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With the rapid development of the transient electromagnetic method (TEM), the numerical simulations of three-dimensional anisotropic media have garnered attention. This study used the rational Krylov method to realise the three-dimensional modelling of the ground magnetic-source airborne transient electromagnetic data for arbitrary anisotropic media. First, based on Maxwell's equations, a time-domain electromagnetic equation was derived for anisotropic media. The finite difference method was employed to discretise the equation, and the electromagnetic field was then expressed as the product of an exponential matrix and a vector related to the source. Subsequently, the rational Arnoldi algorithm was adopted to construct the rational Krylov subspace orthogonal basis vectors, and the electromagnetic fields at different times were obtained based on a model reduction strategy. The accuracy of the proposed algorithm was verified through the comparisons of its results with those of other software packages. We calculated the response for different anisotropic parameters of the typical three-dimensional anisotropic models, and analysed the influence of the anisotropic media on the ground magnetic-source airborne transient electromagnetic data. The proposed three-dimensional forward TEM algorithm provides important technical support for identifying the anisotropic geological bodies and lays the foundation for further inversion research. [ABSTRACT FROM AUTHOR]

Continuously to a previous work on qualitatively investigating the probable electromagnetic interacting process among lithosphere, atmosphere and ionosphere, this work aims to quantitatively establish an electromagnetic coupling model among these three spheres prior to the Wenchuan earthquake. Firstly, a underground finite length electrical dipole in a half-space model has been employed to estimate the possible "energy source" for an observable 1.3 mV m-1 electrical field registered at 1440 km Gaobeidian station during the Wenchuan event. The result shows that the seismo-telluric current covers ~105–106 kA if the measuring frequency f = 0.01–10 Hz considered. The central magnitude of the vertical electrical field caused by the current at 0.01 Hz on the Earth's surface can be up to kV m-1. Then, this vertical field acts as an input into an electric field penetration model. It is shown that this field attenuates quickly at the atmosphere and completely vanishes at the top ionosphere and produces a 0.1 mV m-1 additional electrical field at the ionospheric bottom. Through the TIE-GCM, this additional electrical field causes 0.01 % ionospheric variations on electron density and TEC near the Wenchuan epicenter, as well as near its magnetically conjugated point. Further, the simulations have also been discussively performed on frequencies of 1 Hz and 10 Hz. The results demonstrate that the variations of electron density present their maximum values at the height of ~300–400 km and the varied percentages of ionospheric parameters have been beyond 10 %, the same magnitude as what has been registered during the Wenchuan shock. [ABSTRACT FROM AUTHOR]

In diverse realms of research, such as holographic optical tweezer mechanical measurements, colloidal particle motion state examinations, cell tracking, and drug delivery, the localization and analysis of particle motion command paramount significance. Algorithms ranging from conventional numerical methods to advanced deep-learning networks mark substantial strides in the sphere of particle orientation analysis. However, the need for datasets has hindered the application of deep learning in particle tracking. In this work, we elucidated an efficacious methodology pivoted toward generating synthetic datasets conducive to this domain that resonates with robustness and precision when applied to real-world data of tracking 3D particles. We developed a 3D real-time particle positioning network based on the CenterNet network. After conducting experiments, our network has achieved a horizontal positioning error of 0.0478 μm and a z -axis positioning error of 0.1990 μm. It shows the capability to handle real-time tracking of particles, diverse in dimensions, near the focal plane with high precision. In addition, we have rendered all datasets cultivated during this investigation accessible. [ABSTRACT FROM AUTHOR]

In recent years, the proliferation of wearable healthcare devices has marked a revolutionary shift in the personal health monitoring and management paradigm. These devices, ranging from fitness trackers to advanced biosensors, have not only made healthcare more accessible, but have also transformed the way individuals engage with their health data. By continuously monitoring health signs, from physical-based to biochemical-based such as heart rate and blood glucose levels, wearable technology offers insights into human health, enabling a proactive rather than a reactive approach to healthcare. This shift towards personalized health monitoring empowers individuals with the knowledge and tools to make informed decisions about their lifestyle and medical care, potentially leading to the earlier detection of health issues and more tailored treatment plans. This review presents the fabrication methods of flexible wearable healthcare devices and their applications in medical care. The potential challenges and future prospectives are also discussed. [ABSTRACT FROM AUTHOR]

