Key Seismic Data Issues of Subsalt DHI Analysis

A salt overburden often adversely affects subsalt seismic data. This problem is especially serious in deep water Gulf of Mexico (GOM) under the rugose allochthonous salt canopy. Significant issues need to be addressed before we can make sense of the prestack response even for data with good image and signal-to-noise ratio, which perhaps is a reason why so far there is no GOM subsalt prestack DHI work in the literature. Handling these issues requires innovations. We encountered the following key data issues in our subsalt prestack DHI analysis, regardless of the image quality: 1) Limited reflection angle range, 2) low seismic data resolution, and 3) amplitude distortion from salt overburden. We found that the top-of-salt noise, not the salt-sediment velocity contrast, controls the subsalt angle range in our prospect area. An empirical horizon-based mute strategy is designed to preserve as much angle range as possible. Monochromatic AVO addresses the issue of the low-resolution seismic data in exploration. The amplitude distortion from salt overburden is handled by offset-dependent amplitude balancing using either a stable regional reference event calibrated at well locations or long-window balance.