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Magnetotelluric data were used to study the lithosphere structure of the Southern Oklahoma Aulacogen (SOA). Inversion of the data revealed two low resistivity anomalies beneath the SOA. The first is located in the depth range 0–90 km in the crust and upper lithospheric mantle. The second extends from a depth 100 km to the base of the lithospheric mantle and extends away from the SOA to the ends of the profile. The cause of low resistivity anomalies is discussed in relation to the tectonic evolution of the region and recent laboratory experiments on rock conductivity. The first anomaly is attributed to the combination of (a) water present in mantle minerals and (b) the formation of hydrous mineral phases by interactions between a plume and the lithosphere during rifting. Grain size reduction and fabric alignment from deformation during the Ancestral Rocky Mountain (ARM) orogeny may have also contributed to the low resistivity. This enrichment phase may have mechanically weakened the lithosphere and allowed deformation to occur during the ARM orogeny. The low resistivity of the deeper anomaly is attributed to a fluorine-enriched phlogopite layer that is also coincident with an observed seismic mid-lithosphere discontinuity (MLD). A lithosphere keel of mantle minerals enriched in water underlies this layer and may have formed by accretion of the plume head to the lower lithosphere after rifting, which also rethickened the lithosphere to its present-day depths. The MLD may then reflect a melt layer along a paleo lithosphere-asthenosphere boundary entombed during the accretion.

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]

Estimating subsurface properties from geophysical measurements is a common inverse problem. Several Bayesian methods currently aim to find the solution to a geophysical inverse problem and quantify its uncertainty. However, most geophysical applications exhibit more than one plausible solution. Here, we propose a multimodal variational autoencoder model that employs a mixture of truncated Gaussian densities to provide multiple solutions, along with their probability of occurrence and a quantification of their uncertainty. This autoencoder is assembled with an encoder and a decoder, where the first one provides a mixture of truncated Gaussian densities from a neural network, and the second is the numerical solution of the forward problem given by the geophysical approach. The proposed method is illustrated with a 1-D magnetotelluric inverse problem and recovers multiple plausible solutions with different uncertainty quantification maps and probabilities that are in agreement with known physical observations. [ABSTRACT FROM AUTHOR]

The application of magnetic and geoelectrical methods for archaeological study is presented in this paper. The study was intended to map near-surface structures around the Indrapatra Fort, the 16th-century fortification built during the Early Aceh Sultanate in the north of Sumatra. Some of the structures around the fort have been buried after the 2004 Indian Ocean Tsunami or even by previous paleotsunami deposits. The ground magnetic measurements were performed over an area of 6 750 m2 using a high-resolution Proton Precession Magnetometer G-19 Geometric. Total magnetic field data were acquired as a 3-m grid station along survey lines spaced 3-m apart. Total magnetic field anomalies were calculated by diurnal and International Geomagnetic Reference Field corrections. To clarify the position of objects causing the anomalies, analytical signals and tilt derivative filters have been applied to the total magnetic field anomalies. The method found several structures with contrasting physical properties to those of the surrounding material. Both filters were able to identify the presence of circular, elongated, and square anomalies around the Indrapatra site. The archaeological interpretation of such structures is in terms of wells, canals, and road floors. The structures are made of limestone boulders buried within the uppermost sand layer. The burning process to cement the boulders causes their magnetic properties to be different in contrast to the layer of sand that covers them. Based on magnetic interpretation, three geoelectrical profiles were measured crossing the targeted objects using Supersting R8/IP 56 Electrode Electric Resistivity. Wenner configuration with 1-meter electrodes spacing was applied for a better model resolution. The inverted models of apparent resistivity data show an agreement with magnetic interpretation at shallow depth. The models also imaged the depth and geometry of the objects based on the electrical resistivity properties of subsurface materials. It is expected that the structures preserved tsunamigenic deposits, so the discovery of the structures will be able to contribute to further research on paleotsunami in Aceh. [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) data image the bulk resistivity of the subsurface which can be used to infer magma reservoir conditions beneath volcanoes. The bulk resistivity of magma depends primarily on the melt volume fraction, temperature, and water content. These variables are coupled thermodynamically, yet mixing relations for bulk resistivity implicitly treat them as independent. Here, we use a parameterization of the rhyolite‐MELTS thermodynamic model to constrain relationships between melt fraction, temperature, dissolved water content and bulk resistivity for rhyolitic magmas. This method results in MT interpretations which are (a) thermodynamically consistent at near‐equilibrium conditions, (b) independent of temperature and water content estimates derived from erupted products, and (c) able to consider saturated melts containing a volatile (i.e., aqueous fluid) phase. The utility of the method is demonstrated with three case studies of silicic systems: Mono Basin, Newberry volcano and the Laguna del Maule Volcanic Field (LdMVF). The moderately conductive feature at Mono Basin can be explained by under‐saturated partial melt (6–15 vol%) at <775°C, indicating relatively stable magma storage conditions since the last eruption. However, a relatively resistive feature at Newberry Volcano requires lower temperatures (<750°C) than previous estimates, suggesting that the system has cooled since the last eruption. A conductive feature at the LdMVF cannot be explained by saturated or under‐saturated rhyolitic melt and requires additional conductive phases. These results demonstrate the potential of this new method to reduce uncertainty in MT interpretations and highlight the need for additional coupling strategies between petrology, geophysics, and thermo‐mechanical models to better understand magmatic systems. Plain Language Summary: Determining the magma storage conditions beneath a volcano can give insights into the volcanic hazards. The magnetotelluric (MT) method measures the electrical resistivity of the subsurface, which is valuable because molten rock (i.e., melt) is less resistive than solid rock (or crystals). A mixture of melt and crystals will have a particular resistivity value which means that MT data can infer the relative proportion of melt (i.e., melt fraction). Magmas with higher melt fractions are more mobile and more likely to erupt. In addition, hotter melt with more dissolved water is more conductive than cooler, drier melt, so the electrical resistivity can also be used to infer the temperature and water content. However, there are more unknown variables (temperature, melt fraction, and water content) than known variables (observed resistivity), so more constraints are needed. Thermodynamic models indicate that hotter, wetter conditions have higher melt fractions than cooler, drier conditions, so the unknown variables are not independent. We use thermodynamic models to provide additional constraints on the interpretation of magma reservoir conditions from bulk resistivity, which enables us to provide realistic bounds on the range of plausible conditions and better explain the electrical resistivity observed at three different volcanoes. Key Points: A parameterization of the thermodynamic rhyolite‐MELTS model is coupled to empirical relations for the bulk resistivity of rhyolitic magmaThe coupling can significantly alter previous interpretations of magma reservoir conditions from magnetotelluric dataAllows for a comparison between past magma reservoir conditions inferred from erupted products and current magma reservoir conditions [ABSTRACT FROM AUTHOR]

