Subsalt illumination analysis using RTM 3D dip gathers

Abstract

Summary Reverse Time Migration (RTM) is now the preferred option for subsalt imaging in deep water Gulf of Mexico, and its 3D angle gather output plays an important role in subsalt velocity updating and illumination compensation. We now present recently-developed RTM 3D dip gathers, which are common-image gathers (CIGs) generated through wavefield decomposition and grouped into the subsurface dip-azimuth angle domain. RTM 3D dip gathers offer an opportunity for subsalt illumination analysis. We propose to use sparsely-distributed density point diffractors in the subsalt region for illumination analysis via a modelingmigration procedure. A density model with point diffractors is used for forward modeling, followed by RTM with 3D dip gather output to determine the variable illumination coverage for all subsalt dip and azimuth angles, which can be used for acquisition design, e.g., in deciding the maximum offset. Furthermore, illumination scalars derived from the diffractor-based dip gathers are applied to RTM 3D dip gathers of real data to improve the signal-to-noise ratio (S/N) and compensate for dip-dependent illumination. Because no preconceived information about the subsalt structures is involved, this illumination compensation scheme is not interpretation-driven, resulting in enhanced subsalt images that are not biased towards interpretation preferences. We validate the workflow on a real wide azimuth dataset in the Gulf of Mexico.