Reservoirs of interest for resource exploration, including geothermal and hydrocarbon reservoirs, commonly have an impermeable cap, which traps fluids below. Identifying this boundary is important for resource development. The cap rock for hydrocarbon reservoirs in southwest Iran contains evaporites and thus some geophysical exploration methods, specifically seismic reflection, have faced problems recovering subsurface information in this environment. As an alternative, we generate an electrical resistivity model from magnetotelluric (MT) data. Furthermore, we consider three-dimensional triaxial electrical anisotropy, which is rarely done. The study objectives are to a) define and map the boundary between the cap rock and the principal reservoir, b) characterize geological and tectonic formations in the area, and c) analyze the tectonic factors influencing the evolution of the region. A total of 359 MT measurements were acquired across the Sarab field in an array consisting of five profiles separated by >2000 m with a measurement spacing of >200 m. Transient electromagnetic (TEM) measurements were co-located with the MT measurements at 181 locations and used to correct for static shifts. Isotropic and anisotropic inversions of the MT data were performed, using all impedance tensor elements. The anisotropic electrical resistivity model exhibits both a significantly better alignment with the depths of geological formations known from drilling data and a better fit to the data. Therefore, the boundary between the primary cap rock and principal reservoir, the Gachsaran and Asmari formations, is defined and mapped across the survey area. In addition, major tectonic and fault-related features in the region are identified.

Volcán Uturuncu is a volcano located in the southwestern corner of Bolivia, near the borders with Chile and Argentina. It sits above the Andean subduction zone and is part of the Altiplano-Puna Volcanic Complex (APVC). Volcán Uturuncu is situated on top of the Altiplano-Puna Magma Body (APMB), which is currently the world's largest continental silicic partial melt reservoir. This reservoir is estimated to hold a total volume of 500,000 km3 of 20-30% partial melt and is located about 15 to 20 kilometers below sea level.

Volcán Uturuncu has not produced any eruption during the last 250,000 years, effectively making it an "extinct" volcano. However, the presence of active fumarole fields and the discovery of a consistent uplift pattern suggest that this volcano remains, up until this day, a dynamic system. Hence, numerous geophysical and geochemical surveys have been conducted during the past decades to understand the physical processes behind the recent unrest of this "zombie" volcano. Thay also aimed to shed light on the dynamics between the APMB and the near-surface volcanic-hydrothermal activity. Recent seismological studies worked on constraining the crustal stress distribution, by mapping the faults below Volcán Uturuncu and studying the seismic anisotropy distribution in the surrounding area. Findings from these studies reveal a complex network of fractures with a strong NW-SE-directed seismic attenuation and anisotropy, seeming to indicate the preferential pathway of fluids (Hudson et al. [2022, 2023]).

With this new information in mind, we aim to re-assess the previous electrical resistivity model of Volcán Uturuncu, which was obtained from isotropic inversion of magnetotellurics (MT) data by Comeau et al. [2016]. This model shows a pattern of low resistivity and high resistivity structures, which was interpreted as a series of magmatic dykes. However, this interpretation may overlook the inherent anisotropy of the system. Thus, we aim to generate electrical resistivity models allowing for isotropic and anisotropic zones and assess the results in the context of the newly available scientific data. We will also present preliminary results from the joint inversion of MT and gravity data. Such joint modeling allows us to delineate the density signature of the resistivity anomalies in the subsurface. This can help us in determining whether low resistivity structures represent either saline brines, partial melt or dense sulfide mineralization.

The Tibetan Plateau is undergoing east-west extension manifested by north-trending rifts. Rift dynamics have been attributed to both mantle convection, which induces vertical motion causing general extension, and plate convergence, with northward motion causing along-strike extension, driven by the subducted Indian slab. However, the cause of lithospheric extension remains debated. We carried out electrical resistivity modeling of the entire Tibetan Plateau and present a quantitative interpretation of low-resistivity structures in terms of high fluid fraction and low viscosity. The model reveals low-resistivity features intruding and overlying the resistive lithosphere of Lhasa and Qiangtang. The low-resistivity features show a transition from vertically oriented to horizontally oriented positions at ∼50−70 km depth and appear to be oriented north-south below the Himalaya and Lhasa and east-west below Qiangtang. The anomalies can be explained by partial melts and fluids and may represent the signatures of material migration and locally weakened lithosphere. This material migration must have been significant enough to sustain rifting and drive the rift tips northward, despite the complex tectonic setting of the Tibetan Plateau, which is composed of a number of independent blocks. The results suggest that north-trending rifts were formed in response to fluid flow, after or during lithospheric foundering below Lhasa. Furthermore, fluid flow can explain the surface distribution of rifts in bands and the variations in rift formation and development between Qiangtang and Lhasa, which are attributed to the local rheological differences and specific regimes of vertical and/or horizontal stresses that are induced by fluid migration.

