Amplitude versus offset (AVO) measurements for deep hydrocarbon-bearing sands can be compromised when made in close proximity to a shallow salt piercement structure. Anomalous responses are observed, particularly on low acoustic impedance bright spots. CMP data from key seismic profiles traversing the bright spots do not show the expected Class 3 offset responses. On these CMPs, significant decrease of far trace energy is observed. CMP data from other seismic profiles off-structure do exhibit the Class 3 offset responses, implying that structural complications may be interfering with the offset response. A synthetic AVO gather was generated using well log data, which supports the off-structure Class 3 responses, further reinforcing the concept of structurally-biased AVO responses. Acoustic, pseudo-spectral modeling of the structure substantiates the misleading AVO response. Pseudo-spectral modeling results suggest that signal degradation observed on the far offsets is caused by wavefield refraction--a shadow zone, where the known hydrocarbon-bearing sands are not completely illuminated. Such shadow zones obscure the correct AVO response, which may have bearing on exploration and development.

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