At low frequencies tube or Stoneley waves represent a dominant arrival propagating along boreholes. It was proven in previous studies that 1D effective wavenumber approach provides simple and accurate low-frequency description of tube-wave propagation in open boreholes surrounded by radially homogeneous formation. In this study we apply 1D effective wavenumber approach to radially inhomogeneous media and demonstrate that it still provides excellent description of low-frequency tube-wave propagation. In particular, we focus on the models of cased boreholes: reflection from geological interfaces behind casing, reflection from corroded casing section and reflection from idealized disk-shaped perforation in cased hole. In considered cases frequency-dependent reflection coefficient obtained by 1D effective method and by finite-difference computations show excellent agreement.