Borborema Province is an orogenic belt of Neoproterozoic age part of the Brasiliano/Pan African Orogeny (600 Ma) in the northeast corner of the Brazilian shield. With an area of approximately 380,000 square kilometers, it is composed of fold and thrust belts, progressively stretched on continental scale transcurrent faults, and coeval plutonism. Its north and east flanks are limited by the Atlantic Ocean from a Mesozoic rift, and include passive margins basins and oceanic crust. Southwards, it is adjacent to the São Francisco Craton, while the west limit is overlain by the Paleozoic Parnaíba intracratonic basin. Besides the extensive metasedimentary and magmatic Neoproterozoic crust, it also involves Archean and Paleoproterozoic units. However, those units are obliterated by Brasiliano/Pan African Orogeny that segregates and disperses over the tectonic belts, hampering the interpretation of the architecture of the collisions of paleoplates during the Neoproterozoic. Magnetotelluric geophysical survey acquisitions have been conducted over the last decade by initiative of several Brazilian researchers and have been integrated to other geophysical methods in geotectonics in order to study the deepest parts of the lithosphere. We conducted the integration of twenty resistivity magnetotelluric sections and their interpretations, downplaying the electric response of the upper lithosphere (<100 km deep), aiming to emphasize and map differences in the lithospheric mantle. The resulting map shows high to moderate resistivity cores in the upper mantle, which are interpreted as continental lithospheric relicts, among high conductivity belts, associated to the Brasiliano paleosutures. These results are corroborated by gravimetric and magnetometric data. We propose that the southern limit of the Borborema Province with the São Francisco Craton is located under the Pernambuco lineament region, and that the western boundary with the Parnaíba lithospheric block is located under the Orós-Jaguaribe – Ceará Central Domain. [Display omitted] • Over 20 resistivity sections have been interpreted, using long wavelength data from Magsat and Gravsat as constrains. • Deep geophysics reveal persistent anomalies for lithosphere interpretation. • Neoproterozoic sutures and Archean-Paleoproterozoic blocks mapped. • Extensional regimes of lithospheric significance delimited. [ABSTRACT FROM AUTHOR]

In the northern part of Sumatra Island, Indonesia, the Great Sumatran Fault, which can cause an earthquake, was divided into two segments: the Aceh and Seulimeum. An effort to reduce the risk is mapping the fault area, especially in the region that does not clearly show the sign on the surface, e.g., in the Lamtamot area, Indonesia. Electrical resistivity is widely used to study shallow structures, but the method requires more time when applied in a large area. This research explores the potential of an extremely low frequency (very low frequency-electromagnetic; VLF-EM) method to investigate the shallow fault of the Seulimeum segment. Here, the VLF-EM is compared with other geophysical methods such as resistivity and magnetic methods. For comprehensive results, the geomorphic observation that was conducted covered outcrops of the fault and trenching sites in the geophysical study for validating the model. A similar pattern of the VLFEM and electrical resistivity data has been shown in a two-dimensional profile using data processing. The fault structure can be mapped at a distance of 20-24 m from the profile measurement, which is demonstrated by the low current density associated with the conductive zone from the VLF-EM, and low resistive anomaly in electrical resistivity. The fault can also be confirmed via magnetic intensity, which significantly increases at the same distance (20-25 m) of the VLF-EM and electrical resistivity. The geomorphic observation shows outcrops of fault activity, such as fault scarp, fractures, and faults, in the same direction as the Seulimeum segment, while scrap extends in the northwest direction up to ~20 m around the geophysical surveys. As revealed by the results, the VLF-EM method combined with other geophysical surveys and geomorphic observation can be used as a technique to image the fault that shows the shallow structure of the Seulimeum fault at 20-32 m along the profile. [ABSTRACT FROM AUTHOR]

