BSR AND BLANKING ZONE INFERRED ON SEISMIC PROFIL...
The South China Sea（SCS）is one of the marginal seas of western Pacific with the area of 350×104km2.The average water depth is 1212m and the deepest seabed is at 5377m［8］.The northern margin of the SCS is a rifted margin，where Zhujiangkou，Qiongdongnan，Yinggehai and Beibuwan basin are located.Rich conventional hydrocarbon resources are located in the northern SCS.The Dongsha Islands occur in the eastern part of the northern margin of the SCS.
From February to April 1999，the ODP Leg 184of JOIDES Resolution was made in the South China Sea，the aim of which was to study the sedimentary record of the East Asia monsoon and its global climate impact［9］.The Leg 184shipboard party cored 17holes at Sites 1143-1148in the SCS and exceeded leg objectives by recovering 5463m of sediment.Multi-channel seismic data of SONNE95were also collected（Fig.1），and this data were used to determine the drilling sites in Dongsha Islands area.Multi-channel seismic data were processed for studies of gas hydrates in this area.Muting，Filtering，deconvolution，stacking and migration were implemented in the processing procedure and high quality seismic sections were obtained.Bottom simulating reflectors（BSRs）and amplitude blanking zones were discovered on the seismic profiles.The BSRs are almost parallel to the seafloor with a strong amplitude beneath the blanking zone.A high-amplitude zone usually occurs below the BSR in the eastern part of Dongsha Islands Region，while the BSR in the western part of the Region exhibits obvious negative polarity.The velocity of sediments above the BSRs is higher than that of normal sediments at those depths elsewhere in the word.
Fig.1 The distribution of seismic profiles of SO Leg 95and sites of ODP Leg 184
Site 1144（20°03′N，117°25′E）is located in the Dongsha Islands area at a water depth of 2037m.The maximum depth of the hole is 450m.The seismic section shown in Fig.2is one part of SO95-10through ODP184site 1144.BSR on the profile is the boundary between the blanking zone and high-amplitude zone.At site 1144，the BSR depth is 0.72s of two-way travel time（TWT）and 654meters below the seafloor（mbsf）by time-depth transform.The highamplitude zone below BSR is obvious and is more than 15km long.The thickness of blanking zone and the thickness of high-amplitude zone are both stable and 0.2s or so of TWT.The blanking zone results from gas hydrates-bearing strata and the high-amplitude zone may be related to free gas-bearing sediment.The top of amplitude blanking zone is 0.52s of TWT and about 450mbsf in the region adjacent to ODP site 1144.There are chlorine and methane anomalies at some sites of ODP Leg 184，and these are considered to be related to gas hydrates［10，11］At the east end of the section，there exist pull-ups of upper reflectors and pulldowns of lower reflectors，indicating areas where natural gas may migrate upward.
The seismic reflection profile through projected ODP184site 1144of Dongsha Islands Region is shown in Fig.3，which is one part of SO95-20.BSR on the profile is also the boundary dividing the blanking zone and the high-amplitude zone.The thickness of blanking zone changes along the profile and is about 0.2～0.4s of TWT.At site 1144，the blanking zone is thin and the maximum depth of well is close to the top of the zone.At projected site 1144，the BSR depth is 0.79s of TWT and 730mbsf through time-depth transform.There is a strong event 0.1s below the sea floor，which is parallel to the seafloor relief.Such events can usually be discovered on other profiles within the SCS and are easily explained as BSRs.There is a similar reflector on the reflection profile at Blake Ridge（Fig.16of reference［12］）.The true BSR is the base limit of amplitude blanking zone at Blake Ridge and Dongsha Islans area，consistent with the result of geothermal data from the latter.
Fig.2 The seismic reflection profile through ODP184site 1144of Dongsha Region（SO95-10）(https://www.daowen.com)
Fig.3 The seismic reflection profile through projected ODP184site 1144of Dongsha Region（SO95-20）
Site 1148（18°50′N，116°34′E）is located in the Dongsha Islands area at a water depth of 3294m.And the maximum depth of the hole is 850m.The seismic section shown in Fig.4is one part of SO95-5through ODP184site 1148.The seabed is deep along the section.The amplitude blanking zone is thick and is about 0.4s of TWT.However，the blanking effect is so weak that sequence can almost be seen.BSR is the base limit of the blanking zone，which is parallel to the seafloor and has an obvious negative polarity.There is no high-amplitude zone on the profile.At site 1148the BSR depth is 0.48s of TWT and 475m below the seafloor by using average logging velocity 1.985km/s of the site.
On the north margin of SCS，bedding planes are usually parallel to the seafloor，so it is difficult to recognize BSR.Nevertheless，the blanking zone can be considered as useful information，and the high-amplitude zone and obvious negative event are also helpful.
Fig.4 The seismic reflection profile through ODP184site 1148of Dongsha Region（SO95-5）
Fig.5 Sonic logging data of ODP Leg 184site 1143；（a）site 1144；（c）site 1146；（d）site 1148.
