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The Carpathian belt is one of Europe's major metallogenic provinces, where magmatic ore mineralization is associated with the past subduction environment. The upper crust is mapped for the first time in the Northeast Carpathian Volcanic Arc using magnetotelluric data inversion. The obtained 3-D electrical resistivity model is interpreted in conjunction with geological information and magnetic anomaly data. The model illustrates the deep magmatic plumbing system including kilometer-scale plutonic bodies at a depth of 2–7 km. The model implies that the transport of magma and fluids in the uppermost crust was controlled by pre-existing faults and décollement horizons. Present ore mineralization, mined since historical times, can be attributed to an electrically conductive conduit that is mapped from the surface to a depth of about 30 km. It is suggested that this conduit connected a shallow magmatic chamber to a deep source region in the southeast during late Miocene time. An observed northwest deflection of the deep magmatic conduit at a depth of more than 10 km may explain the spatial gap in the distribution of the Miocene volcanic activity along the Eastern Carpathians., Validerad;2024;Nivå 2;2024-08-08 (hanlid);Funder: Norway Grants 2014‐2021 (12/2020);Full text license: CC BY-NC-ND;Part of special issue: Solid Earth Geophysics as a Means to Address Issues of Global Change

Magnetotelluric data are sometimes accompanied by ‘anomalous’ impedance phases (⁠xy and yx) in the off-diagonal components deviating from the first (0° < xy < 90°) or third (−180° < yx < −90°) quadrant, especially in long-period bands. This phenomenon is called the phases out-of-quadrant (POQ). The POQ poses a challenge in magnetotelluric modelling because simple 1-D or 2-D models cannot explain it. Previous studies have reported that strong inhomogeneity, anisotropy, or particular 3-D structures, such as the L-shaped or cross-shaped conductors, could explain the POQ. Aside from these models, we have discovered that a slanted columnar conductor also generates the POQ. Our systematic investigation through the synthetic forward modelling of an inclined conductive column with a varying geometry showed that the inclination angle and the column length may affect the POQ appearance. We investigated herein the behaviour of the electric currents around the inclined conductive column embedded in a resistive half-space. We found that the induced electric field in the region with the POQ tends to point in the opposite direction to the surrounding vectors. This result can reasonably explain the inverted phase in long-period bands. Furthermore, we confirmed that current is sucked into one end of the column, but discharged from the other end, suggesting that the column works as a current channel. The localized reverse vectors are associated with the current channelling along the inclined conductor, which generates the POQ. A volcanic conduit within a resistive host rock is one of the typical field examples of such an inclined channel. Our study suggests that the POQ is a helpful clue in imaging the geometry of a volcanic magma plumbing system through magnetotelluric surveys.

The Northern Norrbotten ore district hosts a multitude of Sweden’s mineral deposits including world class deposits such as the Malmberget and Kirunavaara Iron oxide apatite deposits, the Aitik Iron oxide copper gold deposit, and a multitude of smaller deposits. Northern Norrbotten has been shaped by tectonothermal events related to the evolution of the Fennoscandian Shield and is a geologically complex environment. Without extensive rock outcropping and with most drilling localized to known deposits the regional to local scale of mineralization is not fully understood. To better understand the evolution and extent of the mineralization’s cross-disciplinary geosciences must be applied, where geophysical methods allow for interpretations of the deep and non-outcropping subsurface. Common earth modelling is a term describing a joint model derived from all available geoscientific data in an area, where geophysical models provide the framework.This study describes the geophysical modeling of two IOA deposits in Norrbotten, the Malmberget deposit in Gällivare and the Per-Geijer deposit in Kiruna. To better put these two deposits into a semi-regional setting magnetotelluric (MT) measurements have been conducted together with LKAB. LTU and LKAB have measured more than 200 MT stations in the two areas from 2016-2023. These measurements have then been robustly processed into magnetic transfer functions (impedances) for the broadband MT frequency spectrum (1000Hz,1000s). Then, all processed data judged to be of sufficient quality have been used for 3D inversion modelling using the ModEM code. The resulting conductivity/resistivity models reveals the local conductivity structure of the area, believed to be closely tied to the mineralization due to the conductive properties of the iron bearing minerals. Both areas yielded believable models which pinpointed known mineralization’s at surface as conductive anomalies and their connections to deeper regional anomalies.During modellin

Forward modelling of electromagnetic induction data simulates the electric and magnetic fields within a computational domain for a given distribution of electromagnetic material properties and a given source of the electromagnetic field. The quantities of interest are the fields at receiver locations at the Earth's surface. Reliable results require high accuracy solutions at the receivers. First and foremost, numerical computations need to be accurate, but ideally they are also resource efficient, i.e., as fast and cheap as possible. Run time and memory demand mainly depend on the size of the numerical problem to be solved. This thesis addresses specific steps within the forward modelling procedure of electromagnetic induction data in order to improve the solution accuracy of forward modelling as well as to reduce computational resources. The solution accuracy is strongly influenced by the spatial discretisation of the computational domain, which directly correlates with the numerical problem size. To optimise the solution accuracy while keeping the numerical problem size as small as possible, a goal-oriented adaptive mesh refinement scheme for three-dimensional controlled-source electromagnetic models is developed. In addition, this thesis investigates the influence of different types of boundary methods on the solution accuracy at the receivers. To replace inhomogeneous boundary conditions in magnetotelluric total-field modelling by perfectly-matched layers (PML), a domain decomposition approach (the total and scattered field decomposition) is adapted for Earth models. By reducing boundary effects, the approach yields superior solution accuracy for specific types of models. The fastest and most memory-efficient way to solve large numerical problems are iterative solution methods. Iterative solvers, however, work poorly for numerical systems arising from domains bounded by PML. In this thesis a preconditioned iterative solution framework that efficiently solves PM

The electrical properties of the boundary beneath the Deccan Volcanic Province and the western Dharwar craton are imaged by using the magnetotelluric method. The magnetotelluric study was carried out along a 150km long WNW-ESE profile from Belgaum (in the Deccan Volcanic Province) to Haveri (in the western Dharwar craton).Data from 19 magnetotelluric stations spaced 10-15km apart were used. The dominant regional geo-electric strike direction obtained is N20ºE. Two-dimensional (2-D) inversion is done by using the non-linear conjugate gradient scheme for both apparent resistivity and phase. The 2-D resistivity model shows a high electrical resistivity character (>10,000ohm-m) in the western Dharwar craton. Two conductive anomalies are mapped in the crustal region. In the WNW side of the profile, a conductive feature (~200ohm-m) is imaged in the mid-lower crust and, in the central part of the profile another conductive feature is mapped in the lower crust. The conductive features robustness is tested using linear and non-linear sensitivity analyses. The conductor mapped in the WNW part of the profile is considered as a deep seated fault representing a boundary or a rift related feature beneath the Deccan Volcanic Province and the western Dharwar craton. A zone of enhanced conductivity (<50ohm-m) at an approximate depth of 10-30km may represent the presence of the rift in the region. This conducting feature on the Western side of the E-W trending Kaladgi Basin can be interpreted as the extension of the Kaladgi Basin further west. A well-correlated geological cross-section is also derived to interpret the resistive features mapped in this study. The electrical resistivity nature of the crust is compared with other regions of the world.

