Seismic velocity increase and deep‐sea gas hydrate concentration above a bottom‐simulating reflector on the northern Cascadia continental slope

The amount of gas hydrates in the accretionary wedge sediments of the northern Cascadia subduction zone off Vancouver Island has been estimated from multichannel seismic (MCS) and Ocean Drilling Program (ODP) data. Detailed semblance velocity analyses and full waveform inversion of MCS data, combined with previously published ODP Sites 889/890 sonic log and vertical seismic profile (VSP) data, show that sediment velocities increase downward more rapidly than the no‐hydrate/no‐gas reference profile from about 1500 m/s near the seafloor to a maximum of 1900 m/s just above the bottom‐simulating reflector (BSR) at a subbottom depth of 224 m. Immediately below the BSR, the MCS velocities drop to ∼1700 m/s. A low velocity of ∼1500 m/s from the VSP data probably represents a thin layer, ∼20 m, containing free gas. The difference between the reference and observed velocities is used to estimate hydrate concentration, which reaches a maximum of 20–30% of the pore space above the BSR. A simple interpretation of the drill core chlorinity dilution data at the ODP site yields a similar hydrate concentration of 35%. The estimated hydrate concentration with subbottom depth represents about 7 m 3 of hydrate per square meter of seafloor. The total methane gas at STP, including hydrate above the BSR and the small amount of free gas below, is about 800 m 3 per square meter of seafloor, or 200 TCF (trillion cubic feet) on the Vancouver Island continental slope. Application of the same method in the Blake‐Bahama region of the eastern U.S. margin indicates that the velocity enhancement and inferred hydrate concentration are very similar.