From the series of articles "Grounding resistance calculation is almost easy".
Like any interstate border, it is very difficult to cross. It must be kept in mind that formula (6) for the potential is true only in unlimited amount of soil. And the ground loop electrodes are rarely deepened for more than 0.5 - 1.0 m into the soil. They are very close to the border with the air. Of course, you can give up on the proximity of the border and carry out the calculation, assuming unlimited ground. Thus, grounding resistance will clearly be less than actual. If you double the value found this way, you get, on the contrary, an overestimated value. The truth is somewhere between these two values, but to clarify exactly where is not the easiest task. The result depends on the configuration of the ground loop. Border cannot be ignored.
The method of mirror reflections will help. It is well known from the tasks in electrostatics. The reflection coefficient for a flat boundary (and our Earth is flat!) for an electric current field is expressed through the soil conductivity g or through its resistivity ρ as
because non-conductive air ρ2 → ∞ for any values of soil resistivity ρ.
Отраженная шина - Reflected bus
Реальная шина - Real bus
Read more "6. How to do that?"