In 2-D magnetotelluric modelling, the standard application of Dirichlet boundary conditions (BC) may severely diminish the solution accuracy, because the unknown scattered part of the electromagnetic field is erroneously reflected at the domain boundary. Therefore, we adapt the total and scattered field decomposition (TSFD) to geophysical modelling, enabling the application of fully absorbing boundary methods, here perfectly matched layers (PML), to the scattered field. Our novel TSFD divides the modelling domain into two regions. In the total-field region containing the area of interest, the solution is computed for the total field. In the scattered-field region containing the boundaries, the solution is obtained for the scattered field, which is fully absorbed by PML at the boundaries. The plane-wave source is excited at the TSFD interface between both regions. Thus, boundary reflections are eradicated leading to superior solution accuracy, and boundaries can be placed closer to the receivers, shrinking the computational problem. Especially for challenging models with strong lateral changes, the solution accuracy of the TSFD is superior to that of the standard Dirichlet approach. Owing to the linearity of Maxwell's equations, the inaccuracy introduced to the electric and magnetic fields by using Dirichlet BC can be expected to partly cancel out in the magnetotelluric transfer functions, for example the impedance tensor. In this work, we quantify this cancellation effect. The inaccuracy is less than typical measurement errors in the vast majority of apparent resistivity and phase data, even, when the primary fields are strongly inaccurate., eSSENCE - An eScience Collaboration

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.

Rosenqvist and Hall (2019), developed a proof-of-concept modeling capability that incorporates a detailed 3D structure of Earth's electrical conductivity in a geomagnetically induced current estimation procedure (GIC-SMAP). The model was verified based on GIC measurements in northern Sweden. The study showed that southern Sweden is exposed to stronger electric fields due to a combined effect of low crustal conductivity and the influence of the surrounding coast. This study aims at further verifying the model in this region. GIC measurements on a power line at the west coast of southern Sweden are utilized. The location of the transmission line was selected to include coast effects at the ocean-land interface to investigate the importance of using 3D induction modeling methods. The model is used to quantify the hazard of severe GICs in this particular transmission line by using historic recordings of strong geomagnetic disturbances. To quantify a worst-case scenario GICs are calculated from modeled magnetic disturbances by the Space Weather Modeling Framework based on estimates for an idealized extreme interplanetary coronal mass ejection. The observed and estimated GIC based on the 3D GIC-SMAP procedure in the transmission line in southern Sweden are in good agreement. In contrast, 1D methods underestimate GICs by about 50%. The estimated GICs in the studied transmission line exceed 100 A for one of 14 historical geomagnetic storm intervals. The peak GIC during the sudden impulse phase of a "perfect" storm exceeds 300 A but depends on the locality of the station as the interplanetary magnetic cloud hits Earth.

In this work two ways of processing controlled-source magnetotelluric (MT) data were tried and compared against each other. The aim was to evaluate the differences between a multivariate-based processing method to a bivariate processing method. The software KMSProTF represents conventional processing using bivariate and robust statistics. MsDEMPCA utilizes a multi-variate Criss-Cross regression scheme to improve the condition of the data-matrix before robustly decomposing it into principal components. Data from the FENICS-19 survey in northern Scandinavia was processed to transfer functions (TF) using the respective method. The TFs were visually interpreted in KMSProTF. There were no significant differences found between the methods. In addition a calibration between instruments was carried out, which caused an exclusion of parts of the data-set.

The Lorca earthquake (May 11th, 2011, Mw 5.2) stands as the most destructive one in Spain over the last 50 years, interpreted as having occurred in an intersegment zone of the strike–slip Alhama de Murcia Fault (AMF) (Eastern Betics, Spain). Magnetotelluric data were acquired along a profile to the SW of Lorca (La Torrecilla profile), to characterize its signature at depth, as part of the multidisciplinary project “INTERGEOSIMA”. Given the short distance between stations, some station pairs were recorded simultaneously, with magnetic sensors in only one of them. In order to properly understand the resulting impedances (called interstation impedances), and the effects of inverting them, we used synthetic models to compare the impedances and the interstation impedances and to analyze the corresponding inversion results, together with the inversion of the quasi-impedance (inversion of the interstation impedances, considering them as impedances). The results are sensitive to the location of the magnetic sensors and the resistivity underneath, but in general the use of the quasi-impedances in the inversion can be considered a valid procedure. Both the 2D and the 3D resistivity models obtained through the inversion allowed us to complement the previous ERT models and represent the continuation of the main fault gouge in depth showing its extension towards the SE.

Geomagnetic perturbations (B-GEO) related to magnetospheric ultralow frequency (ULF) waves induce electric fields within the conductive Earth-geoelectric fields (E-GEO)-that in turn drive geomagnetically induced currents. Though numerous past studies have examined ULF waveB(GEO)from a space weather perspective, few studies have linked ULF waves withE(GEO). Using recently available magnetotelluric impedance andE(GEO)measurements in the contiguous United States, we explore the relationship between ULF waves andE(GEO). We use satellite, ground-based radar,B-GEO, andE(GEO)measurements in a case study of a plasmaspheric virtual resonance (PVR), demonstrating that the PVRE(GEO)has significant spatial variation in contrast to a relatively uniformB(GEO), consistent with spatially varying Earth conductivity. We further show ULF waveE(GEO)measurements during two moderate storms of similar to 1 V/km. We use both results to highlight the need for more research characterizing ULF waveE(GEO).

The subsurface geological conditions of a geothermal system are vital objects to be considered in geothermal exploration. The Magnetotellurics survey was conducted to explore for geothermal potential in Pantar Island. This is to achieve deeper penetration compared to our previous study that adopted resistivity method to determine reservoir zones based on rock resistivity models. The difference in rock resistivity in geothermal systems provides subsurface geological information in the form of low resistivity that associated the clay cap zones (high conductive), the medium resistivity zones associated with the reservoir zones, and high resistivity associated with a heat source. The results of 2D and 3D models from MT data show that the low resistivity value (<20Ωm) as a clay cover zones, this layer from the surface to -1000 meters. Medium resistivity values ​​(20-100 Ωm) starting from depths -1000 meters to -2000 meters associated with reservoirs zones, high resistivity values (> 200 Ωm) starting from depths of -2000 meters are considered as heat source from the Pantar geothermal system.

Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Tectonics, 38(2), (2019):666-686. doi:10.1029/2018TC005246., A magnetotelluric survey in the Barotse Basin of western Zambia shows clear evidence for thinned lithosphere beneath an orogenic belt. The uppermost asthenosphere, at a depth of 60–70 km, is highly conductive, suggestive of the presence of a small amount of partial melt, despite the fact that there is no surface expression of volcanism in the region. Although the data support the presence of thicker cratonic lithosphere to the southeast of the basin, the lithospheric thickness is not well resolved and models show variations ranging from ~80 to 150 km in this region. Similarly variable is the conductivity of the mantle beneath the basin and immediately beneath the cratonic lithosphere to the southeast, although the conductivity is required to be elevated compared to normal lithospheric mantle. In a general sense, two classes of model are compatible with the magnetotelluric data: one with a moderately conductive mantle and one with more elevated conductivities. This latter class would be consistent with the impingement of a stringer of plume‐fed melt beneath the cratonic lithosphere, with the melt migrating upslope to thermally erode lithosphere beneath the orogenic belt that is overlain by the Barotse Basin. Such processes are potentially important for intraplate volcanism and also for development or propagation of rifting as lithosphere is thinned and weakened by melt. Both models show clear evidence for thinning of the lithosphere beneath the orogenic belt, consistent with elevated heat flow data in the region., Funding for MT acquisition and analysis was provided by the National Science Foundation grant EAR‐1010432 through the Continental Dynamics Program. The data used in this study are available for download at the IRIS Data Management Center through the DOI links cited in Jones et al. (2003–2008; https://doi.org/10.17611/DP/EMTF/SAMTEX) and Evans et al. (2012; https://doi.org/10.17611/DP/EMTF/PRIDE/ZAM). We would like to thank the field crew from the Geological Survey Department, Zambia, for their assistance in collecting data. Matthew Chamberlain, David Margolius, and Colin Skinner, formerly of Northeastern University, are also thanked for their field assistance. Data are available from the corresponding author pending their submission to the IRIS DMC repository at which point they will be publically available. This is Oklahoma State University, Boone Pickens School of Geology contribution number 2019‐99., 2019-07-30

