Simulation of grounding systems at high frequencies is of interest in electromagnetic compatibility, especially related to lightning, and in emerging technologies, such as power line telecommunications. This paper presents first results in the effort to extend the electromagnetic field theory based modeling of grounding systems to a two-layer model of earth. A rigorous mathematical model which solution involves numerical solution of Sommerfeld-type integrals is described. Also new more approximate but computationally more efficient solution based on quasi-static image theory is introduced. Comparison between these two solutions led to conclusion related to the applicability of the image theory approach, which may be of interest for practical problems.