) at a depth > 3.5 km below the surface is most likely related to a geothermal heat source. The high resistive zones (roughly 200–4000 Ω.m) are associated with hard-rock geological formations. Although uncertain, a detected medium resistive body (30–200 Ω.m) at a depth of 800 m below the surface possibly indicates the pathway of geothermal fluids forming a convection cell between the hot body and the surface. A lateral resistivity discontinuity is considered a fracture zone acting as a pathway for the fluids to reach the surface forming the hot spring. [ABSTRACT FROM AUTHOR]

Based on the double probe method, we investigated the ionospheric preseismic electric effects for the strong Chile earthquake of magnitude M = 8.8 that occurred on February 27, 2010, at 06:3414 universal time (UT). To correlate the electrical disturbance to natural geophysical activities in the ionosphere associated with the earthquake, we used a new approach to analyze these disturbances a few days before the event, based on DEMETER spacecraft collected data in the ultra low frequency (ULF) band. The double probes method and the calculated preseismic ionospheric electric field resulting from the potential difference between two electrical probes of ICE (Instrument Champ Electrique) onboard the DEMETER satellite are investigated. Once the effect of the satellite motion is canceled, it is found that the disturbance is caused only by the telluric activity. Moreover, the investigation of the collected data illustrates a clear correlation between preseismic electrical disturbances and ionospheric plasma parameters recorded near the future epicenter. The strongest ionospheric electrical disturbances are located near the future epicenter and close to the geomagnetic conjugate points. [ABSTRACT FROM AUTHOR]

The Wuwei Basin is one of the most important oil- and gas-bearing basins in the Meso-Cenozoic basin groups in the Lower Yangtze River region. It has great shale gas resource prospects. The formation mechanism of this basin is poorly studied for lack of constraining data for deep structures. In this paper, a crustal electrical structure model of the Wuwei Basin and the adjacent areas is constructed based on the two-dimensional inversion of a magnetotelluric (MT) sounding profile achieved with the nonlinear conjugate gradient method. The results show that large-scale low-resistance bodies have developed in the underlying middle and lower crust of the Wuwei Basin, and are different from the uplifts on the two sides according to their high-resistance electrical characteristics. The electrical structure and regional geological and geophysical data suggest that the peak zone of the Chuzhou-Chaohu foreland fold-and-thrust belt is located on the east bank of the Yangtze River (Wuhu Section), which, together with the main thrust fault belt in the east, forms a typical thrust structure belt. An early Yanshanian sinistral strike-slip fault developed in the deep part of the Wuwei Basin, which may have controlled the formation and evolution of the basin, whereas the tectonic inversion of the early-developed thrust faults is relatively weak. These findings provide a geophysical basis for future studies of basin tectonic evolution and regional tectonic frameworks. [ABSTRACT FROM AUTHOR]

Various physical processes in association with magnetic reconnection occur over multiple scales from the microscopic to macroscopic scale lengths. This paper reviews multi-scale and cross-scale aspects of magnetic reconnection revealed in the near-Earth space beyond the general global-scale features and magnetospheric circulation organized by the Dungey Cycle. Significant and novel advancements recently reported, in particular, since the launch of the Magnetospheric Multi-scale mission (MMS), are highlighted being categorized into different locations with different magnetic topologies. These potentially paradigm-shifting findings include shock and foreshock transient driven reconnection, magnetosheath turbulent reconnection, flow shear driven reconnection, multiple X-line structures generated in the dayside/flankside/nightside magnetospheric current sheets, development and evolution of reconnection-driven structures such as flux transfer events, flux ropes, and dipolarization fronts, and their interactions with ambient plasmas. The paper emphasizes key aspects of kinetic processes leading to multi-scale structures and bringing large-scale impacts of magnetic reconnection as discovered in the geospace environment. These key features can be relevant and applicable to understanding other heliospheric and astrophysical systems. [ABSTRACT FROM AUTHOR]