New broadband magnetotelluric (MT) data have been acquired along two parallel profiles in the central part of the metallogenic Skellefte district in northern Sweden. The data were recorded as part of the Swedish 4D modelling of mineral belts project and cover an area with several economical and sub-economical deposits. The dimensionality and quality of the data were carefully analyzed and new error floors were systematically determined prior to inverse modelling in 2D and 3D. The algorithms used were EMILIA and WSINV3DMT. For the 2D inversion, only the determinant of the impedance tensor was used, while for the 3D inversion all elements were considered. The obtained models fit the inverted data, and image the main regional features. A detailed comparison reveals the superiority of the 3D model, both in model structures and data fit. After assessing the main features in the model, an interpretation is proposed and refined with the support of previous geophysical studies. The most interesting features are large and medium-sized conductors associated with crustal-scale shear zones and faults within the Skellefte Group rocks. These may be depicting a network of fossil pathways for hydrothermal fluid transport and as such, provide new insight into past processes in the area., VINNOVA 4D modeling of mineral belts

Boat-towed radio-magnetotelluric (RMT) measurements using signals between 14 and 250 kHz have attracted increasing attention in the near-surface applications for shallow water and archipelago areas. A few large-scale underground infrastructure projects, such as the Stockholm bypass in Sweden, are planned to pass underneath such water zones. However, in cases with high water salinity, RMT signals have a penetration depth of a few metres and do not reach the geological structures of interest in the underlying sediments and bedrock. To overcome this problem, controlled source signals at lower frequencies of 1.25 to 12.5 kHz can be utilized to improve the penetration depth and to enhance the resolution for modelling deeper underwater structures. Joint utilization of boat-towed RMT and controlled source audio-magnetotellurics (CSAMT) was tested for the first time at the Aspo Hard Rock Laboratory (HRL) site in south-eastern Sweden to demonstrate acquisition efficiency and improved resolution to model fracture zones along a 600-m long profile. Pronounced galvanic distortion effects observed in 1-D inversion models of the CSAMT data as well as the predominantly 2-D geological structures at this site motivated usage of 2-D inversion. Two standard academic inversion codes, EMILIA and MARE2DEM, were used to invert the RMT and CSAMT data. EMILIA, an object-oriented Gauss-Newton inversion code with modules for 2-D finite difference and 1-D semi-analytical solutions, was used to invert the RMT and CSAMT data separately and jointly under the plane-wave approximation for 2-D models. MARE2DEM, a Gauss-Newton inversion code for controlled source electromagnetic 2.5-D finite element solution, was modified to allow for inversions of RMT and CSAMT data accounting for source effects. Results of EMILIA and MARE2DEM reveal the previously known fracture zones in the models. The 2-D joint inversions of RMT and CSAMT data carried out with EMILIA and MARE2DEM show clear improvement compared wi

Continental break-up is controlled by several parameters and processes (rheology, inherited structures, magmatism, etc). Their impact, chronology and interactions are still poorly known and debated, particularly when rifting interacts with cratons. In order to better understand the rifting initiation in a cratonic lithosphere, we analysed 22 magnetotelluric (MT) soundings collected along two East-West profiles in two different rift segments of the North Tanzanian Divergence. The North Tanzanian Divergence, where the East African Rift is at its earliest stage, is a remarkable example of the transition between magmatic to amagmatic rifting with two clearly identified segments. Only separated by a hundred kilometers, these segments, Natron (North) and Manyara (South), display contrasted morphological (wide versus narrow), volcanic (many versus a few edifices) and seismic (shallow versus deep activity) signatures. Magnetotelluric profiles across the two segments were inverted with a three-dimensional approach and supplied the resistive structure of the upper lithosphere (down to about 70 km). The Natron segment has a rather conductive lithosphere containing several resistive features (Proterozoic Belt), whereas the Manyara segment displays highly resistive blocks probably of cratonic nature encompassing a conductive structure under the axial valley. The joint interpretation of these models with recent local and regional seismological studies highlights totally different structures and processes involved in the two segments of the North Tanzanian Divergence. We identified contrasted CO2 content, magma upwelling or trapping, in depth regarding the Manyara or the Natron branch and the influence of inherited cratonic structures in the rifting dynamics.

Author Posting. © The Authors, 2019. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 218(3), (2019): 1822-1837, doi: 10.1093/gji/ggz253., Joint inversion of multiple electromagnetic data sets, such as controlled source electromagnetic and magnetotelluric data, has the potential to significantly reduce uncertainty in the inverted electrical resistivity when the two data sets contain complementary information about the subsurface. However, evaluating quantitatively the model uncertainty reduction is made difficult by the fact that conventional inversion methods—using gradients and model regularization—typically produce just one model, with no associated estimate of model parameter uncertainty. Bayesian inverse methods can provide quantitative estimates of inverted model parameter uncertainty by generating an ensemble of models, sampled proportional to data fit. The resulting posterior distribution represents a combination of a priori assumptions about the model parameters and information contained in field data. Bayesian inversion is therefore able to quantify the impact of jointly inverting multiple data sets by using the statistical information contained in the posterior distribution. We illustrate, for synthetic data generated from a simple 1-D model, the shape of parameter space compatible with controlled source electromagnetic and magnetotelluric data, separately and jointly. We also demonstrate that when data sets contain complementary information about the model, the region of parameter space compatible with the joint data set is less than or equal to the intersection of the regions compatible with the individual data sets. We adapt a trans-dimensional Markov chain Monte Carlo algorithm for jointly inverting multiple electromagnetic data sets for 1-D earth models and apply it to surface-towed controlled source electromagnetic and magnetotelluric data collected offshore New Jersey, USA, to evaluate the extent of a low salinity aquifer within the continental shelf. Our inversion results identify a region of high resistivity of varying depth and thickness in the upper 500 m of the continental shelf, We gratefully acknowledge funding support from National Science Foundation grants 1458392 and 1459035. We thank the captain and crew of the R.V. Marcus G. Langseth for a successful cruise and the Marine EM Lab at Scripps Institution of Oceanography for providing the instrumentation. We also thank Chris Armerding, Marah Dahn, John Desanto, Jimmy Elsenbeck, Matt Folsom, Keiichi Ishizu, Jeff Pepin, Charlotte Wiman and Georgie Zelenak for participating in the cruise. We gratefully acknowledge Alberto Malinverno for the idea to use a Monte Carlo scheme to estimate the distribution of pore fluid salinity, and William Menke for many constructive conversations and suggestions.

The detection capability of various anomalous phenomena preceding volcanic eruptions has considerably progressed as the geophysical monitoring networks have become denser and multi-disciplinary. However, current eruption forecasting techniques, from a practical perspective, still have much scope for improvement because they largely depend on empirical techniques. In the past decade, three-dimensional modeling based on the electromagnetic sounding methods such as magnetotellurics (MT) have become a practical choice, and its recent applications to active volcanic fields has revealed certain common features among volcanoes. Information about the resistivity structure, especially in ‘wet’ volcanic fields, is useful for the provisional screening of the eruption potential from the viewpoint of the subsurface structure, and, thus, may contribute to the evaluation of eruption imminence in a broad sense. In this study, for evaluation purposes, we present the roles and possible further applications of the subsurface resistivity structure studies by demonstrating the preliminary results and interpretations of an MT survey that we performed in the Kuttara Volcanic Group, northern Japan.

