Examining the significance of the converted-modes beneath an ultra-high-velocity caprock: Physical modeling and a Gulf of Mexico salt proximity VSP surveyстатья
Аннотация:The rugose salt bodies tend to introduce complicated seismic wavefields. In addition to the well-known challenges in seismic imaging around the complex salt structures, we observe that strong converted modes, coupling with the unconverted wavefield, are introduced when an evaporitic caprock (e.g.,anhydrite) deposits over the salt. Strong image artifacts can be caused by a common acoustic imaging strategy. Except forthe better known S-mode conversion, we found that the converted P-modes (converted S-wave in the anhydrite layer, and P-wave elsewhere) can be evident and are more imperceptible in the routine wavefield separation on the receiver end. From the physical experiments and field data examples, we confirm the significant mode-conversion at the top of the anhydrite in a vertical seismic profiling survey geometry. Under the common acoustic assumption of seismic velocity model building and imaging, the complex mode-conversions on top of the salt have long been oversimplified. We analyze the wave partitioning and the pitfall of such oversimplification. The current analysis can be helpful in understanding the physics of wave partitioning in the presence of a thin ultra-high-velocity layer(UHVL).
