Study On The Velocity Model Building Of Pre-stack Reflected Wave

Seismic wave velocity is one of the most critical parameters in seismic data processing.The high-resolution velocity model can be used for seismic data interpretation,can provide a high-precision velocity field for migration imaging and can also provide strong evidence for reservoir identification.How to obtain a precise underground velocity model has become a problem for geophysical researchers today.Due to the requirements of high-precision seismic exploration,traditional velocity model building methods such as stack velocity analysis,migration velocity analysis and reflection tomography cannot often provide high-precision velocity models for seismic exploration in complex formations.The newly developed full waveform inversion(FWI)uses the seismic full wave data,such as phase and amplitude,to continuously update the velocity model to reconstruct the underground velocity structure,therefore owning higher resolution and accuracy than traditional methods.However,due to its own limitations,such as strong dependence on the initial models because of high nonlinearity,low computational efficiency and cycle skipping,the application of FWI in 3D actual seismic data is limited.Studies have shown that if a good initial model is given,FWI will greatly promote its development in the geophysical field.Compared with traditional methods such as travel time inversion,pre-stack waveform inversion(PWI)not only adopts the reflectivity method for forward simulation and genetic algorithm for inversion to get the global optimum solution to improve the computational efficiency,but also uses the seismic full wave data to improve the inversion accuracy.Therefore PWI is a better velocity model building method and can provide a good initial model for FWI.This paper first studies travel time inversion,which performs the inversion using conjugate gradient method on the travel time of reflected wave.However,due to the principle of travel time tomography,the resolution of the inverted velocity model is not very high and it can be used as input to further study PWI in order to provide more accurate initial model for FWI.After in-depth research and discussion,this paper proposes the pre-stack waveform inversion based on adaptive genetic algorithm and uses the result of travel time inversion as the initial model for our method.Finally,we carry out some tests on simulated seismic data and a velocity model with higher accuracy is obtained,which verifies the feasibility of our method to provide a good initial model for FWI.