The sum of all the thickness/resistivity ratios of n - 1 layers which overlie a semi-infinite substratum of resistivity ρ‎_n, such that S × h₁/ρ‎₁ + h₂/ρ‎₂ + h₃/ρ‎₃ +…+ h_n-1/ρ‎_n-1 (mho), where h₁, h₂, etc. are the depths and ρ‎₁, ρ‎₂, etc. the resistivities, of successive layers. A knowledge of h_i/ρ‎_i for the ith layer when it is sandwiched between two layers of much higher resistivity is of importance in resolving the problem of equivalence.