SEISMIC CHARACTERISTICS OF GAS HYDRATES IN THE EAST CHINA SEA

For the purpose of gas hydrate study, multi-channel seismic data were collected by Institute of Oceanology, Chinese Academy of Sciences (IOCAS) using "R/V Science 1" in the East China Sea in 2001. Seismic data from 2001 together with those from 1980s were processed to get the migration profiles. 6 sections of abnormal reflections were determined as BSR based on the migration profiles interpretation. All the BSRs given have strong amplitude and reverse phase as comparing to the sea floor reflector and with buried depth within 0.1 to 0.5 seconds (two way travel time),which agrees with the data from published references. The buried depth of our BSRs increases with the increase of the sea floor depth from 750 m to around 2 000 m.
There is no BSR that cuts the normal sediment layer as the case in other places,and this is because the upper Quaternary sediment layer in the East China Sea is parallel to the sea floor,leading the BSR to being parallel to the sea floor too.Thus it does not mean that our BSR is a faint one. The initial temperature and pressure calculation of our BSRs based on a very simple model for sediment velocity and temperature gradient shows that all our BSRs are within the stability domain in the gas hydrate phase diagram. Assuming that BSR is the base of gas hydrate stability zone in the gas hydrate phase diagram, our BSRs show a similar temperature and pressure condition to that of North Cascadia and Chilian Margin, but different from that of Nankai Trough and Blake Ridge.
Among the whole Okinawa Trough, our BSR is either near main faults(Tokara fault and Miyako fault)or near hydrothermal activity fields distributed along the central graben of Okinawa Trough. On the seismic profile, our BSRs are locally related with tectonic compression and faulting,which can lead to the formation of gas hydrates. Although there is no direct relationship between hydrothermal activity and gas hydrate occurrence, shallow magma in hydrothermal field supplies heat for methane production from organic material in sediment and fault system in the hydrothermal area serves as conduits for the migration of methane from deeper part of the sediment to the gas hydrate reservoir.We believe that hydrothermal environment will help with the formation of gas hydrates.We can see from the seismic characteristics of profiles A1A2 and A14A23 that cold seepage occurs on the seafloor above the BSR of these two profiles. Coherence of the abnormal increase of amplitude ratio and the abnormal decrease of is considered as an evidence of cold seepage on profile A14A23 where BSR occurs.