Salt diapirs are of interest due to their unique properties that make them ideal for secure, long-term subsurface storage, including for CO ₂, natural gas, and radioactive waste. However, their utilization requires an understanding of their structure, which can be achieved with geophysical imaging. It is often a challenge to delineate salt diapirs with seismic reflection methods; therefore, we employ electromagnetic methods. We aim to a) highlight how magnetotellurics can identify the subsurface structure of salt diapirs, b) characterize the key tectonic structures and stratigraphic layers in the area, and c) investigate the role of faults on the distribution of diapirs. To do this we analyze an array of 253 magnetotelluric measurements and generate electrical resistivity models. The study area lies in the Shurab region, Central Iran, where numerous salt diapirs are observed near the surface. Overall, the models show a deformed southwestern zone and an undisturbed northeastern zone. Throughout the area, a thin (∼100 m) surface layer (1–100 Ωm) is underlain by a thick (up to 1000 m) low resistivity (<1 Ωm) layer, interpreted to be sediments of the Upper Red Formation. Below this is a higher resistivity (3–30 Ωm) layer that is complex and variable in depth and thickness, particularly in the southwest, where it shallows. This corresponds to the Lower Red Formation, which is the main salt layer and encompasses the diapirs. The electrical resistivity models successfully determine the locations, boundaries, and depths of salt diapirs within the area. Furthermore, they reveal that the salt diapirs are laterally extended along fault zones. This result provides valuable insights into the area's tectonic evolution and structural framework. Based on these subsurface images and geological information, we conclude that the tectonic activity along the Sen-Sen, Ab-Shirin, and Dehnar faults had a primary role in the formation of the salt diapirs.

Salt diapirs are prominent geological features, formed by the piercing of buoyant salt within overlying strata, with implications for basin evolution, tectonic deformation, and resource accumulation. In this study, we investigate the Shurab salt diapirs in northwestern Central Iran—an area with five known near-surface diapirs—whose subsurface geometries and interconnections at depth remain unclear due to the complex structural settings. To address these challenges, we generated a 3D electrical resistivity model from an array of 183 magnetotelluric (MT) measurements. Phase tensor and resistivity phase tensor analyses confirmed the presence of multidimensional conductivity structures. A range of modeling tests were performed to ensure a robust result, and final models were validated against seismic data and borehole logs, as well as previous 2D electric modeling. The resulting 3D resistivity model provides new insight into the geometry, depth, and interconnectedness of the salt diapirs and superior resolution of diapir flanks compared to seismic data. High resistivity zones at shallow depths correspond to dry salt, while lower resistivity at greater depths indicates brine-saturated regions. Notably, Diapirs No. 4 and 5 were found to be interconnected at depth, sharing a root zone and likely originating from a common evaporite layer. Tectonic analysis suggests that active fault systems—including the Sen-Sen, Ab-Shirin, and Dehnar faults—have played key roles in guiding salt migration and shaping diapir structures. This study highlights the effectiveness of using MT data to image complex salt structures and underscores the importance of integrated geophysical approaches in tectonically active regions.

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibet Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in central Tibet. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model, and combined with the spatiotemporal coupling of the geological, geochemical and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri Ms 6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly related to the detachment of the Indian lithospheric mantle from the Indian crust.

The capital of Mongolia is Ulaanbaatar (UB), which is situated in the central region of the country. Over the past few decades, the city has expanded and developed, establishing itself as the most extensively developed metropolis in Mongolia regarding infrastructure and commerce. While this impact has resulted in development for the nation, it has also led to environmental and social concerns, including traffic congestion and air pollution. The resolution of these issues necessitates a more comprehensive understanding of the geological formation of the region, which can be achieved through the sustainable development of renewable energy and road construction. UB is situated at the confluence of the Tuul River and is enveloped by mountains significantly higher than the surrounding terrain. One is the Bogd Uul intrusive/plutonic granite, situated south of Ulaanbaatar. It spans a 200 km2 area and is believed to have been formed during the Late Triassic (Khishigsuren et al., 2006, 2009) to the early Jurassic period, with an age of 208 Ma. One of the strategies to mitigate the challenges previously identified is to drill Bogd Uul for the purpose of constructing the road.

