Malmberget is located in the northern Norrbotten ore province, Sweden, and hosts an iron oxide–apatite deposit of Kiruna type. The area is actively mined by the state-owned LKAB company, making it an electromagnetically noisy environment. New broadband magnetotelluric data have been collected and analysed, and the 3D electrical conductivity structure has been developed to improve the understanding of, and connection between, the local deposit and more regional structures. To mitigate the influence of noisy data on the inversion results, Huber’s loss function has been introduced and implemented based on an iteratively reweighted least squares scheme. The resulting robust 3D model shares major features such as the mine location and regional anomaly model acquired by standard L2 optimisation but shows differences on a local scale. Sites which received lower weights are generally identified as potentially noisy measurements due to the proximity to sources of anthropogenic noise, emphasising the ability of the scheme to quantitatively identify and deal with measurements contaminated by noise and improving overall data fit. Three main geological conclusions are gained from the 3D resistivity model. Firstly, a large east–west striking conductive anomaly at ~3−4 km depth is present in a large part of the model area. Secondly, three narrow, vertical, conductive structures connect the known mineralisation to the deep conductive anomaly. Thirdly, a second, deep conductive anomaly striking east–west in the south connects to the surface close to the Dundret ultramafic intrusion. Conductivity anomalies in the area are hypothesised to originate from a combination of magnetite mineralisation when close to the surface and iron-rich mafic rocks at depth, with possible presence of sulphides and/or graphite.

Conventional transient electromagnetic (TEM) surveys primarily measure two types of responses: the time derivative of the magnetic induction (db/dt) and the electric field (e). Inversion techniques for these data are well established. In contrast, the magnetic induction (b) response, measurable with low-noise total-field magnetometers (e.g., SQUIDs or fluxgate sensors), remains underutilized in practice, and robust three-dimensional (3-D) inversion methods for its quantitative interpretation are still lacking. Moreover, non-ideal transmitter wave-forms significantly impact all three TEM responses (e, db/dt, and b), and forward modeling and inversion must incorporate these effects properly. To address these issues, this study presents 3-D TEM forward modeling and inversion algorithms that compute responses directly in the time domain with a second-order backward Euler scheme. Waveform effects are accurately integrated via a convolution-based approach, which relaxes time-step constraints and enhances computational efficiency. The inverse problem is solved with the L-BFGS optimization method. By leveraging the convolution approach, we formulate sensitivity computations for e, db/dt, and particularly b responses, fully incorporating wave-form effects. The algorithms are validated through numerical examples and field-data inversion. Sensitivity analyses reveal that b data exhibit a broader spatial sensitivity range than db/dt data, suggesting a greater potential for imaging deeper geological targets. For both db/dt and b data, sensitivity decreases markedly beneath the transmitter loop center, hindering target reconstruction in that region. This work advances the practical applicability of TEM inversion by enabling integrated handling of diverse TEM data types under realistic waveform conditions.

Much of the Norwegian upper crust was shaped by Caledonian thrusting and post-orogenic extension. We investigate the electrical conductivity structure of the crust and associated thrusts and detachments using magnetotellurics to obtain a better understanding Paleozoic tectonics. New magnetotelluric data were collected over an area of c. 60 × 70 km that includes the Gudbrandsdalen Antiform and Atnsjø tectonic window in central Norway.The final 3D electrical conductivity model reveals a highly resistive crustal block, extending E–W along the Gudbrandsdalen Antiform and from the surface to several tens of kilometres depths, imaging the parautochthonous Precambrian basement. No major thrust or detachment has been mapped under the Atnsjø tectonic window, which is therefore considered to most likely represent autochthonous or parautochthonous Baltica basement. Several conductors are imaged along the edge of the magnetotelluric survey, which can be correlated with a major Caledonian thrust and structures in the Precambrian basement.

Integrated earth modelling aims to combine all available geoscientific information to derive one common earth model. However, combining multiple parameters in a meaningful way remains challenging. In this study, we aim to improve mineral exploration workflows through the integration of new magnetotelluric data with other geophysical and petrophysical data to enhance local geological understanding of the northern Kiruna mining district, Norrbotten, Sweden. The area is economically important and geoscientifically interesting. We derived three-dimensional (3D) geophysical models based on new magnetotelluric data and previously collected gravity and magnetic data. The magnetotelluric data and the resulting 3D electrical-resistivity model are described in detail. Additionally, a new petrophysical dataset containing surface and subsurface samples and their density, electrical resistivity and magnetic susceptibility is presented and linked to the 3D geophysical models. The geophysical 3D models are interpreted using information gained from the measured rock properties, allowing for correlation and cross-validation between models and geology. Integration of a clustering approach into the mineral exploration workflow allows for a holistic interpretation of all information available in the area. These interpretations and models improve understanding of the Per Geijer mineral system, its immediate surroundings and the relationships between local rock types and their geophysical and mineralogical signatures. 

One common phenomenon appears in the magnetotelluric models across the Western Carpathian Alpine, Bohemian Hercynian and Scandinavian Caledonian orogenic zones – significant deep subvertical orogen parallel conductive zones in the crust. On the basis of knowledge from the Western Carpathians, we interpret them as shear zones that were formed after the end of oceanic subduction and the subsequent collision between the continental blocks by the transition of the collisional tectonics to transpression. The deep shear zone allowed mineralized fluids and CO₂, which formed a significant conductivity anomaly, to escape from the mantle. The occurrence of such large-scale orogen parallel shear zones in the investigated orogens points to the similarity of the kinematic regime in the orogens from different ages, at least in the Phanerozoic.

