Source biases in midlatitude magnetotelluric transfer functions due to Pc3-4 geomagnetic pulsations

Abstract

The magnetotelluric (MT) method for imaging the electrical conductivity structure of the Earth is based on the assumption that source magnetic fields can be considered quasi-uniform, such that the spatial scale of the inducing source is much larger than the intrinsic length scale of the electromagnetic induction process (the skin depth). Here, we show using EarthScope MT data that short spatial scale source magnetic fields from geomagnetic pulsations (Pc's) can violate this fundamental assumption. Over resistive regions of the Earth, the skin depth can be comparable to the short meridional range of Pc3-4 disturbances that are generated by geomagnetic field-line resonances (FLRs). In such cases, Pc's can introduce narrow-band bias in MT transfer function estimates at FLR eigenperiods (~ 10-100 s). Although it appears unlikely that these biases will be a significant problem for data inversions, further study is necessary to understand the conditions under which they may distort inverse solutions.

Synthesizing Seemingly Contradictory Seismic and Magnetotelluric Observations in the Southeastern United States to Image Physical Properties of the Lithosphere

“…This highly resistive structure (herein referred to as the Piedmont Resistor) is apparent in the raw MT data for this region (Murphy & Egbert, 2017), and other observations from various electromagnetic data sets support the presence of a large, resistive body beneath the SEUS. For example, source biases due to Pc3-4 geomagnetic pulsations, indicative of bulk resistive Earth, are ubiquitous in MT transfer functions in the SEUS (Murphy & Egbert, 2018). Resistive upper mantle beneath the SEUS also appears in the global geomagnetic depth sounding (GDS) results of Sun et al (2015).…”

Validity of the dispersion relations in magnetotellurics: Part I—theory

“…However, departures from this basic assumption are routinely encountered in practice, e.g. : in polar and equatorial regions owing to instabilities in the forms of the source fields (Simpson and Bahr 2005); in some resistive regions of the Earth crust due to short spatial scale source fields from Pc3-4 pulsations (Murphy and Egbert 2018); in any MT data measured at very long periods due to the influence of the diurnal Sq variations (Shimizu et al 2011); etc. In all such cases the source effect (provided that it is strong enough to significantly affect the data) forces an MT response to "switch" at some frequency from being excited by the regular plane-wave source to an input signal of some other type, which naturally manifests itself by violation of the dispersion relations.…”

“…Since the vertical magnetic field component is more sensitive to non-uniform source field contributions than horizontal MT components (Jones and Spratt 2002;Utada 2018), the tipper function Ŵ appears to be particularly vulnerable to the distorting influence of the source effect, as could be seen from two following examples. Figure 6 shows the W zx component estimated by applying the EMTF code of Egbert and Booker (1986) to the 4-week MT measurement taken in September-October, 2012, at the station IAN-37 in Iowa, USA, as part of the EarthScope USArray project (Murphy and Egbert 2018). Figure 7 depicts the W zy component estimated by applying the BIRRP code of Chave and Thomson (2004) to the corrected for the principal Sq signatures 12-month data recorded in 2005-2006 at the geomagnetic observatory CBI in Chichijima, Japan (Shimizu et al 2011).…”

Spatial gradients of geomagnetic temporal variations causing the instability of inter-station transfer functions

“…Frequently, the changes in MT responses and IS-TFs are not caused by the subsurface environment, but by the source field (electric current in the ionosphere or magnetosphere) of natural geomagnetic fluctuations (Egbert et al 2000;Brändlein et al 2012;Romano et al 2014;Vargas and Ritter 2016;Murphy and Egbert 2018;Sato 2020). In particular, IS-TFs may be expected to be strongly affected by the source field because the spatial gradients of the primary geomagnetic fields directly affect both the amplitudes and phases of IS-TFs.…”