Mogod Soum, Bulgan aimag, is located in the eastern part of Khangai Dome. During the winter, the soum is heavily affected by air pollution due to coal burning. Using geothermal resources in the region, manifested by hot springs, could dramatically reduce air pollution. To understand the nature of the geothermal reservoir feeding the hot springs, we conducted Magnetotelluric surveys in the Mogod hot spring region during the fieldwork in 2020, 2021, 2022 and 2024. To obtain a subsurface electrical conductivity model of the hot spring area with magnetotellurics, we inverted data from 60 unique sites. As a tool for inversion, we used a high-order finite element code available to locally refined unstructured meshes to ensure numerical accuracy with a sufficiently fine discretization of the inversion domain while keeping the computational cost feasible. We inverted the full impedance tensor to recover a 3-D electrical conductivity model. The best-fitting model provides important new insights into the subsurface structure of the Mogod region.

Strong earthquakes have been mapped within the Ulaanbaatar region, Mongolia, near the capital city of Ulaanbaatar. From 1994 to 2016, 120 earthquake events were recorded between 3.4 and 5.6, and 978 earthquake events were recorded with a magnitude between 2.5 and 3.4 (Adiya, 2016; Al-Ashkar et al., 2022). Residents of Ulaanbaatar have felt several of these earthquakes.

Historical records dating back to 1905 show that Mongolia as a whole has experienced four major earthquakes with magnitudes larger than 8 and many moderate earthquakes with magnitudes larger than 5.5 (e.g., Adiya, 2016). However, the seismicity in Mongolia is mostly concentrated along the Mongolian-Altai and Gobi-Altai (south and west of the Khangai mountains), the Bulnay fault (north of the Khangai mountains), and around the Mogod area (east of the Khangai mountains) (e.g., Adiya, 2016), which are remote and sparsely populated areas. In contrast, the region around Ulaanbaatar is home to a large population; today, about 1.7 million inhabitants, or half of the country’s total population.

In the west of the Ulaanbaatar region, there are several prominent fault zones, some of which have only recently been identified. The majority of the seismic events in this region are related to three of these: the Khustai, Sharkhai, and Avdar fault zones (Adiya, 2016; Al-Ashkar et al., 2022). Seismicity is typically detected in the upper crust (above ~16 km depth; Ferry et al., 2010; Adiya, 2016). These fault zones are quasi-parallel and are ~100 km long (Figure 1). Historical seismic events are predicted to have produced vertical offsets of up to 10 m, with some sections showing cumulative horizontal offsets of up to 100 m (Al-Ashkar et al., 2022). Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes of magnitude 7+ (Ferry et al., 2010, 2012; Schlupp et al., 2013; Al-Ashkar et al., 2022).

These fault zones pose a serious threat and risk of damage to Ulaanbaatar. Because of this, we aim to characterize the subsurface structure of the active fault zones near Ulaanbaatar in order to better understand them. To do this, we measure magnetotelluric data and generate electrical resistivity models. We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, in addition to geodetic measurements, paleo-seismic trenching, fault mechanical models, and near-surface ground-penetrating radar surveys. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing seismic hazards.

