Geology topics

Greg M. Lucas

Publications and source records attributed to Greg M. Lucas.

16 recordsLinked to original sources

USGS2026

Magnetic storms and geoelectric hazards

Magnetic storms induce geoelectric fields at Earth's surface that can interfere with grounded long-line systems. The September 1859 storm disrupted global telegraph operations, the March 1989 storm caused a blackout in Canada and interfered with electric-power-transmission systems in the United States, and other storms have had related impacts. The geographic and temporal dependence of geoelectric fields are functions of both geomagnetic variation and local surface impedance, which differ considerably across different geological regions. These dependencies can be mapped across the contiguous United States by combining magnetotelluric impedance tensors with ground magnetometer time series. This review illustrates such mapping for the 1989 storm and shows that power-system interference was experienced where surface impedance is high, and when and where geoelectric fields were intense. Statistical analyses indicate that storms comparable to that of March 1989 occur roughly once every four solar cycles. Ongoing developments in numerical modeling and real-time monitoring are anticipated to enable prediction of geoelectric hazards. ▪ Magnetic storms can induced electric fields in the solid Earth that interfere with electric-power-transmission systems. ▪ Geoelectric hazards depend on the storm-time geomagnetic disturbance and the electrical conductivity structure of Earth. ▪ Historically, impacts on telecommunication and power-transmission systems in the United States have been concentrated in the East and Midwest. ▪ The future occurrence of a magnetic superstorm could cause widespread disruption of electric-power-transmission systems.

Annual Review of Earth and Planetary Sciences↗

USGS2025

Mapping a Carrington storm

A map is presented of median 1-min-resolution peak geoelectric-field strength across the United States as would be induced by magnetic storms as intense as the 2 September 1859 Carrington storm. The map is constructed from two data sets: Magnetometer time series from 22 ground-based observatories recording 40 magnetic storms, and surface impedance tensors derived from magnetotelluric measurements acquired at 1616 survey sites across the contiguous United States. Carrington-class storm geoelectric fields are likely to be very strong in the United States East and Midwest; > 5.00 V/km at many places. In Virginia, strengths would likely range from 30.30 V/km, with a 68% confidence interval of [19.44,47.20] V/km, to as low as 0.05 [0.03,0.07] V/km. Comparison of model geopotentials with those measured on 30 long lines, indicates errors of about 18%. A Carrington-class storm would likely induce geoelectric fields with strengths 55% greater than for the 13–14 March 1989 storm.

contiguous United States↗

USGS2025

The impact of the May 1921 superstorm on American telecommunication systems

A compilation is presented of impacts (interference and damage) realized on long-line telegraph and telephone systems across North America during the 13-16 May 1921 magnetic storm. Impacts occurred primarily during local nighttime, after the third of four sudden commencements, and during the storm’s most-prominent main phase. Impacts are attributed to rapid and high-amplitude geomagnetic field variation generated by substorms. This induced potential di erences and between the grounding points of communication networks that were su cient to cause system interference and damage. In the United States, impacts were concentrated in the Midwest and in the East, regions characterized by high electromagnetic surface impedance. Given technological changes, modern telecommunication systems are less exposed to storms like that of May 1921, while power-grid systems are now more exposed to them.

Space Weather↗

USGS2025

Multi-scale intense geoelectric and geomagnetic field perturbations observed after an interplanetary magnetic field turning

Intense geoelectric fields during geomagnetic storms generate geomagnetically induced currents in power grids and other infrastructure, necessitating an understanding of their causes, for example, through coordinated space and ground observations. This study investigates localized intense geoelectric ( E ) and geomagnetic ( B ) field perturbations following an Interplanetary Magnetic Field (IMF) turning during a geomagnetic storm on 25 October 2011. Observations from EarthScope magnetotelluric sites in the upper Midwest United States revealed shorter period ( ∼ 1 min) ultra-low-frequency (ULF) waves superimposed on longer period ( ∼ 10 min) perturbations in both E and B fields. These sites, located at ∼ 19 hr magnetic local time and 56 −57 ° magnetic latitude, recorded large amplitude E and B perturbations. Ground-based all-sky imagers showed auroral brightening with sunward and poleward propagation, while upstream spacecraft linked the perturbations to an IMF turning and solar wind dynamic pressure impulse. The longer-period E and B field perturbations likely stem from localized ionospheric currents tied to substorm auroral activity post-IMF turning. The combination of ionospheric currents, ULF waves, and the Earth's varying conductivity produces intense geoelectric fields of ≥ 2 V/km in the upper Midwest. A comparison using input data and software compatible with the NOAA/USGS geoelectric field nowcast model revealed its limitations in capturing such events due to the temporal and spatial resolution of the underlying data. Using 1-s geomagnetic field data can improve geoelectric field models by capturing short-period and large spatial scale waves, although localized magnetic perturbations remain underestimated due to insufficient ground magnetometer density.