The detection capability of various anomalous phenomena preceding volcanic eruptions has considerably progressed as the geophysical monitoring networks have become denser and multi-disciplinary. However, current eruption forecasting techniques, from a practical perspective, still have much scope for improvement because they largely depend on empirical techniques. In the past decade, three-dimensional modeling based on the electromagnetic sounding methods such as magnetotellurics (MT) have become a practical choice, and its recent applications to active volcanic fields has revealed certain common features among volcanoes. Information about the resistivity structure, especially in ‘wet’ volcanic fields, is useful for the provisional screening of the eruption potential from the viewpoint of the subsurface structure, and, thus, may contribute to the evaluation of eruption imminence in a broad sense. In this study, for evaluation purposes, we present the roles and possible further applications of the subsurface resistivity structure studies by demonstrating the preliminary results and interpretations of an MT survey that we performed in the Kuttara Volcanic Group, northern Japan.

A joint analysis of magnetotelluric and Sp receiver function data, collected along a profile across the central Appalachians, highlights variations in regional lithospheric structure. While the interpretation of each data set by itself is non-unique, we identify three distinct features that are consistent with both the resistivity model and the receiver function image: 1) thin lithosphere beneath the Appalachian Mountains, 2) somewhat thicker lithosphere to the east of the mountains beneath the Coastal Plain, and 3) a lithosphere-asthenosphere boundary that deepens to the west of the mountains. In some regions, the correspondence between seismic velocity discontinuities and resistivity mark the base of the lithosphere, while in other locations we see seismic discontinuities that are contained within the lithosphere. At the western end of our profile a transition from highly resistive lithosphere to more conductive mantle represents the transition across the Grenville front. The thickness of lithosphere beneath the Grenville terrain is ∼140 km. Lithosphere at the eastern end of the profile has a thickness that is not well constrained by our coverage, but is at least 110 km thick. This lithosphere can be associated with a broader region of high resistivity material seen to extend further south. Directly beneath the Appalachian Mountains, lithospheric thickness is inferred to be as thin as ∼80 km, based on observations of elevated mantle conductivities and a westward-dipping seismic converter. Electrical conductivities in the uppermost asthenospheric mantle are sufficiently high (>0.1 S/m) to require the presence of a small volume of partial melt. The location of these elevated conductivities is close (offset ∼50 km to the west) to Eocene volcanic outcrops in and around Harrisonburg, VA. Our observations speak to mechanisms of intraplate volcanism where there is no divergent or convergent plate motion to trigger mantle upwelling or obvious fluid release, either of whic

On 29 June 2015, a small phreatic eruption occurred at Hakone volcano, Central Japan, forming several vents in the Owakudani geothermal area on the northern slope of the central cones. Intense earthquake swarm activity and geodetic signals corresponding to the 2015 eruption were also observed within the Hakone caldera. To complement these observations and to characterise the shallow resistivity structure of Hakone caldera, we carried out a three-dimensional inversion of magnetotelluric measurement data acquired at 64 sites across the region. We utilised an unstructured tetrahedral mesh for the inversion code of the edge-based finite element method to account for the steep topography of the region during the inversion process. The main features of the best-fit three-dimensional model are a bell-shaped conductor, the bottom of which shows good agreement with the upper limit of seismicity, beneath the central cones and the Owakudani geothermal area, and several buried bowl-shaped conductive zones beneath the Gora and Kojiri areas. We infer that the main bell-shaped conductor represents a hydrothermally altered zone that acts as a cap or seal to resist the upwelling of volcanic fluids. Enhanced volcanic activity may cause volcanic fluids to pass through the resistive body surrounded by the altered zone and thus promote brittle failure within the resistive body. The overlapping locations of the bowl-shaped conductors, the buried caldera structures and the presence of sodium-chloride-rich hot springs indicate that the conductors represent porous media saturated by high-salinity hot spring waters. The linear clusters of earthquake swarms beneath the Kojiri area may indicate several weak zones that formed due to these structural contrasts.

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 18 (2017): 4342–4355, doi:10.1002/2017GC007048., Melting beneath mid-ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sublithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the covariation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain-size variability and non-Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain-size and non-Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid-ocean ridges is more profound than their effect on the mantle corner flow., European Research Council; European Union's Seventh Framework Programme Grant Numbers: FP7/2007-2013, 279925; U.S. National Science Foundation; EPSRC Grant Number EP/K032208/1, 2018-12-06

In connection with the Collisional Orogeny in the Scandinavian Caledonides (COSC) project, broad-band magnetotelluric (MT) data were acquired at 78 stations along a recent ca. 55km- long NW-SE directed reflection seismic profile (referred to as the COSC Seismic Profile; CSP), with the eastern end located similar to 30 km to the west of the orogenic Caledonian front. The MT component of the project aims at (i) delineating the highly conductive (similar to 0.1 Omega . m) alum shales that are associated with an underlying main decollement and (ii) calibrating the MT model to borehole logs. Strike and distortion analyses of the MT data show a 3-D structure in the western 10 km of the profile around the 2.5 km deep COSC-1 borehole (IGSN: ICDP5054EHW1001) and a preferred strike angle of N34 degrees E in the central and eastern parts of the profile. 2-D modelling of MT impedances was tested using different inversion schemes and parameters. To adjust the resistivity structure locally around the borehole, resistivity logging data from COSC-1 were successfully employed as prior constraints in the 2-D MT inversions. Compared with the CSP, the model inverted from the determinant impedances shows the highest level of structural similarity. A shallow resistor (> 1000 Omega . m) in the top 2-3 km depth underneath the western most 10 km of the profile around COSC-1 corresponds to a zone of high seismic reflectivity, and a boundary at less than 1 km depth where the resistivity decreases rapidly from > 100 to < 1 Omega . m in the central and eastern parts of the profile coincides with the first seismic reflections. The depth to this boundary is well constrained as shown by 1-D inversions of the MT data from five selected sites and it decreases towards the Caledonian front in the east. Underneath the easternmost part of the profile, the MT data show evidence of a second deeper conductor (resistivity < 1 Omega . m) at > 3 km depth. Based upon the COSC-1 borehole logs, th

Melting beneath mid-ocean ridges occurs over a region that is much broader than the zone of magmatic emplacement that forms the oceanic crust. Magma is focused into this zone by lateral transport. This focusing has typically been explained by dynamic pressure gradients associated with corner flow, or by a sublithospheric channel sloping upward toward the ridge axis. Here we discuss a novel mechanism for magmatic focusing: lateral transport driven by gradients in compaction pressure within the asthenosphere. These gradients arise from the covariation of melting rate and compaction viscosity. The compaction viscosity, in previous models, was given as a function of melt fraction and temperature. In contrast, we show that the viscosity variations relevant to melt focusing arise from grain-size variability and non-Newtonian creep. The asthenospheric distribution of melt fraction predicted by our models provides an improved explanation of the electrical resistivity structure beneath one location on the East Pacific Rise. More generally, we find that although grain-size and non-Newtonian viscosity are properties of the solid phase, their effect on melt transport beneath mid-ocean ridges is more profound than their effect on the mantle corner flow.

The same mathematical formalism of the wave equation can be used to describe anelastic and electromagnetic wave propagation. In this work, we obtain the mathematical analogy for the reflection/refraction (transmission) problem of two layers, considering the presence of anisotropy and attenuation -- viscosity in the viscoelastic case and resistivity in the electromagnetic case. The analogy is illustrated for SH (shear-horizontally polarised) and TM (transverse-magnetic) waves. In particular, we illustrate examples related to the magnetotelluric method applied to geothermal systems and consider the effects of anisotropy. The solution is tested with the classical solution for stratified isotropic media.

