Imaging of Gas Hydrates in the Northernmost South China Sea from Multi-component OBS Data

Abstract

More than 100 Ocean-bottom seismometers (OBS), with a relatively dense spacing of about 500-1000 m, were deployed and recovered for exploring gas hydrates in the northernmost South China Sea. P-wave velocity models have been determined from travel-time inversion of hydrophone and vertical components of OBS data whereas the Poisson’s ratio models are determined from shear waves, converted at the sea floor and the bottom-simulating reflector (BSR), selected from two horizontal components of OBS data. A layer-stripping and Monte-Carlo inversion of the blocky model of the Poisson’s ratio is applied. Furthermore, based on P-wave velocity, Poisson’s ratio and rock physics of the three-phase effective medium, saturation of gas hydrates is estimated. The results show that hydrates are imaged by a relatively low Poisson’s ratio (0.44-0.46) below anticlines and free gas is characterized by a relatively high Poisson’s ratio (0.485-0.5) beneath most of the BSR. We also observe that saturation of hydrates (30%) in the passive continental margin of the South China Sea is greater than that (15-25%) in the active accretionary wedge off SW Taiwan.