Study Of The Joint Tomography With Seismic Firstbreak And Reflection Traveltime

The energy of firstbreak is strong,and it’s easy to identify and pick up on the seismogrom. Firstbreak traveltime tomography is better for near-surface velocity imaging, but beacause of the litimiations of firstbrak’s distribution range, Firstbreak traveltime tomography is poor at imaging deeper medium of multi-layer model. So in order to achieve tomography of greater depth for complex multi-layer structure, we need to take advantage of reflection traveltime information and carry out joint tomography with firstbrak and reflection traveltime.Joint tomography with firstbreak and reflection traveltime includes two parts,that is forward and inverse.The forward part needs to forward the seismic traveltime and raypath,the inverse part needs to solve the combined iterative equation.In this paper,we use the method of linear traveltime interpolation to forward the traveltime and raypath of firstbreak and improve the method by inserting the secondary node in the grid boundary to improve computational efficiency.For the forward of reflection’s traveltime and raypath,we use the method of multi-stage based on LTI.In the inverse part,we use the method of LSQR to slove the combined iterative equation.Next,we design a serise of 2D velocity model for calculation.By calculation,we can get following conclusions: LTI algorithm is precise and fast in computing speed,by inserting secondary node in the grid boundary,we can further improve the accuracy and efficiency in the same size grid,compared to the traditional method of LTI; The multi-stage base on LTI can forward the reflection’s traveltime and raypath; Theoretical results show that tomography with firstbreak traveltime has better imaging results for near-surface velocity model, joint tomography with firstbreak and reflection traveltime can improve the imaging accuracy of the deeper velocity model.