EXPLANATION OF SONIC LOGGING DATA
Site 1143（9°22′N，113°17′E）is located in the Nansha Islands area of the southern SCS at a water depth of 2272m.And the maximum depth of the hole is 500m.The sonic logging velocity is low.It increases from 1.53km/s to 1.80km/s between 140～370m in three stages〔Fig.5（a）〕.and is therefore similar to the velocity of normal sediments worldwide.The sonic logging velocity shows that there is no evidence of hydrate-bearing sediments and the base limit of GHSZ above 370m.
The sonic logging velocity varies between 1.53km/s to 1.75km/s between 110～440m with three stages apparent〔Fig.5（b）〕.The average velocity is 1.62km/s.It shows that there is no evidence of hydrate-bearing sediments above 375m and the base limit of GHSZ is deeper than 440m.Interpretation of the sonic logging data indicates that the BSR depth and the base limit of GHSZ is deep adjacent to site 1144.
Site 1146（19°27′N，116°16′E）is located in the Dongsha Islands area at a water depth of 2092m with the maximum depth of the hole at 600m.The sonic logging velocity is high changing from 1.90km/s to 2.10k/s between 280～590m and shows a complex configuration〔Fig.5（c）〕.The velocity at site 1146is higher than for normal sediments of corresponding depth elsewhere in the world and they are inferred to be gas hydrates-bearing.(https://www.daowen.com)
The sonic logging velocity at site 1148increases from 1.7km/s to 2.1km/s between 200～450m〔Fig.5（d）〕.There is very high velocity zone between 450～475m where the velocity attains 2.4km/s.The velocity increases from 1.9km/s to 2.1km/s between 475～720m.Overall，the velocity at site 1148is higher than normal sediments of corresponding depth.High velocity interval and velocity inverse may be caused by a number of factors，e.g.gravity sedimentation［9］.However，phenomena such as，local headspace methane concentration，low chloride concentrations of interstitial water between 458～475m，high carbonate contents and low or no core recovery［9］are indicative of the presence of gas hydrates.And site 1148475m is the base limit of GHSZ and is 0.46s of TWT by depth-time transform，and is 0.48s of TWT based on average logging velocity，i.e.the same as the seismic result.
There are different characteristics of velocity distribution of sonic logging at ODP184sites.The velocities at sites 1143and 1144are low and are about 1.53～1.80km/s indicating that there is no evidence of gas hydrate.Compared with sites 1146and 1148，the velocity at site 1144is 0.2～0.3km/s lower.Between 200～280m，the velocity at site 1148is about 0.1km/s higher than at site 1143.Between 280～370m，the velocity at site 1144is about 0.15km/s higher than at site 1143and slightly higher than that at site 1148.Between 370～410m and 475～590m，the velocities at site 1146and 1148are similar.And the velocity at site 1148is 0.1～0.4km/s higher than that at site 1146between 410～475m.In particular，the velocity of the very high velocity zone between 450～475m at site 1148is 0.3～0.4km/s higher than that of site 1146.Analyses of sonic logging，geological and geochemistry data shows that there is evidence of gas hydrates in the high velocity interval and velocity inverse intervals.The base limit of GHSZ at site 1148is 475mbsf and this is consistent with the BSR depth.
