Tomography Method Research And Application Of First-arrival Wave Seismic Traveltime

Seismic wave contains abundant information of earth’s internal structure. Usingseismic tomography method, we could effectively utilize the information. The ways toobtain seismic data are generally classified as earthquakes and explosions. Seismictomography is an important geophysical method analyzing the kinematics featuressuch as travel-time and ray path, and dynamics features such as waveform, amplitude,phase and frequency. It can be used to inversely calculate the underground structure,velocity distribution and elastic parameters of the region covered with massseismic-rays.This paper introduced several kinds of seismic tomography classification, andreviewed its research history and current situation at home and abroad. The FirstArrival Seismic Tomography (FAST) method is suitable for processing2D and3Ddata. The shot and receiver locations could be accurately denoted in the model, so theerrors caused by imprecise locations could be avoided. In forward calculation, thefinite difference wavefront tracking method is utilized. In this way the seismic wavefield spread could be effectively and exactly simulated. In inversion calculation, theLSQR method, a variant of the conjugate gradient method, is utilized to solve thelarge linear system of equations. Discernibility matrix and covariance matrix forevaluation of inversion results are offered by the FAST method. We could calculatethe inversion parameters and inversion matrix to obtain these essential information.And we could observe them intuitively by making use of the check-board method.The first-arrival wave travel-time data of Yushu is processed by usingregularization inversion method, which is also a FAST method. The impacts, whichresult from different size of grids, different initial models, and different forward orinversion parameters, on tomography result are analyzed. The resolution of the finalmodel is assessed by using the check-board method. Finally we obtain the fine P wavevelocity structure of the upper crust along the profile. The analysis results are asfollows. From top to bottom the P wave velocity basically represents positive velocitygradient. The near-surface velocity changes smoothly in horizontal. But with theincrease of depth the velocity structure shows remarkable non-homogeneity both inlongitudinal and transverse. Our profile cross the Yushu-Jielong fault in theYushu-Ganzi fault zone. On the section, beneath the Yushu-Ganzi fault zone, velocitydiscontinuities and low velocity abnormality can be identified obviously. TheYush-Jielong fault is made up of two small faults. Maybe the abnormal phenomena iscorresponding to the two faults and loose sediment between them.