Asama Volcano is an andesitic composite volcano and one of the most active volcanoes in Japan. In order to reveal electrical resistivity structure beneath the volcano accurately, we performed a 3-D inversion of dense magnetotelluric survey data. In order to prevent misinterpretation of the subsurface resistivity due to the steep topography around Asama Volcano, we used an unstructured tetrahedral mesh to represent the topography. Furthermore, we reduced the calculation time by transforming the inverse problem from the model space into the data space. Comparison of the new data-space method to the original model-space method showed that the calculation time required to update the model parameterswas reduced as a result of the transformation, whereas the resistivity structure obtained remained unchanged. In the subsurface resistivity structure around Asama Volcano that was estimated from the inversion, resistive bodies were discovered to be located under the old eruption centres. In particular, under the 24 ka collapse caldera to the west of the presently active crater, a spherical resistive body was found to exist in isolation. In addition, there was a widespread conductive layer below the resistive surface layer. By comparison with previous hydrological and geochemical studies, the conductive layer was interpreted as being a high-water-content layer and an overlying layer rich in altered clay minerals. Because the western part of the volcanic conduit was considered to be the resistive area, which is inferred to consist of unfractured rocks with lower permeability than their surroundings, it would appear that the area obstructs the westward flow of the hydrothermal fluid beneath the summit, thereby contributing to higher concentrations of SO4 2− and Cl− in the spring water at the northern and eastern feet as well as the uneven location of a diffuse CO2 anomaly.

The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50–100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern region of Aso volcano (50 km northeast of the mainshock epicenter), and (3) the regions around three volcanoes, Yufu, Tsurumi, and Garan (100 km northeast of the mainshock epicenter). Notably, the zones between these regions are distinctively seismically inactive. The electric resistivity structure estimated from one-dimensional analysis of the 247 broadband (0.005–3000 s) magnetotelluric and telluric observation sites clearly shows that the earthquakes occurred in resistive regions adjacent to conductive zones or resistive-conductive transition zones. In contrast, seismicity is quite low in electrically conductive zones, which are interpreted as regions of connected fluids. We suggest that the series of the earthquakes was induced by a local accumulated stress and/or fluid supply from conductive zones. Because the relationship between the earthquakes and the resistivity structure is consistent with previous studies, seismic hazard assessment generally can be improved by taking into account the resistivity structure. Following on from the 2016 Kumamoto earthquake series, we suggest that there are two zones that have a relatively high potential of earthquake generation along the western extension of the MTL.

One of the requirements to realize electricity production from geothermal energy in the Netherlands is the exploration of the deep subsurface. Currently, detailed geological information below 5 kilometer is sparse. The magnetotelluric (MT) method, a passive electromagnetic method, is a candidate to acquire this information by geophysical surface exploration. Using the MT method, it is possible to image the subsurface resistivity structures. This is achieved by measuring the time-variations in the electromagnetic field at the surface of the Earth. Due to their specific resistivity response related to hydrothermal alteration mineralogy, MT is commonly applied for the exploration of volcanic geothermal systems. Being potentially useful for geothermal exploration, MT has no track record in sedimentary basins such as the Netherlands. Since the resisivity of the subsurface in the Netherlands is mainly controlled by porosity and permeability, MT can be utilized to detect, for example, porous formations or open faults and fractures. Two challenges are face when deploying MT in the Netherlands. First, distortion of the signal by cultural electromagnetic noise. Second, the conductive subsurface, making it hard to distinguish resistivity contrasts. As MT data acquired in the Netherlands are not available, data of geothermal projects, located in Philippines and Turkey are used for processing, modelling and interpretation. For re-processing of the Turkish data, a dedicated processing tool is developed. Of both projects the 3-D resistivity models produced by different inversion algorithms are qualitatively and quantitatively compared and interpreted. The Turkish geothermal system is interpreted as a layered system consisting of a conductive layer of volcanic rocks, an ophiolite complex and a limestone formation which forms the geothermal reservoir. The geothermal system is heated through faults and fractures by pluton some distance away. The geothermal projec

A detailed magnetotelluric survey was conducted in 2013 in the Sehqanat oil field, southwestern Iran to map the geoelectrical structures of the sedimentary Zagros zone, particularly the boundary between the Gachsaran Formation acting as cap rock and the Asmari Formation as the reservoir. According to the electrical well logs, a large resistivity contrast exists between the two formations. The Gachsaran Formation is formed by tens to hundreds of metres of evaporites and it is highly conductive (ca. 1 m-10 m), and the Asmari Formation consists of dense carbonates, which are considerably more resistive (more than 100 m). Broadband magnetotelluric data were collected along five southwest-northeast directed parallel lines with more than 600 stations crossing the main geological trend. Although dimensionality and strike analysis of the magnetotelluric transfer functions showed that overall they satisfied local 2D conditions, there were also strong 3D conditions found in some of the sites. Therefore, in order to obtain a more reliable image of the resistivity distribution in the Sehqanat oil field, in addition to standard 2D inversion, we investigated to what extent 3D inversion of the data was feasible and what improvements in the resistivity image could be obtained. The 2D inversion models using the determinant average of the impedance tensor depict the main resistivity structures well, whereas the estimated 3D model shows significantly more details although problems were encountered in fitting the data with the latter. Both approaches resolved the Gachsaran-Asmari transition from high conductivity to moderate conductivity. The well-known Sehqanat anticline could also be delineated throughout the 2D and 3D resistivity models as a resistive dome-shaped body in the middle parts of the magnetotelluric profiles.

This thesis presents two new algorithms for doing constrained Magnetotelluric (MT) inversion based on an existing Occam 2D inversion program. The first algorithm includes borehole resistivity logs as prior information to constrain resistivity directly in the vicinity of boreholes. The second algorithm uses reflection seismic data as prior constraints to transfer structural information from seismic images to 2D resistivity models. These two algorithms are efficient (proved through tests of synthetic examples) and widely applicable. In this thesis, they have been successfully applied to the COSC (Collisional Orogeny in the Scandinavian Caledonides) MT data. The COSC project aims to study the mountain belt dynamics in central Sweden by drilling two 2.5 km deep boreholes. MT data were collected to locate the main décollement that separates the overlying Caledonian allochthons and the underlying Precambrian basement, as the main décollement is associated with very conductive Alum shale. The previous interpretation based on part of the COSC seismic profile (CSP) was that the main décollement was located along a reflection with depth of 4.5 km underneath Åre and ~3 km underneath Mörsil, in central Jämtland. The MT resistivity model reveals a very conductive layer in the central and western parts of the profile, the top of which coincides with the first seismic reflection. This means that the first conductive alum shale layer occurs at less than 1 km depth, supporting a new interpretation of the main décollement at shallower depth. In a re-interpretation of the CSP data based on the MT model, the main décollement occurs a few hundred metres below the top of the conductor and is coincident with a laterally continuous seismic reflection. Further, the overlying seismic reflections resemble imbricated alum shale of the Lower Allochthon. MT inversion using seismic constraints from CSP gives further support to the new interpretation. Moreover, MT investigations were conducted in

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 16 (2015): 4261–4274, doi:10.1002/2015GC006100., We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics., NSF Grant Number: EAR080-9074; Spanish National Projects Grant Number: CTM2009-07039-E/MAR, CTM2011-30400-C02-02, 2016-06-19

