Introducing inter-site phase tensors to suppress galvanic distortion in the telluric method

Abstract

A common problem when interpreting magnetotelluric (MT) data is that they often are distorted by shallow unresolvable local structures, an effect known as galvanic distortion. We present two transfer functions that are (almost) resistant to galvanic distortion. First, we introduce the electric phase tensor, which is derived from the electric tensor, where the electric tensor relates the horizontal electric fields at a field and base site. The electric phase tensor is only affected by galvanic distortion, if present, at the base site. Second, we introduce the quasi-electric phase tensor, which is derived from the quasi-electric tensor, where the quasi-electric tensor relates the electric field at a field site with the magnetic field at a base site. The quasi-electric tensor is not affected by galvanic distortion. Using a synthetic data-set, we show that the sensitivity of the MT phase tensor, the quasi-electric phase tensor, and the electric phase tensor is comparable for our model under consideration. Furthermore, we demonstrate that stable (quasi-) electric phase tensors can be recovered from a real data-set with the use of existing processing software. In addition, we provide a formalism to propagate the uncertainties from the estimated (quasi-) electric and impedance tensors to their respective phase tensors. The uncertainties of the (quasi-) electric phase tensors are of the same order of magnitude as the uncertainties of the MT phase tensor. From our study, we conclude that the (quasi-) electric phase tensors are an attractive complement to the standard MT responses.

Nowcasting geoelectric fields in Ireland using magnetotelluric transfer functions

“…This has been used extensively to interpret MT data without most of the challenges pertained to galvanic distortion by disregarding the distorted amplitude tensor (Patro et al, 2012;Tietze et al, 2015;Bakker et al, 2015;Samrock et al, 2018)). While the phase tensor only holds information about subsurface induction processes, the amplitude tensor describes galvanic and inductive effects that complement the phase tensor information (Neukirch et al, 2019).…”

Mapping Geoelectric Field Hazards in Ireland

“…The main difference between the galvanic non‐corrected maps with the corrected is a slight increase in amplitude and but significant change in the direction of the vector that is exceeded both for Kp = 8 and Kp = 9. The amplification of the geoelectric field at isolated sites with non‐corrected distortion is a well known effect, due to localized features of galvanic distortions (e.g., Bakker et al., 2015; Samrock et al., 2018). The change in direction likely relates the local subsurface features distorting the perceived direction of the regional geoelectric field (e.g., Ledo et al., 1998; Rung‐Arunwan et al., 2022).…”

Using high-order polynomial basis in 3-D EM forward modeling based on volume integral equation method

“…We start with presenting results for a case in which polynomials are implemented along the vertical direction only. The model taken from Bakker et al (2015) mimics a conducting sedimentary basin surrounded by resistive mountains with a conductive fault beneath the left flank of the basin. Plane and side views of the model are shown in the respective upper and lower plots in Fig.…”