Seismic data acquired on land are susceptible to timeshifts due to rapid lateral variations of the velocity and depth of the near-surface layer. Current methods calculate single timeshifts for each shot and receiver (Schneider, 1971), which are necessarily an averaged correction at each point. The problem is linear and can be solved by least squares (Taner et al., 1974) and Gauss-Seidel (Wiggins et al., 1976). Rothman (Rothman, 1985, 1986) tackles the nonlinearities produced by errors in the correlation of traveltimes using simulated annealing, but still uses surface consistent timeshifts at shots and receivers. In order to interpret the data accurately, timeshifts must be calculated for each raypath. These timeshifts are no longer "static", but "dynamic" (Rothman, 1985). This requires a detailed knowledge of the variations in velocity and depth along the near-surface layer.


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