The controlled source audio-frequency magnetotelluric method (CSAMT) stands out for its economic efficiency and widespread application in geophysical monitoring. However, the separate inversion of time-lapse monitoring data encounters challenges in comparing and identifying abnormal changes due to variations in data fitting. Furthermore, the utilization of a method akin to Cagniard apparent resistivity for inversion necessitates the simultaneous observation of at least two components of the electromagnetic field, making it unsuitable for extensive three-dimensional observations. This paper proposes a 3D time-lapse electric field inversion algorithm for CSAMT, addressing the complexities in geophysical monitoring. The algorithm introduces two regularization factors and defines an objective function with both temporal and spatial constraints. Synthetic testing reveals the stability of the 3D time-lapse electric field inversion algorithm, demonstrating its effectiveness in delineating underground variations. This solution resolves the challenges posed by the independent inversion of time-lapse monitoring data. [ABSTRACT FROM AUTHOR]

At present, the traditional magnetic three-dimensional inversion method has been fully developed and is widely used. Magnetic exploration is a kind of geophysical exploration method that uses the magnetic field changes (magnetic anomalies) caused by the magnetic differences between various rocks in the crust to find useful mineral resources and study the underground structure. Traditional magnetic three-dimensional inversion is relatively inefficient. Moreover, the traditional additive objective function (data fitting difference term plus regularization term and logarithmic obstacle term), which causes the regularization factor selection to be more complicated, is implemented in this method. Therefore, it is necessary to establish a new efficient three-dimensional magnetic inversion algorithm and optimize the selection of regularization factors. In this paper, based on the limited-memory BFGS (L-BFGS) method, the three-dimensional magnetic inversion of a multiplicative objective function is realized. The inversion test is conducted in this paper using both theoretical synthesis data and measured data. The results demonstrate that the limited-memory BFGS method significantly enhances the inversion efficiency and yields superior inversion outcomes compared to traditional magnetic three-dimensional inversion methods. Additionally, the multiplicative objective function-based three-dimensional magnetic inversion method simplifies the process of selecting weight factors for regularization terms. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing plays a crucial role in enhancing reserves and production of unconventional oil and gas resources. Injecting fracturing fluids into the ground to improve reservoirs also introduces the risk of inducing earthquakes; thus, monitoring the migration of these fluids is crucial. The microseismic positioning method determines the fracturing fluid by locating microseismic events generated by the fractured rock strata; however, this method is susceptible to errors. Low-resistivity subsurface fluids can directly change electromagnetic field signals, making the electromagnetic method a technically advantageous approach for monitoring the migration of hydraulic fracturing fluids. The monitoring test data of Well Eyangye-2HF show that the time-lapse controlled-source electromagnetic (CSEM) method is suitable for hydraulic fracturing and has good monitoring effects. The results of CSEM method can also compensate for deficiencies in microseismic monitoring. The electric field (Ex) observed using the CSEM method can directly predict the distribution edge of the fracturing fluid, and the anomalous zone of the Ex rate of change is consistent with the fracturing-fluid injection parameters and microseismic monitoring results. The analysis of field data and forward simulation, based on electrical logging results, led to the conclusion that hydraulic fracturing operations can cause changes in the resistivity of the target layer and surrounding strata. These changes are attributed to the synergistic effects of formation stress, temperature, and the fracturing fluid. The electric-field changes observed using the CSEM method may be caused by multiple factors; however, use of time-lapse CSEM for monitoring hydraulic fracturing is still feasible. [ABSTRACT FROM AUTHOR]

