This study presents global 3‐D electrical conductivity models of the world ocean and marine sediments. Electrical conductivity of the ocean was calculated by invoking the Equation of State of ...Seawater (TEOS‐10) with temperature and salinity data retrieved from the World Ocean Atlas and a series of high‐resolution regional ocean climatology data sets. The resolution of the ocean conductivity atlas varies between 0.1° and 0.25° globally. The conductivity of marine sediments was estimated by using compaction and thermal gradient models constrained by real observations on a 5‐arc‐minute global marine sediment thickness grid. I present numerical simulations of electromagnetic (EM) induction responses that demonstrate a significant effect of 3‐D electrical conductivity of the ocean and marine sediments on EM responses for a broad range of frequencies. I show that both marine and land‐based surveys designed for subsurface conductivity imaging or Space Weather modeling will benefit from inclusion of more realistic conductivity models of the ocean and seabed sediments.
Key Points
New model of 3‐D electrical conductivity of marine sediments
New 3‐D electrical conductivity atlas of the world ocean with combined global and regional data
Ocean and marine sediments have profound effect on electromagnetic responses used for subsurface imaging and space weather impact evaluation
Abstract
This paper reports on new results in the determination of magnetic signals produced by oceanic tides as estimated from satellite magnetic measurements. We find that combining data from the ...past CHAMP (2000–2010) and the present Swarm (since 2013) satellite missions significantly improves the quality of the extracted tidal signals, in particular if along‐track and cross‐track magnetic “gradient” data are utilized. This allows us to determine the magnetic signature not only of the
M
2
tide but also of the much weaker
N
2
and
O
1
tidal constituents. To minimize disturbances from magnetospheric and ionospheric currents, we only use data from the nightside region during geomagnetic quiet conditions and remove core, crustal, and magnetospheric field contributions as given by the CHAOS geomagnetic field model. Despite their small magnitudes, all determined tidal constituents show sensitivity to the electrical conductivity profile of the underlying mantle, enabling imaging the upper mantle below the oceans.
Key Points
Global models of the magnetic signals produced by the
M
2
,
N
2
, and
O
1
oceanic tides have been extracted from satellite magnetic measurements
Combining data from the Swarm and CHAMP missions as well as using field gradient plays key role in the robustness of the estimated models
All tidal constituents show significant sensitivity to the electrical conductivity of the oceanic mantle
Electric currents induced in conductive planetary interiors by time‐varying magnetospheric and ionospheric current systems have a significant effect on electromagnetic (EM) field observations. ...Complete characterization of EM induction effects is difficult owing to nonlinear interactions between the three‐dimensional electrical structure of a planet and spatial complexity of inducing current systems. We present, a general framework for time‐domain modeling of three‐dimensional EM induction effects in heterogeneous conducting planets. Our approach does not assume that the magnetic field is potential, allows for an arbitrary distribution of electrical conductivity within a planet, and can deal with spatially complex time‐varying current systems. The method is applicable to both data measured at stationary observation sites and satellite platforms, and enables the calculation of three‐dimensional EM induction effects in near real‐time settings.
Key Points
Accurate modeling of electromagnetic (EM) induction effects in ground and satellite observations via local and global impulse responses
Including three‐dimensional EM induction effects improves description of the observed magnetic variations during both quiet and disturbed conditions
We provide a data constrained model of external currents for the Swarm era
We present the CHAOS-7 model of the time-dependent near-Earth geomagnetic field between 1999 and 2020 based on magnetic field observations collected by the low-Earth orbit satellites
Swarm
, ...CryoSat-2, CHAMP, SAC-C and Ørsted, and on annual differences of monthly means of ground observatory measurements. The CHAOS-7 model consists of a time-dependent internal field up to spherical harmonic degree 20, a static internal field which merges to the LCS-1 lithospheric field model above degree 25, a model of the magnetospheric field and its induced counterpart, estimates of Euler angles describing the alignment of satellite vector magnetometers, and magnetometer calibration parameters for CryoSat-2. Only data from dark regions satisfying strict geomagnetic quiet-time criteria (including conditions on IMF
B
z
and
B
y
at all latitudes) were used in the field estimation. Model parameters were estimated using an iteratively reweighted regularized least-squares procedure; regularization of the time-dependent internal field was relaxed at high spherical harmonic degree compared with previous versions of the CHAOS model. We use CHAOS-7 to investigate recent changes in the geomagnetic field, studying the evolution of the South Atlantic weak field anomaly and rapid field changes in the Pacific region since 2014. At Earth’s surface a secondary minimum of the South Atlantic Anomaly is now evident to the south west of Africa. Green’s functions relating the core–mantle boundary radial field to the surface intensity show this feature is connected with the movement and evolution of a reversed flux feature under South Africa. The continuing growth in size and weakening of the main anomaly is linked to the westward motion and gathering of reversed flux under South America. In the Pacific region at Earth’s surface between 2015 and 2018 a sign change has occurred in the second time derivative (acceleration) of the radial component of the field. This acceleration change took the form of a localized, east–west oriented, dipole. It was clearly recorded on ground, for example at the magnetic observatory at Honolulu, and was seen in
Swarm
observations over an extended region in the central and western Pacific. Downward continuing to the core–mantle boundary, we find this event originated in field acceleration changes at low latitudes beneath the central and western Pacific in 2017.