The Gaurikund town falls on the way of the famous trekking route to Kedarnath that faced the wrath of the 2013 flood disaster. This fateful event severed more than 5000 casualties, demolished several infrastructures, and shifted the course of Gaurikund spring from its original position. Nevertheless, the Gaurikund geothermal spring system located in the Himalayan Geothermal Belt of the Garhwal region is preeminent for religious beliefs, balneotherapeutic values and a gateway to delve within the geothermal and hydrological characteristics of the area. In this perspective, restoration of Gaurikund geothermal spring system becomes a necessity. A multiparametric approach comprising geospatial, geology, hydrochemistry and geophysics has been used to study and justify these aspects at Gaurikund. The geological studies infer that the geothermal spring gets recharged by the steep, southerly dipping joints in granite gneiss. Subsequently, the deep percolated water heats up due to the high geothermal gradient and then emerge along the Vaikrita Thrust and its sympathetic minor fault-thrust system by advection. Moreover, four spring outlets are inventoried, with discharge varying from 7.46 to 95.54 L/min. The normal emissivity model uses the pre and post-disaster satellite data and generates maximum kinetic temperature images, showing a positive correlation between land surface temperature and spring discharge. Two-dimensional Electrical Resistivity Tomography (Schlumberger, Wenner and Gradient configurations) survey revealed two low resistivity zones proximal to the geothermal spring on the right bank of the Mandakini river. The engineering interventions carried out by bank protection and construction of small gully plugs in the catchment area is recommended along Gaurikund-Sonprayag section on the right bank of Mandakini river. [ABSTRACT FROM AUTHOR]

The tectonic history of southern Africa includes Archean craton formation, multiple episodes of subduction and rifting and some of the world's most significant magmatic events. These processes left behind a compositional trail that can be observed in xenoliths and measured by geophysical methods. The abundance of kimberlites in southern Africa makes it an ideal place to test and calibrate mantle geophysical interpretations that can then be applied to less well‐constrained regions. Magnetotellurics (MT) is a particularly useful tool for understanding tectonic history because electrical conductivity is sensitive to temperature, bulk composition, accessory minerals, and rock fabric. We produced three‐dimensional MT models of the southern African mantle taken from the SAMTEX MT data set, mapped the properties of ∼36,000 garnet xenocrysts from Group I kimberlites, and compared the results. We found that depleted regions of the mantle are uniformly associated with high electrical resistivities. The conductivity of fertile regions is more complex and depends on the specific tectonic and metasomatic history of the region, including the compositions of metasomatic fluids or melts and the emplacement of metasomatic minerals. The mantle beneath the ∼2.05 Ga Bushveld Complex is highly conductive, probably caused by magmas flowing along a lithospheric weakness zone and precipitating interconnected, conductive accessory minerals such as graphite and sulfides. Kimberlites tend to be emplaced near the edges of the cratons where the mantle below 100 km depth is not highly resistive. Kimberlites avoid strong mantle conductors, suggesting a systematic relationship between their emplacement and mantle composition. Plain Language Summary: The present‐day composition of Earth's tectonic plates results from past geological processes. We can learn about Earth's composition from deep rock samples that are carried to the surface during volcanic eruptions and by probing its physical properties, like electrical conductivity, with geophysics. In southern Africa, there are extensive deep rock samples, which have been brought to the surface by kimberlite volcanoes that also host diamonds, and also extensive geophysical data. In this article, we compare the rock compositions with electrical conductivity to learn more about Earth's composition. Our results show that the oldest parts of the plates, which retain compositions similar to their initial composition, appear resistive. On the other hand, regions that have been intruded by deep fluids or molten rock can be resistive or conductive, depending on the types of minerals that were formed during the intrusion. The kimberlite volcanoes mostly erupted through the edges of the most resistive parts of the plates and did not erupt through the conductors. These results will help us to make more accurate interpretations about the composition of parts of the Earth where we do not have deep rock samples. Key Points: Electrical conductivities from 3D magnetotelluric models in southern Africa are compared with compositions from garnet xenocrystsDepleted regions are electrically resistive. All conductors are in metasomatized regions but the style of metasomatism affects conductivityKimberlites tend to surround resistors while avoiding strong conductors, suggesting mantle composition is a control on magmatism [ABSTRACT FROM AUTHOR]

In this paper, we report a preliminary result of the Magnetotelluric investigation of the Mogod area. The Mogod region is one of the most prominent fields for geophysical study since the region includes young and active faults and geothermal activities. We conducted magnetotelluric measurements at 20 sites during geophysical field seasons in 2018-2021 as a pilot survey to understand data property and the electromagnetic noise level for the detailed electromagnetic studies. During the fieldwork, we used Lemi Magnetotelluric instruments and measured all three orthogonal components of the magnetic field and the horizontal components of the electric field. For the data processing, we used Matlab code by using the M-estimate regression method, and estimated the magnetotelluric transfer function with a lownoise level. The electrical resistivity model of the subsurface of survey layout shows us the existing resistivity anomalies at shallow-depth, and thickness of the upper crust approximately 11-17 km. Here, suggest that the thickness of the upper crust is 17 km and crust is 40 km with local magnetotelluric measurements. Additionally, the electric conductor appears in the southwest of Mogod region, we interpret that conductor play as a source of geological activity of Mogod region, and it might be the signature of a remanent fluid. [ABSTRACT FROM AUTHOR]

