Synthetic seismic sections of typical petroleum traps

Interpretation of seismic reflections remains a key problem in seismic exploration because reflections have complex behavior, especially near geologic structures. One method to gain an understanding of this complex behavior is to study synthetic seismic sections of models of typical petroleum traps as computed by zero-offset ray tracing for primary P-waves only. These synthetic sections have features of significant interpretative value to the practicing geophysicist, such as variations in reflection amplitudes and complexities in reflection-time geometries. Asymptotic ray theory was applied to calculate reflection amplitudes, accounting for mode conversion and three-dimensional geometric divergence of ray tubes in the presence of curvilinear interfaces. This suite of synthetic seismic sections illustrates the difficulties in making correct seismic interpretations of geologic structures and suggests three conclusions: (1) The customary assumption that seismic sections are simple images of geologic cross-sections is an oversimplification and can lead to erroneous interpretations. (2) Variations in overlying strata produce marked disturbances in reflection amplitude and traveltime. (3) Critical reflections that provide key structural information are often difficult to recognize and are apt to be ignored, or misidentified, especially if trace processing has been cursory. A fundamental principle of interpretation is underscored by this study: CDP stacked seismic sections must be interpreted so as to be consistent with structural depth models because stacked sections are not simple images of geologic structures.