Numerical Simulation Study On Multiple Waves And Ghost In Ocean Inclined Interface

The study of prediction and attenuation methods of multiples has always been the focus and difficulty in marine seismic data processing.In recent years,many kinds of multiple wave attenuation methods developed at home and abroad can be attributed into two categories-filter methods and wave theory methods.Whether it is a filtering method or a wave theory method,the basic assumptions based on the horizontal interface are established in the marine model.For the properties of the multiples of the inclined interface,it is unclear and perplexed to record the periodic characteristics of multiple waves and the ghost waves in the pre-stacking shot gathers,so that it can't identify or determine whether the deep seismic records of the ocean are deep reflection or multiple waves,and it can't recognize which layer are the primary waves and multiples come from.Pre-stack data processing to suppress multiple waves and ghost waves still remain in the knowledge and awareness that multiples and ghost periodically recurring at the horizontal interface.In the current marine seismic exploration,the theoretical research on the ghost wave is also scarce,and the knowledge of relationship between the time-interval characteristics of the oblique interface and the observation system is not enough.Ghost wave is a kind of interference wave which is unique to ocean exploration whose existence will interfere with the primary wave as a composite wave,which will influence the recognition and imaging of the first wave.The current observation system like variable-depth streamer(VDS)and vertical cable seismic(VCS)have made some attempts to remove the ghost wave of the receiving point,but these methods have neglected the change of the time characteristic of the ghost wave under the inclined interface condition.In order to select a reasonable and effective multi-wave attenuation method to improve the signal to noise ratio and image quality in marine seismic data processing,multiples and ghost in marine inclined interface are simulated and studied in this paper with the time-interval equation of multiples and ghost.Firstly,The time interval characteristics of the surface related multiples and interlayer multiples at different inclinations are obtained by our simulation in both shot gathers and CMP gathers,which provides the theoretical basis for the identification and inversion of the multiples and the ghost wave in the inclined interface.Secondly,on the basis of the study of the time characteristics of multiples and ghost(kinematic characteristics),this paper deduces the analytic expression of the reflection and transmission coefficient of the seabed,and the reflection and transmission coefficients of the submarine interface which changes with the incident angle are obtained.The analysis and discussion of the characteristics are carried out to provide the dynamic characteristics of the submarine interface multiple waves and the simulation of the ghost wave.Finally,we aimed to achieve the simulation and research in the two aspects of both kinematics and dynamic characteristics of ocean inclined multiples and ghost.The results obtained by time-interval simulation show that: firstly,whether surface related multiples or internal multiples,they act as hyperbolic time curves in both shot and CMP gathers.Secondly,the multiples curve in shot gathers are shifted at the vertex of the hyperbolic time curve at the inclined interface,and the degree of the offset is related to the angle of the interface.The stacking velocities of the multiples at the same inclined interface in the CMP gather are different.It can be seen that in both the artillery and the CMP gathers,whether surface related multiples or internal multiples,its periodic and repetitive features in the horizontal interface are no longer present at the tilted interface.The underground interface is assumed to be a horizontal interface in current multiple wave prediction and attenuation methods,but this simple assumption will be a problem once at the inclined interface.Therefore,on the one hand,it is necessary to identify the time interval feature of the multi-wave and the CMP gathers for the prediction and attenuation of the multiples at the inclined interface;on the other hand,the multiples and the ghost wave equation can be used as the theoretical basis of inversion.For the reflection and transmission coefficient at the seabed interface,in this paper we obtained the analytic expression of the reflection and transmission coefficient of the downward incident P-wave with the displacement-stress boundary condition and the energy normalization of the seafloor interface.With the final form of the analytic expression,the reflection and transmission coefficients of seafloor under different parameters are discussed.The variation of the reflection and transmission coefficients with incident angle presents a "segmented" characteristic with the critical angle as the dividing point.The size and type of critical angles are related to the velocity of P-wave and S-wave of seabed layer.At the critical angle,the amplitude and phase angle of the reflection and transmission coefficients change significantly.The difference of velocity of the P-wave and S-wave in different seabed layers leads to different types and sizes of submarine critical angle.Therefore,it seems reasonable to determine the type of critical angle by its characteristic in the reflection and transmission curve,and then determine the properties of submarine interface.This study will help to understand the actual observation design,model prediction and data processing,and improve the understanding of the multiples and the ghost in the ocean inclined interface,which has important scientific significance and application prospect.