Space Weather↗

USGS2023

Correction to “The March 1940 superstorm: Geoelectromagnetic hazards and impacts on American communication and power systems”

In Love et al. (2023), 5 estimated geopotentials on long lines for the March 1989 storm are incorrect. This was the result of incorrect calculation of line integrals. This error affects the two paragraphs of Section 11 and Figure 10 of that paper. The most significant impact of this error is that we overestimated the average ratio of the geopotentials measured the long lines during the March 1940 storm to the estimated geopotentials for the March 1989 storm. Instead of being a factor of 9 higher, the average ratio is only 9% higher. This means the inductive geoeffectiveness of the March 1940 storm was comparable to, or just slightly greater than, the geoeffectiveness of the March 1989 storm. Corrected paragraphs and a corrected figure are provided here.

Space Weather↗

USGS2022

Modeling geomagnetic induction in submarine cables

Submarine cables have become a vital component of modern infrastructure, but past submarine cable natural hazard studies have mostly focused on potential cable damage from landslides and tsunamis. A handful of studies examine the possibility of space weather effects in submarine cables. The main purpose of this study is to develop a computational model, using Python , of geomagnetic induction on submarine cables. The model is used to estimate the induced voltage in the submarine cables in response to geomagnetic disturbances. It also utilizes newly acquired knowledge from magnetotelluric studies and associated investigations of geomagnetically induced currents in power systems. We describe the Python-based software, its working principle, inputs/outputs based on synthetic geomagnetic field data, and compare its operational capabilities against analytical solutions. We present the results for different model inputs, and find: 1) the seawater layer acts as a shield in the induction process: the greater the ocean depth, the smaller the seafloor geoelectric field; and 2) the model is sensitive to the Ocean-Earth layered conductivity structure.

Frontiers in Physics↗

USGS2022

Mapping a magnetic superstorm: March 1989 geoelectric hazards and impacts on United States power systems

A study is made of the relationships 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 March 13-14, 1989 magnetic storm. For this, a 1-minute-resolution sequence of geomagnetic field maps is constructed from magnetometer time series acquired at ground-based observatories. Induced geoelectric field maps are calculated by convolving the geomagnetic maps with magnetotelluric impedance tensors. During the storm, anomalies were concentrated where the lithosphere is electrically resistive, and when and where geoelectric field amplitudes were high. This was particularly true in the Mid-Atlantic, Northeast, and the upper Midwest. Few anomalies were experienced in other parts of the Midwest and across much of the West, where the lithosphere is more conductive, and when and where geoelectric field amplitudes were low. Peak 1-minute-resolution geoelectric field amplitude ranged from 21.66 V/km in Maine and 19.02 V/km in Virginia to < 0.02 V/km in Idaho. Latitude-dependent organization of geoelectric hazards by auroral-zone electrojet currents is detectable, but it is much weaker than geographic organization due to surface impedance. Hazardous geoelectric fields were induced during different storm phases, at different local times, and, by inference, by a variety of ionospheric currents. Compared to geoelectric field amplitudes realized across the United States during March 1989, hazard maps used by utility companies to estimate systems exposure have much less geographic detail and a much smaller maximum-to-minimum range in geoelectric field amplitude. Future research will benet from denser geomagnetic monitoring, additional magnetotelluric surveying, and access to power-system impact data.

Space Weather↗

USGS2021

Down to Earth with nuclear electromagnetic pulse: Realistic surface impedance aﬀects mapping of the E3 geoelectric hazard

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 geoelectric fields are estimated by convolving a standard parameterization of E3 geomagnetic field variation with magnetotelluric Earth-surface impedance tensors derived from wideband measurements acquired across the study region during a recent survey. These impedance tensors are a function of subsurface three-dimensional electrical conductivity structure. Results, presented as a movie-map, demonstrate that localized differences in surface impedance strongly distort the amplitude, polarization, and variational phase of induced E3 geoelectric fields. Locations with a high degree of E3 geoelectric polarization tend to have high geoelectric amplitude. Uniform half-space models and one-dimensional, depth-dependent models of Earth-surface impedance, such as those widely used in government and industry reports informing power-grid vulnerability assessment projects, do not provide accurate estimates of the E3 geoelectric hazard in complex geological settings. In particular, for the Eastern-Midcontinent, half-space models can lead to (order-one) overestimates/underestimates of EMP-induced geovoltages on parts of the power grid by as much as 1,000 volts (a range of 2,000 volts)—comparable to the amplitudes of the geovoltages themselves.

