Anna Kelbert

United States Geological Survey | USGS · Geomagnetism Program

Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with consequences for a range of tectonic and magmatic processes. Most of the fluid is released in the forearc where it plays a critical role in controlling the mechanical properties and seismic behaviour of the subduction megathrust. Here we present results from three-...

A study is made of the relations between geomagnetic and geoelectric field variation, Earth‐surface impedance, and operational interference (“anomalies”) experienced on electric‐power systems across the contiguous United States during the 13–14 March, 1989, magnetic storm. For this, a 1‐min‐resolution sequence of geomagnetic field maps is construct...

Magnetotellurics (MT) is a passive electromagnetic geophysical method that measures variations in subsurface electrical resistivity. MT data are collected in the time domain and processed in the frequency domain to produce estimates of a transfer function representing the Earth's electrical structure. Unfortunately, the MT community lacks metadata...

Intense geoelectric fields during geomagnetic storms drive geomagnetically induced currents in power grids and other infrastructure, yet there are limited direct measurements of these storm‐time geoelectric fields. Moreover, most previous studies examining storm‐time geoelectric fields focused on single events or small geographic regions, making it...

Magnetotellurics (MT) is a passive electromagnetic geophysical method that measures variations in subsurface electrical resistivity. MT data are collected in the time domain and processed in the frequency domain to produce estimates of a transfer function representing the Earth’s electrical structure. Unfortunately, the MT community lacks metadata...

An analysis is made of Earth‐surface geoelectric fields and voltages on electricity transmission power‐grids induced by a late‐phase E3 nuclear electromagnetic pulse (EMP). A hypothetical scenario is considered of an explosion of several hundred kilotons set several hundred kilometers above the eastern‐midcontinental United States. Ground‐level E3...

At present, the most reliable information for inferring storm‐time ground electric fields along electrical transmission lines comes from coarsely sampled, national‐scale magnetotelluric (MT) data sets, such as that provided by the EarthScope USArray program. An underlying assumption in the use of such data is that they adequately sample the spatial...

The hydrology and geology of the Xinzhou geothermal field in South China and its surrounding area have been well studied. Previous studies have indicated that mantle heat, granite radioactive heat, and partial melting are the likely key thermal sources. However, neither the geometry of the geothermal reservoir nor the groundwater circulation patter...

Subduction of hydrated oceanic lithosphere can carry water deep into the Earth, with important consequences for a range of tectonic and magmatic processes. Most fluid is released at relatively shallow depths in the forearc where it is thought to play a critical role in controlling mechanical properties and seismic behavior of the subduction megathr...

Geomagnetic perturbations (BGEO) related to magnetospheric ultralow frequency (ULF) waves induce electric fields within the conductive Earth—geoelectric fields (EGEO)—that in turn drive geomagnetically induced currents. Though numerous past studies have examined ULF wave BGEO from a space weather perspective, few studies have linked ULF waves with...

Geomagnetically induced currents (GICs) are quasi‐direct current (DC) electric currents that flow in technological conductors during geomagnetic storms. Extreme GICs are hazardous to man‐made infrastructure. GICs enter and exit the technological systems, such as the electric power grid, at grounding points, and their magnitudes depend on the curren...

A once‐per‐century geoelectric hazard map is created for the U.S. high‐voltage power grid. A statistical extrapolation from 31 years of magnetic field measurements is made by identifying 84 geomagnetic storms with the Kp and Dst indices. Data from 24 geomagnetic observatories, 1,079 magnetotelluric survey sites, and 17,258 transmission lines are ut...

Geomagnetic disturbances cause perturbations in the Earth’s magnetic field which, by the principle of electromagnetic induction, in turn cause electric currents to flow in the Earth. These geomagnetically induced currents (GICs) also enter man-made technological conductors that are grounded; notably, telegraph systems, submarine cables and pipeline...

Initial processing of magnetotelluric (MT) data collected at a site results in a small data file that defines the MT transfer functions or variants at a discrete set of frequencies. Although not data-intensive, the transfer function represents almost all of the information about Earth's conductivity found in the raw data, and provides a key input t...

S U M M A R Y The electromagnetic (EM) field generated by ocean tidal flow is readily detectable in both satellite magnetic field data, and in ocean-bottom measurements of electric and magnetic fields. The availability of accurate charts of tidal currents, constrained by assimilation of modern satellite altimetry data, opens the possibility of usin...

Geoelectric field time series can be estimated by convolving estimates of Earth‐surface impedance, such as those obtained from magnetotelluric survey measurements, with historical records of geomagnetic variation obtained at magnetic observatories. This straightforward procedure permits the mapping of geoelectric field variation during magnetic sto...

Estimation of ground level geoelectric fields has been identified by the National Space Weather Action Plan as a key component of assessment and mitigation of space weather impacts on critical infrastructure. Estimates of spatially and temporally variable electric fields are used to generate statistically based hazard maps and show promise toward m...

