| Most of the earth’s near-surface layer is loosely deposited and under-compacted,which leads to the viscous nature of the rock layer,which seriously affects wave propagation and seismic wave energy attenuation.When seismic waves propagate in this non-uniform attenuating medium,they will undergo frequency-dependent absorption and attenuation,which limits the energy of the seismic waves to reach the target exploration horizon,which seriously restricts the development of high-resolution exploration.How to suppress the impact of the near-surface on the seismic wave field is one of the core problems to be solved in highresolution reflection seismic exploration.In seismic exploration,the seismic wave propagation medium is generally a non-uniform medium within the seismic frequency band.The physical properties of the medium change with the change of the spatial coordinates,so that the information such as the travel time and wave velocity of the seismic wave changes.By studying the problem of non-uniform near-surface attenuation,the near-surface structure and parameter model with higher accuracy can be obtained,so that the adverse effects of the near-surface on the seismic wave field can be effectively controlled.The physical mechanisms that cause the attenuation effect of seismic waves can be summarized as follows: internal friction loss mechanism between unconsolidated rock particles,electrochemical mechanism and capillary mechanism,and Biot flow and jet flow mechanism.For the first friction loss mechanism in the near-surface rock,a number of attenuationdispersion medium models with quality factor and attenuation function as parameters are established using inelastic medium theory.Furthermore,the wave equation is established by using wave theory to realize the forward problem of seismic exploration.In this paper,through calculation and analysis,it is found that the conventional attenuation-dispersion model is insufficient to describe the strong velocity dispersion phenomenon in the low frequency band of seismic waves.Based on this,this paper corrects an attenuation-dispersion model based on the conventional attenuation-dispersion model.Considering the continuous non-uniform formation,the wave equation of non-uniform attenuation medium is established.Taking into account the relationship between attenuation,dispersion and frequency of seismic waves,the125-point finite difference method in the frequency domain is used to realize the forward problem of the revised attenuation-dispersion model,and the effects of seismic attenuation and velocity dispersion on the propagation law of seismic waves are explored.It will lay a certain theoretical foundation for the subsequent continuous non-uniform attenuation-seismic exploration in the large dispersion area.In this paper,in the case of vertical continuous non-uniform ground medium,the physical mechanism of seismic wave attenuation is analyzed in detail,and the attenuation-dispersion model established for internal friction loss factors is difficult to describe the limitation of large velocity dispersion in low frequency bands.The conventional Azimi second attenuationdispersion model was modified to simulate non-uniform near-surface regions where strong velocity dispersion occurs.Build a 125-point finite difference method in the frequency domain,discrete the scalar wave equation in the frequency domain of the non-uniform attenuation medium based on the wave theory,find the new optimization weighting coefficient,use the Helix spiral transformation to achieve the dimensionality reduction storage of the impedance matrix,and test based on The GMRES iterative method of the ILU preprocessing operator completes the forward simulation of the frequency domain of non-uniform attenuation medium,and analyzes and discusses the seismic wave propagation law in non-uniform attenuation medium. |
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