Seismic Elastic Parameters Inversion In The Elastic Media With The Overlying Fluid Phase

Seismic exploration is an important theoretical foundation and technical support for the exploitation of offshore oil and gas resources.Elastic parameter of middle and deep ocean information not only promote the study of marine basic geological problems such as marine physical environment,strati graphic structure,oceanic crust movement,oceanic lithosphere and upper mantle,but also can promote the development of the theoretical basis research and recognition of sea basin exploration strategy,reduce the risk and increase the economic benefit of deep-water oil and gas exploration.In order to effectively reduce the parameter storage,improve the computational efficiency in the seismic forward modeling of elastic media overlying the fluid phase,acoustic-elastic coupled equations in time and frequency domain are reduced in this paper.A full waveform inversion method for this kind of media in time domain and frequency domain has been implemented.In order to further reduce the strong nonlinearity of full waveform inversion,a full waveform inversion method based on the energy norm for elastic media overlying the fluid phase is developed.The numerical simulation based on the acoustic-elastic coupled wave equation is the foundation of full waveform inversion for elastic medium overlying the fluid phase.In this paper,with the aid of continuity condition of fluid-solid boundary,utilizing first-order acoustic and elastic wave equations in liquid phase and solid phase medium respectively,I establish the acoustic-elastic coupled wave equation of elastic medium overlying the fluid phase based on conventional staggered grid and implement the study of seismic wave forward modeling for this.On these bases,in order to improve the calculation efficiency and reduce the storage pressure in the forward modeling process,using the idea of equivalent staggered grid,I proposed the strategy of transition layer along the fluid-solid interface in time and frequency domain and established the acoustic-elastic coupled equation of time domain and frequency domain,respectively,then realized forward modeling based on equivalent staggered grid.To eliminate the diffraction noise caused by ladder approximation of the undulating seabed interface under conventional Cartesian coordinates system in irregular seabed interface numerical simulation,I reduced the acoustic and elastic wave equations containing PML attenuation boundary in the curved coordinate system and developed fluid-solid coupled medium time and frequency domain seismic wave propagation equation based on the totally equivalent staggered grid,then realized the forward modeling based on these equations.The seismic wave full-waveform inversion of acoustic-elastic equation is an important method and approach to obtain the elastic parameter information of the elastic medium overlying liquid.Based on the seismic wave propagation equation in time domain and in frequency,we utilize the variational principle and the adjoint state method,derive the full-waveform inversion gradient operators of the acoustic-elastic coupled equation in time and frequency domain,and develop the acoustic-elastic coupled full-waveform inversion methods,and realize the single-parameter and dual-parameter inversion for elastic medium overlying liquid,and reveal the use of offshore seismic P-wave data has its potential to obtain the P-and S-wave velocity information of subsurface media.In response to the nonlinear problem faced by multiparameter inversion,the energy norm imaging operator proposed by previous researchers was used to investigate the characteristics of energy reflected by the energy norm to extract reflected wave field based on the analytical solution and the forward simulated scattering wave field,developing a low-frequency large-scale initial model construction method based on energy norm full waveform,describing the ability of full-waveform inversion based on energy norm to extract low-frequency large-scale reflected energy.On the basis of the rule processing and low-frequency filtering,the paper has realized the study of the full waveform inversion method in the real P-wave data in deep-water survey.In summary,the acoustic-elastic coupled equation in time and frequency domain are established for the elastic medium overlying the fluid,and it improves the computational efficiency and reduces the storage burden.On this basis,we develop full waveform inversion based on the acoustic-elasitc coupled media,and achieve the elastic parameters inversion in the elastic media overlying the fluid phase.Because of the strong non-linear problem of full waveform inversion,we propose a full waveform inversion based on energy-norm in the elastic media overlying the fluid phase.There are still many problems in the full waveform inversion of the fluid-solid coupled media,such as stable inversion of density,computational efficiency,and actual data collection preprocessing,etc.It needs a long time to research,and it is believed that multi-parameters full waveform inversion in the elastic media overlying the fluid phase will play an increasingly important role in practical applications.