Maps are presented of extreme-value geoelectric field amplitude and horizontal polarization for the Northeast United States. These maps are derived from geoelectric time series calculated for sites across the Northeast by frequency-domain multiplication (time-domain convolution) of 172 magnetotelluric impedance tensors, acquired during a survey, wi...

Magnetic and electric field measurements at Earth’s surface provide information on Earth's interior and on space weather. An open-source central repository of these data has received a major update.

Maps of extreme value, horizontal component geoelectric field amplitude are constructed for the Pacific Northwest United States (and parts of neighboring Canada). Multidecade long geoelectric field time series are calculated by convolving Earth surface impedance tensors from 71 discrete magnetotelluric survey sites across the region with historical...

A review is given of the present feasibility for accurately mapping geoelectric fields across North America in near-realtime by modeling geomagnetic monitoring and magnetotelluric survey data. Should this capability be successfully developed, it could inform utility companies of magnetic-storm interference on electric-power-grid systems. That real-...

Commonly, one-dimensional (1D) Earth impedances have been used to calculate the voltages induced across electric power transmission lines during geomagnetic storms under the assumption that much of the three-dimensional structure of the Earth gets smoothed when integrating along power transmission lines. We calculate the voltage across power transm...

Maps of extreme-value geoelectric field amplitude are constructed for the Mid-Atlantic United States, a region with high population density and critically important power-grid infrastructure. Geoelectric field time series for the years 1983-2014 are estimated by convolving Earth-surface impedances obtained from 61 magnetotelluric survey sites acros...

Geoelectric fields at the Earth's surface caused by magnetic storms constitute a hazard to the operation of electric-power grids and related infrastructure. The ability to estimate these geoelectric fields in close to real time and provide local predictions would better equip the industry to mitigate negative impacts on their operations. Here, we r...

In support of a multi-agency project for assessing induction hazards, we present maps of extreme-value geoelectric amplitudes over about half of the continental United States. These maps are constructed using a parameterization of induction: estimates of Earth-surface impedance, obtained at discrete geographic sites from magnetotelluric survey data...

Long period global scale electromagnetic induction studies of deep Earth conductivity are based almost exclusively on magneto-variational methods, and require accurate models of external source spatial structure. We describe approaches to inverting for both the external sources and three-dimensional (3D) conductivity variations, and apply these met...

NOTE: The electric resistivity model file can be found at: URL：(https://ds.iris.edu/ds/products/emc-mcrmtyang-etal2015resistivity/) We present initial results from three-dimensional inversion of long period EarthScope magnetotelluric (MT) transportable array data from 232 sites covering the north-central US. The study area covers the 1.1 Ga Mid-Co...

EarthCube cyberinfrastructure will accelerate geoscientists ability to characterize and understand complex Earth systems, from the Space environment to Earth’s core, from the top of the atmosphere to the bottom of the sea, and throughout Earth History, by providing enhanced access to data, and new technologies and methods to integrate, analyze and...

In September, I participated in a general scientific discussion regarding the U.S. National Science Foundation Directorate for Geosciences (NSF GEO) Priorities and Frontiers 2015–2020 document. One of the key issues raised in conjunction with this document was the issue of science versus infrastructure. Although there was overwhelming agreement on...

We describe implementation of a modular system of computer codes for inversion of electromagnetic geophysical data, referred to as ModEM. The system is constructed with a fine level of modular granularity, with basic components of the inversion ─ forward modeling, sensitivity computations, inversion search algorithms, model parametrization and regu...

Global electromagnetic (EM) induction studies have been the focus of increasing attention during the past few years. A primary stimulus for this interest has been increased quality, coverage and variety of the newly available data sets especially from recent low-Earth-orbiting satellite missions. The combination of traditional ground-based data wit...

Results from a three-dimensional inversion of broadband magnetotelluric (MT) data collected at 68 stations, deployed along six intersecting profiles, are evaluated for accuracy and potentiality for yielding tectonic information. The MT profiles have different orientations from one another, making the data distribution over the area unusual for 3-D...

Combining long-period magnetotelluric data from the spatially uniform EarthScope USArray and higher-resolution profiles, we obtain a regional three-dimensional electrical resistivity model in the Snake River Plain and Yellowstone areas (Idaho and Wyoming, United States), and provide new constraints on the large-scale distribution of melt and fluids...

The Jacobian of the non-linear mapping from model parameters to observations is a key component in all gradient-based inversion methods, including variants on Gauss-Newton and non-linear conjugate gradients. Here, we develop a general mathematical framework for Jacobian computations arising in electromagnetic (EM) geophysical inverse problems. Our...

Providing new constraints on the origin of the Yellowstone / Snake River Plain (YSRP) hotspot system is an important contribution enabled the EarthScope program. This age-progressive track of rhyolitic volcanism has long been hypothesized as resulting from a deep mantle plume. Here we present an integrated view of new results from EarthScope seismi...