We report a transport study on Pd3In7 which displays multiple Dirac type-II nodes in its electronic dispersion. Pd3In7 is characterized by low residual resistivities and high mobilities, which are consistent with Dirac-like quasiparticles. For an applied magnetic field (μ0H) having a non-zero component along the electrical current, we find a large, positive, and linear in μ0H longitudinal magnetoresistivity (LMR). The sign of the LMR and its linear dependence deviate from the behavior reported for the chiral-anomaly-driven LMR in Weyl semimetals. Interestingly, such anomalous LMR is consistent with predictions for the role of the anomaly in type-II Weyl semimetals. In contrast, the transverse or conventional magnetoresistivity (CMR for electric fields E⊥μ0H) is large and positive, increasing by 103−104 % as a function of μ0H while following an anomalous, angle-dependent power law ρ xx ∝ ( μ 0 H) n with n(θ) ≤ 1. The order of magnitude of the CMR, and its anomalous power-law, is explained in terms of uncompensated electron and hole-like Fermi surfaces characterized by anisotropic carrier scattering likely due to the lack of Lorentz invariance. [ABSTRACT FROM AUTHOR]

Hydraulic stimulation of enhanced deep geothermal reservoirs commonly targets pre-existing joint networks with the goal of increasing reservoir permeability. Here, we study the permeability and strength of joint-free and jointed Buntsandstein sandstones from the EPS-1 exploratory borehole at the Soultz-sous-Forêts geothermal site (France). The studied jointed samples contain naturally formed fractures that are variably filled with secondary mineralisation. We find that the permeability of these rocks is more sensitive to the presence and orientation of bedding than to the presence of joints at the scale of the samples: permeability is lowest in samples where bedding is oriented perpendicular to the direction of fluid flow. While well-sealed joints can act as barriers to fluid flow, partially filled joints neither inhibit nor promote fluid flow with respect to their joint-free counterparts. These samples were then deformed under triaxial conditions to assess (1) whether deformation reactivates pre-existing joints, and (2) how permeability changes as a result of deformation. We find that the mechanical response of the rocks depends on the extent to which joints are sealed. Well-sealed joints locally increase rock strength and experimentally induced fractures do not exploit pre-existing joint surfaces; partially sealed joints, by contrast, act as planes of weakness that localise strain. Although the permeability of all samples increased during deformation, permeability increase was largest in samples with poorly filled joints. We conclude that hydraulic stimulation operations must carefully consider the extent to which targeted joint networks are filled. Partially sealed joints are ideal targets for stimulation: these features act as planes of weakness within the rock mass and their reactivation can result in significant increases in permeability. By contrast, well-sealed joints may increase rock strength locally and may never reactivate during stimulation, making them poor targets for permeability enhancement. [ABSTRACT FROM AUTHOR]

On September 12, 2016, an ML 5.8 earthquake occurred near Gyeongju City in the southeastern part of the Korean peninsula. Two representative major faults, the Yangsan Fault (YSF) and Ulsan Fault (USF), are close to the earthquake hypocenter. However, the actual location of the hypocenter was reported to be outside of these fault zones. To identify an earthquake-generating fault and analyze its precise mechanism, it is necessary to understand the deep geological structure of the fault system. In this study, an audio-magnetotelluric (AMT) survey was conducted to analyze the subsurface structure of the fault system and geological characteristics in the Gyeongju area of Korea. AMT data were acquired from 301 stations around the earthquake epicenter and processed using robust processing with the remote reference technique. Inversion profiles were designated as either parallel (N-S) or perpendicular (E-W) to the regional fault strikes. The geoelectrical structure was obtained using two-dimensional (2-D) inversions of the joint set of transverse electric (TE) and transverse magnetic (TM) modes. The vertical sections along the E-W and N-S profiles that passed through the epicenter distribution contain a low resistivity zone at a depth of 2–3 km. The horizontal section of the resistivity distribution confirms that a high resistivity zone is located around the epicenter distribution. In contrast, a low resistivity zone is located at the center of the epicenter distribution. The high resistivity zone is located in the Yucheon Group (pyroclastic rocks) and granites, which are located on the western part of the YSF. In contrast, the low resistivity zone was observed in the alluvium to the northwest. The Hayang Group (clastic sedimentary rocks) and granites are located between the YSF and USF, and the observed high resistivity zone is coincident with the location of the granites. Paleogene and Neogene sediments distributed on the eastern part of the USF contained low resistivity zones. Compared with a magnetic anomaly map, the pattern of the magnetic anomalies is similar to that of the resistivity. This indicates that the resistivity model presented in this study is consistent with the geological characteristics of the study area. [ABSTRACT FROM AUTHOR]

