The determination of the correct velocity structure of the near surface is a crucial step in seismic data processing and depth imaging. Generally, first-arrival traveltime tomography based on refraction data or diving waves is used to assess a velocity model of the subsurface that best explains the data. Such first-arrival traveltime tomography algorithms are very attractive for land data processing because early events in the seismic records are very often dominated by noise, and reflected events are very difficult or even impossible to identify. On the other hand, first arrivals can generally be identified quite clearly and are very often the only data available to reconstruct the near-surface velocity structure.

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