Earth and Space Sciences↗

USGS2021

Magnetotelluric sampling and geoelectric hazard estimation: Are national-scale surveys sufficient?

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 heterogeneity of the surface relationship between geomagnetic and geoelectric fields. Here, we assess the degree to which the density of MT data sampling affects geoelectric hazard assessments. For electrical transmission networks in each of four focus regions across the contiguous United States, we perform two parallel band-limited (10 1 –10 3 s) hazard analyses: one using only USArray-style (∼70-km station spacing) MT data, and one incorporating denser (≪70-km station spacing) MT data. We find that the use of USArray-style MT sampling alone provides a useful first-order estimate of integrated geoelectric fields along electrical transmission lines. However, we also find that the use of higher density MT data can in some areas lead to order-of-magnitude differences in line-averaged electric field estimates at the level of individual transmission lines and can also yield significant differences in subregional hazard patterns. As we demonstrate using variogram plots, these differences reflect short-spatial-scale variability in Earth conductivity, which in turn reflects regional lithotectonic structure and history. We also provide a cautionary example in the use of electrical conductivity models to predict dense MT data; although valuable for hazard applications, models may only be able to reproduce surface geoelectric fields as captured by the MT data from which they were derived.

AGU Space Weather↗

USGS2020

Modified GIC estimation using 3-D Earth conductivity

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 currents that flow underground. They are, therefore, a function of the Earth's electrical conductivity, represented at ground level as Earth impedances, as well as the resistance parameters of the power network. Traditional GIC estimation practices are based on Earth impedances obtained from laterally homogeneous or piecewise layered-Earth models. We refer to these methods, collectively, as the 1-D approximation. However, GIC hazard mitigation can be improved with more accurate GIC modeling that takes the spatially heterogeneous Earth's conductivity into account. Here, we propose a modified approximation for GIC estimation that is very similar to the 1-D approximation but is instead derived from empirical 3-D Earth impedances. Our formulation sets up the computation of static, frequency-dependent power line telluric response functions, which, once computed, may be considered part of the power grid system model. These response functions may then be used for historical scenario analysis of GIC hazards and for simplified real-time, albeit approximate, GIC estimation in a power grid. This modest modification to the simpler local field formulation approach avoids real-time integration of geoelectric fields along power lines while taking the realistic 3-D Earth into account in a rigorous manner. Once implemented, the method provides a power grid operator with the benefits of convenience and computational speed for a first look real-time operational GIC hazard assessment. We estimate that the proposed modified 3-D GIC modeling approach produces GIC values that are well within 50% of those obtained with the full-scale power line integration of spatially variable geoelectric fields, for storms comparable in scale to the 2003 Halloween storm, all geological structures, and power lines located in the contiguous United States and other low- to middle-latitude regions.

Space Weather↗

USGS2020

A 100-year geoelectric hazard analysis for the U.S. high-voltage power grid

A once-per-century geoelectric hazard map is created for the United States 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, 1079 magnetotelluric survey sites, and 17,258 transmission lines are utilized to perform a geoelectric hazard analysis with the most comprehensive data publicly available. With this data we estimate once-per-century geoelectric fields at the magnetotelluric survey sites and calculate the theoretical voltages within transmission lines in the United States power grid. Once-per-century geoelectric field strengths span more than three orders of magnitude from a minimum of 0.02 V/km at a site in Idaho to a maximum of 26.8 V/km at a site in Maine, with nearly 30% of the surveyed land area ex- ceeding 1 V/km. We show the influence that geoelectric field polarization has on geoelec- tric hazards when viewed on a power transmission network. The calculated transmission line voltages can exceed 1000 V in some transmission lines. Four regions in the United States with particularly notable geoelectric hazards are identified and discussed: the East coast, Pacific Northwest, upper Midwest, and the Denver metropolitan area.

Space Weather↗

USGS2019

Empirical estimation of natural geoelectric hazards

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 storms. Statistical analysis of the time series allows extrapolation to extreme‐value amplitudes, such as might be realized during an intense magnetic storm in the future. The development of these products is illustrated for the Mid‐Atlantic United States, using impedances obtained from EarthScope survey data and geomagnetic variation records obtained at the Fredericksburg observatory operated by the U.S. Geological Survey. For this region, 100‐year geoelectric exceedance amplitudes have a range of almost three orders of magnitude (from 0.04 V/km at a site in southern Pennsylvania to 24.29 V/km at a site in central Virginia), and they have significant geographic granularity, which is due to site‐to‐site differences in surface impedance (and subsurface electrical conductivity structure). Maps of 100‐year exceedance amplitudes resemble those of geoelectric amplitudes for the March 1989 magnetic storm, and, in that sense, the March 1989 storm resembles what might be loosely called a “100‐year” event.