Abstract—This paper discusses the results of magnetic variation soundings at two sites in the eastern Arctic, in the Chaun Bay region, at the Pevek and Valkarkai weather stations, by using the ModEM program to perform a three–dimensional (3D) inversion of tippers. The inversion has produced a geoelectric model of the region in a subsurface area of 300 × 300 × 200 km. The moduli of tippers at both sites have values between 0.2 and 0.85, reaching the maximum ones in a period of 1000 s. At the Pevek weather station, the real induction arrow in the Parkinson convention is oriented to the west throughout the range of periods. At the second site, its azimuth changes from 30° to the NE to –30° to the NW as the period of variations increases. Throughout the range of depths, conductive inhomogeneities are located to the west and north of the Chaun folded zone. They extend as a narrow strip under the western and northern coastal parts of the zone at depths of 8–12 km. As the depth increases, they are split into blocks, which reach their maximum size in the horizontal plane at depths ranging between 20 and 30 km. The most prominent among them is the conductive block beneath the southern part of the Chaun Bay. The roots of these blocks are distinguishable at depths of up to 100 km. The Chaun folded zone is represented by a high–resistance block down to a depth of 150 km with an electrical resistivity of more than 1000 Ω m. Comparison of the obtained geoelectric model with the geophysical studies previously conducted here reveals a correlation between the location of conductive formations and the location of weakly positive anomalies in the gravity field in the Bouguer and isostatic reductions in the coastal water area. [ABSTRACT FROM AUTHOR]

A shallow well was drilled in the East Siberian Sea for the first time with the purpose of geological study of the suture zone of deep waters of the Arctic Ocean and marginal structures of the continental shelf. The works have been carried out at the initiative of the Federal Agency on Mineral Resources (Rosnedra) in August 2022. The well was drilled by the Russian Geological Research Institute (VSEGEI) and Rosgeologia with the assistance of Rosneft and was linked to the Rosneft's long-term stratigraphic drilling program. Drilling was accompanied by continuous seismic-acoustic profiling and well thermometry. The maximum depth of the well was 472 m from the bottom of the sea floor; 147.5 m of core was taken to a depth of 268 m. Two sedimentary sequences were identified in the section: the Lower Jurassic–Lower Cretaceous gently deformed sequence and the Upper Neogene–Quaternary horizontally lying sequence. The sequences are separated by a thick unsampled interval with horizons of confined waters. The Jurassic–Lower Cretaceous sequence composes a northwestern-dipping monocline broken by faults and probably belongs to the complex filling the Zhokhov marginal trough of the Verkhoyansk–Chukotka orogen on the Eastern Arctic shelf. [ABSTRACT FROM AUTHOR]

Magnetooptical spectroscopy is an effective method for studying the magnetic microstructure of homogeneous and heterogeneous magnets. This review is devoted to analysis of numerous factors affecting the intensity and spectral dependence of a magnetooptical signal of the equatorial Kerr effect in nanocomposites "ferromagnetic metal–dielectric" in the visible and near infrared spectral regions. Examples of the influence of the metal concentration, nanoparticle size and shape, the substrate, the material of the dielectric, the amorphization of grains, the deposition method, and other factors on the magnetooptical spectrum are considered. The differences in the magnetooptical spectra for the superparamagnetic, superferromagnetic, and ferromagnetic states are demonstrated. It is noted that in the presence of fractions with different field dependences of the magnetization in a nanocomposite, the magnetooptical signal is not proportional to the total magnetization. Examples of enhancement and sign inversion of the magnetooptical signal in nanocomposites are considered. The possibility of the description of magnetooptical spectra using the methods of the effective medium (the Bruggeman method and the Maxwell–Garnett symmetrized approximation) is discussed. [ABSTRACT FROM AUTHOR]

The magnetotelluric (MT) investigation carried out in Koyna Seismogenic Zone (KSZ), an intra-plate earthquake region in Western India, along an E–W profile brings out moderately conductive (~ 700–1000 Ωm) near vertical features within the very high resistive (> 20,000 Ωm) granite/granite-gneiss basement. Occurrences of these anomalous moderate conductors are corroborated with sensitivity analysis. The alignment of earthquake hypocenters along the resistive–conductive boundary signifies the moderate conductor as basement fault. The conversion of resistivity values to the ratio of seismic P- to S-wave velocity (vp/vs) suggests that the moderate conductivity of the fault zone (as compared to the surrounding basement) appears due to the presence of fluid in the fault zone. Geophysical evidences reveal ~ 2.5–3.6 vol% fluid in the fault zone with ~ 1.8–2.6% interconnected porosity, which migrates along the structural boundary and invades the mechanically strong basement to nucleate the brittle failure within it. The present study proposes two mechanisms for the seismicity in the Koyna region. First: the meteoric water circulation due to the loading–unloading of nearby Koyna reservoir acts as potential fluid source for this triggered seismicity, which has also been suggested by previous studies. Second: the fluid circulation due to a deep-seated source. The present MT study brings out a conductive feature below 20 km depth which is thought to be emerged due to the dehydration of amphibole bearing rocks. The fluid generated from dehydration might act as a probable source to the triggered seismicity; since the conductive feature has a linkage to the upper crust. [ABSTRACT FROM AUTHOR]

