An application of joint first-break and reflection traveltime tomography in the complex mountainous area of western China

The near-surface structures in western China are complex and the vertical and horizontal velocity variations are significant in complex mountainous areas. The gully in the mountain area is vertical and horizontal, the gully is deep and the slope is steep, the fault system is developed, the middle and shallow detachment faults are developed, the deep faults are complex, the regularity is poor, and the propagation path of seismic waves is extremely complex. The conventional tomographic cannot accurately describe the spatial velocity variations, and it is difficult to meet the needs of high-precision imaging in this area. In order to solve this problem, the study proposes a joint first-break and reflection traveltime tomography makes full use of the characteristics that the first-arrival wave is sensitive to the velocity and thickness of the near-surface weathering layer, and the full-azimuth reflected wave has a good imaging effect on the underground structure and formation interfaces. Using the first-arrival wave information of shot gathers and full-azimuth reflection gathers, the uniqueness and reliability of inversion results are improved by joint tomography and the underground velocity variations are more accurately described. The application shows that this method can effectively improve the accuracy of velocity model in complex mountain areas, enhance the imaging quality of complex structures and faults, and provide strong technical support for oil and gas exploration in complex mountain areas.