Objectives: The main goal was to investigate the regional setting in Eastern Borborema to study the uplift of the Araripe Basin. Another goal of the BODES experiment was to build integrated geophysical images of the Borborema Province’s crust and lithospheric mantle in order to improve our understanding of its geodynamic and tectonic evolution. Conclusions: We have interpreted the high conductivities under the Araripe Basin as an indication of shallow, enriched asthenospheric mate- rial, involving small amounts of fluids and melt. We have proposed that the emplacement of a shallow asthenosphere would have orig- inated through extension and passive upwelling at the time of basin formation, and explained its preservation through geologic time by invoking lateral asthenospheric flow from a distant source. We have proposed that the hot, buoyant asthenospheric mate- rial under the basin provided a vertical push under the Araripe Basin that helped the tectonic inversion of the basin. Additionally, the presence of this asthenospheric material under the Araripe Basin – and the correspondingly raised geotherm – might have weakened the lithospheric mantle, caused regional compressional stresses to concentrate on the brittle crust, and contribute to basin uplift. The high resistivities flanking the conductive anomaly north and south of the Araripe Basin have been interpreted as thickened lithosphere representing lithospheric material that was eroded from under the basin and attached to the flanks through a roll-down flow process. Shear zones have been mapped as conductive anomalies cross- ing the crust in the Borborema region, suggesting the presence of conductive minerals or graphite, precipitated by fluids flow and shear zone reactivation. To the south of the profile, the Riacho do Pontal fold belt is observed to be flanked by conductive structures, which have been interpreted as relict subduction material

Hontomín (N of Spain) hosts the first Spanish CO2 storage pilot plant. The subsurface characterization of the site included the acquisition of a 3-D seismic reflection and a circumscribed 3-D magnetotelluric (MT) survey. This paper addresses the combination of the seismic and MT results, together with the available well-log data, in order to achieve a better characterization of the Hontomín subsurface. We compare the structural model obtained from the interpretation of the seismic data with the geoelectrical model resulting from the MT data. The models correlate well in the surroundings of the CO2 injection area with the major structural differences observed related to the presence of faults. The combination of the two methods allowed a more detailed characterization of the faults, defining their geometry, and fluid flow characteristics, which are key for the risk assessment of the storage site. Moreover, we use the well-log data of the existing wells to derive resistivity-velocity relationships for the subsurface and compute a 3-D velocity model of the site using the 3-D resistivity model as a reference. The derived velocity model is compared to both the predicted and logged velocity in the injection and monitoring wells, for an overall assessment of the computed resistivity-velocity relationships. The major differences observed are explained by the different resolution of the compared geophysical methods. Finally, the derived velocity model for the near surface is compared with the velocity model used for the static corrections in the seismic data. The results allowed extracting information about the characteristics of the shallow unconsolidated sediments, suggesting possible clay and water content variations. The good correlation of the velocity models derived from the resistivity-velocity relationships and the well-log data demonstrate the potential of the combination of the two methods for characterizing the subsurface, in terms of its physical properties (velo

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project "Magnetotellurics in the Scandes" (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 simultaneous long period and 220 broad-band MT sites were occupied in the summers of 2011 to 2013. The 3-D inversion of the MaSca data was obtained using the ModEM 3-D code. The full impedance and tipper data were used for the inversion. The rocks of Archaean and Proterozoic basement towards east and the Caledonian nappes towards west are modelled as resistive structures. In the central and southern parts, the whole crust is resistive and reflects the Trans-Scandinavian Igneous Belt granitoids. The middle to lower crust of the Svecofennian province is conductive. An uppermost crustal conductor is revealed in the Skelleftea Ore District. The south end of the Kittila Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. A map of the crustal conductance for the north-west Fennoscandian Shield is presented.

New magnetotelluric (MT) data in north-west Fennoscandia were acquired within the framework of the project "Magnetotellurics in the Scandes" (MaSca). The project focuses on the investigation of the crustal and upper mantle lithospheric structure in the transition zone from stable Precambrian cratonic interior to passive continental margin beneath the Caledonian orogen and the Scandinavian Mountains in western Fennoscandia. An array of 59 synchronous long period and 220 broad-band MT sites was occupied in the summers of 2011 to 2013. We estimated MT transfer functions in the period range from 0.003 to 10(5) s. The Q-function multi-site multi-frequency analysis and the phase tensor were used to estimate strike and dimensionality of MT data. Dimensionality and strike analyses indicate generally 2-D behaviour of the data with 3-D effects at some sites and period bands. In this paper we present 2-D inversion of the data, 3-D inversion models are shown in the parallel paper. We choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Seven crustal-scale and four lithospheric-scale 2-D models are presented. The resistive regions are images of the Archaean and Proterozoic basement in the east and thin Caledonian nappes in the west. The middle and lower crust of the Svecofennian province is conductive. The southern end of the Kittila Greenstone Belt is seen in the models as a strong upper to middle crustal conductor. In the Caledonides, the highly conductive alum shales are observed along the Caledonian Thrust Front. The thickest lithosphere is in the Palaeoproterozioc Svecofennian Domain, not in the Archaean. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west.

I implemented and tested the method of Non-Linear Conjugate Gradients (NLCG) to invert magnetotelluric (MT) and radiomagnetotelluric (RMT) data in two dimensions. The forward problem and the objective function gradients were computed using finite-difference methods. The NLCG algorithm was applied to three field data sets to test the performance of the code. It was then compared to the inversion techniques of Occam and damped Occam considering the quality of the output resistivity models and the computation times. The implemented code was further investigated by testing two line search techniques to reduce the objective function along a given search direction. The first line search procedure was constrained to the first Wolfe condition, leading to a rather inexact line search. The second, more thorough line search, was additionally constrained to the second Wolfe condition. Three preconditioners were applied to the NLCG algorithm and their performance was analysed. The first preconditioner was set to the diagonal of the approximate Hessian matrix and updated every 20-th iteration. Preconditioners two and three were updated with the Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm using the identity matrix and the diagonal of the approximate Hessian matrix as start preconditioners, respectively. The tests showed that the method of NLCG is more efficient pertaining to computation times compared to the Gauss-Newton (GN) based techniques (Occam and damped Occam). For the two smaller data sets that were inverted, the NLCG inversion was two to four times faster than Occam and damped Occam. For the larger data set, the NLCG inversion converged more than one order of magnitude faster than the GN based inversion techniques. This is because GN methods require to evaluate the entire sensitivity matrix to update the model, whereas NLCG only needs to compute a matrix-vector product of the Jacobian. Moreover, expensive operations such as matrix products and direct inversions of l

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 15 (2014): 4230–4253, doi:10.1002/2014GC005509., Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc., Phil Wannamaker and Virginie Maris gratefully acknowledge funding by the U.S. National Science Foundation under grants EAR08–43725 and EAR08–38043 through the Earthscope and Geophysics programs. The 2D inversion capability received development support under U.S. Department of Energy contract DE-PS36-04GO94001. Rob Evans was supported through Earthscope grant EAR08–44041 and Shane McGary through a National Defense Science and Engineering Graduate (NDSEG) fellowship. Fieldwork in Canada was made possible by an NSERC Discovery Grant and a Canadian Foundation for Innovation award to Martyn Unsworth., 2015-05-11

We present results of marine MT acquisition in the Alboran sea that also incorporates previously acquired land MT from southern Spain into our analysis. The marine data show complex MT response functions with strong distortion due to seafloor topography and the coastline, but inclusion of high resolution topography and bathymetry and a seismically defined sediment unit into a 3-D inversion model has allowed us to image the structure in the underlying mantle. The resulting resistivity model is broadly consistent with a geodynamic scenario that includes subduction of an eastward trending plate beneath Gibraltar, which plunges nearly vertically beneath the Alboran. Our model contains three primary features of interest: a resistive body beneath the central Alboran, which extends to a depth of ∼150 km. At this depth, the mantle resistivity decreases to values of ∼100 Ohm-m, slightly higher than those seen in typical asthenosphere at the same depth. This transition suggests a change in slab properties with depth, perhaps reflecting a change in the nature of the seafloor subducted in the past. Two conductive features in our model suggest the presence of fluids released by the subducting slab or a small amount of partial melt in the upper mantle (or both). Of these, the one in the center of the Alboran basin, in the uppermost-mantle (20–30 km depth) beneath Neogene volcanics and west of the termination of the Nekkor Fault, is consistent with geochemical models, which infer highly thinned lithosphere and shallow melting in order to explain the petrology of seafloor volcanics.