Many active volcanoes and various types of seismic activities exist in the southern part of the Northeast Japan subduction zone. One of the geologically most interesting features in this area is the sequential explosive eruptions of a group of volcanoes. The group consists of Mt. Azuma and Mt. Adatara on the volcanic front line, Mt. Bandai west of the volcanic front, and Mt. Numazawa on the back-arc side. A previous petrological study on the eruption products regarded Mt. Numazawa as an anomalous back-arc volcano because its lavas are similar to those of volcanoes on the volcanic front. The reason behind this unique connection was unclear, and hence, this study was intended to understand the deep fluids distribution beneath the area. For this purpose, a 3-D regional electrical resistivity structure was estimated from a series of wide-band magnetotelluric surveys, with 45 observation points deployed from the fore-arc to the back-arc sides. The most important feature of the resistivity structure is a large conductive zone in the central part of the area, spanning from the upper mantle to the lower crust. Interestingly, the lateral elongation of the conductor is oblique to the volcanic front line and consistent with the spatial distribution of the group of volcanoes and the low-frequency earthquake clusters around them. Therefore, the conductor most likely represents a large, elongated magmatic fluid reservoir beneath the volcanoes. Hydrous partial melting might be the cause of the enhanced conductivity. [ABSTRACT FROM AUTHOR]

As volcanoes are closely related to the living environment of humans, in particular via natural hazards and geothermal energy, scientific studies on volcanic edifice structures are required. Ulleung Island is a Quaternary volcanic island located in the mid-western East Sea (Sea of Japan). In this study, we conducted an audio-frequency magnetotelluric (AMT) survey to image the substructure of the Ulleung volcanic edifice. In addition, the electrical structure was interpreted from the geothermal system perspective, geochemical compositions of volcanic rocks, and the possibility of the presence of a magma reservoir. AMT data were obtained from 25 stations and processed using the remote reference technique. Then, the three-dimensional (3-D) approach was reasonably adopted according to dimensionality analysis. Before conducting the 3-D inversion, the effects on topography and ocean were analyzed using a simplified 3-D synthetic model, because Ulleung Island is surrounded by sea and the topography is undulating. Most AMT stations on Ulleung Island are distorted by topographical and oceanic effects; in particular, oceanic effects are significant at frequencies lower than 10 Hz. The 3-D inversion was conducted with full impedance components and vertical magnetic transfer functions in a frequency range of 10,000–1 Hz. The results show that the Ulleung volcanic edifice is characterized by two layers of electrical structures as follows: near-surface resistive anomalies and an underlying conductive layer. Considering the high geothermal gradient on Ulleung Island, we suggest that the conductive layer of the volcanic edifice is due to hydrothermal alteration of basaltic rocks, with a potential heat source underneath. Based on the geochemical characteristics of the Ulleung volcanic rocks, the possibility of heat transfer from a trachytic magma reservoir within the shallow crustal is suggested. In summary, this study presents a geological interpretation of shallow volcanic structures beneath Ulleung Island and the possibility of an active magma reservoir as a potential heat source for the volcanic edifice. [ABSTRACT FROM AUTHOR]

The magnetotelluric method relies on variations of natural electromagnetic fields, which in the vicinity of human settlements are persistently distorted by anthropogenic electromagnetic noise. A large source of noise to the magnetotelluric response is caused by the harmonic oscillations of the power network utility frequency centered on 50/60 Hz along with the associated higher harmonics. Removing this type of noise is essential for high frequency magnetotelluric measurements used for shallow surveys. There are a large number of approaches for how to treat power line noise in magnetotelluric signals, however, commonly used methods do not take into account time variations/instabilities of the utility frequency. That is not serious problem in vicinity of well balanced grid networks, but can cause issues in regions with larger utility frequency variations. Under such conditions, commonly used methods loose more of the natural signal, which is undesirable especially in case of very noisy datasets. Hence, we adopted approach for removing of power line noise with respect to time variations of the utility frequency and applied it to magnetotelluric signals to preserve more of natural signal. The method is based on modelling of the grid network harmonic oscillations by the optimum utility frequency and its integer multiples. The resulting sum of sinusoidal signals is subsequently subtracted from recorded data and only particular noise frequencies are removed from the original signal with high precision, while frequency ranges around power line harmonics are cleaned. [ABSTRACT FROM AUTHOR]

