high resolution seismic imaging for deep-water gas hydrate exploration; a case study offshore japan

High resolution seismic surveys are needed for accurate subsurface modeling and imaging of gas hydrate resources. vertical cable seismic (vcs) method, with a set of buoyed vertical array of receivers which are anchored to the seafloor, can acquire such a high resolution data. the close distance to the shallow targets below seafloor and the silent deep-water recording environment provide higher resolution and better signal to noise ratio. the downgoing receiver ghost reflections are separated from the entire seismic wavefield and imaged for an extended subsurface coverage. reverse time migration (rtm) method with a free surface top boundary condition is used for imaging complex geological features. because of the asymmetrical reflection point between the source and receiver, conventional velocity analysis methods based on normal move-out correction are not applicable to the vcs survey. therefore, seismic interferometry is used to develop a velocity model for imaging. a case history from umitaka spur in sea of japan is presented to delineate the corresponding gas hydrate system. high resolution resulting images suggest that a large gas chimney exists beneath a pockmark and is responsible for transferring methane gas from a deep hydrocarbon source to the shallow sediments through a nearly vertical faulting regime.