A new method is proposed for inverting single-station surface-wave seismograms. Instead of inverting directly the time-signal, use is made of secondary observables built up from a set of time-frequency images. This allows us to enhance the signal-to-noise ratio and to make more linear the relationship between the model parameters and the inverted observables. The feasibility of the method is checked on multimode synthetics prior to present the inversion of a long-period multimode seismogram related to the Pacific Ocean. ¿ American Geophysical Union 1987