Characteristics of Gas Hydrate and Free Gas Offshore Southwestern Taiwan: A Combined Seismic Reflection/Refraction Analysis

Offshore southwestern Taiwan, BSRs have widely been observed in water depths ranging from 600 to 3000 m. The highest concentration of BSR is located in the northwestern portion of the Manila accretionary prism where rapid deposited terrigenous sediments may have relatively high amounts of organic carbon, thereby providing a source for the methane. Special efforts have been made to derive the best velocity information from the seismic reflection and OBS refraction data. A series of pre-stack depth migrations in a streamlined Deregowski loop was implemented in a layer stripping approach: During each iteration, residual move-out in the image gathers are analyzed and the migration velocity field is update from the resulting Depth/Depth error. In order to account for the 2D complexity of the surveyed area, about 15 iterations were necessary. Manual picking was indispensable during the early iterations in order to stabilize the inversion processing by integrating the interpreter's knowledge. A spatially continuous residual velocity analysis automatic picking has proven efficient for refinement of the velocity model in the later stage. These acoustic velocities are then tested in a ray-tracing scheme on the vertical and hydrophone components of the OBSs. A damped least-square to the linearized inverse problem, using the partial derivatives of the travel times with respect to the model parameters and the travel time residuals better constrains the acoustic velocities in the vicinity of the OBS. Poisson Ratio values we estimated at the vicinity of the 3 OBS stations analyzed in this study will be presented. The modeling of the complex seismic waves recorded by the 4 component stations allows us to propose estimates of the elastic properties of sediments bearing gas hydrate and free gas and derive hydrate saturation values offshore southwestern Taiwan.