I propose a method for estimating gas hydrate and free gas concentrations versus depth in marine sediments, using Gassmann’ s equation. The theory models gas hydrate- and free gas-bearing sediments partially saturated with water. A qualitative estimate of the concentrations can be obtained by comparing the theoretical velocity for full-water saturation to the experimental velocity, evaluated by a tomographic analysis or available from well logging. Positive anomalies indicate the presence of gas hydrate and negative anomalies indicate the presence of free gas. A quantitative estimate of concentrations is obtained by fitting the theoretical velocity to the experimental velocity. The method has been tested against sonic log and VSP data and indirect estimations of gas hydrate concentration from chloride content in core logs.