Joint Oceanographic Institutions, United States

Three successful vertical seismic profiles (VSPs) were acquired during Ocean Drilling Program (ODP) Leg 204 at South Hydrate Ridge. The data confirm earlier results from ocean bottom seismometer data and analysis of moveout from common midpoint reflection data that the average velocity between the seafloor and the bottom-simulating reflector (BSR) is <1600 m/s throughout the region and is lowest near the summit, where the amount of hydrate is greatest. This result supports the conclusions that free gas and hydrate coexist beneath the summit and that the average amount of gas hydrate present elsewhere is low. The data also indicate that low-velocity zones (LVZs) resulting from free gas beneath the BSR must be thin and stratigraphically controlled. The only LVZ resolvable from traveltime analysis of the VSP data is associated with Horizon A, which has been interpreted to be the primary conduit transporting free gas to vents at the summit of South Hydrate Ridge. Thin LVZs associated with Horizons B and B', however, are indicated by sonic logs as well as by strong negative polarity reflections in the multichannel seismic data. This limited distribution of sub-BSR free gas contrasts with previous results at North Hydrate Ridge (Leg 146) and Blake Ridge (Leg 164), which indicate the presence of free gas zones several hundred meters thick that result in distinct LVZs in the VSP data from those earlier ODP legs.