The Iberian Chain is a Cenozoic intraplate thrust belt located within the Iberian plate. Unlike other belts in the Iberia Peninsula, the scarcity of geophysical studies in this area results in a number of unknowns about its crustal structure. The Iberian Chain crust was investigated by means of a NE-SW refraction/wide-angle reflection seismic transect and two magnetotelluric profiles across the chain, oriented along the same direction. The seismic profile was designed to sample the crust by means of three shots designed to obtain a reversed profile. The resulting velocity-depth model shows a moderate thickening of the crust toward the central part of the profile, where crustal thickness reaches values above 40. km, thinning toward de SW Tajo and NE Ebro foreland basins. The crustal thickening is concentrated in the upper crust. The seismic results are in overall agreement with regional trends of Bouguer gravity anomaly and the main features of the seismic model were reproduced by gravity modeling. The magnetotelluric data consist of 39 sites grouped into two profiles, with periods ranging from 0.01 s to 1000 s. Dimensionality analyses show significant 3D effects in the resistivity structure and therefore we carried out a joint 3D inversion of the full impedance tensor and magnetic transfer functions. The Mesozoic and Cenozoic basins along the Chain are well characterized by shallow high conductive zones and low velocities. Elongated conductors reaching mid-crustal depths evidence the presence of major faults dominating the crustal structure. The results from the interpretation of these complementary geophysical data sets provided the first images of the crustal structure of the Iberian Chain. They are consistent with a Cenozoic shortening responsible of the upper crust thickening as well as of the uplift of the Iberian Chain and the generation of its present day topography.

We use data from two magnetotelluric profiles, ToSca10 and ToSca'09, over the Scandinavian Mountains to study the crustal structure in southern Norway. The profiles cross the major tectonic structures of the Caledonian orogen as well as the western margin of the Precambrian Baltica. Dimensionality and strike analyses indicate generally 3-D behavior of the data. However, the majority of the used data distinguishes a preferable strike direction, which is supported by the geology of the region. Hence, we employ 2-D inversion and choose to invert the determinant of the impedance tensor to mitigate 3-D effects in the data on our 2-D models. Magnetotelluric data from both profiles are inverted using a damped least squares solution based on a singular value decomposition. We improved the solution by defining the inverse model covariance matrix through gradient or Laplacian smoothing operators. The two-dimensional inversion models of the ToSca'09 and ToSca'10 field data from southern Norway derived from the damped least squares scheme with the Laplacian inverse model covariance matrix are presented. Resistive rocks, extending to the surface, image the autochthonous Southwest Scandinavian Domain and the allochthonous Western Gneiss Region. Near-surface conductors, which are located between the resistive Caledonian nappes and Precambrian basement, delineate highly conductive shallow-sea sediments, so called alum shales. They exhibit a decollement along which the Caledonian nappes were overthrust. A deeper, upper to midcrustal conducting layer in the Southwest Scandinavian Domain may depict the remnants of closed ocean basins formed during the accretions and collisions of various Sveconorwegian terranes. In ToSca'10, the Caledonian nappes, the conducting alum shales and the deeper conductor are terminated in the west by the Faltungsgraben shear complex which represents a crustal scale boundary between the Western Gneiss Region in the west and the Southwest Scandinavian Domain

Zagros is a relatively young and active fold-thrust belt, which has formed due to convergence between the Eurasian and Arabian plates. Magnetotelluric (MT) soundings along a transect were carried out to determine the crustal structure in the collision zone of the two Palaeocontinents. MT data were analysed and modelled using 2-D inversion schemes. The models show clear conductive and resistive domains along the MT profile consistent to a great extent with documented tectonic features and surface geology. The models obtained from the joint inversion of transverse electric and transverse magnetic modes as well as the inversion of the determinant data show similar features along the profile. The new MT results reveal that the transition between two continents at the surface coincides with the western boundary of Sanandaj-Sirjan Zone (SSZ) at the Main Zagros Thrust (MZT). Along the profile towards northeast the conductors at top indicate massive Neogene sediments of the central domain (CD) while the very thick, shallow-located, resistive body (5-25 km thick and 100 km long) beneath is unlikely to be of oceanic affinity, but continental. Another main feature along the profile is the main resistive and conductive parts of the Arabian Plate, which coincide with the tectonic events of High Zagros Fault and Mountain Front Fault. Two highly conductive thick zones are recognized at the southwest part and in the middle of the profile apparently extending to a depth of about 50 km, possibly related to a downward smearing effect due to the presence of thick sedimentary columns in the upper crust. Along the profile, conductive features are recognized at the metamorphic SSZ and Urumieh-Dokhtar Magmatic Assemblage units as well as at CD. Below site 31 along the surface trace of the MZT, the transition between the two continents is distinguished by a complex sequence of conductive and resistive zones both varying laterally as well as vertically. The main difference between the two do

Author Posting. © Author, 2014. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 198 (2014): 622-636, doi: 10.1093/gji/ggu121., The robust statistical model of a Gaussian core contaminated by outlying data that underlies robust estimation of the magnetotelluric (MT) response function has been re-examined. The residuals from robust estimators are systematically long tailed compared to a distribution based on the Gaussian, and hence are inconsistent with the robust model. Instead, MT data are pervasively described by the alpha stable distribution family whose variance and sometimes mean are undefined. A maximum likelihood estimator (MLE) that exploits the stable nature of MT data is formulated, and its two-stage implementation in which stable parameters are first fit to the data and then the MT responses are solved for is described. The MLE is shown to be inherently robust, but differs from the conventional robust estimator because it is based on a model derived from the data, while robust estimators are ad hoc, being based on the robust model that is inconsistent with actual data. Propriety versus impropriety of the complex MT response was investigated, and a likelihood ratio test for propriety and its null distribution was established. The Cramér-Rao lower bounds for the covariance matrix of proper and improper MT responses were specified. The MLE was applied to exemplar long period and broad-band data sets from South Africa. Both are shown to be significantly stably distributed using the Kolmogorov–Smirnov goodness of fit and Ansari-Bradley non-parametric dispersion tests. Impropriety of the MT responses at both sites is pervasive, hence the improper Cramér-Rao bound was used to estimate the MLE covariance. The MLE is shown to be nearly unbiased and well described by a Gaussian distribution based on bootstrap simulation. The MLE was compared to a conventional robust estimator, establishing that the standard errors of the former are systematically smaller than for the latter and that the standardized differences between them exhibit excursions that are both too frequent and too large to be d, This work was supported by NSF grant EAR0809074.