Poleward of 45-50 degrees (geomagnetic) observatory data are influenced significantly by auroral ionospheric current systems, invalidating the simplifying zonal dipole source assumption traditionally used for long period (T > 2 days) geomagnetic induction studies. Previous efforts to use these data to obtain the global electrical conductivity distr...

Long period (10-20,000 s) magnetotelluric (MT) data are being acquired in a series of temporary arrays deployed across the continental United States through the EMScope component of EarthScope. MT deployments in 2006-2011 have acquired data at 325 sites on an approximately regular grid, with the same nominal spacing as the USArray broadband seismic...

A spherical decomposition of the Biot-Savart law can be obtained by taking the quasi-static limit of the well-known free-space magnetic Green's function in spherical coordinates. Using this decomposition, the toroidal spherical harmonic coefficients of an arbitrary current system are explicitly given in terms of the physical currents, and the contr...

We have performed a set of three dimensional inversions of magnetotelluric data in the Snake River Plain and Yellowstone areas. We used a total of 73 sites from Earthscope MT Transportable Array (Idaho, Montana and Wyoming areas) and a subset of 19 sites from an earlier long-period MT survey in the Snake River Plain (SRP). Data for 14 periods from...

We have developed a modular system of computer codes for inversion of electromagnetic (EM) geophysical data (ModEM). ModEM allows for rapid adaptation of inversion algorithms developed for one purpose (e.g., three-dimensional magnetotellurics (MT)) to other EM problems (e.g., controlled source EM). The modular approach can also simplify maintenance...

Long period (10-20,000 s) magnetotelluric (MT) data are being acquired in a series of temporary arrays deployed across the continental United States through EMScope, a component of EarthScope, a multidisciplinary decade-long project to study the structure and evolution of the North American Continent. MT deployments in 2006-2010 have so far acquire...

Electrical conductivities of transition zone minerals have been shown to be highly sensitive to water content, with concentrations of the order of 1 wt% increasing conductivity by roughly an order of magnitude. Remote sensing of conductivity with electromagnetic geophysical methods thus has the potential to provide useful constraints on deep mantle...

Small amounts of water can significantly affect the physical properties of mantle materials, including lowering of the solidus, and reducing effective viscosity and seismic velocity. The amount and distribution of water within the mantle thus has profound implications for the dynamics and geochemical evolution of the Earth. Electrical conductivity...

We discuss the opportunities for efficient inversion of large magnetotelluric (MT) data sets in the scope of our flexible modular system for gradient based inversion of electromagnetic data (ModEM). This general electromagnetic (EM) inversion system has been developed in the past couple of years using an "object oriented" approach. It has been spec...

EMScope, the MT component of the Earthscope project has completed its final year of infrastructure construction, and its third annual campaign of regional magnetotelluric array operations in the western USA. Seven semi-permanent "backbone" MT observatories have been installed in California, Oregon, Montana, New Mexico, Minnesota, Missouri and Virgi...

We develop a non-linear conjugate gradient inversion for global long period electromagnetic induction studies. The scheme requires computation of derivatives of the regularized penalty functional. We derive analytical and numerical expressions for these derivatives, and the associated Jacobian, and show how these can be efficiently implemented by g...

We are developing a general modular system for gradient based inversion of electromagnetic (EM) data. The inversion code has been designed using an object oriented approach, with the highest levels of functionality independent of problem specifics. In particular, top level modules manipulate abstract data objects (such as data vectors, model parame...

As part of the EarthScope project in 2006-07 approximately 100 long period (5-15000 s) magnetotelluric (MT) sites were occupied covering Oregon and Washington. In contrast to traditional MT surveys where sites are concentrated along one or a few profiles, in the EarthScope project sites have been occupied on a quasi-uniform 75 km grid. Both the sit...

We address the non-linear ill-posed inverse problem of reconstructing the global three-dimensional distribution of electrical conductivity in Earth's mantle. The authors have developed a numerical regularized least-squares inverse solution based on the non-linear conjugate gradients approach. We apply this methodology to the most current low-freque...

The case for substantial heterogeneity in mantle conductivity has stimulated the development of methods for solving Maxwell's equations in a heterogeneous conducting sphere. A global 3-D frequency domain forward solver has been devised (Uyeshima & Schultz, 2000), accurate and efficient enough to be an attractive kernel of a practical inverse method...

We present initial results from application of a 3D spherical Earth non-linear conjugate gradients inversion code to existing catalogues of geomagnetic observatory response functions with periods in the range 5 - 107 days. Inverting several different subsets of these data, with varying regularization assumptions and model parametrizations, results...

We have performed a set of three dimensional inversions of magnetotelluric data in the Snake River Plain and Yellowstone areas. We used a total of 73 sites from USArray MT Transportable Array (Idaho, Montana and Wyoming areas) and a subset of 19 sites from an earlier long-period MT survey in the Snake River Plain (SRP). Data for 14 periods from 7.3...