The resistivity method and time-domain-induced polarization (TDIP) are two branches of electric exploration that are used to solve problems in mineral exploration, hydrogeology and engineering geology. In recent years, integrating different physical parameters for joint inversion to improve the accuracy of inversion results has been extensively examined; however, three-dimensional joint inversion of the two methods above has not been realized. To further address this issue, in this research, we used the limited-memory BFGS (L-BFGS) method to develop a three-dimensional joint inversion algorithm of the resistivity method and TDIP based on the cross-gradient constraints. In the new algorithm, the resistivity method and TDIP inversion were iteratively updated alternately to ensure that the inversion results can simultaneously meet the two conditions of obtaining minimum data misfits and finding structural similarity. The three-dimensional synthetic dataset inversion results showed that the models obtained by joint inversion are more accurate in the recovery of both the boundaries and the values of the anomalies. Especially in the background of high noise, joint inversion has higher resolution for the target body. The joint inversion algorithm was also successfully applied to a groundwater detection practice in Beijing, China, in which the practicability of the algorithm was confirmed. [ABSTRACT FROM AUTHOR]

Many rocks often have the electrical anisotropy property that produces quite different responses from the electrical isotropic rocks. We applied an advanced modeling method to calculate the apparent resistivity responses over an anisotropic target in a homogenous isotropic backgound and an isotropic target embedded in different anisotropic host rocks. We demonstrated comprehensive modeling experiments to characterize the responses of surface pole-pole surveys to the buried targets in different geological backgrounds, which vary in the principal resistivities and Euler angles of the targets or the host rocks. Our modeling results show that: (i) the location of current injection controls the apparent resistivity images in value and shape; (ii) almost circle-contours or more or less elongated contours appear in the apparent resistivity images for the anisotropic targets; (iii) the isotropic target embedded in the anisotropic host rock generates some radial patterns from the current injection point; (iv) the anisotropy strike angles can be obtained from the symmetry axis of the apparent resistivity contours; and (v) multiple locations of current injections throughout the detection region are necessary for the surface pole-pole survey to avoid misleading data interpretation. [ABSTRACT FROM AUTHOR]

This document discusses recent advances in forward modeling and inversion of geophysical electromagnetic data. The research community studying electromagnetic methods has grown in recent years, with applications expanding to include airborne, marine, and borehole scenarios. However, there are still challenges to accurate modeling and inversion, such as the computational requirements and noise in the data. The document presents eight articles on topics including numerical modeling, inversion, signal processing, and case studies. While 3D inversion is a widely discussed topic, this special issue does not include any articles specifically addressing it, suggesting that it may not yet be practical for real-life situations. [Extracted from the article]

ZTEM refers to the Z-axis tipper electromagnetic method. The ZTEM method is an airborne magnetotelluric sounding method based on the difference in rocks' resistivity using the native electromagnetic field. The method is effective in exploring large-scale structures when the ground is fluctuant. The paper introduces the inversion algorithm of 2D ZTEM named the conjugate gradient method. This method, which avoids solving the Jacobi matrix, is very effective but not effective enough when the model is divided into a big grid. This study can perform further computation using parallel computation and then receive the processed data. We compare the results of the serial algorithm with the result of the parallel algorithm, which proves that the parallel algorithm is correct. When the number of processes is between three and six, the speedup ratio is between 1.74 and 3.19. It improves the effectiveness of the parallel algorithm of 2D ZTEM. [ABSTRACT FROM AUTHOR]