Continental rifting is initiated by a dynamic interplay between tectonic stretching and mantle upwelling. Decompression melting assists continental breakup through lithospheric weakening and enforces ...upflow of melt to the Earth's surface. However, the details about melt transport through the brittle crust and storage under narrow rift‐aligned magmatic segments remain largely unclear. Here we present a crustal‐scale electrical conductivity model for a magmatic segment in the Ethiopian Rift, derived from 3‐D phase tensor inversion of magnetotelluric data. Our subsurface model shows that melt migrates along preexisting weak structures and is stored in different concentrations on two major interconnected levels, facilitating the formation of a convective hydrothermal system. The obtained model of a transcrustal magmatic system offers new insights into rifting mechanisms, evolution of magma ascent, and prospective geothermal reservoirs.
Plain Language Summary
The Earth's continental plates can be broken apart by tectonic forces to form rift systems that may extend over thousands of kilometers. Continental rifts are often associated with numerous volcanoes that are fed by magma, which forms deep in the Earth's mantle and rises up along pathways of the fractured tectonic plate. A detailed understanding of the volcanic systems is important for hazard assessment and for geothermal energy production. We analyzed geophysical data that were measured at a volcanic field in the Ethiopian Rift Valley. The data consist of time series of the natural electric and magnetic fields. These fields are sensitive to the electrical conductivity structure of the Earth and can be used to image the subsurface electrical properties down to depths of many kilometers. Magma, for example, has typically very high electrical conductivities, which is why this method is ideal to detect melt reservoirs in the Earth's crust. Our obtained model shows, in great detail, how magma is transported through the crust and how it is stored below the volcanic system. Therefore, it provides new insights into rift‐associated volcanism, risk assessment, and geothermal energy production.
Key Points
Magmatic system in continental rift setting extends over entire crust
Shallow crustal partial melt zone drives convective hydrothermal system
Three‐dimensional magnetotelluric phase tensor inversion is an ideal tool for imaging volcanic systems and for geothermal exploration
This paper reports on new results in the determination of magnetic signals produced by oceanic tides as estimated from satellite magnetic measurements. We find that combining data from the past CHAMP ...(2000–2010) and the present Swarm (since 2013) satellite missions significantly improves the quality of the extracted tidal signals, in particular if along‐track and cross‐track magnetic “gradient” data are utilized. This allows us to determine the magnetic signature not only of the M2 tide but also of the much weaker N2 and O1 tidal constituents. To minimize disturbances from magnetospheric and ionospheric currents, we only use data from the nightside region during geomagnetic quiet conditions and remove core, crustal, and magnetospheric field contributions as given by the CHAOS geomagnetic field model. Despite their small magnitudes, all determined tidal constituents show sensitivity to the electrical conductivity profile of the underlying mantle, enabling imaging the upper mantle below the oceans.
Key Points
Global models of the magnetic signals produced by the M2, N2, and O1 oceanic tides have been extracted from satellite magnetic measurements
Combining data from the Swarm and CHAMP missions as well as using field gradient plays key role in the robustness of the estimated models
All tidal constituents show significant sensitivity to the electrical conductivity of the oceanic mantle
We present a robust and scalable solver for time-harmonic Maxwell's equations for problems with large conductivity contrasts, wide range of frequencies, stretched grids and locally refined meshes. ...The solver is part of the fully distributed adaptive 3-D electromagnetic modelling scheme which employs the finite element method and unstructured non-conforming hexahedral meshes for spatial discretization using the open-source software deal.II. We use the complex-valued electric field formulation and split it into two real-valued equations for which we utilize an optimal block-diagonal pre-conditioner. Application of this pre-conditioner requires the solution of two smaller real-valued symmetric problems. We solve them by using either a direct solver or the conjugate gradient method pre-conditioned with the recently introduced auxiliary space technique. The auxiliary space pre-conditioner reformulates the original problem in form of several simpler ones, which are then solved using highly efficient algebraic multigrid methods.
In this paper, we consider the magnetotelluric case and verify our numerical scheme by using COMMEMI 3-D models. Afterwards, we run a series of numerical experiments and demonstrate that the solver converges in a small number of iterations for a wide frequency range and variable problem sizes. The number of iterations is independent of the problem size, but exhibits a mild dependency on frequency. To test the stability of the method on locally refined meshes, we have implemented a residual-based a posteriori error estimator and compared it with uniform mesh refinement for problems up to 200 million unknowns. We test the scalability of the most time consuming parts of our code and show that they fulfill the strong scaling assumption as long as each MPI process possesses enough degrees of freedom to alleviate communication overburden. Finally, we refer back to a direct solver-based pre-conditioner and analyse its complexity in time. The results show that for multiple right-hand sides the direct solver-based pre-conditioner can still be faster for problems of medium size. On the other hand, it also shows non-linear growth in memory, whereas the auxiliary space method increases only linearly.
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