Alexander Grayver

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Grayver

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Alexander

ORCID

0000-0003-1132-3705

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Publications 1 - 10 of 18

Multi-scale imaging of 3-D electrical conductivity structure under the contiguous US constrains lateral variations in the upper mantle water content
Munch, Federico D.; Grayver, Alexander (2023)
Earth and Planetary Science Letters
The magnetotelluric (MT) component of the USArray survey is a unique data set that enables imaging the electrical conductivity distribution from the surface down to astenospheric depths. Here, we present a new 3-D electrical conductivity model (MECMUS-2022) derived by inverting data from 1291 USArray MT stations covering ∼80% of the contiguous United States on a quasi-regular 70-km grid. The inversion was performed using a novel multi-scale imaging approach that can consistently incorporate a large range of spatial scales and perform 3-D modeling directly in the spherical frame, completely avoiding the conventional flat-Earth assumption. Furthermore, the use of locally refined meshes allows us to take into account the complex coastline and mimic the natural resolution footprint of broadband MT transfer functions. We find conductivity variations that correlate with known continental structures such as due to the active tectonic processes within the western United States (e.g., Yellowstone hotspot, Basin and Range extension, and subduction of the Juan de Fuca slab) as well as the presence of deep roots beneath cratons. We further interpreted conductivity variations in terms of the upper mantle water content by coupling electrical conductivity with constrains on mantle thermo-chemical structure derived from the analysis of seismic data (in the form of P-to-s and S-to-p receiver functions). Our results suggest the existence of a relatively dry upper mantle beneath the United States, with a weak trend in the upper mantle water content from North to South.
The geophysical signature of a continental intraplate volcanic system: From surface to mantle source
Comeau, Matthew J.; Becken, Michael; Grayver, Alexander; et al. (2022)
Earth and Planetary Science Letters
The structure of continental intraplate volcanic systems — which occur far from tectonic boundaries, unlike the majority of Earth's volcanism — is enigmatic and not fully understood, as are the underlying mechanisms responsible, due in part to a lack of high-resolution geophysical data. Central Mongolia contains Quaternary–Neogene aged alkaline basalt flows and volcanic cones, thousands of kilometres from active tectonic margins, in addition to an abundance of geochemical and petrological data — making this a natural laboratory to study intraplate volcanism. Using a recently collected, high-resolution, multi-scale, magnetotelluric dataset acquired across central Mongolia, we generate and analyze electrical resistivity models of the structure beneath the Tariat and Chuluut volcanic zones with the goal of imaging the volcanic system from surface to mantle source. The models reveal narrow, subvertical, lower resistivity anomalies in the middle-upper crust that are conspicuously located beneath surface expressions of volcanism. The lower crust (depths of 25–50 km) is characterized by the widespread distribution of isolated low-resistivity zones. A local low-resistivity zone is imaged in the mantle (depths of 60–90 km) above a broad, homogenous, doming low-resistivity feature. Considering the available evidence, we propose that the low-resistivity anomalies in the middle-upper crust are the remnant signatures of past transient magma pathways (or collection of pathways), caused by metasomatic alteration during the ascent of hot magmatic fluids. The lower crustal anomalies are interpreted to be domains of saline fluids in a thermally perturbed lower crust. In the mantle, the low-resistivity structure is explained by a broad mantle upwelling and thermal anomaly with a local zone of low-percent partial melt — the source for intraplate volcanism. The geophysical images are consistent with geochemical and petrological evidence from erupted lavas that indicates a single common mantle source region, limited crustal contamination, and rapid direct ascent, making crustal magma storage unlikely. Thus the geophysical models show remarkable and unique translithospheric images of a continental intraplate volcanic system, from surface to mantle source, with the results relevant to other continental regions.
Aluto-Langano Geothermal Field, Ethiopia: Multiscale Image of Trans-crustal Magmatic System by Combining Local 3D and Regional 2D Magnetotelluric Soundings
Dambly, Marie Luise Texas; Samrock, Friedemann; Grayver, Alexander; et al. (2021)
Proceedings World Geothermal Congress 2020+1
Geothermal energy has the potential to support Ethiopia in its attempt to overcome energy poverty and provide sustainable energy for all. The formation and distribution of geothermal energy resources in Ethiopia is associated with magmatic activity along the Main Ethiopian Rift (MER). Until now, geothermal exploration mainly concentrated on shallow magmatic-hydrothermal systems underneath volcanoes in the central part of the MER. At the geothermal field of the Aluto volcano local 3-D magnetotelluric (MT) surveys successfully imaged the geothermal reservoir leading to the placement of productive geothermal wells. However, due to the limited aperture (15 km) of the surveys it was not possible to image the lower crustal magmatic plumbing system of the volcano. A regional MT study along a 120 km long profile transecting the rift and Aluto volcano was conducted within the project RiftVolc. They imaged an off-rift axis magmatic system in the Silti-Debre Zeyit Fault Zone (SDFZ) west of the central rift. However, the feeding zone of Aluto’s magmatic system remained enigmatic. In this study we combined the two existing datasets, resulting in a selection of 165 stations located at Aluto and 26 stations along the 120 km long RiftVolc cross-rift profile. The preliminary 3-D model revealed the existence of a deep conductor in about 35 km of depth on the western side of the rift, indicating that partial melt is present.
