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Current approaches to seismic data processing and interpretation ignore the change in reflection coefficient with incident angle. The justification is typically based on fluid earth models or models in which the ratio of compressional velocity to shear velocity (Cp/Cs) is assumed constant. In clastic basins, normal incidence reflectivity is low, and variations in Cp/Cs can be significant. Under these conditions the change in reflection coefficient with incident angle can be very significant. Replacement of the plane wave normal incidence synthetic seismogram with the point source solid earth synthetic seismogram is likely to lead to important changes in our approach to acquisition, processing, and interpretation of seismic refle tion data.
The accompanying figure shows a sample solid earth synthetic seismogram. In this example, a change from brine to gas produces a dim spot on the near traces, a bright spot on the far traces, and a polarity reversal on the stacked section. The CDP stacked trace should be different from our conventional normal incidence synthetic seismogram.
In general, in a clastic basin, we might expect to see a different set of rocks emphasized on partial stacks from different offset ranges. The sensitivity of our solid earth synthetic seismogram to changes in Poisson's ratio is such that we suggest that conventional reflection data in clastic basins should permit the extraction of band-limited shear impedance logs as well as compressional impedance logs. Attempts to work only with the CDP stacked data should yield a "hybrid impedance log;" a mixture
of the effects of compressional and shear velocity variations.
The expected variation in reflectivity with offset in clastic basins suggests that we should seriously question our conventional processing and interpretation assumptions in these areas. It also suggests that conventional seismic reflection data might yield a shear impedance image and an improved compressional impedance image rather than the currently employed hybrid impedance image.
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