Seismic traveltime tomography is widely used to estimate interval velocities from seismic data. Tomographic techniques can be categorized in two classes: those operating on traveltimes (Bishop et al, 1985 ; Harlan et al, 1991; Stork and Clayton, 1991) and those operating on stacking velocity fields (Toldi, 1985, 1989). Toldi's technique is more robust in the presence of noise and works directly with the stacking-velocity field that is readily available during processing of seismic data. In this paper we study the effect of a simple geologic structure underlying a complex velocity overburden as expressed in dip moveout (DMO) and prestack time-migration velocity spectra and demonstrate that both produce spectra that are similar to ordinary stacking velocity spectra. Surprisingly, though Toldi's operator strictly is applicable only to normal moveout (NMO) derived velocities, model studies indicate that it can be extended to DMO and prestack time-migration derived velocity fields. Moreover, in the case of more complex structure, interval-velocities from Toldi's inversion provide a good initial estimate for more powerful techniques based on depth migration .


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