Dimensionality and distortion analysis of the measured magnetotelluric (MT) data are challenging procedures to conclude the main properties of regional geo-electric structure (e.g., its strike direction) and quantify the influence of superficial distorting bodies. In this paper, we applied several analysis methods to the same data set from southwest Zagros to determine the dimensionality and degree of the inherent distortion in the data and to recover the regional responses. Results show that the regional 2D responses can be better retrieved in a two-step procedure: First, the dimensionality and distortion are classified by impedance invariants and then, the Groom-Baily (GB) method is applied to remove distortion effects and to estimate the regional strike and principal impedances. An audio-magnetotelluric (AMT) data set acquired at 19 stations along a profile in the southwest of Simply Folded Belt (SFB) of the Zagros Mountains is investigated. The Bahr and WAL invariants confirm regional 1D and 2D structures with local galvanic distortion at most periods. The GB decomposition of MT impedance data reveals period-independent distortion parameters. The strike analysis on all data whose principal phase differences are greater than 5° shows a regional trend of about 30°. 2D inductive effects were also retrieved by removing distortion effects from the measured data. [ABSTRACT FROM AUTHOR]

Study region: The study area is located at Kapunda, South Australia with a semi-arid climate and comprises a fractured rock aquifer. Established in 1842, Kapunda was the first mining town in Australia and its open-pit mine was a key economic driver for the whole continent. The mine was closed in 1879 and the remnant low-grade copper was considered sub-economic, hence the mine has been developed into a tourist attraction. With the recent advances in In-Situ Recovery (ISR), the feasibility of recovering the remnant copper is under consideration again. Study focus: A field trial involving lixiviant injection and extraction is under development. This study uses a combination of stochastic groundwater modeling and multiple geophysical techniques to investigate (1) the flow paths of injectant and whether it will reach the Light River, and (2) the injectant residence time in the aquifer. New hydrological insights for the region: Depending on the effective porosity of the aquifer, the modeling suggests a probability of 1 %–5 % for the injectant to escape the site. There is a large uncertainty in the simulated injectant residence time, ranging from 200 to beyond 1000 days. These analyses are conservative and do not consider the biodegradability of the injectant, which can achieve a 90 % degradation over 28 days. Our modeling shows no evidence that the injectant can reach the Light River within this timeframe.

One objective of a lander mission to Jupiter's icy moon Europa is to detect liquid water within 30 km as well as characterizing the subsurface ocean. In order to satisfy this objective, water within the ice shell must also be identified. Inductive electromagnetic (EM) methods are optimal for water detection on Europa because even a small fraction of dissolved salts will make water orders of magnitude more electrically conductive than the ice shell. Compared to induction studies by the Galileo spacecraft, measurements of higher-frequency ambient EM fields are necessary to resolve the shallower depths of intrashell water. Although these fields have been mostly characterized by prior missions, their unknown source structures and plasma properties do not allow EM sounding using a single surface magnetometer or the orbit-to-surface magnetic transfer function, respectively. Instead, broadband EM sounding can be accomplished from a single surface station using the magnetotelluric (MT) method, which measures horizontal electric fields as well as the three-component magnetic field. We have developed a prototype Europa Magnetotelluric Sounder (EMS) to meet the measurement requirements in the relevant thermal, vacuum, and radiation environment. EMS comprises central electronics, a fluxgate magnetometer on a mast, and three ballistically deployed electrodes to measure differences in surface electric potential. In this paper, we describe EMS development and testing as well as providing supporting information on the concept of operations and calculations on water detectability. EMS can uniquely determine the occurrence of intrashell water on Europa, providing important constraints on habitability. [ABSTRACT FROM AUTHOR]

The resistivity method in geophysics is used to solve various geological and engineering problems. Recently, this nondestructive method has been used on trees to investigate possible infections within the trunks by scanning resistivity variations. In this study, the electrical resistivity method has been aimed to be applied on various trees in Istanbul, Turkey to test whether the method applies to trees via regular resistivity measurement devices used in geophysics. Firstly, a multi-channel resistivity device, that is designed to automatically take measurements on the ground, is modified to carry out the measurements on trees. The measured data are processed using two different approaches. The first approach is to prepare a program in MATLAB, which is capable of adapting measurement points into a circular profile via interpolation. The data processed with this program are then gridded to prepare resistivity contour slices. The detection and handling of faulty measurements are discussed briefly in this section. The second approach is to use an open-source electrical tomography program (BERT V2) to apply inversion to the collected resistivity data. Finally, all the results and conclusions are interpreted considering the resistivity distribution within tree trunks, including sample slices from two trees that are known to have defected beforehand. As a conclusion of our studies, we have found that a regular resistivity measurement device used in earth sciences is applicable to an extent on trees to investigate possible defects within their trunks. [ABSTRACT FROM AUTHOR]