Electrical anisotropy is increasingly recognized as an important aspect of the resistivity models required to explain magnetotelluric (MT) observations. However, a limited number of practical MT inversion algorithms that can consider anisotropy have been published to date. To address this problem, we have developed a three‐dimensional (3‐D) MT inversion algorithm that recovers a 3‐D resistivity model that considers arbitrary electrical anisotropy. The inversion uses the same inversion algorithm as the widely used ModEM inversion algorithm, and a novel forward modeling algorithm to consider the anisotropic Earth. The algorithm was tested on both synthetic and field MT data. Inversions considered both a completely general anisotropy tensor with six components and approximations with less parameters. Synthetic inversions show that the two horizontal components of resistivity and the anisotropy strike can be well recovered, while the vertical component of resistivity is poorly resolved, primarily because current flow in MTs is dominantly horizontal. The synthetic examples confirm the limitation of the axial anisotropic inversion technique when applied to MT data produced by a resistivity model with arbitrary anisotropy. The synthetic inversions also showed that inversion of data from an isotropic model will not result in an artificially anisotropic model. Compared to the isotropic inversion model of the real MT data, the anisotropic model clearly shows some features that are consistent with the mapped geology. As expected, the results showed that a given data set can be fit by a range of models, with an inherent trade‐off from 3‐D heterogeneity to 3‐D anisotropy. This uncertainty can be reduced with the use of prior information in the inversion. Plain Language Summary: The resistivity model of real Earth generally contains anisotropic features. Field magnetotelluric (MT) data may be significantly influenced by these features. To interpret such data, inversion algorithms which can consider electrical anisotropy are highly demanded. However, a limited number of practical MT inversion algorithms that can consider anisotropy have been published to date. To address this problem, we have developed a three‐dimensional (3‐D) MT inversion algorithm that recovers a 3‐D resistivity model that considers arbitrary electrical anisotropy. The algorithm was tested to be effective on both synthetic and field MT data. Compared to the isotropic inversion models of the tested MT data, the resolved anisotropic models are more reliable. Key Points: A three‐dimensional non‐linear conjugate gradient inversion algorithm is implemented for the inversion of magnetotelluric (MT) data to produce an electrical resistivity model with arbitrary anisotropyInversion examples of synthetic and real MT data validate the usefulness of the algorithmReal MT data inversion obtains an anisotropic model which is more consistent with the mapped geology than the corresponding isotropic model [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) and wide‐angle seismic reflection/refraction surveys play a fundamental role in understanding the crustal rheology and lithospheric structure of the Earth. In recent years, the integration of the two methods in order to improve the robustness of the inversion has started to gain attention. We present a new approach for joint 3‐D inversion of MT and wide‐angle seismic reflection/refraction data to accurately determine crustal structures and Moho depth. Based on H‐κ stacking of teleseismic receiver functions, we estimate an initial reference Moho. This is used as input for the subsequent MT/seismic joint inversion, where the Moho interface is updated and crustal structures are added to the model. During the joint inversion process, structural similarity is facilitated through the cross‐gradient constraint. Synthetic model tests show an improvement of the inversion results over separate inversions. In particular, the tests based on two geologically realistic models demonstrate that the crustal structure and even the trade‐off between velocity and Moho interface can be sufficiently resolved by combined MT and seismic data sets when using the estimates from analysis of receiver functions. These results show that the new method can provide useful constraints on crustal structures including their geophysical properties and discontinuities. Key Points: A new coupled algorithm is developed for 3‐D imaging of magnetotelluric and wide‐angle seismic reflection/refraction dataTwo strategies are presented to mitigate the problem of accurately determining crustal structures and Moho interface simultaneouslyThe inclusion of teleseismic receiver functions is proved to be effective in the new algorithm [ABSTRACT FROM AUTHOR]

3D controlled-source audio frequency magnetotelluric (CSAMT) responses can be distorted strongly by topography and should be accounted for in data inversion and interpretation. In this paper we present a scheme to incorporate topographic distortions into the inversion instead of correcting them. This approach has been verified by comparing the modelling results with 2D FEM CSAMT solutions and synthetic inversion examples. Compared with the responses generated from a half-space model with flat surface, it is found that not only the topography in the survey area but also that at the source position may strongly distort the CSAMT responses. The field example indicates that results with topography are much better than those without considering topography to map the distribution of coal seam underground, which also illustrates the effectiveness of our approach. [ABSTRACT FROM AUTHOR]

