ABSTRACT

We present two methods to compute angle-domain common image gathers (ADCIG) by downward-continuation migration, and we analyze their amplitude response versus reflection angle (AVA). A straightforward implementation of the two methods leads to contradictory, and thus obviously inaccurate, amplitude responses. The amplitude problem is related to the fact that downward continuation migration is the ad-joint of upward-continuation modeling, but it is a poor approximation of its inverse. We derive the weighting operators, diagonal in the frequency-wavenumber domain, that makes migration a good approximation to the inverse of modeling. After weighting, the ADCIGs computed by the two methods become consistent. Other important factors degrading the accuracy of AVA in practical situation are the limited sampling and offset range, and the bandlimited nature of seismic data.

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