Joint Tomography With Seismic First-break And Reflection Under Rugged Topographical Conditions

Numerical simulation of seismic wave field is undoubtedly an important way to study the propagation of seismic waves in underground media.At the same time,it can also collect,interpret and process seismic data in complex areas.Seismic exploration is carried out against the geological background of sand dunes and loess tableland in the west of China.Due to the highly undulating surface topography and complex and diverse underground structures,it is quite difficult to conduct static correction,seismic wave identification and seismic imaging for seismic exploration.Therefore,it is of great practical significance to study the numerical simulation algorithm of seismic wave field and make it better applicable to the environment of complex structures and media.Seismic tomography has always been an important technique in seismic exploration.With the help of model reconstruction algorithm,known seismic wave information can be used to invert relevant data such as underground medium structure and elastic parameters.For the initial arrival wave of an earthquake,it has the characteristics of simple tracking and identification,and has obvious advantages in near-surface velocity analysis or undulating surface statics.However,in tomography inversion,velocity modeling with only initial arrival wave cannot accurately reconstruct the underground velocity information,especially for the imaging effect of multi-layer model deep medium is very poor.The reflected wave carries more information than the first arrival wave and has a larger detection depth,so the reflected wave also plays a crucial role in tomography.In this study,the complex undulating surface medium is taken as the object,and the seismic wave field forward modeling,initial arrival wave and reflection wave travel tomography are studied respectively.It is expected to make a breakthrough in tomographic inversion and serve for data processing,velocity analysis and other aspects of seismic exploration in the complex media region with undulant surface.In order to obtain accurate seismic wave travel time and computational efficiency is also taken into account in the background of surface undulating and complex underground structure.In this paper,previous research results are used to carry out forward ray tracing simulation,that is,LTI ray tracing algorithm based on mixed mesh subdivision.The core of the algorithm is to fuse the irregular quadrilateral with the regular rectangular grid and get a new formula for calculating local travel.The numerical results show that this method can not only deal with the velocity interface flexibly,but also has high precision in the calculation of ray path and travel time.The travel time information obtained by forward simulation calculation is taken as the observation value.In the inversion,the widely used LSQR algorithm is used to solve the tomography equations,and combined with the model parameter weighting and regularization,multi-scale gradual inversion and other technical means to achieve the undulation surface initial arrival wave and reflection wave tomography.The validity and applicability of the proposed method are verified by model test.