Interpreting Artifacts of Body Wave Interferometry

Abstract:

Modeling of virtual source gathers (VSG) and reflection profiles produced by body wave interferometry of geometrically biased P-wave source distributions is used here to identify and interpret coherent seismic arrivals that do not correspond to conventional seismic phases. These seismic arrivals are often labeled as “artifacts” (or “spurious arrivals”) due to their unrealistic apparent velocities and non-causal arrival times. We discuss the appearance in both reflection profiles produced by auto-correlation as well shot gathers generated using cross-correlations of seismograms from sources with various depth distributions. We show that relatively deep sources (below the imaged interface) produce stronger and more coherent "conventional" reflections, especially at small offsets. Shallow sources are more effective in retrieving direct and refracted arrivals. We also show how energy associated with critically refracted energy can be shifted during cross-correlation with corresponding loss of information about the depth of velocity gradient causing the refraction. Furthermore, we describe how the contribution of such artifacts can be minimized by proper design of recording arrays, and how useful information can be extracted from the artifacts themselves by modeling.