Measuring soil resistivity and estimating soil equivalent resistance in a long distance
What you have is a resistivity field.  Depending on how complicated it it, and what your boundary assumptions need to be, there may be closed-form solutions, or you might need finite element analysis techniques to get a grip on your situation.  

It's been many moons since I've done math like this (propagation of signals in nervous system tissue), so its a bit over my head these days.  https://archive.epa.gov/esd/archive-geophysics/web/html/resistivity_methods.html might serve as a good intro, and resistivity field would be a good search term.