Improved subsalt images with least-squares reverse time migration

Abstract

Complex overburdens often distort reservoir images in terms of structural positioning, stratigraphic resolution, and amplitude fidelity. One prime example of a complex overburden is in the deepwater Gulf of Mexico, where thick and irregular layers of remobilized (i.e., allochthonous) salt are situated above prospective reservoir intervals. The highly variant salt layers create large lateral velocity variations that distort wave propagation and the illumination of deeper reservoir targets. In subsalt imaging, tools such as reflection tomography, full-waveform inversion, and detailed salt interpretation are needed to derive a high-resolution velocity model that captures the lateral velocity variations. Once a velocity field is obtained, reverse time migration (RTM) can be applied to restore structural positioning of events below and around the salt. However, RTM by nature is unable to fully recover the reflectivity for desired amplitudes and resolution. This shortcoming is well-recognized by the imaging community, and it has propelled the emergence of least-squares RTM (LSRTM) in recent years. We have investigated how current LSRTM methods perform on subsalt images. First, we compared the formulation of data-domain versus image-domain least-squares migration, as well as methods using single-iteration approximation versus iterative inversion. Then, we examined the resulting subsalt images of several LSRTM methods applied on the synthetic and field data. Among our tests, we found that image-domain single-iteration LSRTM methods, including an extension of an approximate inverse Hessian method in the curvelet domain, not only compensated for amplitude loss due to poor illumination caused by complex salt bodies, but it also produced subsalt images with fewer migration artifacts in the field data. In contrast, an iterative inversion method showed its potential for broadening the bandwidth in the subsalt, but it was less effective in reducing migration artifacts and noise. Based on our understanding, we evaluated the current state of LSRTM for subsalt imaging.

Analysis of Salt Stratification Modeling Influences over Reverse Time Migration Considering and not Considering Least-Squares Schemes

“…We can also note that when we add the detailed velocity model (with the stratification insertion) the results are even better. This last statement confirms that least-squares schemes in the velocity models associated with detailed velocity models has potential to deliver more accurate amplitude responses for the pre-salt targets in the Santos Basin, as pointed in some previous works (Ji et al, 2011;Wang et al, 2017;Fonseca et al, 2018;Dias et al, 2019;Shadrina et al, 2020).…”

“…This does not come as a surprise to us once the quality of seismic images is strongly dependent on the velocity models used for the migration (Huang et al, 2017), particularly in complex areas such as salt flanks and subsalt zones (Zhang and Sun, 2009;Jones and Davison, 2014). Therefore, building geologically constrained velocity models, accounting for salt stratification, is imperative when imaging the pre-salt targets in the Santos Basin.…”

Geological Characterization of Evaporitic Sections and Its Impacts on Seismic Images: Santos Basin, Offshore Brazil

“…Inside the PSDM's border-limit, a myriad of methods is available, ranging from Kirchhoff PSDM to Reverse-Time Migration (RTM). Completing the toolbox of state-of-the-art processing techniques are also Full-Waveform Inversion (FWI) (Ben-Hadj-Ali et al 2008;Barnes & Charara (2009); Operto et al, 2013;Vigh et al, 2014) and Least-Squares Migration (LSM) as per discussed in (Nemeth et al, 1999;Hu et al, 2001;Dias et al, 2017;Wang et al, 2017;Dias et al, 2018).…”

Application of Reverse Time Migration to Faults Imaging in Rakhine Basin, Myanmar

“…Complex subsurface structures with strong lateral velocity variations are particularly challenging for obtaining highresolution images with correct amplitudes. Several authors have tried different approaches such as least-squares RTM (LS-RTM) which uses iterative least-squares inversion to correct migration errors [17][18][19] or combinations of the leastsquares inverse problem and RTM [20][21][22][23][24]. While LS-RTM produces sidelobes around reflectors and high-wavenumber migration artifacts, its results in the image resolution, reducing artifacts, and compensating amplitudes are auspicious.. Seismic diffraction is another approach that has been used for imaging faults and steep dips, yielding improved subsurface imaging with better representations of structural topographies at high spatial resolutions [25][26][27][28].…”