Acoustic Character of the Bottom-Simulating Reflector (BSR) in the Ulleung Basin, East Sea (Japan Sea)

Abstract

The East Sea (Japan Sea) is a back-arc sea behind the Japan Arc. It is principally comprised of three deep basins (the Japan, Ulleung, and Yamato Basins) separated by rifted continental fragments. Earlier studies have confirmed the presence of gas hydrates in basement highs in the eastern margin of the Japan Basin by direct sampling; these hydrates are present in shallow sediments or even on the sea bottom [1]. Gas hydrates occur worldwide in oceanic sediments under low temperature and high pressure condition. Gas hydrates are of major interest as an energy resource for the future, a factor in global climate change and geological hazards.A BSR is a prime indicator of gas hydrate occurrences, marking the base of the hydrate stability zone (BHSZ). Acoustic impedance will decrease across the BHSZ from sediments containing gas hydrates to unconsolidated sediments below with free gas. The gas hydrate BSR is a reflection at the BHSZ caused by a negative acoustic impedance contrast and thus has reversed polarity with respect to seafloor reflection. In this study we analyzed seismic character of the BSR revealed in MCS (multichannel seismic) profiles from the plain of the Ulleung Basin. We computed amplitude and polarity of seismic reflection signals using a stochastic inversion method for sparse spike deconvolution of MCS data. We also estimated the effect of free gas below the BHSZ on the BSR amplitude using the instantaneous frequency.