Abstract

The locally converted shear wave is often neglected in ray-trace modeling when reproduction of the AVO response of potential hydrocarbon reservoirs is attempted. Primaries-only ray-trace modeling in which the Zoeppritz equations describe the reflection amplitudes is most common. The locally converted shear waves, however, often have a first-order effect on the seismic response. This fact does not appear to be widely recognized, or else the implications are not well understood.Primaries-only Zoeppritz modeling can be very misleading. Interference between the converted waves and the primary reflections from the base of the layers becomes increasingly important as layer thicknesses decrease. This interference often produces a seismogram that is very different from one produced under the primaries-only Zoeppritz assumption. For primaries-only modeling of thin layers, synthetic seismograms obtained by use of a linearized approximation to the Zoeppritz equations to describe the reflection coefficients are more accurate than those obtained by use of the exact Zoeppritz reflection coefficients.A real-data example consisting of an assemblage of very thin layers has recently been discussed in the literature. Inferences as to the true earth properties based on the predicted amplitude variation with offset are in error because the primaries-only assumption is invalid. For one of the models, primaries-only modeling predicts an amplitude increase of approximately a factor of three from the near trace to the far trace. Reflectivity modeling predicts an amplitude decrease with offset.The O'Doherty-Anstey effect suggests that transmission loss for primary reflections should not be included in normal-incidence synthetic seismograms if the short-period reverberations are not also included. The same principle holds for prestack modeling. Similarly, the Zoeppritz equations should not be used for synthetic seismograms without including the locally converted shear wave.

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