An acoustic reflection tomography method was applied to multichannel seismic reflection data acquired close to the Ocean Drilling Program Leg 164 Blake Ridge bottom-simulating reflector (BSR) transect, to reconstruct the local velocity field associated with gas hydrates and free gas in the sediment pores and to qualitatively assess gas hydrate and free gas abundances. The interval velocities obtained with the tomography analysis indicate deviations from a typical velocity-depth profile in normally consolidated marine sedimentary sections, with lateral velocity gradients. Below the BSR, a sharp decrease in velocity (1300-1650 m/s) has been detected in a layer bounded by the BSR and a quite discontinuous normal-polarity reflector consistently found approximately 80 ms below it, clearly indicating the presence of free gas in the sediment pores. A theoretical model for elastic porous media has been applied to the sonic logs at Leg 164 sites to quantify amounts of gas hydrate. The resulting theoretical velocity curves have been compared to downhole logging data and laboratory measurements conducted on the three sites, and a quantitative estimation of gas hydrates trapped in the sediment pores has been made. The proposed method furnished results that qualitatively agree with the in situ measurements, and it can be extrapolated in areas where no experimental data are available.