Velocity estimations derived by tomographic inversion of traveltimes conducted on two multichannel seismic reflection profiles located close to the Blake Ridge BSR transect, allowed us to identify: (1) the occurrence of lateral velocity anomalies associated with the presence of gas hydrates in the sediments, and (2) the presence of a low velocity zone, indicating a free gas-bearing sediment layer beneath the BSR.
For this specific case, the low signal-to-noise ratio of the seismic response, the unsuitable seismic acquisition geometry for the tomographic method, and the intrinsic low reflectivity of the sedimentary section in this area (very homogeneous clay formation) that prevents picking of an adequate number of horizons, all combine in reducing the tomography resolution limit. The velocity resulting from the tomography method is similar to that found from VSP measurements, with the presence of an abrupt velocity decrease below the BSR at Sites 995 and particularly at Site 997, where free gas concentrations are supposed to be more substantial (Holbrook et al., 1996). The reasons for the discrepancy in thickness of the low-velocity zone resulting from the two methods is mainly because of the fact that the MCS line is not exactly positioned along the BSR transect, and that significant gas accumulation variations can be hypothesized in this area, as well as different gas hydrate saturation.
A theoretical approach to calculate gas hydrates and free gas concentrations, derived from the elastic theory for porous media, has furnished values comparable to those obtained with indirect techniques for gas hydrate abundances (chloride ion contents), and has revealed the presence of free gas in the sedimentary pore space.
In conclusion, this method appears appropriate to reveal and quantify the presence of clathrates in marine sediments, and can be applied in areas where no direct data (downhole logging, samplings) are available.