Fig. 1

Graphical illustration of the infinite chain of invariants and of the missing first link: a apparent resistivity curves; b phase curves. The longer arrows indicate the direction toward convergence of the infinite chain. The shorter arrows indicate the direction toward the missing first links given by the solution of the quadratic equation. The TE and TM resistivities coincide with \(\rho_{ - } \;{\text{and}}\;\rho_{ + }\), respectively, but it could be the other way around