Seismic velocity estimation using time-reversal focusing

Seismic velocity estimation is a challenging task, especially when no initial model is present. In most cases, a traveltime tomography approach is used as a significant part of the workflow. However, it requires noise-sensitive, time-consuming picking and uses a ray approximation of the wave equation. Time reversal (TR) is a fundamental physical concept, based on the wave equation's invariance under TR operation. If the recorded wavefield is reversed and back-propagated into the medium, it will focus at its original source location regardless of the complexity of the medium. We use this property for seismic velocity analysis, formulated as an inversion problem with focusing at the known source location and onset time as the objective function. It is globally solved using competitive particle swarm optimization and an adequate model parameterization. This approach has the advantages of using the wave equation, being picking-free, handling low signal-to-noise ratio and requiring neither information on the source wavelet nor an initial velocity model. Although the method is discussed in the framework of direct source-receiver path acquisition, the foundations for its use with conventional reflection data are laid. We have determined the method's usefulness and limitations using synthetic and field crosshole acquisition examples. In both cases, inversion results are compared with a standard traveltime tomography approach and illustrate the advantages of using TR focusing.