Imaging silicic systems using geophysics is challenging because many interrelated factors (e.g., temperature, melt fraction, melt composition, geometry) can contribute to the measured geophysical anomaly. Joint interpretation of models from multiple geophysical methods can better constrain interpretations of the subsurface structure. Previously published resistivity and shear wave velocity (Vs) models, derived separately from magnetotelluric (MT) and surface wave seismic data, respectively, have been used to model the restless Laguna del Maule Volcanic Field, central Chile. The Vs model contains a 450 km3 low‐velocity zone (LVZ) interpreted as a region with an average melt fraction of 5–6%. The resistivity model contains a conductor (C3) interpreted as a region with a melt fraction >35%. The spatial extents of the LVZ and C3 overlap, but the geometries and interpretations of these features are different. To resolve these discrepancies, this study investigates the resolution of the MT data using hypothesis testing and constrained MT inversions. It is shown that the MT data are best fit with discrete conductors embedded within the larger LVZ. The differences between the MT and seismic models reflect resolution differences between the two data sets as well as varying sensitivities to physical properties. The MT data are sensitive to smaller volumes of extractable mush that contain well‐connected crystal‐poor melt (C3). The seismic data have lower spatial resolution but image the full extent of the poorly connected crystal‐rich magma storage system. The combined images suggest that the LdMVF magma plumbing system is thermally heterogeneous with coexisting zones of warm and cold storage. Key Points: A published velocity model derived from surface wave inversion is used to constrain the 3‐D magnetotelluric inversion at Laguna del MauleDiscrepancies between the velocity and resistivity models are likely due to magma storage conditions rather than inversion nonuniquenessThe MT images a crystal‐poor melt volume within a larger crystal‐rich mush imaged by seismic indicating a thermally heterogeneous system [ABSTRACT FROM AUTHOR]

A quasi 3D electrical resistivity (ERT) survey was undertaken at a UNESCO World Heritage site, Saqqara, Giza, Egypt, during a joint archaeological-geophysical mission from Cairo University. The main objective is to detect the locations of the subsurface archaeological tombs/or crypts and to allocate any possible archaeological bodies/features buried underneath the study area. In this survey, SYSCAL Pro system with 24 electrodes and a multi-core cable is used for automatic data acquisition of profiling data. The dipole{ dipole array was used to enhance resolution, 14 resistivity lines are conducted during this Survey. The processed data were analysed in order to produce resistivity tomography (ERT) for qualitative and quantitative interpretations. Inversion of the ERT data identified variation of resistivity values and the expected locations of the underground galleries and highlight the presence of regular shape structures probably due to features of archaeological interest. Excavations made accordingly in the study area led to an interesting discovery of a tomb of the Great Army General, Iwrhya. The tomb is approximately 2000 years old as it covers the reigns of both Kings Seth I and Ramesses II. Using the 3D resistivity tomography with such a multi-electrode technique proved its efficiency and applicability for non-invasive archaeo-geophysical prospecting. [ABSTRACT FROM AUTHOR]

The phase tensor of magnetotellurics is analysed in terms of its invariants with regard to axes rotation. These invariants are displayed as conic sections (ellipses), eigenvectors, and as Mohr diagrams. Attention is drawn to a supplementary ellipse which may be constructed to complement the usual phase tensor ellipse. The two ellipses together help convey the full information available from phase tensor analysis. For the cases of 1D and 2D regional structure, the ellipses, eigenvectors and Mohr diagrams show distinctive features, such as no preferred strike direction in the case of 1D, and a consistent strike (or across-strike) direction in the case of 2D. In the general case of 3D regional structure a "closest 2D strike" direction may be apparent, though there is a range of possibilities for this quantity. The range includes Bahr's regional strike estimates, which are shown to be given by the eigenvectors of the phase tensor. Generally in this paper, the phase tensor will be referred to as the CBB tensor, in recognition of its discoverers Caldwell, Bibby and Brown. [ABSTRACT FROM AUTHOR]

Monitoring dynamics of volcanic eruptions with geophysics is challenging. In August and November 2012, two small eruptions from Mount Tongariro provided a unique opportunity to image subsurface changes caused by the eruptions. A detailed magnetotelluric survey of the Tongariro volcanic complex completed prior to the eruption (2008–2010) provides the preeruption structure of the magmatic system. A subset of the initial measurement locations was reoccupied in June 2013. Significant changes were observed in phase tensor data at sites close to the eruptive center. Although subsurface electrical resistivity changed, the geometry of the preeruptive reservoir did not. These subsurface resistivity variations are interpreted as being predominantly caused by interaction of partial melt and the overlying brine layer causing volume reduction of the brine layer through phreatic eruption. The ability to detect significant changes associated with the magma reservoir suggests that magnetotellurics can be a valuable volcano monitoring tool. Plain Language Summary: Preeruption and posteruption electromagnetic magnetotelluric measurements are used to determine the variation in subsurface electrical properties resulting from changes in the Tongariro volcanic center magma chamber associated with the 2012 eruptive cycle. The observed electrical property changes are related to the physical eruption properties (e.g., eruptive volume, style, and composition), revealing the state of the magmatic system both prior to and following the eruption. Knowing both the preeruption and posteruption states of the magmatic system and the surface eruptive properties enables reconstruction of the subsurface eruption mechanism. Successful identification of preeruption and posteruptive states of the volcano is evidence for the usefulness of continuous magnetotelluric monitoring of volcanoes to identify variations within magmatic systems that may be indicative of imminent eruption. Key Points: We identify significant temporal change in preeruption and posteruption magnetotelluric soundingsAn eruption mechanism model is developed consistent with both geophysical and geological observationsMagnetotellurics is a viable additional monitoring tool for active volcanic systems [ABSTRACT FROM AUTHOR]

