A software for the casing and tubing strings’ thickness determining in the multi-string wells was developed<br>considering magnetic permeability and electric conductivity of a metal on a base of the direct and<br>inverse problem solutions.<br>A direct problem solution was based on a spectral approach using a Fourier integral. The inverse<br>problem is solved by calculations of variations The results of the mathematical modeling of the measurements,<br>the examples of the model and actual down-hole data interpretation were provided.