AVO modeling and the locally converted shear wave

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.

One-way and one-return approximations (de wolf approximation) for fast elastic wave modeling in complex media

“…There are several modeling algorithms available for AVO calculation. The reflectivity method is one of the most common methods used for modeling AVO responses in layered media (Fuchs and Müller, 1971;Simmons and Backus, 1994;Wapenaar et al, 1999). It can generate an exact AVO response in arbitrarily layered medium, but cannot be used in structures with lateral velocity variations.…”

“…It can generate an exact AVO response in arbitrarily layered medium, but cannot be used in structures with lateral velocity variations. Ray methods based on various approximations of the Zoeppritz equations are also very common for AVO analysis (Widess, 1973;Simmons and Backus, 1994;Bakke and Ursin, 1998). However, in the presence of thin layers, primaries-only Zoeppritz modeling may produce incorrect results.…”

“…The reservoir is thick enough so that the reflections from the top and base are separated in seismograms. Simmons and Backus (1994) used these models to investigate AVO responses associated with angles of incidence, different type interfaces, and porosities of interest. In this section we will use these models to show the accuracy of the thin-slab method for modeling reflections including primary reflections, conversions and diffractions.…”

Feasibility of acoustic imaging for in-situ characterization of subsurface soil injected with fresh mortar

“…Here, AVO studies are not conducted to characterize the interface as in oil seismic reflection (Simmons and Backus, 1994) or fracture GPR characterization (Deparis and Garambois, 2009). They are only performed to evaluate the expected amplitude levels after wave energy splitting (Castagna, 1993).…”

Resolution enhancement in shallow 3D-PS seismic reflection

“…However, in practice, meaningful analysis of coal-seam response requires more detailed consideration of the full package of events from the seam. This has been demonstrated by Simmons Jr and Backus (1994), who used reflectivity modelling to determine the character of the total reflection package for a thin coal seam. Their work focused primarily on the zone within the critical angle.…”