THE CHARACTER OF BSR ON DIFFERENT PROFILES
Reprocessing was focused on obtaining better images of sub-bottom shallow structure，and then to investigate the seismic characteristics of BSR distribution.The processing includes geometry definition，common mid-point（CMP）sort，velocity analysis，normal moveout（NMO）correction，stack and migration.No filter and gain are used in the processing to preserve the original amplitude information.Complex trace analyses are conducted and instantaneous profiles are obtained.
The near-offset profile of NT62-2is shown in Fig.2.Sediments of the western Nankai Trough，subducting oceanic crust and the decollement above are clearly indicated on the south-eastern part of the near-offset profile.Because BSR is the research target of this study and there is no BSR at the Nankai Trough axis region，further data processing has never been conducted on this part.BSRs are mainly distributed in western Nankai accretionary prism.There are some indications of BSR on the near-offset profile，but the image is not good.On the single-trace profile it is difficult to find the real distribution of BSRs on the slope of the western Nankai Trough.The analyses of the near-offset profile show that there is BSR near 5.8s at trace 744（shot 879）.There are two dipping reflectors between 5.2～5.6s near trace 905（shot 1040），and the upper one is BSR which will be demonstrated by analysis of following data processing results.BSRs at both shots on common shot gathers are shown in Fig.3.
Fig.2 Near-offset profile of NT62-2
Geometry is defined according to navigation data，and common-midpoints（CMP）are obtained and sorted.Because the average shotpoint interval is 44m and it is not integral multiple of the group interval，near CMPs are incorporated.Two CMP gathers are shown in figure 4，which are corresponding to the above two shot gathers.Compared with shot gathers，reflectors on CMP gathers are regular and can be easily used for velocity analysis.Velocity analyses are conducted on some CMP gathers（one CMP gather is selected for every 40gathers）and stacking velocities are obtained.Normal moveout is corrected by using stack velocities and the stack profile is obtained（Fig.5）.Compared with the near-offset profile，the resolution and signalversus-noise ratio on stack profile have been improved.And more BSRs are clearly distinguished.Two dipping reflectors are imaged clearly and analysis indicates that the upper one is BSR.
Fig.3 Shot gathers of NT62-2
Fig.4 CMP gathers of NT62-2(https://www.daowen.com)
Stolt F-K migration is conducted on the stack profile using 90%of the water velocity as the migration velocity and the migration profile is shown in Fig.6.Compared with stack profile，seafloor and BSRs are imaged well.BSRs are more clear and continuous，which are mainly distributed between CMP50-340and CMP430-750.It shows that BSR，especially the seaward BSR，is not continuous and its amplitudes vary laterally.
Fig.5 Stack profile of NT62-2
Fig.6 Migration profile of NT62-2
Complex trace analysis has been conducted on a single-channel profile at the Blake Ridge by Taylor et al.［9］.Instantaneous profiles show the character of high amplitude and low frequency beneath BSR and along fault path.It indicates the attenuation of free gas on seismic waves.Complex trace analyses are also conducted on stack profile and migration profile in this paper.Although there is no clear indicator of free gas on the instantaneous frequency profiles，we have found that BSRs are clearly mapped on the instantaneous phase profile〔Fig.7（a）〕and high-amplitude and lateral amplitude variation of BSRs are shown on the instantaneous amplitude profile（Fig.7（b）〕.Instantaneous profiles can be used as common tools for analysis for the character of gas hydrate BSRs.
Fig.7 （a）Instantaneous phase profile and（b）instantaneous amplitude profile of NT62-2
