SUMMARY
Central Mongolia is a prominent region of intracontinental surface deformation and intraplate volcanism. To study these processes, which are poorly understood, we collected magnetotelluric ...(MT) data in the Hangai and Gobi-Altai region in central Mongolia and derived the first 3-D resistivity model of the crustal and upper mantle structure in this region. The geological and tectonic history of this region is complex, resulting in features over a wide range of spatial scales, which that are coupled through a variety of geodynamic processes. Many Earth properties that are critical for the understanding of these processes, such as temperature as well as fluid and melt properties, affect the electrical conductivity in the subsurface. 3-D imaging using MT can resolve the distribution of electrical conductivity within the Earth at scales ranging from tens of metres to hundreds of kilometres, thereby providing constraints on possible geodynamic scenarios. We present an approach to survey design, data acquisition, and inversion that aims to bridge various spatial scales while keeping the required field work and computational cost of the subsequent 3-D inversion feasible. MT transfer functions were estimated for a 650 × 400 km2 grid, which included measurements on an array with regular 50 × 50 km2 spacing and along several profiles with a denser 5–15 km spacing. The use of telluric-only data loggers on these profiles allowed for an efficient data acquisition with a high spatial resolution. A 3-D finite element forward modelling and inversion code was used to obtain the resistivity model. Locally refined unstructured hexahedral meshes allow for a multiscale model parametrization and accurate topography representation. The inversion process was carried out over four stages, whereby the result from each stage was used as input for the following stage that included a finer model parametrization and/or additional data (i.e. more stations, wider frequency range). The final model reveals a detailed resistivity structure and fits the observed data well, across all periods and site locations, offering new insights into the subsurface structure of central Mongolia. A prominent feature is a large low-resistivity zone detected in the upper mantle. This feature suggests a non-uniform lithosphere-asthenosphere boundary that contains localized upwellings that shallow to a depth of 70 km, consistent with previous studies. The 3-D model reveals the complex geometry of the feature, which appears rooted below the Eastern Hangai Dome with a second smaller feature slightly south of the Hangai Dome. Within the highly resistive upper crust, several conductive anomalies are observed. These may be explained by late Cenozoic volcanic zones and modern geothermal areas, which appear linked to mantle structures, as well as by major fault systems, which mark terrane boundaries and mineralized zones. Well resolved, heterogeneous low-resistivity zones that permeate the lower crust may be explained by fluid-rich domains.
In December 2019, the International Association of Geomagnetism and Aeronomy (IAGA) Division V Working Group (V-MOD) adopted the thirteenth generation of the International Geomagnetic Reference Field ...(IGRF). This IGRF updates the previous generation with a definitive main field model for epoch 2015.0, a main field model for epoch 2020.0, and a predictive linear secular variation for 2020.0 to 2025.0. This letter provides the equations defining the IGRF, the spherical harmonic coefficients for this thirteenth generation model, maps of magnetic declination, inclination and total field intensity for the epoch 2020.0, and maps of their predicted rate of change for the 2020.0 to 2025.0 time period.
For 3-D inversion of controlled-source electromagnetic (CSEM) data, increasing availability of high-performance computers enables us to apply inversion techniques that are theoretically favourable, ...yet have previously been considered to be computationally too demanding. We present a newly developed parallel distributed 3-D inversion algorithm for interpreting CSEM data in the frequency domain. Our scheme is based on a direct forward solver and uses Gauss-Newton minimization with explicit formation of the Jacobian. This combination is advantageous, because Gauss-Newton minimization converges rapidly, limiting the number of expensive forward modelling cycles. Explicit calculation of the Jacobian allows us to (i) precondition the Gauss-Newton system, which further accelerates convergence, (ii) determine suitable regularization parameters by comparing matrix norms of data- and model-dependent terms in the objective function and (iii) thoroughly analyse data sensitivities and interdependencies. We show that explicit Jacobian formation in combination with direct solvers is likely to require less memory than combinations of direct solvers and implicit Jacobian usage for many moderate-scale CSEM surveys. We demonstrate the excellent convergence properties of the new inversion scheme for several synthetic models. We compare model updates determined by solving either a system of normal equations or, alternatively, a linear least-squares system. We assess the behaviour of three different stabilizing functionals in the framework of our inversion scheme, and demonstrate that implicit regularization resulting from incomplete iterative solution of the model update equations helps stabilize the inversion. We show inversions of models with up to two million unknowns in the forward solution, which clearly demonstrates applicability of our approach to real-world problems.