Since the Cenozoic, a series of extensional south-north normal faults and gneiss-granite domes evolved in the southern Tibetan Plateau, the formation mechanism of which is of scientific interest and which has implications for the tectonic dynamics of the plateau. Typical of such features are the Xainza-Dinggye rift and the Mabja gneiss dome, which are located in the Xainza-Xietongmen-Dinggye region in the central Tibetan Plateau. In this study, Magnetotelluric measurements across this region are used to generate a high-resolution 3-D electrical resistivity model of the subsurface and to analyze the cause of the conductive zones. The large-scale conductive zones identified in the middle-lower crust may result from aqueous melt partial melting, whereas the smaller-scale conductive zones in the upper-middle crust may result from saline fluids, possibly with varying minor volumes of melts. Subsequently, based on the electrical resistivity model and combined with the spatiotemporal coupling of the geological, geochemical, and geophysical data, the state and migration features of crustal materials are discussed. The results show that the upwelling of mantle materials along subduction channels and slab-windows related to the tearing of the Indian lithospheric plate contributed to the partial melting of the middle-lower crust in the Lhasa terrane. Furthermore, partial melting of the upper-middle crust in the Tethys-Himalaya terrane resulted from southern extrusion of crustal materials in the Lhasa terrane. These two mechanisms can significantly reduce the effective viscosity. We speculate that the deformation of the brittle upper crust that is controlled by large-scale ductile layers characterized by weak rheology is the main dynamic mechanism of rift evolution. Meanwhile, the metamorphism and anatexis in the upper-middle crust of the Tethys-Himalaya terrane related to the southern extrusion of materials contributed to the evolution of the Mabja gneiss dome. During the middle Miocene, the southern extrusion of crustal materials may have been influenced by the cooling events beneath the Mabja gneiss dome, which can explain why the deep areas beneath the Mabja gneiss dome have middle-high resistivity. In addition, our study region is located in the Mediterranean-Himalayan seismic belt, and mainly includes shallow-focus earthquakes and intermediate-depth earthquakes. In the north, shallow-focus earthquakes are mainly controlled by the accumulation of stress in the brittle layer of the overlying crust related to the ductile layer of the middle and lower crust. In the south, shallow-focus earthquakes (e.g., Dingri MS6.8 earthquake) mainly occur in the rigid, resistive block, which is surrounded by conductive zones, possibly because fluid migration may be hindered by these resistive blocks. The intermediate-depth earthquakes are mainly controlled by the response in the subsurface area, which is related to the detachment of the Indian lithospheric mantle from the Indian crust.

The Qitianling pluton in southern Hunan, China, has spatially and genetically influenced the formation and distribution of a series of polymetallic deposits, including Xintianling, Baoshan, Huangshaping, and Furong. These deposits host a variety of tungsten- and tin-related deposits, often regarded as strategic and critical rare metals, and comprise one of the most prominent reserves globally. A thorough understanding of the structure of the Qitianling pluton is essential for insights into the development and evolution of the metallogenic system in southern Hunan. Working towards the goal of investigating regional structural features and magma emplacements model, we have generated three-dimensional (3-D) electrical resistivity models of the Qitianling pluton and its surrounding areas to upper-crustal depth using magnetotelluric (MT) data that range from 1000 Hz to 0.001 Hz. The results reveal that the upper-crust of southern Hunan is mainly characterized by high resistivity with multiple unique conductive zones. The high-resistivity anomalies (>1000 Ω·m) are interpreted to represent the Qitianling pluton. In addition, they correspond very well to a negative residual Bouguer gravity anomaly. Moreover, the morphology of the feature aligns with low-velocity obtained by modelling reflected seismic waves. Conductive anomalies (<30 Ω·m) near the sides of the pluton that extend through the upper crust likely indicate the presence of the Chenzhou-Linwu deep-seated fault system, which may have served as a pathway for the upward migration and emplacement of magma/hydrothermal fluids. Conductive features (<30 Ω·m) beneath the Qitianling pluton are inferred to represent ancient magma reservoirs where assimilation and mixing processes occurred before magma emplacement. Based on the geophysical models and the available geological data, a multi-stage magma emplacement model of the Qitianling pluton is proposed, which provides new insights into the W-Sn polymetallic mineralization system and the regional magmatic evolution within southern Hunan.

The whole-lithosphere architecture controls the genesis, evolution, and transport of ore-forming fluids. Transient tectonic and geodynamic processes, occurring at various spatial and temporal scales, control the structure of the lithosphere. However, there remains questions about the source mechanism for ore-forming fluids and their depth of genesis. Thus knowledge of the deep structural framework can advance understanding of the development and emplacement locations of mineral systems. Deep geophysical exploration studies carried out with this in mind may be crucially important for targeting new ore deposits in unexplored and underexplored regions.

As a case study, we investigate a gold-copper metal belt located at the margin of an Archean-Paleoproterozoic microcontinent in central Mongolia. We explore three-dimensional models of the electrical resistivity generated from a regional-scale array of magnetotelluric data. In addition, we examine models of shear-wave velocity throughout the lithosphere.

