Multiple arrivals

When a grid point has multiple arrivals, three phases of wavefront propagations give different results of the attributes of wavefronts. The following rules are used to select a specific arrival.

• First arrival: minimum t_o.
• Most energetic arrival: minimum J_o.
• Left-going arrival: minimum u_o.
• Right-going arrival: maximum u_o.
• Up-going arrival: minimum w_o.
• Down-going arrival: maximum w_o.

Because these rules are defined locally, they don't generally ensure the global selections. The first arrival is an exception. Selecting minimum traveltimes at all steps of local wavefront propagations yields global minimum traveltimes. This result can be explained by the fact that traveltime is a monotonically increasing function along a ray. Other attributes of wavefronts do not have this property. For example, the geometrical spreading factor J may increase or decrease along a ray. Thus, the method may give locally most energetic arrivals, instead of globally most energetic arrivals.