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Darcy Cordell
PhD Geophysics
dcordell@ualberta.ca

using electromagnetic geophysical methods for a wide array of applications

Research interests include:

volcanic hazards
earthquake hazards
space weather hazards
subduction zones
geothermal energy
and more
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Current Research Projects

My main area of expertise uses magnetotelluric methods to image the electrical conductivity of the Earth for a wide array of applications
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Space Weather Hazards in Alberta I am currently an NSERC Postdoctoral Fellow at the University of Alberta investigating hazards to the electric power grid during geomagnetic storms.
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Estimating Magma Reservoir Conditions Estimating magma reservoir conditions (e.g. temperature, water content, melt fraction) using electrical conductivity and petrological-thermodynamic models.
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Earthquakes and Fluids in Costa Rica Imaging subduction zone fluids and their relationship to slow slip and earthquakes on the Nicoya Peninsula, Costa Rica. In collaboration with the Electromagnetic Geophysics Lab at Georgia Tech .

Education and Experience

BSc Geophysics (Hon), University of Calgary (2013)

PhD Geophysics, University of Alberta (2020)

Postdoctoral Fellow, Georgia Institute of Technology (2022)

Postdoctoral Fellow, University of Alberta (2023-current)

Publications

Lee, B., Unsworth, M.J., Finley, T., Kong, W., Cordell, D., in review. Electrically anisotropic structure of the Rocky Mountain Trench near Valemount, British Columbia: Implications for geothermal exploration: submitted to Canadian Journal of Earth Sciences, Manuscript ID: CJES-2023-0086.
Cordell, D., Naif, S., Evans, R., Key, K., Constable, S., Shillington, D., Bécel, A., 2023. Forearc seismogenesis in a weakly coupled subduction zone influenced by slab mantle fluids: Nature Geoscience, v. 16, doi: 10.1038/s41561-023-01260-w.
Cordell, D., Naif, S., Troch, J., Huber, C., 2022. Constraining magma reservoir conditions by integrating thermodynamic petrological models and bulk resistivity from magnetotellurics: Geochemistry, Geophysics, Geosystems, v.23, doi:10.1029/2022GC010455.
Cordell, D., Hill, G., Bachmann, O., Moorkamp, M., Huber, C., 2022. Estimating melt fraction in silicic systems using Bayesian inversion of magnetotelluric data: Journal of Volcanology and Geothermal Research, v.423, doi:10.1016/j.jvolgeores.2022.107470.
Cordell, D., Unsworth, M.J., Lee, B., Hanneson, C., Milling, D., Mann, I.R., 2021. Estimating the geoelectric field and electric power transmission line voltage during a geomagnetic storm in Alberta, Canada using empirical magnetotelluric impedance data: The influence of three-dimensional electrical structures in the lithosphere: Space Weather, v.19, doi:10.1029/2021SW002803.
Slezak, K., Diaz, D., Araya-Vargas, J., Cordell, D., Reyes-Cordova, F., 2021. Magnetotelluric image of the Chilean subduction zone in the Salar de Atacama region (23°S - 24°S): insights into factors controlling the distribution of volcanic arc magmatism: Physics of the Earth and Planetary Interiors, v.318, doi:10.1016/j.pepi.2021.106765
Cordell, D., Unsworth, M.J., Lee, B., Diaz, D., Bennington, N.L., Thurber, C.H., 2020. Integrating magnetotelluric and seismic images of silicic magma systems: A case study from the Laguna del Maule Volcanic Field, central Chile: Journal of Geophysical Research Solid Earth, v.125, doi:10.129/2020JB020459
Lee, B., Unsworth, M.J., Arnason, K., Cordell, D., 2020. Imaging the magmatic system beneath the Krafla geothermal field, Iceland: A new 3-D electrical resistivity model from inversion of magnetotelluric data: Geophysical Journal International, v.220, doi:10.1093/gji/ggz427
Cordell, D., Unsworth, M.J., Diaz, D., Reyes-Wagner, V., Currie, C., Hicks, S.P., 2019. Fluid and melt pathways in the central Chilean subduction zone near the 2010 Maule earthquake (35°S-36°S) as inferred from magnetotelluric data: Geochemistry, Geophysics, Geosystems, v.20, doi:10.1029/2018GC008167
Wespestad, C.E., Thurber, C.H., Andersen, N.L., Singer, B.S., Cardona, C., Zeng, X., Bennington, N.L., Keranen, K., Peterson, D.E., Cordell, D., Unsworth, M.J., Miller, C., Williams-Jones, G., 2019. Magma reservoir below Laguna del Maule Volcanic Field, Chile imaged with surface-wave tomography: Journal of Geophysical Research: Solid Earth, v.124, doi:10.1029/2018JB016485
Cordell, D., Unsworth, M.J., Diaz, D., 2018. Imaging the Laguna del Maule Volcanic Field, central Chile using magnetotellurics: Evidence for crustal melt regions laterally-offset from surface vents and lava flows: Earth and Planetary Science Letters, v.488, doi:10.1016/j.epsl.2018.01.007
Reyes-Wagner, V., Diaz, D., Cordell, D., Unsworth, M.J., 2017. Regional electrical structure of the Andean subduction zone in central Chile (35°-36°S) using magnetotellurics: Earth, Planets and Space, v.69:142, doi:10.1186/s40623-017-0726-z
Comeau, M.J., Unsworth, M.J., Cordell, D., 2016. New constraints on the magma distribution and composition beneath Volcan Uturuncu and the southern Bolivian Altiplano from magnetotelluric data: Geosphere, v.12, doi:10.1130/GES01277.1