Directly beneath the metal belt, and the surface expressions of known mineral deposits and occurrences, the electrical resistivity model reveals narrow, vertical, finger-like low-resistivity features within the high-resistivity upper-middle crust, which are connected to a large low-resistivity zone in the lower crust. A broad low-resistivity zone is imaged in the lithospheric mantle. This is well aligned with a zone of low shear-wave velocity. We carry out a quantitative correlation analysis between electrical resistivity and shear-wave velocity and observe a close correlation within the zones of interest.

In the upper-middle crust, the low-resistivity signatures give evidence for ancient pathways of fluids below the metal belt constrained by structure along a tectonic boundary. In the lower lithosphere, the low-resistivity and low-velocity signatures are interpreted to represent a fossil fluid source region. We propose that these signatures are caused by a combination of factors. In particularly, factors related to refertilization and metasomatism of the lithospheric mantle by long-lived subduction at the craton margin, possibly including iron enrichment, F-rich phlogopite, and metallic sulfides, are analysed and discussed.

Continental intraplate volcanic systems, with their locations far from plate tectonic boundaries, are not well understood: the crustal and lithospheric mantle structure of these systems remain enigmatic and there is no consensus on the mechanisms that cause melt generation and ascent. The Cenozoic saw the development of numerous volcanic provinces on the African plate, including within the Central Hoggar, located in Northwest Africa, part of the Tuareg shield. The magmatic activity began at approximately 34 Ma and continued throughout the Quaternary. In order to understand the origins and potential mechanisms that generated the intraplate volcanic activity in the Central Hoggar we aim to image the subsurface architecture, in terms of electrical resistivity, from the surface to the lithospheric mantle. To do so we use magnetotelluric measurements from 40 locations to generate a 3-D electrical resistivity model, over an area of about 100 km by 160 km. Low-resistivity features (i.e., conductors) are observed in the crust that are narrow, linear structures congruent with the boundaries of terranes and prominent fault zones (e.g., Azrou N’Fad). They likely reflect the Pan-African mega-shear zones, which were reactivated throughout the tectonic evolution of the region. The model reveals that these faults are lithospheric-scale. The low-resistivity features likely represent the signatures of past fluid pathways and mineralization. A deeper low-resistivity feature is observed in the upper lithospheric mantle directly beneath the Manzaz and Atakor volcanic districts. It may represent local, small-scale metasomatism of the sub-continental lithospheric mantle, and low-percent partial melting, that sits above a regional, large-scale asthenospheric upwelling associated with the Hoggar swell. It is likely the origin point of the fluids responsible for the overlying anomalies. The results highlight the control of the lithospheric-scale, mega-shear zones on the spatial distribution of the recent Cenozoic volcanic activity, which was influenced by the location of pre-existing structural weaknesses.

The Mongol-Okhotsk suture and the Adaatsag ophiolite belt are associated with the closure of the Mongol-Okhotsk paleo-ocean and are located within the Central Asian Orogenic Belt (CAOB) and Mongolia. The suture zone is flanked by volcanic-plutonic belts that host significant metallogenic zones, containing deposits of copper and gold. The tectonic evolution of this region is not fully understood and the lithospheric structure has been poorly studied. We analyze magnetotelluric data and generate a model of the electrical resistivity distribution across this region. Whereas the northern segment has a sharp transition from a high-resistivity upper crust to a low-resistivity lower crust, as observed beneath the Hangai Dome, the southern segment does not show this transition. A wide, low-resistivity zone (1–100 Ωm) imaged in the crust and lithospheric mantle is coincident with the Mongol-Okhotsk suture and ophiolite, revealing a clear and significant lithospheric-scale feature. Across the profile, numerous narrow, vertically oriented, low-resistivity features (1–100 Ωm) are spatially associated remarkably well with the proposed boundaries of tectonic domains. These results confirm ideas about the development of the CAOB. Some of these low-resistivity features are beneath the surface locations of large mineral zones, and likely represent fossil fluid pathways. We show congruent seismic velocity models for comparison and the results show a large-scale low-velocity anomaly (decrease of 2%–3%) that correlates with the location of the low-resistivity anomaly below the Mongol-Okhotsk suture. The geophysical results, combined with geological and geochemical data, provide insights into the structure of this region and help shed light on unanswered questions.

