Prismatic wave imaging with dual flood RTM

Abstract

In deep water Gulf of Mexico (GOM), defining accurate salt geometry is essential for imaging geological structures beneath complex salt bodies. Reverse time migration (RTM) handles complex wave propagations in any direction without dip limitation, and is now the standard imaging algorithm in salt model building. With increasing computing capacity and integrated imaging and interpretation, our ability to use RTM imaging as an interactive tool for delineating salt geometry has been significantly improved. The conventional RTM salt model building is a top-down approach which consists of various steps of migration and interpretation: sediment flood RTM and picking the Top of Salt (TOS), salt flood RTM and picking the Base of Salt (BOS), and overhang RTM and interpretation as needed. However, in the areas with complex salt geometry, such as narrow mini-basin with surrounding salt bodies, the conventional top-down salt building approach may not be conducive to correct salt body interpretation and can lead to poor images beneath the salt. In this paper, we demonstrate the application of RTMbased prismatic wave imaging in complex salt model building, in particular the introduction of dual flood RTM to evaluate the prism waves and correctly interpret the salt horizon. A 2D synthetic case study is utilized to demonstrate the prismatic wave work flow. The technique is then applied to a marine wide azimuth dataset in Walker Ridge, Gulf of Mexico, in order to improve subsalt imaging in the complex structural setting.

Steeply dipping structure imaging with prismatic waves incorporating wavefield decomposition

“…(2007) assumed primary and prismatic reflections can be separated due to the large difference in move out; Li et al. (2011) applied a model‐dependent dual‐flood reverse time migration (RTM) flow, which propagates the source and receiver wavefields in two different models; Sun and Fei (2021) modified the imaging condition in the de‐primary RTM method to accommodate a composite wavefield. Although this class of methods can use prismatic waves in the recorded data to recover steeply dipping flanks, it is difficult to obtain the prior information about basement interfaces in the migration velocity model in field data applications.…”

“…The migration of prismatic waves has been widely studied in recent years and can be summarized into two classes. The first class requires the exact location of basement interfaces embedded in the accurate migration velocity model (Farmer et al., 2006; Jin et al., 2006; Jones et al., 2007 ; Li et al., 2011; Link et al., 2007; Marmalyevskyy et al., 2005; Sun & Fei, 2021). Therefore, these methods can accurately predict primary wavefields in the migration velocity model and correlate these wavefields with receiver backward wavefields of prismatic waves to depict steeply dipping interfaces.…”

“…Therefore, these methods can accurately predict primary wavefields in the migration velocity model and correlate these wavefields with receiver backward wavefields of prismatic waves to depict steeply dipping interfaces. To enhance the image of vertical structure, Marmalyevskyy et al (2005), Jin et al (2006) and Link et al (2007) assumed primary and prismatic reflections can be separated due to the large difference in move out; Li et al (2011) applied a modeldependent dual-flood reverse time migration (RTM) flow, which propagates the source and receiver wavefields in two different models; Sun and Fei (2021) modified the imaging condition in the de-primary RTM method to accommodate a composite wavefield. Although this class of methods can use prismatic waves in the recorded data to recover steeply dipping flanks, it is difficult to obtain the prior information about basement interfaces in the migration velocity model in field data applications.…”

Joint least-squares reverse time migration of primary and prismatic waves

“…Migration of prismatic waves for delineation of the steeply dipping structures has been studied by many researchers and can be summarized into two categories. The first category is to explicitly embed the sharp reflectors into the migration velocity model (Marmalyevskyy et al, 2005;Farmer et al, 2006;Jin et al, 2006;Jones et al, 2007;Li et al, 2011). However, accurate positioning of the sharp reflectors adds complexity to the already difficult migration velocity model building process.…”

Imaging vertical interfaces using acoustic time reversal

“…Malcolm et al (2009) followed a similar approach to Jin et al (2006) but inverted for the abrupt interfaces in the velocity model starting from a smooth version of the velocity prior to imaging. These doubly-scattered waves are also called prismatic waves and have been used extensively to image salt flanks and narrow mini-basin areas (predominantly vertical interfaces) once we have the prior location of at least one reflecting interface (Farmer et al, 2006;Zhang and Sun, 2009;Cavalca and Lailly, 2005;Liu et al, 2011;Li et al, 2011). However, obtaining a smooth version of the velocity model is a far simpler task than obtaining accurate estimates of abrupt, reflecting interfaces in the model (Noble et al, 1991;Symes and Carazzone, 1991;Chavent and Jacewitz, 1995;Sava and Biondi, 2004).…”