To estimate casing thickness with litho-density logging in cased hole

the ratio of low energy counts NS to high energy counts NH in γ spectrum and the relationships between the ratio and photoelectric absorption coefficient Pe as well as casing thickness HC in different mineral compositions are calculated with Monte Carlo program MCNP.The results show that litho-density logging is insensitive to formation of lithology

so that Pe cannot be obtained from the ratio NS/NH.However

since there is a good linear relation between log NS/NH and logHC

the HC can be estimated through the ratio NS/NH.