Summary
We present new transfer functions (TFs) that can handle external electromagnetic (EM) sources of complex geometry. These TFs relate global expansion coefficients describing the source with a ...locally measured EM field. In this study, the new TFs concept was applied to the daily magnetic variations measured at the ground. The parameterisation of the source in terms of spherical harmonics was adopted. We used nearly 20 years of data from 125 mid-latitude observatories and explored how the results are affected by (I) solar activity conditions, (II) the choice of the prior conductivity model used for the source coefficient estimation, and (III) the presence of ocean tidal magnetic signals. We found that choosing magnetically quiet periods is beneficial due to simpler source morphology, and the choice of prior conductivity model may significantly affect the source coefficients and TFs at short periods. We further observed significant contributions by ocean tidal magnetic signals at coastal and island observatories and corrected for them. Finally, the estimated TFs were inverted for the mantle conductivity at several locations representing different geological settings.
We present a novel approach to investigate variations in upper mantle and transition zone (MTZ) water content based on the joint analysis of electromagnetic (EM) signals originating in the ionosphere ...and magnetosphere. We invert EM signals (period range 6 hr to 85 days) to probe the electrical conductivity structure underneath 20 geomagnetic observatories, accounting for the complex spatial structure of the ionospheric and magnetospheric sources. The joint inversion of EM data for the daily and long‐period bands leads to significantly improved resolution in the upper mantle and MTZ. The conductivity profiles reveal significant lateral variability, which we interpret in terms of mantle water content by coupling electrical conductivity with constrains on mantle thermochemical structure derived from the analysis of seismic data. Our results suggest the existence of a relatively dry MTZ beneath Europe and a water‐enriched MTZ underneath North America and northern Asia.
Plain Language Summary
The amount of water trapped in the Earth's interior has a strong effect on the evolution and dynamics of the planet, which ultimately controls the occurrence of earthquakes and volcanic eruptions. However, the distribution of water inside the Earth is not yet well understood. To study the Earth's deep interior, we make use of changes in the Earth's magnetic field to detect variations in electrical conductivity inside the planet. Electrical conductivity is a characteristic of a rock that varies with temperature and water content. Here, we present a novel methodology to estimate the amount of water in different regions of Earth's mantle. Our analysis suggests the presence of small amounts of water in the mantle underneath Europe, whereas larger amounts are expected beneath North America and northern Asia.
Key Points
Joint inversion of daily and long‐period geomagnetic variations results in better resolved mantle conductivity structure
Incorporation of seismic constraints helps isolate mantle water content
We find a relatively dry mantle beneath Europe and a water‐enriched transition zone underneath North America and northern Asia
SUMMARY
Silicic volcanic complexes in the Main Ethiopian Rift (MER) system host long-lived shallow magma reservoirs that provide heat needed to drive geothermal systems. Some of these geothermal ...systems in Ethiopia appear to be suitable for green and sustainable electricity generation. One such prospect is located at the Corbetti volcanic complex near the city of Awassa. High-resolution imaging of the subsurface below Corbetti is of imminent importance, not only because of its geothermal potential, but also due to reported evidence for an ongoing magmatic intrusion. In this study, we present a new subsurface 3-D electrical conductivity model of Corbetti obtained through the inversion of 120 magnetotelluric stations. The model elucidates a magmatic system under Corbetti and reveals that it is linked to a magma ponding zone in the lower crust. Magma is transported through the crust and accumulates in a shallow reservoir in form of a magmatic mush at a depth of $\gtrapprox 4\, {\rm km b.s.l.}$ below the caldera. The imaged extent and depth of the shallow magma reservoir is in agreement with previous geodetic and gravimetric studies that proposed an ongoing magmatic intrusion. Interpreting our model with laboratory-based conductivity models for basaltic and rhyolitic melt compositions suggests that Corbetti is seemingly in a non-eruptible state with ∼6–16 vol. per cent basaltic melt in the lower crust and ∼20–35 vol. per cent rhyolitic melt in the upper crust. With these observations, Corbetti’s magmatic system shares common characteristics with volcanic complexes found in the central MER. Specifically, these volcanic complexes are transcrustal two-stage magmatic systems with magma storage in the lower and upper crust that supply heat for volcano-hosted high-temperature geothermal systems above them. According to the presented subsurface model, a cross-rift volcano-tectonic lineament exerts first-order controls on the magma emplacement and hydrothermal convection at Corbetti. Our study depicts hydrothermal convection pathways in unprecedented detail for this system and helps identify prospective regions for future geothermal exploration. 3-D imaging of both the Corbetti’s magmatic and associated geothermal systems provides key information for the quantitative evaluation of Corbetti’s geothermal energy potential and for the assessment of potential volcanic risks.