Continental intraplate volcanic systems, with their location far from plate tectonic boundaries, are not well understood: the crustal and lithospheric mantle structure of these systems remain enigmatic and there is no consensus on the mechanisms that cause melt generation and ascent. The Cenozoic saw the development of numerous volcanic provinces on the African plate. This includes the Hoggar volcanic province, located in Northwest Africa, part of the Tuareg shield. It is composed of several massifs with contrasting ages and eruptive styles. The magmatic activity began at around 34 Ma and continued throughout the Neogene-Quaternary. Phonolite and trachyte domes as well as scoria cones and necks are found in the Manzaz and Atakor volcanic districts. In order to image the crustal and lithospheric mantle structure of this region, and to understand the origins and potential mechanisms of the continental intraplate volcanic activity in the Central Hoggar and specifically the Atakor/Manzaz area, we acquired magnetotelluric (MT) measurements from 40 locations and generated a 3-D electrical resistivity model. The model covers an area of about 100 km by 200 km. Images of the subsurface architecture, in terms of electrical resistivity, from the near-surface to the lithospheric mantle, allow us image the deep plumbing system of the volcanic system. Low resistivity features (i.e., conductors) in the crust that are narrow, linear structures trending approximately north-south, are revealed along the two boundaries of the Azrou N’Fad terrane, in the Manzaz area. They likely reflect the Pan-African mega-shear zones, which were reactivated throughout the tectonic evolution of the region. The model reveals that these faults are lithospheric-scale. In addition, the low-resistivity features likely represent the signatures of past fluid flow. The location of the recent Cenozoic volcanic activity was likely influenced by the pre-existing structure. A deep feature of moderate conductivity is located in the upper lithospheric mantle directly beneath the Manzaz and Atakor Volcanic Districts. It may represent the origin of the overlying anomalies and may suggest metasomatism of the sub-continental lithospheric mantle.

New evidence worldwide has linked the surface locations of mineral deposits and their crustal-scale electrical conductivity footprint. We examine the relationship between the Gangdese Miocene porphyry copper deposits, Tibetan Plateau, and the electrical conductivity signature from a three-dimensional model generated from 311 magnetotelluric measurements. The distribution of electrical resistivity throughout the crust and the conductance within the mid-lower crust (depth range of 25–70 km) is analyzed. The results clearly show that the large and ultra-large Miocene porphyry copper deposits coincide spatially with conductive zones and areas of very-high conductance (>10,000S) in the mid-lower crust. Computations are undertaken to determine the influence of water-bearing silicate melts and alkali-bearing (Na+ and K+) fluids on conductivity. Based on this, the bulk conductivity is interpretated to be caused by a system of alkali-rich volatile-rich partial melt. The alkali-rich volatile-rich magmatic-hydrothermal fluids facilitate the migration and concentration of metal ions originating in deep areas. The volumes necessary are much less than partial melt alone and can thus help to reconcile large conductivity variations with small seismic velocity variations. The electrical structure indicates the magma source area of anatexis in the lower crust, a multi-stage magmatic system with large mid-crustal and small upper-crustal magma reservoirs, and complex pathways related to rift zones. We determine that the conductive zones in the mid-lower crust have an influence on the development of the mineralization and the location of the mineral belt.

We carried out long-term measurements of the electric potential in the laboratory and in the field and assessed the stability and temperature sensitivity of the recordings.

In the field, a robust design for long-term telluric recordings including a redundant parallel dipole so that consecutive stable time windows are more likely to be recorded was implemented in the Sauerland region of Germany (more than three months). Field testing is complicated by the fact that the system is no longer in isolation. However, the stability of the electric potential measured in the laboratory was a reasonable predictor of the stability of electric potential measured in the field. Nevertheless, instabilities in the form of spikes in the potential, steps, and spontaneous jumps (on the order of 1 mV) of unknown origin were observed.

The field measurements included a temperature-logging device. The temperature was monitored at two locations: a) the bottom-hole temperature at a depth of 80 cm below the surface, where the electrode was planted, and b) the top-hole temperature at a depth of 5 cm below the surface. The recorded temperatures in the electrode hole can be compared to the air temperature (as recorded in the nearby village). The results clearly show that planting the electrode deeper avoids the daily variations of temperature, which, in this case, were appreciable (up to 7°C), and which can affect the electric potential recordings. The bottom-hole temperature variation follows the long-term seasonal trend (e.g., 1–2°C/10 days), but is insensitive to short-term variations. Furthermore, installing electrodes at such depths can insulate them and avoid problems associated with the temperature going below the freezing point.

