A-36 FIRST-ARRIVAL TOMOGRAPHY FOR NEAR- SURFACE MODEL BUILDING Summary 1 A new method has been developed for tomographic determination of the near-surface velocity structure from first arrivals. It treats the first arrivals as direct body waves propagating along turning rays and represents the velocity structure with a grid model. Estimation of the grid’s node velocities is formulated as an iterative regularized linear least-squares problem. The traveltimes and raypaths required for the inversion are calculated by a highly efficient grid raytracing technique. Experiments with both synthetic and real data show that the new tomographic method is accurate and capable of recovering