We present a new global electrical conductivity model of Earth's mantle. The model was derived by using a novel methodology, which is based on inverting satellite magnetic field measurements from ...different sources simultaneously. Specifically, we estimated responses of magnetospheric origin and ocean tidal magnetic signals from the most recent Swarm and CHAMP data. The challenging task of properly accounting for the ocean effect in the data was addressed through full three-dimensional solution of Maxwell's equations. We show that simultaneous inversion of magnetospheric and tidal magnetic signals results in a model with much improved resolution. Comparison with laboratory-based conductivity profiles shows that obtained models are compatible with a pyrolytic composition and a water content of 0.01 wt% and 0.1 wt% in the upper mantle and transition zone, respectively.
Magnetic sounding is a powerful tool to explore the interior of planetary bodies through the electrical conductivity structure. The electrical conductivity structure of the lunar mantle has ...previously been derived from surface magnetic field measurements as part of the Apollo 12 mission and concurrent magnetometer data acquired from orbit through the Explorer 35 satellite. Here, we derive the first global conductivity structure of the upper and midmantle using only satellite magnetometer data collected by the recent Lunar Prospector and Kaguya Selene satellite missions. We show that the field in the geomagnetic tail exhibits a simple geometrical structure and can be well described by a single spherical harmonic of degree and order one. Employing this information about the inducing field geometry and assuming a potential representation of the field in the geomagnetic tail, we derive a frequency‐dependent transfer function and invert it for a one‐dimensional (1‐D) electrical conductivity profile of the lunar upper and midmantle. Our global transfer function shows striking similarity with the local one obtained from joint analysis of Apollo 12 and Explorer 35 magnetometer data. This indicates the lack of local variations at the Apollo 12 landing site compared to the globally averaged upper to midmantle electrical conductivity structure.
Plain Language Summary
Exploring the interior structure of planetary bodies is exceptionally difficult. However, traditionally geophysical methods have allowed us to gain insight by using various techniques, including magnetic sounding. In this study, we use satellite magnetic field data to constrain the electrical conductivity structure of the Moon. Electrical conductivity is an intrinsic material property that is sensitive to temperature, composition, and volatile content. Magnetic sounding relies on the fact that time‐varying external magnetic fields induce electric currents and thus secondary magnetic fields in the subsurface, both of which can be measured by a lander or satellite. While it is challenging to separate the inducing field from the induced response, we find that when the Moon is in the geomagnetic tail, the organized nature of the inducing field allows us to get a magnetic transfer function and invert it for global conductivity with depth. Our model suggests that the lunar upper mantle at the Apollo landing site is representative of the average global structure.