The article discusses the 25th Electromagnetic Induction Workshop organized by the Division VI of IAGA. The workshop brought together over 500 scientists from around the world to exchange views on the latest developments in electromagnetic geophysics. The workshop covered various topics, including instrumentation, theory, exploration, tectonics, marine EM, and global and planetary studies. Three review papers were published in a special issue of Surveys in Geophysics, focusing on research on the electrical resistivity of the Earth, exploration for hydrocarbon reservoirs, and subsurface resistivity modeling. The article expresses gratitude to the editor and reviewers involved in the publication process. [Extracted from the article]

A CO2-based Enhanced Geothermal System (CO2-EGS) has dual benefits of heat extraction and CO2 storage. Mineralization storage of CO2 may reduce reservoir permeability, thereby affecting heat extraction. Solutions require further research to optimize and balance these two benefits. In this study, CO2 storage and heat extraction were simulated by alternating cyclic injection of water and supercritical CO2 into fractured granite. By analyzing the changes of ion composition in water samples and the minerals of fracture surface, the mechanisms controlling the fracture permeability with and without proppant were obtained. The results suggest that monticellite and vaterite were formed besides montmorillonite, calcite and illite after increasing the injection cycles. This promotes mineralization storage of CO2 but reduces reservoir permeability. Without proppant, the permeability decreased in three stages and the reduction rate exhibited a sharp-slow–fast–slow trend. While the use of proppant caused an increase of two orders of magnitude in permeability. Therefore, increasing the non-contact area of the main fracture and the CO2 flow velocity can avoid a large decrease in permeability, which will increase the heat extraction and mineralization storage of CO2. The findings provide solutions for the CO2 emission reduction and the efficient exploitation of hot dry rock. [ABSTRACT FROM AUTHOR]

A liquid nitrogen cold-shock fracturing method is proposed to resolve the problem of single heat-exchange cracks generated by hydraulic fracturing in an enhanced geothermal system. A complex crack network can be constructed from the great thermal difference between cryogenic fluids and hot rocks. In this work, damage characterization that combined resistivity, ultrasonic wave, and nuclear magnetic resonance (NMR) was undertaken. The matrix continuity was analyzed ultrasonically, the fracture channel was analyzed by resistivity, and the pore structure was analyzed by NMR. The high-temperature granite damage after liquid nitrogen cold-shocking was evaluated quantitatively. The ultrasonic velocities of the granite cores decreased with high temperature, especially over 400 °C. The resistivity decreased and the largest drop was 81.86% for granite C at 600 °C. According to the NMR T2 spectra, liquid nitrogen cold-shocking expanded the pore volume and increased the pore number. The porosity after cold-shocking increased exponentially with heating temperature, where the porosities of granites A, B, and C reached 7.21%, 4.29%, and 3.10%, respectively. The damage variables calculated by wave velocity and resistivity increased with heating temperatures. The correlation of porosity with resistivity and wave velocity was established by multiple linear regression. Liquid nitrogen cold-shocking induced a developed pore-fracture network in hot dry rocks. [ABSTRACT FROM AUTHOR]

The structure, magnetic, magnetothermal, and magnetoelastic properties of Gd5Si Ge In2x (x = 0–0.1) intermetallic compounds have been studied in the region of magnetostructural phase transitions. It is shown that introduction of indium creates the effect of negative pressure, leading to a change in the critical temperature of the magnetic phase transition in the monoclinic phase of the compounds studied and to partial separation of the magnetic and structural phase transitions in them. [ABSTRACT FROM AUTHOR]

The results of experimental studies of electric field disturbances generated by sources of an ionospheric origin in the coastal zones of the White and Barents seas are presented. The data recording was performed by geophysical stations GI-MTS-1. The horizontal electric field was recorded by telluric lines ~50 m long. The Ex component was set orthogonally to the coastline in the direction from the shore, and the Еy component was directed along the coastline. In the coastal strip of the sea–land contact, a zone of anomalous increase in the amplitude of ULF variations (F < 1 Hz) of the Ex component adjacent to the water's edge was revealed. An additional narrow zone of anomalous amplification of the ULF variations of the electric field is observed at a distance of ~30 m from the water's edge. The coastal effect reaches its maximum near the water's edge in the coastal zone and decreases with distance from the seashore. [ABSTRACT FROM AUTHOR]

