This document presents the general problem of electromagnetic inversion in the frequency
domain. The 1-dimensional analysis of a stratified medium is performed at different frequencies;
the data measured on the surface (the admittance or the apparent electric conductivity) are compared
with the solution of the system obtained from a Lagrangian formulation which minimizes
a suitable cost function. The information acquired at different frequencies is integrated in the inversion
procedure; two approaches are presented and compared: the first method independently
solves the problem for each frequency and then integrates the contributions, while the second
method solves a global problem which involves all the contributions at the same time. The conjugate
gradient algorithm has been used as iterative minimization strategy for both procedures.
Numerical examples validate this analysis.