1. A method of analyzing traces collected along a plurality of adjacent sail lines in a marine seismic survey area, comprising: selecting a plurality of trace groups, at least one trace in each group sharing a common midpoint with at least one trace from another group; determining an initial zero-offset travel time for each trace in the trace groups; and generating a plurality of updated zero-offset travel times and a plurality of time corrections for the trace groups using a pre-selected function of the initial zero-offset travel times.