Simultaneous inversion for source field and mantle electrical conductivity using the variable projection approach
Min, Jingtao; Grayver, Alexander (2023)
Earth, Planets and Space
Time-varying electromagnetic field observed on the ground or at a spacecraft consists of contributions from (i) electric source currents, such as those in the ionosphere and magnetosphere, and (ii) corresponding fields induced by source currents within the conductive Earth’s interior by virtue of electromagnetic induction. Knowledge about the spatio-temporal structure of inducing currents is a key component in ionospheric and magnetospheric studies, and is also needed in space weather hazard evaluation, whereas the induced currents depend on the Earth’s subsurface electrical conductivity distribution and allow us to probe this physical property. In this study, we present an approach that reconstructs the inducing source and subsurface conductivity structures simultaneously, preserving consistency between the two models by exploiting the inherent physical link. To achieve this, we formulate the underlying inverse problem as a separable nonlinear least-squares (SNLS) problem, where inducing current and subsurface conductivity parameters enter as linear and nonlinear model unknowns, respectively. We solve the SNLS problem using the variable projection method and compare it with other conventional approaches. We study the properties of the method and demonstrate its feasibility by simultaneously reconstructing the ionospheric and magnetospheric currents along with a 1-D average mantle conductivity distribution from the ground magnetic observatory data.
Interior Heating of Rocky Exoplanets from Stellar Flares with Application to TRAPPIST-1
Grayver, Alexander; Bower, Dan J.; Saur, Joachim; et al. (2022)
The Astrophysical Journal Letters
Many stars of different spectral types with planets in the habitable zone are known to emit flares. Until now, studies that address the long-term impact of stellar flares and associated coronal mass ejections (CMEs) assumed that the planet’s interior remains unaffected by interplanetary CMEs, only considering the effect of plasma/UV interactions on the atmosphere of planets. Here, we show that the magnetic flux carried by flare-associated CMEs results in planetary interior heating by ohmic dissipation and leads to a variety of interior–exterior interactions. We construct a physical model to study this effect and apply it to the TRAPPIST-1 star whose flaring activity has been constrained by Kepler observations. Our model is posed in a stochastic manner to account for uncertainty and variability in input parameters. Particularly for the innermost planets, our results suggest that the heat dissipated in the silicate mantle is both of sufficient magnitude and longevity to drive geological processes and hence facilitate volcanism and outgassing of the TRAPPIST-1 planets. Furthermore, our model predicts that Joule heating can further be enhanced for planets with an intrinsic magnetic field compared to those without. The associated volcanism and outgassing may continuously replenish the atmosphere and thereby mitigate the erosion of the atmosphere caused by the direct impact of flares and CMEs. To maintain consistency of atmospheric and geophysical models, the impact of stellar flares and CMEs on atmospheres of close-in exoplanetary systems needs to be studied in conjunction with the effect on planetary interiors.
Probing the state of felsic magma reservoirs through integrated magnetotelluric-petrological modelling
Samrock, Friedemann; Grayver, Alexander; Bachmann, Olivier; et al. (2021)
AGU Fall Meeting Abstracts
Geophysically guided well siting at the Aluto-Langano geothermal reservoir
Samrock, Friedemann; Grayver, Alexander; Dambly, Luise; et al. (2023)
Geophysics
Volcano-hosted high-temperature geothermal reservoirs are powerful resources for green electricity generation. In regions where such resources are available, geothermal energy often provides a large share of a country’s total power generation capacity. Sustainable geothermal energy utilization depends on the successful siting of geothermal wells, which in turn depends on 1 prior geophysical subsurface imaging and reservoir characterization. Electromagnetic resistivity imaging methods have proven to be a key tool for characterizing magma-driven geothermal systems, since resistivity is sensitive to the presence of melt and clays that form through hydrothermal alteration. Special emphasis is often given to the clay cap, which forms on top of hydrothermal reservoirs along the flow paths of convecting geothermal fluids. As an example, the Aluto-Langano volcanic geothermal field in Ethiopia was covered with 178 densely spaced magnetotelluric stations. The 3-D electrical conductivity model derived from the magnetotelluric data images the magma body that acts as a heat source of the geothermal system, controlling geothermal convection and formation of alteration zones (commonly referred to as”clay cap”) atop the