The Jiamusi and Songnen blocks converged in the easternmost segment of the Central Asian Orogenic Belt as a result of the subduction and subsequent closure of the Mudanjiang oceanic plate during the Permian–Jurassic. The Mudanjiang suture zone was later directly affected by subductions of the Paleo-Pacific plate and Pacific plate and is therefore an ideal place to study the subduction polarity and later transformation of a paleo-suture zone. Using three-dimensional inversion of magnetotelluric data collected along a 160-km-long profile across the Mudanjiang suture zone, we established a resistivity model of the suture zone and adjacent area. Our results reveal the subduction polarity and subduction trace of the Mudanjiang oceanic plate and provide geoelectrical evidence for reactivation of the Mudanjiang suture zone induced by the (Paleo-)Pacific plate subduction. The suture zone shows a complex conductive structure. The west-dipping crustal-scale conductor beneath the Songnen-Jiamusi collision zone represents the fossil subduction zone and indicates the westward subduction polarity of the Mudanjiang oceanic plate. Furthermore, the Mudanjiang fault identified by surface geology does not fully represent the deep structure of the Mudanjiang suture zone. The definition of the suture zone should be extended to the whole conductive region with a lateral extent of ∼70 km. Solid conductive minerals beneath the arc in front of the subduction zone were exhumated up from deep to the upper crust. The "chimney"-shaped conductor connected with the mantle represents the intrusive pathways of mantle-derived materials, suggesting that the Mudanjiang suture zone was reactivated by subductions of the Paleo-Pacific plate and Pacific plate, leading to remelting of the cooled and crystallized materials in the pathways. Therefore, subduction of the (Paleo-)Pacific plate destroyed the lithospheric structure of the paleo collision zone in the eastern segment of the Central Asian orogenic belt, and the large-scale crustal conductor beneath the suture zone reflects reactivation of the paleo-suture zone. [ABSTRACT FROM AUTHOR]

Dykes play a key role in various geologic environments. Here, we show how we employed profile magnetic data to perceive and identify the number of dykes and their parameters by a particle swarm optimiser scheme. The results deduced from this inversion include identification and estimation of the magnetic dipole moment, depth, half-width, magnetisation angle, and origin location (dyke parameters) of dykes. Two synthetic models demonstrate this scheme: the first model tests the impact of a third-order regional anomaly and the second model considers the effect of adjacent structures. Besides, we provide two case studies related to mineral exploration, viz., the Parnaiba basin from Brazil and the Pishabo Lake example from Canada. The subsurface dyke parameters estimated from drilling information and the available literature were consistently matched with those achieved from our inversion scheme, thereby demonstrating the effectiveness of the particle swarm optimisation scheme. [ABSTRACT FROM AUTHOR]

A geological model related to coal-rock electromagnetic parameters is constructed in this work using coal-rock media and coal-rock interfaces as the research objects for electromagnetic wave propagation response. An in-depth analysis was conducted to examine the response characteristics of the electromagnetic wave detection technology for coal and rock geological orientations while drilling in coal mines. We also studied the detection and identification methods and applicability of coal and rock interfaces. Next, we simulated the response characteristics of the instrument in the coal seam and established a calculation method for the orientation of the interface between the roof and floor of the coal seam and the distance between the instrument and the coal seam's interface. The quasi-Newton method was used to perform inversion calculations on the theoretical model of the response of the borehole-while-drilling azimuth electromagnetic wave resistivity tool, and the simulation data were inverted in real time, thereby reducing the number of calculations of the Jacobian matrix, improving the inversion speed, and decreasing the level of dependence on the initial value. Furthermore, we obtained the signal response law of electromagnetic wave detection in low-resistivity aquifers and abnormal geological bodies (e.g., collapse columns and gangue interlayers) based on forward and inversion numerical simulation. Finally, by investigating the directional electromotive force simulation imaging method of the orthogonal coil, we verified the feasibility of the application of azimuth electromagnetic wave resistivity in high-resistivity coal seams. In this way, we can provide the basis for the detection of coal seam interface, geological anomaly body, and geological guidance application. [ABSTRACT FROM AUTHOR]

