  An accurate knowledge of the per-unit length impedance of power cables is
necessary to correctly predict electromagnetic transients in power systems. In
particular, skin, proximity, and ground return effects must be properly
estimated. In many applications, the medium that surrounds the cable is not
uniform and can consist of multiple layers of different conductivity, such as
dry and wet soil, water, or air. We introduce a multilayer ground model for the
recently-proposed MoM-SO method, suitable to accurately predict ground return
effects in such scenarios. The proposed technique precisely accounts for skin,
proximity, ground and tunnel effects, and is applicable to a variety of cable
configurations, including underground and submarine cables. Numerical results
show that the proposed method is more accurate than analytic formulas typically
employed for transient analyses, and delivers an accuracy comparable to the
finite element method (FEM). With respect to FEM, however, MoM-SO is over 1000
times faster, and can calculate the impedance of a submarine cable inside a
three-layer medium in 0.10~s per frequency point.
