One common method of searching for seismic anomalies associated with increased porosity is to integrate the stacked seismic traces to obtain an “acoustic impedance” section or, if a suitable relation between density and velocity is known, a seismic interval velocity section. Lateral variations in the color-plotted displays can then be interpreted in terms of porosity variation associated with inferred velocity variation. Buried in this procedure is the assumption that the stacked section represents reflectivity at normal incidence.
In the developing technique of “seismic lithology,” where variations in reflection amplitude with source receiver offset are used to infer lithologic properties, examples are accumulating that show dramatic changes in reflection amplitude with offset within common depth point gathers. Consequently, after stacking, the stacked section will be poorly correlated with normal incidence reflectivity.
Recent development in seismic lithologic analysis have led to a procedure that allows the normal incidence reflectivity to be estimated from the offset variation. The results of this procedure show greater continuity and geologic interpretability. More important, upon trace integration, the resultant impedance sections show greater stability leading to a higher confidence level in potential porosity anomalies.