Articles | Volume 36, issue 2
Regular paper
07 Mar 2018
Regular paper |  | 07 Mar 2018

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

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

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.

Short summary
Passive electromagnetic induction soundings in the far field live on the fact that natural electromagnetic field variations (their source signals) fulfill the plane-wave assumption. In the frequency range of geomagnetic pulsations some signals (upstream waves) do and others (field line resonances) do not meet this condition. The consequences for several branches of such soundings are shown. A consequence is that a remote reference site may be situated in another continent than the local site.