Seismic monitoring is often used to check hydrocarbon movements during production. On the site of Céré-la-Ronde, an underground water-bearing gas storage facility developed by Gaz de France, a methodology has been implemented for estimating gas column height and saturation with time lapse seismic. The methodology is based on the Gassmann model, on the measurement of time-shifts and on the computation of a specific seismic function. The time-shifts obtained by cross-correlation techniques are used to derive the residual time-shift Δt induced by the presence of gas within the reservoir. A specific function is used to estimate both the gas bubble extension and the gas time thickness Ht. It is shown that knowledge of Ht and Δt and the use of a Gassmann model can lead to an estimation of the gas saturation. The methodology has been applied successfully on conventional seismic profiles as well as on vertical seismic profiles (VSP). The results obtained with seismic data are in good agreement with those obtained by logging (sonic and neutron logs) but with a lateral investigation, which is much larger than the one given by logging techniques. In our field example, the gas anomaly has been checked over a distance of 300 m with VSP data and over 2 km with surface seismic data.