DISCUSSION

Gas hydrates are believed to occur in the marine sediments of the South China Sea and the East China Sea（see Fig.5）［10～12］.Preliminary identifications of BSRs have been carried out in the northern margin（north of Xisha Trough，south of Dongsha Islands and south of Xisha Islands）and southern margin（Offshore Sabah）of the south China Sea and Okinawa trough.But further investigation and study should be done to discuss the exactness of identified BSRs and understand the distribution and amounts of gas hydrates in China marginal seas.

Fig.5　World-wide locations of observed submarine gashydrates［12］，gas hydrates in mid-Okinawa Trough［11］1and 2indicate sea-floor seepage-associated and non-associated gas hydrates，respectively

Seismic inversion is an important problem in theoretical and applied seismology.Because higher-dimensional wave equation inversion is much difficult to study in theory and practice，research on inversion for elastic parameters of stratified media attracts numerous mathematicians and geophysicists.So far single parameter inversion based on one-dimensional acoustic equation has been solved well theoretically by previous researchers and recently is being applied in seismic exploration.But multi-parameter inversion based on vector wave equation with non-impulsive wavelet response need to be studied further［13～14］.One and half-dimensional elastic coefficients inversion will be developed and applied in the studies of gas hydrates and other fields.(https://www.daowen.com)

The methods have been presented for simultaneous inversion of two parameters（compressional wave velocity and shear velocity）and three parameters（density，compressional wave velocity and shear velocity）of stratified isotropic media from finite bandwidth data of P-P and P-SV plane wave seismogram［13～14］.They are based on elastic wave equation，so the methods can deal with multiple reflections and conversions between shear and compressional modes.It is proved by two parameters and three parameters inversion experiments for band-limited seismogram with and without noise that the quasi-characteristic band inversion method based on layerstripping principle and optimization theory has some advantages of high computation speed，good stability and anti-noise ability（see Fig.6）.

Gas hydrates are very important because of their vast resources potential，their role as submarine geohazard，and their effects on global climate in the world.Gas hydrate deposits are identified principally by seismic method.Numerous studies involving forward modeling and inversion of seismic reflection have been carried out.These studies are calculations of interval velocities from RMS stacking velocities，travel time inversion，poststack acoustic impedance inversion，amplitude-versus-offset analysis and one-dimensional single parameter full waveform inversion of wide-angle reflection data.So it is very crucial to develop seismic methods to acquire elastic parameters structure of gas hydrate deposits by prestack seismic inversion based on vector wave equation，and then to estimate gas hydrate concentration and discuss its formation mechanism.

Fig.6　Simultaneous inversion results of three elastic parameters in stratified isotropicmedia（a）bandwidth inversion results using 20traces of noise-free plane wave seismogram；（b）bandwidth inversion results using 20traces of plane wave seismogram with 30%random noise①represents compressional wave velocity；②represents shear wave velocity.（Model parameters profiles are plotted with solid lines，and inverted parameters profiles are plotted with dashed lines.）