The polar regions are host to fundamental unresolved challenges in Earth studies. The nature of these regions necessitates the use of geophysics to address these issues, with electromagnetic and, in particular, magnetotelluric studies finding favour and being applied over a number of different scales. The unique geography and climatic conditions of the polar regions means collecting magnetotelluric data at high latitudes, which presents challenges not typically encountered and may result in significant measurement errors. (1) The very high contact resistance between electrodes and the surficial snow and ice cover (commonly MΩ) can interfere with the electric field measurement. This is overcome by using custom-designed amplifiers placed at the active electrodes to buffer their high impedance contacts. (2) The proximity to the geomagnetic poles requires verification of the fundamental assumption in magnetotellurics that the magnetic source field is a vertically propagating, horizontally polarised plane wave. Behaviour of the polar electro-jet must be assessed to identify increased activity (high energy periods) that create strong current systems and may generate non-planar contributions. (3) The generation of 'blizstatic', localised random electric fields caused by the spin drift of moving charged snow and ice particles that produce significant noise in the electric fields during periods of strong winds. At wind speeds above ~ 10 m s−1, the effect of the distortion created by the moving snow is broad-band. Station occupation times need to be of sufficient length to ensure data are collected when wind speed is low. (4) Working on glaciated terrain introduces additional safety challenges, e.g., weather, crevasse hazards, etc. Inclusion of a mountaineer in the team, both during the site location planning and onsite operations, allows these hazards to be properly managed. Examples spanning studies covering development and application of novel electromagnetic approaches for the polar regions as well as results from studies addressing a variety of differing geologic questions are presented. Electromagnetic studies focusing on near-surface hydrologic systems, glacial and ice sheet dynamics, as well as large-scale volcanic and tectonic problems are discussed providing an overview of the use of electromagnetic methods to investigate fundamental questions in solid earth studies that have both been completed and are currently ongoing in polar regions. [ABSTRACT FROM AUTHOR]

The article discusses a study which mapped the crust and upper mantle structure of central Mongolia to be able to understand the underlying mechanisms of intraplate processes and its implications for intraplate surface uplift and volcanism. Topics covered include the insights provided by thermo-mechanical numerical modeling, t potential explanations for intraplate surface uplift, and the results of the multi-scale magnetotelluric modeling.

Abstract The most computationally intensive step in 3D magnetotelluric (MT) inversion is the calculation of the forward response. This fact makes any modelling which requires many function evaluations, including genetic algorithms and Markov chain Monte Carlo inversion, extremely time consuming. Using Artificial Neural Networks (ANNs) it is possible to approximate these expensive forward functions with rapidly evaluated alternatives. Using a limited subset of resistivity models created in a simple parameterisation, this work is the first to apply ANNs to approximate the 3D MT forward function. Training data are generated with a compute time of two weeks, and after training the ANN is able to reproduce forward responses at arbitrary site locations with accuracy similar to the level of typical data errors. To evaluate the accuracy of the models, we show that these forward responses may be used to successfully invert MT data in an evolutionary framework. Examples are shown in both synthetic and real-world scenarios, and results are compared with those from traditional inversion algorithms. We conclude that the trained ANN inversion has a fraction of the run-time of a traditional inversion and is successful at modelling the space of its limited parameterisation. Highlights • Neural network has been trained to reproduce MT forward function. • Used a compact parameterisation of resistivity and flexible station locations. • Can be successfully used in the inversion of synthetic structure. • Has been applied to the inversion of real MT data. [ABSTRACT FROM AUTHOR]

Çanakkale is the second province, after Istanbul, that has territories in both Asia and Europe. The city, also known as Dardanelles, is located on the Gallipoli peninsula in the northwest of Turkey, and the Biga peninsula which is an extension of Anatolia at the west. The region is tectonically quite active and it has high earthquake generation potential. The city has experienced a significant earthquake (Mw = 6.8) once again that occurred on May 24, 2014. In addition to these challenges, remarkable part of the city's territory and also potential new settlement areas are located on thick alluvium. Thus, mentioned-above disadvantages clearly increase the importance of this study, which focuses on determining the areas having different earthquake hazard potentials with regard to local site conditions by performing seismic risk assessments, as well as providing a basis for the preparation of settlement suitability maps on different scales that will lead knowledge for zoning plans. To that end, an integrated geophysical and geotechnical study was performed in a detailed manner. Multi-channel analysis of surface waves (MASW) and microtremor (MT) measurements were carried out at properly distributed 110 stations representing the whole survey area. The geotechnical study, planned to contribute geophysical interpretation, involved the drilling of 110 boreholes that are close to geophysical measurement stations. In addition to examining Vs30 variations based on the MASW, the ground dominant vibration period maps were also produced through MT time series analysis. In particular, the mean Vs velocity (Vs30) up to the first 30 m and the ground oscillation period change based on spectral amplitude ratios (HVSR) yielded substantial information that helped generate the seismic microzonation maps and also make the ground classification. These outputs contributed to exhibit the risk zones in the coastal city that has a dense and narrow settlement plan. • First order site characterization of Çanakkale city by geophysical and geotechnical data • MASW and Microtremor data were used to evaluate geophysical site conditions. • Geophysical results were integrated with geotechnical data (mechanical drilling data). • Seismic micro hazard zonation of the city was estimated. • We provided guidelines for urban planning with respect to possible earthquake threats. [ABSTRACT FROM AUTHOR]

