Examining the significance of the converted-modes beneath an ultra-high-velocity caprock: physical modeling and a Gulf of Mexico salt proximity VSP survey Jingjing Zong ∗ , Yukai Wo † , Jizhong Yang ‡ , and Nikolay Dyaur § ∗ University of Electronic Science and Technology of China, † Southwest Petroleum University, ‡ Tongji University, § Schmidt Institute of Physics of the Earth, RAS SUMMARY The rugose salt bodies tend to introduce complicated seismic wavefields. In addition to the well-known challenges in seis- mic 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 for the better known S-mode conversion, we found that the con- verted 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 com- mon 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 parti- tioning 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). INTRODUCTION Imaging around and beneath the salt structures is one of the most challenging tasks in seismic exploration (O’Brien, 2005; Li et al., 2019; Wang et al., 2019). Salt structures distribute ex- tensively in the Gulf of Mexico (GoM) region and elsewhere around the globe, playing important roles in the tectonic move- ment, underground storage, hydrocarbon and carbonate-dioxide capture, mining resources and so on (Davison et al., 1996; Warren, 2006; Jackson and Hudec, 2017). The plastic defor- mations during the halokinesis histories give rise to the rugose salt geometries, posing great challenges to seismic methods on mapping and characterizing the salt structures (Hudec et al., 2011; Jones and Davison, 2014). Many studies report that the large impedance contrast across the salt boundaries can be associated with strong energy partitioning effects which chal- lenges the routine seismic imaging (Ogilvie and Purnell, 1996; Leveille et al., 2011). While most studies focus on the en- ergy conversions introduced by the rock salt itself (essentially, halite), little attention has been paid to the evaporitic caprock of the rock salt bodies. Williamson Beckman and Williamson (1990) and Hamlin Hamlin (2006) record an average thickness of caprock (anhydrite, gypsum, calcite and etc.) of around 146 m (481 ft) of 36 shallow salt domes studied in the northeast Gulf Coast. Although very thin, we analyzed the GoM well logs and find that the evaporite rock can demonstrate signif- icantly higher impedance than both the encasing formations and the rock salt. As the rock salt is often referred as the ‘high- velocity-layer’ (HVL), we use the ‘ultra-high-velocity-layer’ (UHVL) to describe the evaporitic caprock. Thus, we study the wavefield characteristics in the presence of an ultra-high-velocity layer (UHVL) on top of the salt bodies, which have been over-simplified under the common acoustic seismic imaging routine. Among various geophysical meth- ods, the vertical seismic profiling (VSP) survey is advanta- geous in providing a direct measurement of the wave propa- gation characteristics with the increased depth (Stewart et al., 2002; Hornby et al., 2006; Gaiser, 2016). Hence, we take ad- vantage of the VSP geometry and design a physical experi- ment featuring a geological unit consisting of the salt body overlaid by the UHVL. Strong S-mode conversion at the top of the UHVL is observed in both numerical and physical mod- eling results, even at the near-offset range where mode conver- sion is often neglected at the nearly normal incidence angle. The converted S-mode further generates a family of P- and S- waves in the salt bodies, which deserve to be fully understood to be differentiated from the primary P-wavefield. A single- offset salt proximity VSP field survey is further analyzed and it validates the observations from the numerical and physical modeling experiments. We are reminded from the modeling and field examples of the importance of understanding the con- verted wavefield before choosing the processing and imaging strategy. Figure 1: Well logging suit acquired from a well drilled in the north GoM coast. PHYSICAL MODELING We first obtain the elastic properties from a well logging suit acquired in a well drilled in the northern GoM coast, as dis- 10.1190/image2022-3745750.1 Page 3589 Second International Meeting for Applied Geoscience & Energy © 2022 Society of Exploration Geophysicists and the American Association of Petroleum Geologists Downloaded 08/20/22 to 54.237.88.230. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/image2022-3745750.1