This thesis deals with the problem of a point source of harmonic current in a multilayer horizontally complex medium, where the first layer is air, while the other layers belong to a multilayer soil. Special attention is paid to the case when the source and the observation point are in the same layer. The first chapter of the thesis is introductory and provides an introduction to the problem of a point source of transient current in a multilayer medium. The second chapter describes a multilayer horizontally complex model of the medium in which the distribution of transient potential is calculated. Most of the transient electromagnetic models developed so far are limited to homogeneous soil, or to a two-layer medium (soil and air). Only in the last few years have transient electromagnetic models appeared that take into account multilayer soil. The third chapter presents Maxwell’s differential equations for an alternating harmonically varying electromagnetic field. For a quasi-static field distribution, the Helmholtz differential equation is reduced to the Poisson differential equation, and then the Poisson differential equation is solved and the potential retardation is subsequently introduced. The fourth chapter describes the retardation factor and the calculation for the special case when the modulus of the retardation factor from the calculation turns out to be equal to unity. The fifth chapter presents examples of the analysis of the retardation factor when the source and the observation point are located in the same and in different layers of the medium. The sixth chapter presents a three-dimensional presentation of the modulus of the retardation factor in the observed four-layer.