Research On Reflection Full-Waveform Inversion

With the development of oil and gas exploration,the exploration targets are becoming more and more complex.Generally,complex geological structures are characterized by drastic changes in velocity in both horizontal and vertical directions.It is difficult to obtain accurate information of subsurface structures by prestack time migration.Therefore,prestack depth migration imaging for complex structures is particularly important.With the continuous development of prestack depth migration technology,the requirement for modeling accuracy of migration velocity in depth domain is becoming higher and higher.In recent years,fullwaveform inversion method has gradually become a research hotspot.It uses full-waveform information of seismic waves to build high-precision velocity model for complex geological structures.Due to the limited offset,the transmission wave used in the initial stage of traditional full waveform inversion can not reach the deep layer,resulting in poor inversion effect.This paper studies reflection full-waveform inversion.The transmission path of reflected wave is used to supplement the deep information.And reflection full-waveform inversion is used to enhance the inversion accuracy of the traditional full waveform inversion in the deep layer.In this paper,the absorption attenuation boundary,the establishment of reflectivity model,the gradient of objective function,and computational efficiency in reflection full-waveform inversion are studied in depth.The main research progress is as follows:(1)The finite difference high precision forward simulation of acoustic equation is studied and realized.Optimal forward parameters are selected by testing,at the same time avoid dispersion and ensure computational efficiency.The attenuation of seismic wave with different absorbing layer thickness is analyzed to ensure the accuracy of seismic wave field simulation,and the method of improving calculation efficiency is studied.(2)The least-squares reverse time migration is studied and realized.The least-squares reverse time migration gradient and the analytical step size are derived.The least-squares reverse time migration imaging results can meet the requirements of reflectivity model for demigration.(3)The full-waveform inversion in time domain is studied and realized.The deep inversion effect under different iteration times is analyzed,and the influence of low frequency absence of seismic data on inversion results is tested.(4)The reflection full-waveform inversion is studied and realized.According to Born perturbation theorem,the inversion gradient is deduced by Green's function method and Lagrange multiplier method.GPU acceleration technology is used to improve the computational efficiency of reflection full-waveform inversion.The results of model test show that the reflection full-waveform inversion can improve the accuracy of velocity modeling.This study has important significance for promoting the development of full-waveform inversion.At the same time,it can provide high precision velocity model for prestack depth migration,improve migration imaging quality and promote the development of seismic exploration.