Simultaneous Traveltime Inversion Using Multi-phase Fresnel Volume Rays

Seismic traveltime tomography has been widely used for its advantages of stability,high efficiency and less computing resource requirement.It has become one of the main methods to deduce and study the internal structure of earth.However,the realistic problems that can not be ignored are the small number of natural earthquakes and their frequent occurrence at the specific tectonic zones.In addition,there have no enough seismic stations to record them.In this case,the number of rays is small and unevenly distributed,and the resolution of traveltime tomography is also limited,making it difficult to meet the needs of fine inner structure of earth.In view of this situation,the Fresnel volume ray with a certain width was introduced in traveltime tomography,and proposes to improve the imaging resolution with the existing data.Seismic ray theory is based on the infinite high-frequency hypothesis,and only the velocity structure along the raypath affects the traveltime.However,the fact that the actual seismic wave is not infinitely high frequency but band-limited makes the velocity structure in the radial zone will also affects the seismic traveltime.The use of Fresnel volume ray with a certain width instead of single ray is more realistic in the actual situation.At the same time,it can improve the coverage of the model space and reduce the non-uniqueness of the solution so as to improve the imaging resolution and quality.Based on the shortest path method and combined with multistage computational scheme,the idea of stepwise superposition of time-field was proposed,and the forward calculation of Fresnel volume ray of multi-phase in Cartesian coordinate system was realized.In order to adapt to the forward calculation under different scalar ranges and combined with the non-spherical characteristics of the earth itself,the multi-phase ray tracing algorithm under the ellipsoidal coordinate system was realized.The algorithm is suitable for any ellipsoid(or sphere)model,and can calculate a variety of complex seismic phases with high accuracy.Further,the above algorithm was introduced into anisotropic TI media.The multi-phase Fresnel volume ray was applied to simultaneous invert the velocities and reflector geometry of isotropic media in Cartesian and ellipsoidal coordinate system,respectively.The numerical simulation results show that the Fresnel volume ray tomographymethod using appropriate frequency or frequency range,is superior to the conventional ray method in both the Cartesian and the ellipsoidal coordinate system.The actual seismic data imaging results show that the Fresnel volume ray tomography in the ellipsoidal coordinate system can well adapt to the actual data and has stable and reliable imaging quality.The main innovative achievements of the dissertation are:(1)Considering the actual shape of the earth,the multi-phase seismic ray tracing algorithm in ellipsoid coordinate system is proposed.The method can be used to calculate the global traveltime in isotropic and anisotropic media,and to execute ellipsoid correction;(2)The calculation of multi-phase Fresnel volume ray in complex(anisotropic)media under various coordinate systems is realized;(3)The method of simultaneous inversion of two parameters(velocity and reflector geometry)using Fresnel volume ray in different(Cartesian,ellipsoid)coordinate systems was proposed and undertaken.