Key Points
We study the time‐varying magnetic field environment in lunar orbit using Lunar Prospector and Kaguya Selene magnetometer data
We derive the first global radial electrical conductivity profile of the lunar upper and midmantle
Estimated electrical conductivities in the lunar midmantle are similar to local Apollo‐based models
The Main Ethiopian Rift is accompanied by extensive volcanism and the formation of geothermal systems, both having a direct impact on the lives of millions of inhabitants. Although previous studies ...in the region found evidence that asthenospheric upwelling and associated decompression melting provide melt to magmatic systems that feed the tectono‐magmatic segments in the rift valley, there is a lack of geophysical models imaging these regional and local scale transcrustal structures. To address this challenge, we use the magnetotelluric method and image subsurface electrical conductivity to examine the magmatic roots of Aluto volcano, quantify and interpret the melt distribution in the crust considering established concepts of continental rifting processes and constrain the formed geothermal system. Specifically, we combined regional (maximum 30 × 120 km2) and local (15 × 15 km2) magnetotelluric data sets and obtained the first multi‐scale 3‐D electrical conductivity model of a segment of the central Main Ethiopian Rift. The model unravels a magma ponding zone with up to 7 vol. % melt at the base of the crust (30–35 km b.s.l.) in the western part of the rift and its connection to Aluto volcano via a fault‐aligned transcrustal magma system. Melt accumulates at shallow crustal depths (≥4 km b.s.l.), thereby providing heat for Aluto's geothermal system. Our model suggests that different volcano‐tectonic lineaments in the rift valley share a common melt source. The presented model provides new constraints on the melt distribution below a segment of the rift which is important for future geothermal developments and volcanic hazard assessments in the region.
Plain Language Summary
Continental rifting is a fundamental process of plate tectonics that breaks continents apart to ultimately form new oceans. The landscape of the Main Ethiopian Rift is characterized by abundant volcanism and hot springs, which indicate the presence of geothermal resources formed by magmatic heating of subsurface fluids. Here we present a new 3‐D subsurface electrical conductivity image of the magmatic system and geothermal reservoir beneath the Aluto volcano in the Main Ethiopian Rift. The model allows us to estimate the amount and distribution of magmatic melt. This is the first model that provides a high‐resolution image of the entire magmatic system below a central part (maximum 30 × 120 km2) of the Main Ethiopian Rift from the deep magmatic melt source up to the surface. The new model shows that the geothermal reservoir under Aluto has been formed as a consequence of rifting‐related volcanic activity thereby providing a geophysical illustration of fundamental geological processes. These results also have a high societal relevance by providing a basis for volcanic risk assessment and contributing to a better understanding of how the sustainable green geothermal energy resources form.
Key Points
3‐D multi‐scale magnetotelluric model imaging the entire transcrustal tectono‐magmatic system across a segment of the Main Ethiopian Rift
A lower crustal magma ponding zone feeds a fault‐aligned magmatic mush column supplying heat for the geothermal system of Aluto volcano
Eastern and western volcano‐tectonic lineaments likely share a lower crustal magma source with an estimated melt fraction of 7 percent
This review presents the progress made in the last decade in the field of large-scale electromagnetic (EM) induction with natural sources, which fluctuate at periods from seconds to years and ...originate in oceans, ionosphere and magnetosphere. These mechanisms produce field variations that can be used to image subsurface electrical structure of Earth and planets across scales and depths from the shallow crust to the lower mantle. In the last decade, we have seen a substantial progress made in different areas related to methods, observations and 3-D numerical modelling of EM phenomena at crustal and mantle scales. Specifically, new methods for handling complex ionospheric and magnetospheric sources were proposed, accompanied by more efficient forward and inverse modelling tools that allowed us to combine several broadband sources and constrain electrical conductivity on multiple scales simultaneously. Magnetic signals due to oceanic tides were established as a new source to probe conductivity of the sub-oceanic upper mantle. Further, the launch of ESA Swarm satellites in 2013 and their successful ongoing operation have marked a new era in the field of large-scale EM induction, unlocking a set of new opportunities, but also posing new challenges. These developments were backed by new lab measurements of electrical conductivity for mantle minerals at temperatures and pressures that are getting closer to the relevant pressure and temperature conditions in the mantle, alleviating the need for inaccurate extrapolations. The latter enabled more plausible quantitative estimates of water content, melt fractions and temperature in the mantle. In parallel, crust and mantle conductivity models along with developed modelling techniques have become an integral part of geomagnetic field and geomagnetically induced currents (GICs) modelling workflows, establishing new inter-disciplinary knowledge domains.
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