Controlled-source audio magnetotellurics, which is a common technology in geophysical surveys, typically uses the multichannel mode of data acquisition. Often, a capacitive coupling effect occurs among the multiple receiving wires and receiving electrodes and the earth. This effect causes the distortion of the observed apparent resistivity and phase curves. The capacitive coupling of the observation mode is simulated using an equivalent circuit model, and the characteristics of the influence of the length of the receiving wire and grounding resistance of the electrode on capacitive coupling are investigated via the forward simulation of several typical models. The capacitive decoupling of a device for controlled-source audio geomagnetic observation is studied and applied to process the measured data from the Hongtoushan mining area in Liaoning Province, China. This approach effectively weakens the capacitance coupling effect and improves observation quality, and the inversion results match well with known geological information. This study examines the capacitive decoupling technique and offers a scientific foundation for the standardization of the controlled-source audio geomagnetic data gathering technology. [ABSTRACT FROM AUTHOR]

The 2011 Tohoku-oki earthquake (Mw 9.0) was characterized by a huge fault slip on the shallowest part of the plate interface, where fault behavior had been believed to be aseismic. In this study, we modeled the two-dimensional resistivity distribution across the slip area based on ocean-bottom electromagnetic measurements to understand the physical properties around the plate interface controlling fault rupture processes. The optimal 2D resistivity model showed a conductive area around the shallowest plate interface where the huge coseismic slip was observed, whereas the deeper plate interface where the fault rupture was nucleated was relatively more resistive. The shallowest plate interface was interpreted to have a high pore seawater fraction, whereas the deeper interface was interpreted as a dry area. These findings are consistent with the hypothesis that aseismic frictional conditions changed to conditions enhancing fault rupture when the rupture propagated to the wet, clay-rich shallowest plate area. The optimal resistivity model also revealed a conductive area under the outer-rise area of the Pacific Plate. This finding supports the existence of a hydrated oceanic crust that supplied aqueous water to the subduction zone, including to the huge fault slip area. [ABSTRACT FROM AUTHOR]

The forward calculation of magnetotelluric (MT) responses is generally assumed to be sufficiently accurate compared with typical observational errors in practical modeling and inversion studies. Although the uncertainty of the forward calculation may be examined by comparison with analytical or other numerical solutions for some simple models, such an examination does not guarantee that the uncertainty is similar for more realistic complex structures. In this study, I propose a simple method to evaluate the uncertainty of MT forward modeling for practical three-dimensional (3D) conductivity structure models in a Cartesian coordinate system. The method is based on the idea that the horizontal coordinate system can be selected arbitrarily for a general 3D structure. The synthesized MT responses are ideally identical irrespective of the selection but are different because of the difference in discretization angles, boundary values, and numerical errors. By synthesizing MT responses to the model in several different coordinate systems, the mean, standard deviation, and coefficient of variation can be calculated. These statistics provide quantitative information on how stably the forward calculations synthesize MT responses under the given conditions of the structure model, observation array, periods, numerical algorithm for the forward modeling, and mesh design. The proposed method was applied to two practical situations of seafloor MT arrays in the northwestern Pacific and southern Atlantic and a land MT array in Hokkaido, Japan. The results show that the uncertainty is comparable to real observation errors and is significantly dependent on the MT impedance element, period, site, structure model, and horizontal coordinate system. The uncertainty of the forward calculation should be considered for each element, period, and site to quantitatively evaluate how well a given model explains the data. I propose a new root-mean-square in which the residuals are normalized by both the standard errors of the MT responses observed and synthesized. This would help avoid overfitting data in the inversion analysis by ignoring the uncertainty of the forward calculation. This method is also useful for testing the appropriate selection of the coordinate system and mesh design. [ABSTRACT FROM AUTHOR]

In this paper, we have studied the electrical and magnetotransport properties of strontium-deficient Pr0.7Sr0.3 − xMnO3 polycrystalline manganites prepared by solid-state reaction. The temperature dependence of resistivity shows the presence of a metal–insulator transition at Tρ with the presence of resistivity minimum at very low temperatures. The Tρ values shift from 265 K for x = 0 to 157 K for x = 0.1 with increasing strontium deficiency. We have identified the transport mechanisms responsible for magnetotransport properties. Some peculiarities were obtained for x = 0.1 sample, which can be linked to the half-doped nature of this specimen. Colossal values of magnetoresistance (MR) were recorded for all our samples near Tρ (for µ0H = 2 T, negative MR values are 58%, 89% and 88% for x = 0, x = 0.05 and x = 0.1, respectively). –MR increases with increasing strontium deficiency. Colossal MR values suggests the possibility of using our samples for technological applications like magnetic field sensing and spintronics. [ABSTRACT FROM AUTHOR]

