The USArray EarthScope program supported by the National Science Foundation (NSF) consists of a continent-wide survey of seismic and magnetotelluric sites conducted throughout the USA since 2006. This study considers a collection of impedances at discrete geographic locations. 1Hz geomagnetic vector field data were collected via fluxgate magnetometers, and 1Hz geoelectric field data were collected via pairs of lead–lead chloride nonpolarizing electrodes arranged in a standard magnetotelluric array (Schultz 2010). We selected four sites for comparison in this study, two sites each within two physiographic zones PT-1 and SU-1 (Fig. 1). Each impedance was derived from a measured geoelectric and geomagnetic time series at 30 discrete periods from \(T = 7.3143\) to \(T=1.8725\cdot 10^4\) s (Egbert 2007).
We consider two impedances located 125 km apart in Minnesota, USA: RED36 (Fig. 2) and MNB36 (Fig. 3), with MNB36 almost directly north of RED36. The geologic setting in this area is controlled by a mid-continent rift system, which produced a series of intracontinental sedimentary basins juxtaposed against older cratonic rock. These juxtapositions are further exaggerated by faulting that places conductive, sedimentary rock characteristic of basin fill against more resistive igneous and metamorphic cratonic rock (Bally and Palmer 1989). MNB36 is located within the Archean Superior province to the north while RED36 is located within the Animikie Basin to the south. We also considered two impedances located 200 km apart in Virginia, USA: VAR57 and VAQ58, with VAR57 100 km west-southwest of VAQ58. Both VAR57 and VAQ58 are in the Piedmont province of the southern Appalachian Mountains, which is mainly composed of accreted sedimentary material thrust upon the North American plate.
Figure 2 shows the impedance for RED36 which, as we shall see corresponds to smaller estimations of E(x, y, t). Here, the real components of the complex impedance tensor are represented in blue while imaginary components are represented in red. Figure 2a–d corresponds to the \(Z_{xx}\), \(Z_{xy}\), \(Z_{yx}\), and \(Z_{yy}\) components of the impedance tensor, respectively. Figure 3 represents the impedance for MNB36 similarly, which corresponds to the larger geoelectric response (note the difference in y scale compared to Fig. 4). In Fig. 2, we see that the diagonal components of the impedance response (\(Z_{xx}\) and \(Z_{yy}\)) are very small, indicating that the conductivity distribution of the subsurface is close to 1-dimensional. The off-diagonal components (\(Z_{xy}\) and \(Z_{yx}\)), though unique from one another, only reach 2 \(\Omega\) in each component at shorter periods. Comparing Figs. 2 to 3, MNB36 demonstrates a much stronger transfer function compared with RED36. The plots for MNB36 further show a much larger difference in \(Z_{xx}\) and \(Z_{yy}\), indicating that the subsurface conductivity distribution is more representative of a 3-dimensional conductivity model.
Discrete time series of the near-surface geomagnetic field (Love and Chulliat 2013) were obtained through the Brandon (BRD) Magnetic Observatory located in Brandon, Manitoba, operated by National Resources Canada (NRCan), and the Fredericksburg (FRD) Geomagnetic Observatory in Fredericksburg, Virginia, operated by the U.S. Geologic Survey (USGS). These observatories were considered due to their proximity to the EarthScope impedance tensors (Fig. 1). The geomagnetic data gathered at these observatories have a sensitivity of 0.1 nT and have undergone digital and analog filtering. The magnetic storm we selected for this study occurred on June 22, 2015, and was recorded at both observatories at a sampling frequency of 1 Hz over 3 days beginning at 00:00 on June 22, 2015. This time interval encompasses the magnetic storm from its onset at approximately 18:00 on June 22, 2015, to its cessation at approximately 20:00 on June 25, 2015.
Figure 4a shows the geomagnetic field measured during the June 22, 2016, storm at BRD and the corresponding estimations of the geoelectric field calculated via Eq. 3. In Fig. 4a, \(B_{x}(t)\) is represented in black and \(B_{y}(t)\) in green. This geomagnetic times series incorporates the entire evolution of the geomagnetic storm over the course of three full days beginning at 00:00 of June 22, 2016. The storm reaches its largest magnitude of 1207 nT in \(B_{x}(t)\) and 725 nT in \(B_{y}(t)\) during the first day of the storm, with the largest geomagnetic variation occurring between 18:00 on June 22, 2016, to 12:00 on June 23, 2016. After 18:00 on June 24, 2016, geomagnetic activity concludes and the geomagnetic field returns to a baseline value. Figure 5a shows the geomagnetic field measured during the June 22, 2016, storm at FRD. Though we show the same June 22, 2016, magnetic storm over the same time interval, the response of \(B_{x}(t)\) and \(B_{y}(t)\) measured at FRD is smaller in magnitude, reaching 213 nT in \(B_{x}(t)\) and 150 nT in \(B_{y}(t)\) during the first day of the storm. As with BRD, the largest geomagnetic variations occur between 18:00 on June 22 , 2015, and 12:00 on June 23, 2015. The magnitude of the geomagnetic field at the Virginia sites is reduced compared to the Minnesota sites, likely due to the difference in latitude between the two magnetic observatories: BRD is located within the auroral oval at \(49.8^\circ\) latitude (\(50^\circ\) magnetic latitude), while FRD is located below the auroral oval at \(38.2^\circ\) latitude (\(40^\circ\) magnetic latitude).