The aim of the present study is to examine whether seismologic diagnostic techniques may be used to improve the models of the solar chromosphere. We are mainly concerned by oscillations observed in the cores of strong lines such as the resonance lines of Ca II or Mg II. It is intuitive, and confirmed by numerical simulations (Gouttebroze and Leibacher, 1980), that the displacements of the cores of these lines are strongly correlated with the vertical velocity of the atmospheric layer where the line center optical depth is equal to 1. With classical atmospheric models such as those of Vernazza et al. (1981) (hereafter VAL), these line cores are formed in the chromospheric plateau, around 1500 km for the Ca II lines and 1900 km for the Mg II ones.