Internal structure imaging of the Earth, along with determining basin structure, can aid in evaluating potential seismic hazards. However, the high operating cost limits the current geophysical exploration methods; moreover, it is difficult to apply these techniques over a large area, which limits our understanding of the Quaternary structure and the development of earthquake prevention science. A combination of dense array observation technology and ambient noise surface wave tomography is being rapidly developed as a high-resolution urban detection method. Here, we report the ambient noise imaging results of a high-density array experiment. In the ambient noise surface wave tomography method (e.g., surface wave tomography; Eikonal tomography), the signal is assumed to be a single mode. However, several multimode signals were detected in this dataset. With the use of traditional methods to measure the dispersion, mode confusion occurs and the extracted dispersion curve jumps. To solve this problem, by combining the advantages of phase-matched filtering and dispersion compensation, we realized the automatic pickup of fundamental group velocity using reference phase velocity. From this, a Rayleigh wave group velocity map was obtained. The regional average phase velocity information was included in the inversion steps to reduce the uncertainty in the inversion of shear wave velocity. Finally, an S-wave velocity structure model was obtained within a depth of 500 m. The velocity structure was roughly layered and grew with depth. In the depth range of 240–320 m, obvious decreases in the S-wave velocity were observed. Compared with geothermal drilling data, this was speculated to be the reflection of a water-rich (confined water) sand layer. This study provides a technical approach for and a processing example of a high-density array, and its velocity model can be used as a reference for urban subsurface structure, underground space utilization, and earthquake disaster prevention and control. [ABSTRACT FROM AUTHOR]

Inversion of magnetotelluric (MT) responses has been used to explore the electrical conductivity distribution of the Earth's interior. In three-dimensional (3-D) inversions, it is significant to use a good initial model, because final model obtained by most 3-D inversion methods is influenced by the initial model. Although uniform initial models are widely used in 3-D inversions, one-dimensional (1-D) initial models are alternatives, which could more appropriately represent the actual conductivity variations in the Earth's interior. This study presents a two-step 3-D inversion method, especially for marine cases. This inversion method first concentrates on obtaining a 1-D initial model and then inverts for 3-D conductivity structures with it, in both of which the 3-D topography is carefully taken into consideration. This method was tested by synthetic models of different topography variations (depression-shaped, smoothly varying, channel-shaped and square-shaped plateau topography) and of heterogeneous layers with different checkerboard-type anomalies (sharp or smooth lateral conductivity variations) embedded in 1-D model of depth-dependent conductivity. The comparisons were done about obtaining an initial model by the proposed method and that inverted from the corrected responses. The results of 3-D inversions by using the method of this study were also compared to that with different uniform initial models. Results of synthetic tests and comparisons were discussed by using directional information of newly introduced model-vector parameters. The performance and validity of this method was verified. It also revealed that some of the newly introduced model-vector parameters could be used to show the convergence of inversions and help to select inverse model. [ABSTRACT FROM AUTHOR]

Our multiyear studies have enabled us to make maps showing magnetic anomalies ΔTa for major geothermal systems in the Pauzhetka area of southern Kamchatka. Magnetic fields possess both general characteristics and individual features for each object of study. The Nizhne-Koshelev vapor-dominated geothermal field is identified on the basis of a set of linear negative magnetic anomalies confined to thermal-controlling tectonic faults. The Pauzhetka geothermal field is characterized by an inhomogeneous structure of the anomalous magnetic field ΔTa: the northwestern area has a quiet, slightly negative magnetic field, showing that this part of the field is dominated by lateral spreading of hydrothermal fluids from the upper aquifer; the southeastern part has numerous sign-varying magnetic anomalies of high intensity confined to acid–intermediate subvolcanic bodies. The South Kambalnyi group of thermal fields is characterized by a lower absolute value of magnetic induction Т compared with the Pauzhetka and the Nizhne-Koshelev geothermal fields, showing that the rocks of the Kambalnyi volcanic range are more intensely altered by hydrothermal and metasomatic processes, probably as a result of a long-continued action of convective heat flow. [ABSTRACT FROM AUTHOR]