Forward Modeling And Reverse Time Migration For Rugged Acoustic-Elastic Coupling Media

With the development and application of submarine cable technology,marine seismic exploration can make full use of the P and S wave information for imaging complex structures,which is a huge breakthrough in marine seismic exploration.Usually submarine cable technology a single elastic equation is adopted to calculate the wave field in the imaging process,however the single equation has the disadvantages of excessive calculation and poor stability.Therefore,it is necessary to study the wavefield extrapolation operator of acoustic-elastic coupling media.Meanwhile,the rugged seabed interface put forward a challenge for seismic imaging in the marine environment.In the complex sea area of China,severely rugged seabed structures have been a thorny issue in seismic data processing and imaging,which have strong impact on the imaging accuracy of the underlying strata.In conventional processing,elevation static is usually used to eliminate the influence of seismic wave travel time.This method based on the assumption of surface consistency.However,the assumption of surface consistency is not satisfied by the large variation of height and velocity in the rugged seabed structure region,which makes the elevation static extremely difficult and inaccurate.Aiming at the rugged seabed interface environment,this paper proposes a undulating acoustic-elastic coupling medium imaging method.At first,the principle of body-fitted grid generation based on fluid ideas is introduced,it can generate grids based on complex underground structures and ensure the orthogonality and smoothness of the generated curved grids on the boundary.Based on the body-fitted grid,the acoustic wave and elastic wave equations in the curvilinear coordinate system are derived,and the wave equation is discretized with a fully staggered grid and improved the false scattering caused by the stepped grid.According to the acoustic wave and elastic wave equations in the curvilinear coordinate system,the forward modeling of the rugged acoustic-elastic coupled medium is realized,Acoustic and elastic wave equations are used in the fluid phase and solid regions respectively,and the body-fitted grid and coordinate transformation techniques are used to transform the rugged acoustic-elastic coupling interface into a horizontal seabed interface in a curvilinear coordinate system.The continuity condition between acoustic pressure and stress is used at the seabed boundary to achieve a stable transformation of the acoustic wave field to the elastic wave field.Ultimately,the reverse time migration technique for the rugged acoustic-elastic coupled medium was successfully realized,and introduce the Helmholtz wavefield decomposition method based on wavefield extrapolation into the elastic wave equation in curvilinear coordinates to obtain high-precision imaging results for the PP and SS components of the rugged acoustic-elastic coupled equation.The model calculation results show that the rugged acoustic-elastic coupling medium reverse time migration method proposed in this paper can obtain accurate imaging results of the complex ocean environment.