Signals received by low-frequency multibeam echosounders are strongly affected by sound penetration inside the upper sediment layers and by backscattering from buried layers down to depths of a few meters; this may lead to serious ambiguities and misinterpretations of experimental data. These phenomena are modeled here using a concept of equivalent input backscattering strength (EIBS), based on a combination of classical models of local backscattering strength and propagation inside fluid layered media. The local backscattering strength at a buried interface is expressed first to account for the impedance adaptation due to the overlying layers, for the angular refraction effects due to the velocity profile, and for the layered structure of the underlying medium. It is then transferred to the upper water-sediment interface, accounting fur propagation inside the layered stack; the transfer coefficient is obtained from the classical theory of plane wave propagation in layered media. The volume backscattering effects are processed in the same way and account for the finite thickness of the layers. The various contributions are finally summed to give the backscattering strength, at the upper interface, that features the various effects of propagation and attenuation inside the layered structure.