The METI (Ministry of Economics, Trade and Industry) Sado-oki Nansei 3D seismic survey was carried out in deep water southwest off Sado Island, Japan Sea. The survey area covered the Umitaka Spur, which features mounds and pockmarks. Other surveys sampled large masses of methane hydrate beneath the sea floor, discovered methane bubble plumes rising into the water, and investigated high resistivity anomalies below the sea floor.
We applied 3D pre-stack time migration and continuous velocity analysis method to 3D seismic data, and investigated the geological and P-wave velocity structure below the sea floor of the Umitaka Spur and its surrounding area.
This revealed high velocity anomalies, suggesting the methane hydrate occurrence below mounds and pockmarks. On the other hand, P-wave velocities below the sea floor in areas of the Umitaka Spur were lower than propagated in water layers. Therefore, it was suggested that concentrations of methane hydrates were limited below mounds and pockmarks, although some gas may be contained in low-velocity zones.
The situation may be caused by the localization of methane supplies from deep layers.