The controlled-source audio-frequency magnetotelluric (CSAMT) method has become an important method in geophysical electromagnetic exploration. However, traditional CSAMT only gathers a single set of orthogonal electric and magnetic data, which cannot describe the whole subsurface geological structure. Due to increasingly complex geological targets, the drawbacks of traditional CSAMT have gradually become more significant, promoting the need for tensor CSAMT. Tensor CSAMT can gather richer information, but the 3D forward and inversion models of this method have developed slowly since it was first proposed. The common method for inverting the data of the tensor CSAMT is still magnetotelluric (MT). This paper adopts a staggered-grid finite difference method to realise the 3D forward modelling of the tensor CSAMT. On this basis, we adopt a limited-memory Broyden-Fletcher-Goldfarb-Shanno (LBFGS) method to implement a 3D inversion with full impedance data. Through inverting synthetic and real data, we prove that: (1) directly using an MT method to invert the data of the tensor CSAMT will obtain an incorrect result, (2) the inversion result of tensor CSAMT is more reliable than that of the traditional CSAMT, and (3) LBFGS is more efficient than the nonlinear conjugate gradient (NLCG) for tensor CSAMT.Our research shows that 3D tensorCSAMTinversion with LBFGS is very useful and practical for electromagnetic exploration. [ABSTRACT FROM AUTHOR]

Controlled-source audio-frequency magnetotellurics (CSAMT) has developed rapidly in recent years and are widely used in the area of mineral and oil resource exploration as well as other fields. The current theory, numerical simulation, and inversion research are based on the assumption that the underground media have resistivity isotropy. However a large number of rock and mineral physical property tests show the resistivity of underground media is generally anisotropic. With the increasing application of CSAMT, the demand for probe accuracy of practical exploration to complex targets continues to increase. The question of how to evaluate the influence of anisotropic resistivity to CSAMT response is becoming important. To meet the demand for CSAMT response research of resistivity anisotropic media, this paper examines the CSAMT electric equations, derives and realizes a three-dimensional (3D) staggered-grid finite difference numerical simulation method of CSAMT resistivity axial anisotropy. Through building a two-dimensional (2D) resistivity anisotropy geoelectric model, we validate the 3D computation result by comparing it to the result of controlled-source electromagnetic method (CSEM) resistivity anisotropy 2D finite element program. Through simulating a 3D resistivity axial anisotropy geoelectric model, we compare and analyze the responses of equatorial configuration, axial configuration, two oblique sources and tensor source. The research shows that the tensor source is suitable for CSAMT to recognize the anisotropic effect of underground structure. [ABSTRACT FROM AUTHOR]

The eastern terminations of the Himalayan orogeny, named Namche Barwa, are considered a vital natural laboratory in the Tibetan plateau for geodynamics due to its distinctive geological and geomorphological characteristics. Magnetotelluric (MT) data measured at 83 sites around the Namche Barwa are imaged by three-dimensional (3-D) inversion to better reveal the crustal structure of the eastern Himalaya. The results show a complex and heterogeneous electrical structure beneath the Namche Barwa. The electrical conductors distributed in the middle and lower crust around the Namche Barwa provide additional evidence for the 'crustal flow' model if they are considered as some parts of the flow in a relatively large-scale region. The near-surface resistivity model beneath the inner part of Namche Barwa conforms with the locations of hot spring and fluid inclusions, the brittle-ductile transition, and the 300°C-400°C isotherm from previous hydrothermal studies. Relatively resistive upper crust (>800 Ωm) is underlain by a more conductive middle to lower crust (<80 Ωm). The electrical characteristics of the thermal structure at shallow depth indicate an accumulation of hydrous melting, a localized conductive steep dipping zone for decompression melting consistent with the 'tectonic aneurysm' model for explaining the exhumation mechanism of metamorphic rocks at Namche Barwa. The results also imply that both surface processes and local tectonic responses play a vital role in the evolution of Namche Barwa. An alternative hypothesis that the primary sustained heat source accounts for the local thermal-rheological structure beneath Namche Barwa is also discussed. [ABSTRACT FROM AUTHOR]