A robust magnetotelluric (MT) inversion algorithm has been developed on the basis of quantile-quantile (q-q) plotting with confidence band and statistical modelling of inversion residuals for the MT response function (apparent resistivity and phase). Once outliers in the inversion residuals are detected in the q-q plot with the confidence band and the statistical modelling with the Akaike information criterion, they are excluded from the inversion data set and a subsequent inversion is implemented with the culled data set. The exclusion of outliers and the subsequent inversion is repeated until the q-q plot is substantially linear within the confidence band, outliers predicted by the statistical modelling are unchanged from the prior inversion, and the misfit statistic is unchanged at a target level. The robust inversion algorithm was applied to synthetic data generated from a simple 2-D model and observational data from a 2-D transect in southern Africa. Outliers in the synthetic data, which come from extreme values added to the synthetic responses, produced spurious features in inversion models, but were detected by the robust algorithm and excluded to retrieve the true model. An application of the robust inversion algorithm to the field data demonstrates that the method is useful for data clean-up of outliers, which could include model as well as data inconsistency (for example, inability to fit a 2-D model to a 3-D data set), during inversion and for objectively obtaining a robust and optimal model. The present statistical method is available irrespective of the dimensionality of target structures (hence 2-D and 3-D structures) and of isotropy or anisotropy, and can operate as an external process to any inversion algorithm without modifications to the inversion program.

Five magnetotelluric (MT) profiles have been acquired across the Cascadia subduction system and transformed using 2-D and 3-D nonlinear inversion to yield electrical resistivity cross sections to depths of ∼200 km. Distinct changes in plate coupling, subduction fluid evolution, and modes of arc magmatism along the length of Cascadia are clearly expressed in the resistivity structure. Relatively high resistivities under the coasts of northern and southern Cascadia correlate with elevated degrees of inferred plate locking, and suggest fluid- and sediment-deficient conditions. In contrast, the north-central Oregon coastal structure is quite conductive from the plate interface to shallow depths offshore, correlating with poor plate locking and the possible presence of subducted sediments. Low-resistivity fluidized zones develop at slab depths of 35–40 km starting ∼100 km west of the arc on all profiles, and are interpreted to represent prograde metamorphic fluid release from the subducting slab. The fluids rise to forearc Moho levels, and sometimes shallower, as the arc is approached. The zones begin close to clusters of low-frequency earthquakes, suggesting fluid controls on the transition to steady sliding. Under the northern and southern Cascadia arc segments, low upper mantle resistivities are consistent with flux melting above the slab plus possible deep convective backarc upwelling toward the arc. In central Cascadia, extensional deformation is interpreted to segregate upper mantle melts leading to underplating and low resistivities at Moho to lower crustal levels below the arc and nearby backarc. The low- to high-temperature mantle wedge transition lies slightly trenchward of the arc.

The lithospheric conductivity in the westernmost Fennoscandia has been studied using magnetotelluric (MT) data. The western margin of Fennoscandia was significantly affected in Paleozoic by the Caledonian orogeny and later by the rifting of Laurentia and the opening of the Atlantic Ocean c. 80 Ma ago. Magnetotelluric studies have been carried out in two target areas in southern Norway and in western Fennoscandia. The first study resulted in 2-D geoelectric models of two profiles stretching from Oslo to the Norwegian coast. The interpretation suggests that the basement is in general very resistive with a few conductive upper crustal layers, representing the alum shales, and middle crustal conductors possibly imaging the remnants of the closed ocean basins. A more extensive MT study was performed within the project “Magnetotellurics in the Scandes”. Measurements were carried in summers of 2011 to 2013, resulting in an array of 279 MT sites. The data allowed us to derive 2-D geoelectric models for the crust and upper mantle as well as 3-D models for the crust. The inversions revealed a resistive upper crust and a conductive lower crust, two upper crustal conductors in the Skellefteå and Kittilä districts, highly conducting alum shales in the Caledonides and a conductive upper crust beneath the Lofoten peninsula. The thickness of the lithosphere is around 200 km in the north and 300 km in the south-west. The Palaeoproterozoic lithosphere is the thickest, not the Archaean, on contrary to a generally accepted hypothesis. A better image of the lithosphere will help to evaluate the proposed mechanisms of the exhumation of the Scandinavian Mountains. The theoretical part of this study is the development of a new multi-resolution approach to 3-D electromagnetic (EM) modelling. Three-dimensional modelling of MT data requires enormous computational resources because of the huge number of data and model parameters. The development of the multi-resolution forward solver, Tiivistelmä Olemme tutkineet litosfäärin sähkönjohtavuutta Fennoskandian länsiosassa magnetotelluurisen (MT) menetelmän avulla. Fennoskandian länsireuna muokkautui merkittävästi paleotsooisena aikana Kaledonidien vuorijonopoimutuksessa sekä myöhemmin mesotsooisena aikana Laurentia-mantereen repeytyessä ja Atlantin valtameren syntyessä noin 80 miljoonaa vuotta sitten. MT-tutkimukset tehtiin Etelä-Norjassa ja Fennoskandian luoteisosassa. Ensimmäisessä tutkimuksessa kallioperän sähkönjohtavuutta kuvattiin kaksiulotteisilla (2-D) johtavuusmalleilla, jotka ulottuvat Oslosta Norjan rannikolle. Mallien tulkinta viittaa siihen, että maan kuori on pääosin hyvin eristävä lukuun ottamatta muutamaa kuoren ylä- ja keskiosassa olevaa johdekerrosta. Yläkuoren johteet edustavat alunaliuskeita ja keskikuoren johteet todennäköisesti suljetuissa merialtaissa syntyneitä hiilipitoisia sedimenttikerrostumia. Laajempi MT-tutkimus tehtiin ”Magnetotellurics in the Scandes” -hankkeessa. Mittauksia tehtiin 279 mittauspisteessä kesinä 2011–2013. Saadun aineiston avulla voitiin laatia 2-D inversiomallit kuoresta ja ylävaipasta sekä 3-D inversiomalli kuoresta. Tulosten mukaan täällä kuoren yläosa on eristävä kun taas kuoren alaosa on sähköä hyvin johtava. Edellisen lisäksi malleissa näkyy yläkuoren johtavat muodostumat Skellefteån ja Kittilän alueilla, korkean johtavuuden alunaliuskeet Kaledonidien alueella sekä johde Lofoottien alla. Litosfäärin paksuus on noin 200 km mittausverkon pohjoisosassa ja noin 300 km lounaassa. Tämän mukaan litosfääri on paksuin varhaisproterotsooisen litosfäärin alueella, ei arkeeisen litosfäärin alueella vastoin yleistä hypoteesia. Tutkimuksen teoreettisessa osassa kehitettiin sähkömagneettiseen mallinnukseen uusi monitasoiseen diskretisointiin perustuva menetelmä. MT-aineiston 3-D käänteisongelman ratkaisu ja siihen liittyvä suora mallintaminen vaativat suuren laskennallisen kapasiteetin, koska havaintojen ja mallin kuvaamiseen tarvittavien parametrien määrä on eri

Author Posting. © American Geophysical Union, 2013. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geochemistry, Geophysics, Geosystems 14 (2013): 1685–1692, doi:10.1002/ggge.20103., Our knowledge of magma dynamics would be improved if geophysical data could be used to infer rheological constraints in melt-bearing zones. Geophysical images of the Earth's interior provide frozen snapshots of a dynamical system. However, knowledge of a rheological parameter such as viscosity would constrain the time-dependent dynamics of melt bearing zones. We propose a model that relates melt viscosity to electrical conductivity for naturally occurring melt compositions (including H2O) and temperature. Based on laboratory measurements of melt conductivity and viscosity, our model provides a rheological dimension to the interpretation of electromagnetic anomalies caused by melt and partially molten rocks (melt fraction ~ >0.7)., We acknowledge partial support under NASA USRA subaward 02153–04, NSF EAR 0739050, and the ASU School of Earth and Space Exploration (SESE) Exploration Postdoctoral Fellowship Program., 2013-12-12