Inversion of first‐arrival seismic traveltimes without rays, implemented on unstructured grids

SUMMARY We develop a method for inverting first‐arrival seismic traveltimes without using ray tracing in the forward solution, nor to calculate sensitivity information. We consider unstructured 2‐D triangular and 3‐D tetrahedral grids for discretizing the subsurface velocity distribution. Unstructured grids provide some computational advantages when dealing with complicated shapes that are difficult to represent with rectilinear grids. However, we stress that the key details of our inversion approach that avoid ray tracing are equally relevant to rectilinear grids. The forward problem is solved using the Fast Marching Method, an efficient numerical algorithm that can be used for propagating first‐arrival seismic wave fronts through a velocity distribution. Our minimum structure inversion algorithm uses sensitivity information calculated directly during the forward solution. This calculation relies on explicit symbolic differentiation of the forward modelling equations and, as such, our inversion approach does not require ray tracing and uses sensitivity information that is numerically consistent with the forward solution. Our method is applied to 2‐D and 3‐D geologically realistic synthetic scenarios based on the Voisey's Bay massive sulfide deposit in Labrador, Canada.