geothermal reservoir. Detailed 3-D imaging of the clay cap topography can provide a direct insight into hydrothermal flow patterns and helps identify potential upflow zones. At Aluto all productive geothermal wells were drilled into zones of clay cap thinning and updoming, which is indicative for underlying hydrothermal upflow zones. In contrast, non-productive wells were drilled into zones of clay cap thickening and lowering, which is an indicator for underlying outflow zones and cooling. This observation is linked to fundamental characteristics of volcano-hosted systems and can likely be adapted to other geothermal fields where sufficiently detailed MT surveys are available. Therefore, high-resolution 3-D electromagnetic imaging of hydrothermal alteration products (”clay cap”) can be used to infer the hydrothermal flow patterns in geothermal reservoirs and contributes to derisking geothermal drilling projects.
Constraining the size and state of magma reservoirs through a quantitative approach combining MT, lab measurements and petrological modelling
Samrock, Friedemann; Grayver, Alexander; Bachmann, Olivier; et al. (2022)
Abstract, 25th EM Induction Workshop
Magnetotelluric measurements are a powerful tool to image the subsurface under active volcanic regions. 3-D models, computed from magnetotelluric data, nowadays provide detailed multi-scale images of the electrical conductivity distribution. Since electrical conductivity is predominantly controlled by the presence of melt and fluid phases, conductivity models proved to be highly suitable for mapping the distribution of melt and for constraining melt fractions. Melt fractions can be estimated by combining laboratory models for melt electrical conductivity and mixing laws to derive the bulk electrical conductivity of multiphase systems. Melt electrical conductivity depends on the composition of the melt, the amount of dissolved water as well as temperature and pressure conditions. However, estimates of melt fractions are often based on arbitrary combinations of these parameters, and they do not consider the dependance of melt interconnectedness on melt fraction. Here we present an interdisciplinary approach to interpret electric conductivity models from three volcanoes in the Main Ethiopian Rift. We use rhyolite-MELTS to model magma crystallization and storage conditions constrained by petrological analyses of on-site erupted products. To derive the electrical conductivity of melt during fractional crystallization we derive an expanded melt electrical conductivity model by interpolating between existing models for rhyolite, dacite and andesite melt. Thereby we obtain a generalized model that describes the electrical conductivity of melt in dependence on the SiO2 and H2O content, pressure and temperature. These parameters are given by rhyolite-MELTS. Decreasing melt connectedness with diminishing melt fraction is considered by varying the cementation exponent, m, in the generalized Archie’s law whilst taking into account conservation of connectedness. Furthermore, we describe the magma reservoirs as three-phase systems consisting of crystals, melt and magmatic volatiles. The results show that this approach enables us to constrain the current state of magma reservoirs in terms of melt fraction, temperature and free volatile abundance. The latter is of eminent importance when discriminating between the two major mechanisms that drive volcanic unrest: magma on the move or increased degassing of a crystallizing magmatic system, so-called second boiling. With this study we demonstrate the great capability of the presented interdisciplinary solution approach that combines geophysical observations, petrological probes and laboratory models to capture the current state of volcanoes. The outcome is of major importance when it comes to realistic volcanic hazard assessment and geothermal energy applications that require a detailed understanding of magmatic heat sources that sustain geothermal reservoirs.
Assimilation of Ground and Satellite Magnetic Observations Unravels the Ionospheric, Magnetospheric, and Induced Fields During the May and October 2024 Geomagnetic Storms
Grayver, Alexander; Min, Jingtao; Olsen, Nils; et al. (2025)
Geophysical Research Letters
The geomagnetic storms of May and October 2024 were the strongest storms of solar cycle 25, with the (Formula presented.) index as low as −412 nT and auroras down to low latitudes. To investigate these extreme events, we developed a global magnetic field model of magnetospheric, ionospheric, and Earth-induced contributions obtained by assimilating magnetic observations. To leverage both ground observatory and multi-satellite magnetic data from the Swarm, CryoSat-2, GRACE-FO, and MSS-1 satellites, we used a new modeling approach to separate the magnetospheric and ionospheric sources, and to model both external and the Earth-induced fields with a 1-hr time resolution. The high spatio-temporal resolution of the model enables investigating the coupling between the mid-latitude ionospheric and magnetospheric field sources during all phases of the geomagnetic storms. The model reveals the total magnetic field disturbance at satellite and ground altitudes, providing critical input for space weather and a more accurate representation of the geomagnetic field.
The formation of geothermal systems in the context of magma-assisted continental rifting: Magnetotelluric models from the Main Ethiopian Rift (MER)
Dambly, Luise; Samrock, Friedemann; Grayver, Alexander; et al. (2022)
25th Electromagnetic Induction Workshop (EMIW 2022) Book of Abstracts

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