Zhang L. and Claerbout J. Traveltime tomographic inversions of seismic data usually require picking the arrival times of certain events recorded by receivers. We have developed an algorithm that picks the traveltime of an event according to the phase properties of the wavelet of the event. If the event has a minimum phase wavelet, the algorithm picks the first break of the wavelet. If the wavelet of the event has a zero phase spectrum, the central point of the wavelet is picked. For any mixed phase wavelet, the algorithm picks the first break of the causal, minimum phase part of the wavelet.
Numerical experiments show that, for isolated, noise-free events, the algorithm performs well.
However, for field data with strong, spiky noises, the algorithm fails to pick the correct traveltimes. The computational tasks involved in the nonlinear tomographic traveltime inversion can be efficiently accomplished by using finite difference methods. The traveltimes can be computed by solving the eikonal equation. The derivatives of traveltimes with respect to the parameters of a slowness model can also be computed by solving a first-order linear partial differential equation. This new approach greatly reduces the computational cost imposed by the conventional ray tracing method.