Interferometric velocity analysis using physical and nonphysical energy

Abstract

In controlled-source seismic interferometry, waves from a surrounding boundary of sources recorded at two receivers are crosscorrelated and summed to synthesize the interreceiver Green’s function. Deviations of physically realistic source and receiver geometries from those required by theory result in errors in the Green’s function estimate. These errors are manifested as apparent energy that could not have propagated between receiver locations — so-called nonphysical energy. We have developed a novel method of velocity analysis that uses both the physical and nonphysical wavefield energy in the crosscorrelated data generated between receiver pairs. This method is used to constrain the root-mean-square (rms) velocity and layer thickness of a locally 1D medium. These estimates are used to compute the piece-wise constant interval velocity. Instead of suppressing multiple energy as in conventional common midpoint velocity analysis, the method uses the multiply reflected wavefield to further constrain the rms velocity and layer-thickness estimates. In particular, we determined that the nonphysical energy contains useful physical information. By using the nonphysical energy associated with the truncation of the source boundary and the crosscorrelation of reflected waves, a better-defined estimate of the rms velocity and layer thickness is achieved. Because this energy is excited far from the receiver pair, the technique may be ideally suited to long-offset seismic reflection data. We found that interferometric velocity analysis works best to characterize the first few layers beneath a receiver array. We have considered an acquisition configuration that can be used in a marine seismic setting.