Travel Time Tomography Method With Fresnel Volume And Its Applications

Correct reconstruction of the velocity distribution of the near earth's surface is very important in solving static corrections problem of the seismic data.The usual travel time tomography method is based on geometrical ray theory,however,it assumes that the seismic wave has very high frequency,which often makes the tomographic inversion have ill posed problem because of the sparse ray distribution in the model space and finally affects the reliability and resolution of the tomographic results.Actually,seismic wave is band limited in frequency,and its energy comes from one Fresnel volume connecting source point and receiving point instead of a geometrical ray,so it's supposed that travel time tomography which is based on Fresnel volume have much better tomographic results in contrast with what of the ray-based tomography.In order to overcome the limitation of the travel time tomography based on geometrical ray theory and improve the reliability and resolution of the tomographic results,we mainly develop a lot of research on travel time tomography method with Fresnel volume in this paper,and have got some achievements.Firstly,we study the calculation method of Fresnel volume and influence coefficient of the points in the volume to the travel time.Then,we study the tomographic inversion method based on influence weight coefficient of the points in the Fresnel volume,and finally develop the travel time tomography method with two-dimensional Fresnel volume.Numerical experiment results of several models prove that the method is correct,and it increases the coverage and its number of times of the model space,improves inversional rate of convergency,efficiency of the tomography calculation,resolution and reliability of the tomographic results. Secondly,in order to resolve the problems such as great calculation account and memory space of three-dimensional tomography,we study an approximate calculation method of three-dimensional Fresnel volume.We study the calculation function of influence weight coefficient of the points in the Fresnel volume by many experiments. We study a parallel algorithm of tomographic inversion and finally develop the travel time tomography method with three-dimensional Fresnel volume and homologous parallel algorithm.Numerical experiment results of the models prove this tomograpthy method has all the excellent features on the travel time tomography method with two-dimensional Fresnel volume.Besides,we analyze the influence on the tomographic results and inversional rate of convergency with different frequencies of the Fresnel volume tomography,and then develop a method by using variational frequencies in the processing of tomography inversion with Fresnel volume. Numerical experiment results of the models prove it can properly improve the inversional rate of convergency,the resolution and reliability of the tomographic results.Based on the theory and research work above,the softwares of travel time tomography with two-dimensional and three-dimensional Fresnel volume are developed by using C++ programming language,which contain simulation model construction,ray tracing and travel time calculation,tomography inversion calculation, and displaying of model and velocity data.We processe actual seismic data of different complex areas using the softwares,and get their tomographic velocity results. Then,we use the velocity results to calculate the static corrections and nestification processing,which proves that the tomography method with Fresnel volume has better results of static corrections in contrast with what of the field and ray-based tomography.