Fermat's interferometric principles for multiple reflection tomography

Interferometric forms of Femat's principle are derived that converts traveltimes of free-surface multiples into those for lower-order events such as primary reflections, These primary reflection traveltimes can then be used to tomographically invert for layer velocities more accurately than standard traveltime tomography applied to multiples. As an example, Format's interferrometric principle converts VSP (vertical-seismic-profile) traveltimes of receiver-side ghosts and direct, waves to those for CDP (common-depth-point) primary reflections. These CDP traveltimes can be used to estimate, e.g., statics velocities for a 3D survey, which is impossible by conventional VSP tomography applied to primary traveltimes. This procedure is exact in the high frequency approximation, and it, can be extended to multiples in CDP and earthquake data. Fermat's interferometric principles presents new opportunities for imaging multiples and usefully extends the illumination area of conventional VSP. CDP and earthquake traveltime tomography.