S-wave Velocity Derivation Near the BSR Depth of the Gas-hydrate Prospect Area Using Marine Multi-component Seismic Data

???????????????????????? ????????? ????????? ????????? ?????? ?????? ????????? S??? ?????? ????????? ????????? ??????????????? ????????? ??????????????? ???????????? ????????? ??????. ?????? ????????? ?????? ??????????????????????????? ??????????????? ??? ???????????? P??? ????????? S??? ????????? ????????? ???????????? ??????, ??? ????????? ??????????????? ???????????? ?????? P?????? ????????? ????????????. ????????? S?????? ?????? ????????? ????????? ?????? ????????? ????????? ?????? ?????? ???????????? ????????? ????????? ??????????????? ?????????????????? ?????? ????????? ?????????????????? ??????????????? ?????? ??? ????????? ????????? ????????? ????????? ????????? ???????????? ??? ????????? ????????????. ??? ??????????????? ????????? ????????? ???????????? ?????? ??????????????? ???????????????????????? ???????????? ??? ????????? ??????????????? BSR(????????? ?????? ?????????)??? ????????? ???????????? ??? ???????????? ?????????????????????????????? 2009??? 5?????? OBS(????????? ????????? ?????????)??? ???????????? ????????? ????????? ????????? ????????? ????????? ???????????? ???????????????????????? ?????? ?????? ????????? P??? ??? ???????????? S?????? ????????? ????????????.OBS??? ???????????????(hydrophone) ????????? ????????? P??? ????????? ???????????? ????????? ?????? ???????????? ???????????? P??? ?????? ??? ?????? ????????? ???????????????. ??????????????? ????????? 2?????? ????????? ?????? ????????? ?????? ??????????????? ?????? ?????? ??????????????? ?????? ??????????????? ??????????????? ?????????. ????????? ?????? 2?????? ?????????(geophone)??? ????????? ????????? ?????? ????????? ?????? S?????? ???????????? ????????? ?????? radial ?????? ???????????? ???????????? ????????? ????????? ?????? S??? ???????????? ????????? ????????? ????????? ??? ??????????????? ???????????? OBS??? ????????? ??????????????? ????????? ?????? S??? ??????????????? ??????????????? ???????????????. ??? ????????? ?????? ??????????????? ???????????????????????? ????????? ?????? BSR ????????? ?????? P??? ?????? ?????? ????????? P????????? ?????? BSR ???????????? ????????? ????????? S?????? ????????? ?????? ???????????? ????????? ?????? ??????????????? ?????????????????? ????????? ?????? BSR ???????????? ????????? ??? ??????????????? ???????????? ???????????????. S-wave, which provides lithology and pore fluid information, plays a key role in estimating gas-hydrate saturation. In general, P- and S-wave velocities increase in the presence of gas-hydrate and the P-wave velocity decreases in the presence of free gas under the gas-hydrate layer. Whereas there are very small changes, even slightly increases, in the S-wave velocity in the free gas layer because S-wave is not affected by the pore fluid when propagating in the free gas layer. To verify those velocity properties of the BSR (bottom-simulating reflector) depth in the gas-hydrate prospect area in the Ulleung Basin, P- and S-wave velocity profiles were derived from multi-component ocean-bottom seismic data which were acquired by Korea Institute of Geoscience and Mineral Resources (KIGAM) in May 2009. OBS (ocean-bottom seismometer) hydrophone component data were modeled and inverted first through the traveltime inversion method to derive P-wave velocity and depth model of survey area. 2-D multichannel stacked data were incorporated as an initial model. Two horizontal geophone component data, then, were polarization filtered and rotated to make radial component section. Traveltimes of main S-wave events were picked and used for forward modeling incorporating Poisson's ratio. This modeling provides S-wave profiles and Poisson's ratio profiles at every OBS site. The results shows that P-wave velocities in most OBS sites decrease beneath the BSR, whereas S-wave velocities slightly increase. Consequently, Poisson's ratio decreased strongly beneath the BSR indicating the presence of a free gas layer under the BSR.??? ????????? ???????????????????????? ?????? ???????????? 2011??? ‘???????????????????????? ?????????????????? ??????’ ????????? ???????????? ????????????????????