Magnetotelluric (MT) survey results from the Late Archean Manica greenstone belt, an extension of the Odzi-Mutare greenstone belt of the Zimbabwe Craton, are presented. A total of 33 MT stations were acquired on an irregular grid with an average station spacing of approximately 5 km. This data set represents the first MT survey in Mozambique. The results from the 3-D modeling indicate a conductive mid-crustal structure in the central part of the surveyed greenstone belt and the presence of narrow sub-vertical conductive structures connecting the mid-crustal conductor with shallow structures. These sub-vertical conductive structures are tentatively interpreted as marking the location of fluid pathways for mineralizing fluids associated with gold occurrences. The modeled mid-crustal conductor mapped in the Manica greenstone belt does not have a western continuation toward the adjacent Odzi-Mutare greenstone belt in Zimbabwe.

The northern Norrbotten ore province, situated within the Fennoscandian Shield, constitutes a major metallogenic domain in Europe and hosts more than 40 iron oxide-apatite (IOA) deposits, including the Kiirunavaara deposit, the type locality for this deposit type. Although extensive mapping, geochemical, geochronological, and structural studies have refined the conceptual understanding of IOA systems at deposit and district scales, a fully integrated three-dimensional framework that synthesizes this knowledge with regional geological and geophysical datasets has not yet been developed. This study presents an integrated 3D model of the northern Norrbotten ore province that incorporates lithological, structural, and petrophysical constraints with inversions of magnetic and gravity data, as well as available magnetotelluric conductivity models. The geological models delineate the architecture of intrusive suites, supracrustal belts, crustal-scale shear zones, and subordinate brittle–ductile second-order structures, implemented in a voxel-based domain for quantitative spatial integration. Mafic to ultramafic intrusions previously linked locally to IOA genesis are consistently coincident with high-density, magnetically anomalous domains and spatially associated with vertically extensive electrical conductors. These features are interpreted as the upper-crustal expression of a deeper transcrustal conduit system that facilitated magma ascent, fluid migration, and heat transfer. IOA deposits are located within second-order oblique structures intersecting or splaying from first-order structures, and their geometry and extent correlate with diagnostic magnetic signatures characterized by steep gradients, localized highs, and adjacent negative side lobes. These findings refine the metallogenic framework for IOA mineralization in the northern Norrbotten ore province and provide a robust 3D geoscientific basis for testing mineral system hypotheses.

The Woodfjorden area in northwestern Svalbard hosts the world's northernmost onshore thermal springs, extinct Quaternary volcanoes (the Bockfjorden Volcanic Complex), late Miocene lava flows (the Seidfjellet Formation) that overlie a Devonian sedimentary basin, and a long-lived regional fault zone. To contribute to a better understanding of subsurface fluid flow in this volcanic region, we conducted a 3D magnetotelluric (MT) survey at 12 sites, combined with water and gas sampling at the thermal spring clusters (Trollkjeldene and Jotunkjeldene). While the MT survey has a limited aperture due to the remoteness of the region and complex logistics, the identified structural features put several constraints on the evolution of this poorly studied hydrothermal system. The electrically conductive zones (1–10 Ωm), under Trollkjeldene and the Halvdanpiggen volcanic center, are interpreted as signatures of deep hydrothermal fluids or mineralized zones; the resistive anomalies (1–10 kΩm) at 1–4 km depth under the Sverrefjllet volcano indicate buried magmatic or basement rocks. Stable isotope data indicate both meteoric and crustal components for thermal waters. Carbon and noble gas isotope ratios support a mixed gas source involving crustal, atmospheric, and mantle components. Helium isotope ratios suggest a mantle-derived fraction of 6%–8%. These findings suggest extinct magmatic activity and ongoing hydrothermal circulation, likely prolonged due to fluid compartmentalization at depth by low-permeability magmatic or metamorphic rocks.

The Late Cretaceous obduction of the Semail Ophiolite onto the rifted passive Arabian margin and the Cenozoic collisional tectonics with the final closure of the Neo-Tethys Ocean, are major contributors to the present-day crustal architecture of the northern United Arab Emirates (UAE). We acquired the first 3D grid magnetotelluric observations from 73 stations in the northern UAE in order to image deep crustal electrical structures associated with the two significant compressional episodes. Inversion of the broadband magnetotelluric data reveals the subsurface geometry of the high-resistivity ophiolite blocks, the underlying conductive thrust sheet (Haybi-Hawasina nappe), a “wedge-shaped” conductive foreland basin flanking the Hajar mountain ranges, and at depth, a high-resistivity structure associated with a fold-thrust belt adjacent to the allochthonous units, or Proterozoic crystalline basement. The ophiolite along the eastern coast (from cities Khor Fakkan to Fujairah) is more than 20 km thick and dips eastward. Across the Wadi Ham fault, the resistive ophiolite lies against less resistive materials that may represent Bani Hamid metamorphic rocks. The thin-skinned thrust sheets of the proximal-distal Tethyan sedimentary units (Haybi-Hawasina complexes) exhibit low resistivity in the Dibba zone. In contrast to its gently east-dipping geometry in the Dibba zone, the conductive Haybi-Hawasina structure in the southern portion of the study area appears nearly vertically (>20 km) beneath the dense Khor Fakkan and Aswad ophiolite blocks. This result suggests extensive deformation of the thrust sheets beneath the southerly dense ophiolite blocks.