Understanding of the first arrivals in the shape of a Christmas tree

Abstract

In land seismic data processing, the first arrivals are often picked to image the near surface by applying a traveltime inversion method for statics corrections. In many areas, however, the first arrivals of a shot gather exhibit a pattern like a Christmas tree: the first arrivals quickly disappear over offset and then another branch nearly parallel to the previous one appears with a constant time shift, and the pattern may be repeated further. An initial thought is that these Christmas tree branches are the refractions associated with different layers and imbedded low velocity layers in the near surface cause the delay. To confirm our understanding on such shot gathers, we establish a few numerical models and apply a 1-D elastic modeling program to simulate seismic data. We found that the first refraction quickly fading away because of its propagation through a thin high-velocity layer. The later Christmas tree branches are the wide-angle reflection and its multiples from a low velocity zone right below the thin high-velocity layer. Except for the shallow direct wave and the first refraction branch, all later Christmas tree branches at long offset are not really the “first arrivals.” Numerical experiment indicates that using the traveltimes of these Christmas tree branches for the first arrival traveltime inversion produces false near surface models. For this type of data, a more sophisticated method such as waveform inversion should be applied for near surface imaging.