Abstract 3-D inversion of geophysical data has become a practical, commonly used tool in the interpretation of magnetotelluric (MT) data. However, 3-D inversion is a non-linear and non-unique task and requires adopting additional assumptions. In this paper, we analyze the influence of selection of inversion parameters in the case of previously published, real model for Pomerania region (Poland). This zone is a regional scale 2-D structure, which locally becomes 3-D. The analyzed model was accomplished by inversion of full impedance and tippers from 31 MT stations with the period range from 10 s to 10,000 s. The inversion parameters were chosen to provide the best possible agreement of the model with well-known geophysical and geophysical data. The purpose of the presented work is to determine how the change of some inversion parameters influences previously obtained model, and therefore we check which structure features remain in spite of the change of the implied parameters. A number of tests were performed employing an implementation of the freely available ModEM inversion package. These included the choice of an appropriate initial model and a model covariances evaluation as a means to control smoothness between adjoining model areas. Additionally, we compared the models calculated from all transfer functions (impedances and tippers) and from subsets (only tippers and off-diagonal components of the impedance tensor and tippers). Moreover, the tests with the model coordinate system rotation were performed. Our analysis has shown that the values of covariance matrix coefficients have an evident effect on the results. However, the multiple use of the covariance matrix with small coefficients gives a similar result as its single use with higher coefficients. The initial model and the selection of the transfer function for inversion can significantly change the model. However, changing the orientation of the coordinate system from geographic to the one that is aligned with a regional strike direction does not cause noticeable changes in the solution obtained. It can be only noted that for the rotated coordinate system the obtained model becomes closer to the model of a 2-D regional structure. Highlights • 3-D inversion of different magnetotelluric real data sets with various model parameterizations • Examination of the choice of MT transfer functions set on the 3-D inversion solution. • Our tests prove the necessity of careful and appropriate selection of inversion parameters, model space, and type of transfer functions [ABSTRACT FROM AUTHOR]

Abstract Large earthquakes away from plate boundaries pose a significant threat to human lives and infrastructure, but such events typically occur on previously unknown faults. Most cases of intra-plate seismicity result from compression related to far-field plate boundary stresses. The April 2017 M W 6.5 earthquake in central Botswana, and subsequent events, occurred in a region with no previously known large earthquakes, occurred away from major present day tectonic activity, and accommodate extension rather than compression. Here, we present results from an integrated geophysical study that suggests the recent earthquakes may be a sign of future activity, controlled by the collocation of a weak upper mantle and weak crustal structure, between otherwise strong Precambrian blocks. Magnetotelluric data highlights Proterozoic continent accretion structure within the region, and shows that recent extension and seismicity occurred along ancient thrust faults within the crust. Our seismic velocity and resistivity models suggest a weak zone in the uppermost mantle, that does not persist to greater depths, and is therefore unlikely to represent mantle upwelling. The Botswana events may therefore be indicative of top-down extension as a response to large scale extensional forces. Highlights • Current intra-plate seismicity is facilitated by the inter-play between ancient tectonic structures and weak upper mantle. • We observe a strong low velocity anomaly in the upper mantle that we interpret as a remnant of previous deformation. • Lack of deep thermal anomaly suggests a top down process rather than thermal weakening from below. [ABSTRACT FROM AUTHOR]

Abstract In the evaluation of low- to medium-enthalpy geothermal resources on the island of Ireland, some of the most interesting targets are the deep sedimentary basins of Northern Ireland. The deepest of these is the Rathlin Basin, where Permian and Triassic reservoir sediments are known to exist to at least 2300 m depth. Two deep boreholes within the basin provide evidence of elevated temperatures at depth that are atypical within Ireland, prompting further geophysical exploration of the basin as one component of the IRETHERM project. The magnetotelluric (MT) method was selected as the investigative geophysical tool as it is capable of sensing and defining electrically conductive porous sediments beneath overlying resistive strata, in this case flood basalt sequences. MT data were acquired on a rectangular grid of 39 sites across almost half of the onshore basin to investigate the composition and spatial variation of the basin's formations. One-dimensional stochastic inverse modelling of the observed MT data was with a reversible-jump Markov chain Monte Carlo 1D inversion code, resulting in ensembles of models for each site. The use of model ensembles rather than single models avoids the pitfall of over-reliant interpretation on non-unique resistivity models, increasing the robustness of the interpretation. Interpreted models compare very favourably with nearby deep borehole records, and interpolation of the complete set of ensemble interpretations results in a conservative reservoir volume of approx. 32 km3 of combined Permian and Triassic sandstones beneath the MT survey. Based upon new, high quality temperature data available in the Ballinlea 1 borehole, an approximate estimation of thermal energy in place as a function of final reservoir temperature has been performed for the interpreted MT resistivity model volume. A final minimum temperature of 25 °C (being the temperature that comparable estimates have been made for adjacent geothermal prospects) results in a minimum estimated Indicated Geothermal Reserve (IGR) of 2.9 × 1018 J beneath the MT survey area. The modelling results suggest that exploitation of the maximum volume of sediments would occur for a final temperature of ≈55 °C. [ABSTRACT FROM AUTHOR]

The methods and results of magnetotelluric sounding in the range of 0.0001-1000 s and more with the use small portable magnetotelluric stations are considered. Data on the distribution of electrical conductivity at depths beneath the Avacha-Koryak zone of contemporary volcanism are obtained. The obtained data are interpreted using the modern techniques and methods. [ABSTRACT FROM AUTHOR]