In order to attain good quality transfer function estimates from magnetotelluric field data (i.e., smooth behavior and small uncertainties across all frequencies), we compare time series data processing with and without a multitaper approach for spectral estimation. There are several common ways to increase the reliability of the Fourier spectral estimation from experimental (noisy) data; for example to subdivide the experimental time series into segments, taper these segments (using single taper), perform the Fourier transform of the individual segments, and average the resulting spectra. To further reduce the bias of spectral estimation, a multitaper approach can be adopted. In this approach, a number of orthogonal taper functions are used to generate independent estimates, from the same time segment, which are subsequently averaged. We apply multitaper spectral analysis to magnetotelluric time series data and we show examples of responses from field data. The results clearly show that this approach improves the transfer function responses, often significantly, particularly at the long periods.

Within the Ulaanbaatar region, Mongolia, 120 earthquake events were recorded with a magnitude between 3.4 and 5.6 and 978 earthquake events had a magnitude between 2.5 and 3.4, for the period from 1994 to 2016. Several of these have been strongly felt by residents of Ulaanbaatar. Historical records, since 1905, show that Mongolia as a whole has experienced four major earthquakes with magnitudes larger than 8 and many moderate earthquakes with magnitudes larger than 5.5. The seismicity in Mongolia is concentrated along the Mongolian-Altai and Gobi-Altai, the Bulnay (north of the Khangai mountains), and around Mogod (east of the Khangai mountains).

In the west of the Ulaanbaatar region there are several prominent fault zones, some only identified very recently. The majority of the seismic events in this region are related to the Khustai, Sharkhai, and Avdar fault zones. Seismicity is typically detected in the upper ~16 km of the crust. These fault zones are 100+ km long and historical events are predicted to have produced vertical offsets of up to 10 m; some sections show a cumulative horizontal offset of up to 100 m. Based on paleo-seismic surveys, it is estimated that these fault zones could produce earthquakes up to magnitude 7. Therefore, these faults pose a serious threat and risk of damage to Ulaanbaatar.

In this presentation we aim to characterize the active fault zones near Ulaanbaatar with electrical resistivity models generated from magnetotelluric data. In mid-2024 we carried out measurements across the Khustai, Sharkhai, and Avdar fault zones and modeled the local features near the fault traces and the regional crustal features of the region. Preliminary models show several low-resistivity features (approximately <100 Ωm) in the near-surface. The upper crust (0-25 km depth) appears to have a generally high-resistivity (~10,000 Ωm), whereas the lower crust (25–50 km depth) appears to have a lower resistivity (approximately <100 Ωm).

We aim to give an integrated interpretation of the electrical conductivity structure of the subsurface with geomorphological and geological knowledge, geodetic measurements, paleo-seismic trenching, and near-surface ground-penetrating radar surveys. We also aim to discuss the relation with fault mechanical models and local fault damage zones, and the relevance of the low slip rate. Understanding the subsurface structure of the region and characterizing the active faults is an important step for assessing the seismic hazard.

Three-dimensional (3-D) modelling of magnetotelluric (MT) data is standard practice nowadays, with various 3D inverse solvers being available for commercial and scientific usage. Three approaches are commonly used to numerically solve Maxwell’s equations in practice: finite-differences, finite elements, and volume integral equations. Most standard forward and inverse MT solvers/approaches have been benchmarked against each other and tested on several synthetic data/models. However, there has been few comparisons of the electrical conductivity models recovered by different solvers from real datasets.

In this presentation, we tackle this issue by generating inverse models from MT impedances taken from a subset of a large regional array in Central Mongolia using different codes: MODEM, based on finite differences; GEMMIE, based on integral equations; and FEMALY, a solver based on finite elements. In addition, we compare the recovered models with a published model, which was obtained by the finite elements code GOFEM. We will discuss the obtained models considering the underlying fundamentals of each method, the different inversion strategies, and the corresponding inversion parameters used, such as mesh discretization and regularization.