The purpose of statics for seismic data is to remove the disturbing effects of the near-surface low-velocity layer (LVL) on the continuity of deep seismic reflections. The traveltimes of waves at the bottom of the LVL can often be obtained by picking first breaks on shot records. Since the LVL often has irregular velocities and thicknesses both vertically and laterally, we develop a vertically and laterally inhomogeneous velocity and thickness model with the help of refraction traveltimes, from which the statics can be obtained. Then we describe a method for computing accurate statics from first breaks according to the model. It is based on a linearizing principle for first break traveltimes and leads to the algorithm that is widely and successfully applied within the framework of seismic tomography. In this method, the differences between observed traveltimes on field records and those computed by ray tracing from an initial model of the LVL are minimized in the least-squares sense by iterative techniques to obtain accurate statics. The accuracy of the method lies in dividing the LVL into small finite blocks, each having particular values of the P and S velocities and of thickness, first obtaining accurate velocities and thicknesses for these blocks, then obtaining accurate statics by the method. The potential importance of this refraction-tomography statics method is shown through an application of the method to the vertical- and radial-component data (P-P and P-SV) from the Line EUE001, Slave Lake, northern Alberta.
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