An efficient and safe algorithm for calculating dispersion curves has been developed. On currently available personal computers (e.g. with a processor Athlon 2.2 GHz), the time needed to calculate one sample is about 5 microseconds for a two-layer model. If the dispersion curve of the fundamental mode is sampled with 30 points, more than 6500 models can be calculated in only one second. It has been implemented in a command line program ( os_forward ) and in an inversion tool ( os_na ). This algorithm is also exploited to calculate other spectral properties of the ground model like the ellipticity and the auto-correlation, detailed in the next sections.