Full-Waveform Inversion For Irregular Topographic Models

Full-waveform inversion(FWI)is a method to find the optimal solution in the sense of a predefined norm.Since FWI can effectively use the waveform information of seismic data,it can improve the accuracy of inversion.Therefore,this method has been widely studied.The basic idea of full-waveform inversion is to measure the misfit between synthetic seismic records and actual seismic records by using a mathematic norm,and then iteratively update the velocity model through local optimization algorithms to reduce the value of the norm;consequently it approaches the real model.In practical applications,there are several difficulties,such as the non-linearity between seismic records and model parameters,the need to process massive amounts of data,and the requirement of a highly-accurate initial model.To use full-waveform inversion in actual exploration effectively and efficiently,further research is needed.The main calculation of full-waveform inversion is wavefield simulation.At present,the most commonly used method is the finite-difference method because this method is easy to be implemented and has relatively smaller calculation than other methods.However,if the undulating surface is discretized with a regular mesh,it will be represented by a step-like surface.When performing wavefield simulations,these step-like approximations will produce artificial diffraction waves and travel-time errors,affecting the results of wavefield simulation.To use finite-difference algorithms for simulating the undulating surface,a body-fitted grid is used as an aid in this study.This method can transform the irregular physical domain into a regular computing domain,which facilitates the use of the finite-difference method.The curved grid generated by using the body-fitted method coincides with the undulating surface,which can simplify the processing of the surface.The research in this paper shows that the wavefield simulation method based on the body-fitted grid can effectively deal with the undulating surface,and the method can be further used for full-waveform inversion to improve its applicability.