On the relevance of source effects in geomagnetic pulsations for induction soundings

Anne Neska,Yuri Petrovich Sumaruk,Jan Tadeusz Reda,Mariusz Leszek Neska

doi:10.5194/angeo-36-337-2018

Abstract

Abstract. This study is an attempt to close a gap between recent research on geomagnetic pulsations and their usage as source signals in electromagnetic induction soundings (i.e., magnetotellurics, geomagnetic depth sounding, and magnetovariational sounding). The plane-wave assumption as a precondition for the proper performance of these methods is partly violated by the local nature of field line resonances which cause a considerable portion of pulsations at mid latitudes. It is demonstrated that and explained why in spite of this, the application of remote reference stations in quasi-global distances for the suppression of local correlated-noise effects in induction arrows is possible in the geomagnetic pulsation range. The important role of upstream waves and of the magnetic equatorial region for such applications is emphasized. Furthermore, the principal difference between application of reference stations for local transfer functions (which result in sounding curves and induction arrows) and for inter-station transfer functions is considered. The preconditions for the latter are much stricter than for the former. Hence a failure to estimate an inter-station transfer function to be interpreted in terms of electromagnetic induction, e.g., because of field line resonances, does not necessarily prohibit use of the station pair for a remote reference estimation of the impedance tensor.

Highlights

We motivate our study by adducting a number of contradictions and ambiguities that are encountered both within the literature on electromagnetic induction soundings and between literature and empirical findings
The first exception concerns certain regions in which all LMT source signals are inhomogeneous according to, e.g., Bahr and Simpson (2005). These regions are situated beneath a polar or the equatorial electrojet. We focus on the latter here because this region is more important for our study
From both pictures it is evident that there exist pulsations that cannot originate from field line resonances since they are present at the Equator and have a quasi-global coherence length

Summary

Introduction

We motivate our study by adducting a number of contradictions and ambiguities that are encountered both within the literature on electromagnetic induction soundings and between literature and empirical findings. These are demonstrated on data which will be documented in the Data section and which have been processed by methods introduced in the Method section. The precondition for the proper functioning of such a modeling is that transfer functions depend only on electromagnetic induction in the subsurface This means in particular that the geometry of the source field, i.e., of the incident electromagnetic wave which drives that induction, can be neglected. This is an elementary consequence of a basic equation in electrodynamics – the telegrapher’s equation which describes the damped propagation of an electromagnetic wave in an electrically conducting medium

Methods

Results

Conclusion