mtcode is a MATLAB package for analyzing magnetotelluric data, building and editing conductivity models, and preparing inversion files

Available on GitHub

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Frequently Asked Questions

What are "electromagnetic geophysical methods" and how do they work?
Electromagnetic (EM) geophysical methods are a bunch of different types of techniques used to image the electrical conductivity underground. All electromagnetic geophysical methods work on the same basic principle. A source (or primary) electromagnetic (EM) wave propagates into the Earth and induces a secondary EM wave in the Earth which propagates back to the surface. At the surface, the total field can be measured. The way EM waves are induced in the ground is dependent primarily on the electrical conductivity of the ground. Conductors induce stronger currents in the ground than resistors. So the fundamental property that we measure with EM methods is electrical conductivity and this can be related to different Earth materials because some materials (e.g. brine, magma, copper) are extremely conductive compared to other materials (e.g. dry granite).
What is "magnetotellurics"?
Magnetotellurics is a six-syllable mouthful which we usually shorten to "MT" for short. MT is a specific EM geophysical method which is different from the other methods because it measures natural EM fields as a primary source. This allows it to measure a very broad range of frequencies to image conductors over a broad range of depths from the upper 10s of meters to depths >100 km. However, its major limitation is that it requires you to measure the magnetic field and the electric field whereas other EM methods can get by measuring only the magnetic field. Magnetic fields are relatively easy to measure. Electric fields are more finicky, especially when they are on the order of mV/km. For reference, your average toaster produces an electric field on the order of 100 million mV/km. So these are very, very, very weak electric fields which we have to measure.
What is "space weather" and why is it hazardous?
Space weather describes phenomena related to perturbations of the Earth's near-space environment, particularly the geomagnetic field and ionosphere. Perturbations to the geomagnetic field can be driven by various physical processes, but most are related to changes in the solar wind. If there is a large eruption of charged particles from the sun during a solar flare (or "coronal mass ejection"), then these charged particles hit the Earth's geomagnetic field and cause it to bend and warp. At its most basic level, this is a changing magnetic field and (from Maxwell's equations), a changing magnetic field will induce a current inside a conductor. The Earth is relatively conductive (compared to the vacuum of space!) and so this "space weather" drives currents in the ground. The reason this can be hazardous is because the currents will preferentially flow into good conductors and people have built really good conductors such as power transmission lines and pipelines. Unwanted currents flowing into the power grid are known as "geomagnetically induced currents" (or GICs) and these can cause damage to electrical infrastructure and even cause blackouts. The most well-known disruption due to GIC occured in March 1989, when the entire Hydro-Quebec network was taken offline for 6 hours. 
What do you do during field work?
Look for a good spot. Dig some holes. Repeat. Each MT site location requires at least 5 holes to be dug: four electrodes and either one hole for a fluxgate magnetometer, or 2-3 holes for induction coil magnetometers. Electrodes are oriented in a large 100 meter X, with one electrodes at the north, east, south, and west directions. All the cables are connected to a data logger at the center of the X. Each electrode pair (north-south and east-west) records a voltage, which is converted into an electric field value based on the distance between electrodes (i.e. the electric field is just the voltage divided by the distance). The electric field and magnetic field are recorded simultaneously by the data logger usually with sample rates anywhere between 8 Hz to 1500 Hz. We leave the sites to record for anywhere from 1 day to 1 month, depending on the geological target. We then return to the site, and remove all the equipment. Every project is different. If it is a regional project, then a lot of time can be spent driving, talking to landowners, and getting permissions. If it is a local project, then more sites can be installed more quickly.
Copyright, 2023