Book chapter↗

USGS2018

Geoelectric hazard maps for the Pacific Northwest

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 1‐min (2‐min Nyquist) geomagnetic variation time series obtained from two nearby observatories. After fitting statistical models to 1‐min geoelectric amplitudes realized during magnetic storms, extrapolations are made to estimate threshold amplitudes that are only exceeded, on average, once per century. One hundred‐year geoelectric exceedance amplitudes range from 0.06 V/km at a survey site in western Washington State to 9.47 V/km at a site in southeast British Columbia; 100‐year geoelectric exceedance amplitudes equal 7.10 V/km at a site north of Seattle and 2.28 V/km at a site north of Portland. Systematic and random errors are estimated to be less than 20%, much less than site‐to‐site differences in geoelectric amplitude that arise from site‐to‐site differences in surface impedance. Maps of 100‐year exceedance amplitudes are compared with the peak geoelectric amplitudes realized during the March 1989 magnetic superstorm; it is noted that some storms of relatively modest intensity can generate localized geoelectric fields of relatively high amplitude. The geography of geoelectric hazard across the Pacific Northwest is closely related to known geologic and tectonic structures.

Pacific Northwest↗

USGS2018

Calculation of voltages in electric power transmission lines during historic geomagnetic storms: An investigation using realistic earth impedances

Commonly, one-dimensional (1-D) 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 transmission lines in the mid-Atlantic region with both regional 1-D impedances and 64 empirical 3-D impedances obtained from a magnetotelluric survey. The use of 3-D impedances produces substantially more spatial variance in the calculated voltages, with the voltages being more than an order of magnitude different, both higher and lower, than the voltages calculated utilizing regional 1-D impedances. During the March 1989 geomagnetic storm 62 transmission lines exceed 100 V when utilizing empirical 3-D impedances, whereas 16 transmission lines exceed 100 V when utilizing regional 1-D impedances. This demonstrates the importance of using realistic impedances to understand and quantify the impact that a geomagnetic storm has on power grids.

Space Weather↗

USGS2018

Geoelectric hazard maps for the Mid-Atlantic United States: 100 year extreme values and the 1989 magnetic storm

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 across the Mid‐Atlantic with historical 1 min (2 min Nyquist) measurements of geomagnetic variation obtained from a nearby observatory. Statistical models are fitted to the maximum geoelectric amplitudes occurring during magnetic storms, and extrapolations made to estimate threshold amplitudes only exceeded, on average, once per century. For the Mid‐Atlantic region, 100 year geoelectric exceedance amplitudes have a range of almost 3 orders of magnitude (from 0.04 V/km at a site in southern Pennsylvania to 24.29 V/km at a site in central Virginia), and they have significant geographic granularity, all of which is due to site‐to‐site differences in magnetotelluric impedance. Maps of these 100 year exceedance amplitudes resemble those of the estimated geoelectric amplitudes attained during the March 1989 magnetic storm, and, in that sense, the March 1989 storm resembles what might be loosely called a “100 year” event. The geoelectric hazard maps reported here stand in stark contrast with the 100 year geoelectric benchmarks developed for the North American Electric Reliability Corporation.

Geophysical Research Letters↗

USGS2018

Extreme‐value geoelectric amplitude and polarization across the northeast United States

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, with decades‐long, 1‐min resolution time series of geomagnetic variation, acquired at three magnetic observatories. The maps show that, during intense magnetic storms, high geoelectric amplitude hazards are realized across electrically resistive, igneous and metamorphic rock of the Appalachian Mountains and the New England Highlands, while low geoelectric hazards are realized across electrically conductive, sedimentary rock of the Appalachian Plateau and the Mid‐Atlantic Coastal Plain. From statistical extrapolation, once‐per‐century (100‐year) geoelectric amplitudes are highest at a site in Virginia at 25.44 V/km (followed by a site in Maine at 21.75 V/km and a site in Connecticut at 19.39 V/km); 100‐year geoelectric amplitude exceeds 10 V/km at 12 sites across the northeast; geoelectric amplitude is lowest at a site in Virginia at 0.05 V/km. Average errors for these values are estimated to be about 38%, or much less than the more than 2 orders of magnitude range seen in geoelectric amplitudes from one survey site to another across the northeast. It is noteworthy that geoelectric fields tend to be most (least) polarized at locations with high (low) geoelectric hazard. Furthermore, geoelectric fields over the Appalachians tend to be polarized southeast‐to‐northwest, or generally in a direction orthogonal to the southwest‐to‐northeast geological strike. Results reported here inform utility companies in projects for evaluating and managing the response of power grid systems to the deleterious effects of geomagnetic disturbance.

Connecticut, Delaware, Maine, Maryland, Massachuse↗