Four kinds of array of induced polarization (IP) methods (surface, borehole-surface, surface-borehole, and borehole-borehole) are widely used in resource exploration. However, due to the presence of large amounts of the sources, it will take much time to complete the inversion. In the paper, a new parallel algorithm is described which uses message passing interface (MPI) and graphics processing unit (GPU) to accelerate 3D inversion of these four methods. The forward finite differential equation is solved by ILU0 preconditioner and the conjugate gradient (CG) solver. The inverse problem is solved by nonlinear conjugate gradients (NLCG) iteration which is used to calculate one forward and two “pseudo-forward” modelings and update the direction, space, and model in turn. Because each source is independent in forward and “pseudo-forward” modelings, multiprocess modes are opened by calling MPI library. The iterative matrix solver within CULA is called in each process. Some tables and synthetic data examples illustrate that this parallel inversion algorithm is effective. Furthermore, we demonstrate that the joint inversion of surface and borehole data produces resistivity and chargeability results are superior to those obtained from inversions of individual surface data. [ABSTRACT FROM AUTHOR]

New evidence concerning the middle and upper crustal structure of the Longmen Shan range where the May 12, 2008 M7.9 Wenchuan earthquake occurred is presented on the basis of seismic reflection surveys. The results of seismic reflection profiles clearly show a relatively complete stratigraphic sequence of the Upper Yangtze Basin since the Cambrian. Folding deformation, along with movements along numerous detachment layers, took place in the strata among the Wenchuan–Maoxian Fault (WMF), the Yingxiu–Beichuan Fault (YBF), and the Guanxian–Anxian Fault (GAF). The WMF is steep in the shallow strata, turning gentler gradually from a depth of 4 km, and more so at depths beyond 7 km. The WMF imbricates with the YBF and GAF, and they do not converge within the top 15 km of the crust. This interpretation disagrees with the viewpoint that the YBF and GAF extend with subvertical trend to a depth of 20 km with a robust relationship between the hypocenters and the subsurface faults and fold geometries. We conclude that the sub-horizontal detachment layers where most aftershocks occurred are the major causative fault, instead of the YBF. The YBF might be an earthquake-propagating fault according to our seismic reflection data and the distribution of aftershocks. The overlying rigid and rootless Pengguan complex (∼3–4 km) might aggravate the damage to the YBF. Furthermore, there exists a new high-angle, listric structures vertical coseismic fault, which is able to link the seismogenic detachment layer and the shallow YBF together and resulted in huge damage of the YBF. [ABSTRACT FROM AUTHOR]

The Luobusa chromite mine in Tibet, the largest chromite deposit discovered in China to date, is hosted in the Luobusa ophiolite in the eastern section of the Yarlung Zangbo suture zone. A combined seismic reflection and magnetotelluric survey was carried out on the mantle peridotites in which the chromitite deposits occur in order to investigate their origin and to prospect for other deposits in the area. The survey indicates that the Luobusa chromitite-bearing ultramafic block is a tectonic slice thrust onto the southern margin of the Gangdese batholith by multiple south-dipping faults. The maximum thickness of the ultramafic rock mass is 4 km, and it is underlain by the upper crust of the Indian Plate. Low-resistivity bodies in the upper crust of the Luobusa area lie at a depth of up to 5 km, and strata at a depth of 7 km cross the Yarlung Zangbo suture zone at a relatively small angle. Between depths of 20–50 km, there are a number of oval low-resistivity bodies, the lower parts of which might have been close to the Moho at a depth of approximately 65 km near the Yarlung Zangbo River. It is inferred that such low-resistivity bodies arise from partially molten bodies in the middle and lower crust and are related to deep tectonic activity and mantle material upwelling in the Yarlung Zangbo suture zone. [ABSTRACT FROM AUTHOR]