Seismic character of the BSR in the Ulleung Basin plain, East Sea (Japan Sea)

A bottom simulating reflector (BSR) was identified in multichannel seismic reflection profiles collected in the Ulleung Basin plain, East Sea (Japan Sea). The BSR, occurring in densely stratified sediments, is observed very close to the boundary between two sequences interpreted as low-energy turbidites/hemipelagic sediments and mass-transport deposits. The essential characteristics of the BSR include its cross-cutting relationship to strata at a small angle, strong amplitude, and reverse polarity with respect to the seafloor reflection. Stochastic inversion of seismic data indicates that the BSR reflection coefficients reach maximally the range from 0.23 to 0.26 and are 1.5∼1.7 times that of the seafloor reflection. A decrease in interval velocities, disrupted seismic signals, and noticeable drop of the frequency content are recognized below the BSR. These features strongly suggest that the BSR represents the base of gas hydrate stability zone (BHSZ) in the Ulleung Basin plain and a free gas zone is present below the BSR with high amplitude. Inversion also discriminates the lower amplitude BSR that is not easily identified from interfering reflection signals, suggesting that gas hydrates exist more extensively in the Ulleung Basin than recognized in seismic profiles.