Velocity Models and Pre-stack Depth Migration of Gas Hydrates on Taiwan Accretionary Prism

Reflection images of the accretionary prism along seismic profiles collected from R/V Maurice Ewing in 1995 and from R/V Marcus G. Langseth in 2009 off SW Taiwan are presented for charactering gas hydrates in this study. Pre-stack depth migrations of the seismic profiles indicate the thickness of sediments about 2-3 km and the depth of the bottom simulated reflector (BSR) about 300 m below the sea floor. Layers of gas hydrate and free gas are about 100 m thick and 80 m thick, respectively. Horizontal velocity analysis of the seismic profiles show a gas-hydrate layer above BSR with a P-wave velocity of about 2100 m/s and a free-gas layer below BSR with a P-wave velocity of about 1800 m/s. Most BSRs are non-continuous in the accretionary prism that may result from the gas dissipation from thrust faults, fractures and severe deformation due to subduction. Gas migrated from thrust faults and accumulated beneath the lower slope of the accretionary prism may result in more clear and continuous BSRs in the lower slope than those in the upper slope of the accretionary prism. The difference of BSRs may be associated with the organic sediments came from the river of Taiwan.