Quasi-Stationary Approximation in the Problem of Excitation of Low-Frequency Electromagnetic Fields in Lithosphere

Excitation of electromagnetic field at ultralow, extremely low, and lower frequencies by a horizontal grounded antenna in a two-layer medium is considered. Approximate analytical formulas are derived to describe field evolution in the low-frequency range. Accuracy of the quasi-stationary approximation is analyzed. Substitution of the impedance of a plane wave that is normally incident on the interface for a real surface impedance is estimated. Frequency limits of the approximate approach in the estimation of the Earth conductivity are determined for several experiments performed on the Kola Peninsula in the framework of the FENIСS-2014 and FENICS-2019 International Projects.