We present results of the geoelectric studies in Bulgaria, carried out with the GEOMAG-02 equipment at the geophysical observatories of the Bulgarian Academy of Sciences. Compared with previous long-period electromagnetic investigations carried out with an analogue device by Russian and Bulgarian geophysicists in the 1980s the application of modern equipment and processing methods makes it possible to obtain interpretive options for the periods from a few seconds up to 3 hr and study the geoelectric structure at shallower depths. The recorded data together with the previous long-period data were analysed and the 1-D, 2-D and quasi-3-D inversion have been employed to model the conductivity distribution within the Earth's crust of Bulgaria. Some of the revealed conductivity anomalies seem to spatially coincide with the regional fault zones—Maritsa and Sub-Balkan. Joint interpretation of the 2-D and quasi-3-D inversion results suggests the relation of one of these conductivity anomalies near the PLN observatory to the Maritsa fault zone. The association of some of revealed conductivity anomalies with the fluid transport processes is supported by the fracture-type hydrothermal reservoirs widespread in the studied area south of the Sub-Balkan fault (and especially south of the Maritsa fault) and by the audiomagnetotelluric case study in the Bourgas geothermal basin in the eastern part of Bulgaria. A comparison of the distribution of conductivity anomalies, determined from the quasi-3-D inversion, and seismicity shows that most of the earthquakes occur outside the high conductivity areas. In seismoactive regions such effect may indicate possible association with metamorphic processes in the crust with the participation of fluids within the fractured media. [ABSTRACT FROM AUTHOR]

Abstract: Inversion of self‐potential data for source current density, js, in complex volcanic settings, yields hydrological information without the need for a prior groundwater flow model; js contains information about pH, pore saturation, and permeability, from which we infer the distribution of liquid and vapor phases. To understand the hydrothermal flow dynamics and hydraulic connectivity between surface thermal features at Mount Tongariro volcano, New Zealand, we undertook a reconnaissance scale self‐potential survey and developed an inversion routine for js, constrained by an existing 3‐D conductivity model from magnetotelluric measurements. The 3‐D distribution of js at Mount Tongariro reveals a discontinuous zero js zone interpreted as vapor or residually saturated pore space, surrounded by low to moderate js interpreted as circulating condensate liquid. Bounding faults act as conduits for down flowing groundwater or condensate, as well as barriers for the hydrothermal system. Localized small‐scale circulation associated with individual surface thermal features, rather than a single circulating system, accounts for the lack of widespread anomalous geochemical observations prior to the 2012 Te Maari eruption. [ABSTRACT FROM AUTHOR]

Magnetotelluric (MT) soundings were conducted in a 4 × 4 km area at the Aquistore CO 2 sequestration site at Estevan, Saskatchewan, Canada in 2013, 2014 and 2015, prior to CO 2 injection. The MT response is locally one-dimensional but spatially variable at short periods (<0.01 s), regionally one-dimensional at intermediate periods (0.01 s to 10 s), and two-dimensional at long periods (>30 s). Responses corresponding to Williston Basin rocks are uniform across the area. A representative MT response was inverted using constraints derived from a resistivity well-log. Beneath the Jurassic Watrous to Vanguard formations (at depths greater than 1240–1600 m), the resistivity of the inversion model is consistently 20–30% lower than in the reference well-log model, consistent with the MT method sensing the longitudinal resistivity of the more strongly stratified units at these depths. Electromagnetic noise in the MT data set includes high-frequency odd harmonics of the 60 Hz source and a broad-band source, spatially associated with the CO 2 pipeline, observed only in 2014. Within the period range 10 −4 to 10 2  s, but outside the bands influenced by the broad-band noise, the off-diagonal MT impedance response at Aquistore can be measured with an RMS repeatability of 1% or better. [ABSTRACT FROM AUTHOR]

ABSTRACT The electromagnetic response of a horizontal electric dipole transmitter in the presence of a conductive, layered earth is important in a number of geophysical applications, ranging from controlled-source audio-frequency magnetotellurics to borehole geophysics to marine electromagnetics. The problem has been thoroughly studied for more than a century, starting from a dipole resting on the surface of a half-space and subsequently advancing all the way to a transmitter buried within a stack of anisotropic layers. The solution is still relevant today. For example, it is useful for one-dimensional modelling and interpretation, as well as to provide background fields for two- and three-dimensional modelling methods such as integral equation or primary-secondary field formulations. This tutorial borrows elements from the many texts and papers on the topic and combines them into what we believe is a helpful guide to performing layered earth electromagnetic field calculations. It is not intended to replace any of the existing work on the subject. However, we have found that this combination of elements is particularly effective in teaching electromagnetic theory and providing a basis for algorithmic development. Readers will be able to calculate electric and magnetic fields at any point in or above the earth, produced by a transmitter at any location. As an illustrative example, we calculate the fields of a dipole buried in a multi-layered anisotropic earth to demonstrate how the theory that developed in this tutorial can be implemented in practice; we then use the example to examine the diffusion of volume charge density within anisotropic media-a rarely visualised process. The algorithm is internally validated by comparing the response of many thin layers with alternating high and low conductivity values to the theoretically equivalent (yet algorithmically simpler) anisotropic solution, as well as externally validated against an independent algorithm. [ABSTRACT FROM AUTHOR]