Dirty salt velocity inversion: The road to a clearer subsalt image

Abstract

For deep-water Gulf of Mexico, accurate salt geometry is critical to subsalt imaging. This requires the definition of both external and internal salt geometries. In recent years, external salt geometry (i.e., boundaries between allochthonous salt and background sediment) has improved a great deal due to advances in acquisition, velocity model building, and migration algorithms. But when it comes to defining internal salt geometry (i.e., intrasalt inclusions or dirty salt), no efficient method has yet been developed. In common industry practices, intrasalt inclusions (and thus their velocity anomalies) are generally ignored during the model building stages. However, as external salt geometries reach higher levels of accuracy, it becomes more important to consider the once-ignored effects of dirty salt. We have developed a reflectivity-based approach for dirty salt velocity inversion. This method takes true-amplitude reverse time migration stack volumes as input, then estimates the dirty salt velocity based on reflectivity under a 1D assumption. Results from a 2D synthetic data set and a real 3D Wide Azimuth data set demonstrated that the reflectivity inversion scheme significantly improves the subsalt image for certain areas. In general, we believe that this method produces a better salt model than the traditional clean salt velocity approach.

Dirty salt velocity model building with constrained iterative tomography: Methodology and application

“…This method can be used for smaller inclusions or, as further described herein, to assign initial velocity for larger inclusions which will be further updated using tomographic methods. Li et al (2011) proposed using RMO information at the boundary of the inclusion to update dirty salt velocity with tomography in the 3D angle domain. This might be effective when RMO information at the boundary of the inclusion is available and has good quality.…”

“…An early attempt to understand how salt inclusions affect the image was presented by Haugen et al (2008). Ji et al (2011) proposed a reflectivity inversion method to update dirty salt velocity. While this method is simple, it is based on a strong assumption that the velocity -density relationship obeys Gardner's relation.…”

“…While this method is simple, it is based on a strong assumption that the velocity -density relationship obeys Gardner's relation. Li et al (2011) proposed to use reverse time migration (RTM) angle gathers in tomography (Woodward et al, 2008) to update dirty salt velocity. While this approach has advantages over surface offset gather tomography, it only uses inclusion boundary residual moveout (RMO), which may not be practical for real cases because the gather inside salt is normally very noisy.…”

Staining algorithm for seismic modeling and migration

“…Liu et al (2011) modify conventional RTM and use the multiples as constructive energy for imaging. Velocity model building plays an important role to enhance the subsalt visibility and resolution (Fliedner et al, 2007;Wang et al, 2008;Ji et al, 2011). A target-oriented strategy using a data set obtained by generalized Born modeling based on a single scattering approximation to the full wave equation can use wavefield-based velocity estimation to focus on improving velocities in subsalt regions (Tang and Biondi, 2011).…”

Iterative reflectivity-constrained velocity estimation for seismic imaging

“…A scheme to combine MVA with FWI was also suggested in Biondi & Almomin (2012) to improve the resolution. Incorporating the information of migration into the tomography process for velocity estimation improves the final depth image (Mathewson et al 2012), where a dirty salt velocity was estimated via the reflectivity, which is computed by true-amplitude RTM, under a 1D assumption (Ji et al 2011). With a similar reasoning, Maciel et al (2015) applied nonlinear filters from the field of morphological image processing to address this challenge and to enhance the contrast of the JMI velocity solution.…”