3-D tomographic imaging of nearsurface velocity for complex near surface statics calculation

Aimed to an approach that derives the 3-D nearsurface velocities,on which the reasonable field static values for complex nearsurface can be calculated, from the arrival time of first break by using tomography technology. The approach is two steps included the forward and inversion. Shortest Path Ray Tracing is used in the forward step to calculate first break time as well as ray path and SIRT (Simultaneous Iterative Reconstruction Technology) in the inversion step to determine the adjustment to previous near-surface velocity model. A new velocity model is constructed by adding the adjustment to previous one. The final velocity model will be reached by iteration of the two steps and is independent of initial model. The example of theoretical model, containing highly variable near-surface velocities in 3 directions, shows that the sharp lateral variations of 3-D near-surface velocities can be reconstructed appropriately by the approach.Results demonstrate that the reliable 3-D near-surface velocities and also static values can be achieved by using the approach from the south China mountainous area.