In this First Break article from March 2024, Allon Bartana, Jeff Codd and David Kessler (SeismicCity), Joakim Blanch, Drew Eddy, Ramses Meza, Varendra Rambaran and J. P. Blangy (Woodside Energy) and Yi Huang (TGS) demonstrate the advantages of full elastic imaging using the OBN dataset acquired by Woodside Energy in 2019 to better image the sub-salt field.
Summary
Over the years Shenzi field in deep-water Gulf of Mexico became a test bed for evaluation of new seismic acquisition and processing technologies. These include 3D narrow azimuth streamer data, 3D-rich azimuth streamer data, and recently 3D OBN data (Mifflin et al, 2021). On the processing side, many types of processing techniques were used to image the various data types recorded over Shenzi field, from 3D ray-based Kirchhoff summation PSDM to Acoustic RTM PSDM and Acoustic FWI imaging. In the work presented here we demonstrate the advantages of Full Elastic imaging using the OBN dataset acquired by Woodside Energy in 2019 to better image the sub-salt field.
Introduction
In the past, computer capacity and cost led to the use of various approximations in the implementation of geophysical applications. These included use of ray-based velocity estimation (Stork 1992, Kosloff et al., 1996) and prestack depth migration (PSDM) algorithms (Gray et al., 2006), as well as the use of the acoustic assumptions as the basis for both velocity estimation and PSDM. With today’s computing strength and cost, these limitations can be removed. In the last decade the industry has moved to wave equation applications for velocity estimation by means of full waveform inversion (FWI) (Tarantola 2005, Vigh et al., 2016) and two way wave equation reverse time migration (RTM) (Baysal et al., 1983) as the leading method for application of PSDM. We can now progress further and move from using acoustic approximations to the use of full Elastic applications for both velocity estimation and PSDM.
Seismic imaging applications based on acoustic approximations have been in use for the past 40 years as an industry standard. This assumption considers the earth a fluid which supports propagation of P waves only and is justified on the basis that acoustic and elastic media produce the same travel times for pure P wave propagation. However, even for marine acquisition where only P waves are recorded, some of the reflected events propagate as S waves (through P-S mode conversions) along part of the propagation path in the subsurface. These shear waves supply important information about the subsurface geology and can help resolve subsurface rock properties as well as in-situ stress. In addition, anisotropy is only approximated with acoustic imaging, a fact which causes artifacts from slowly propagating quasi-shear waves. Today we are able to use the full elastic wave equation as the basis for PSDM and velocity estimation in an economic way. This is done without any approximations, enabling the use of both P and S waves in imaging (i.e., the full wavefield).
Another recent industry change in the application of depth imaging is to introduce FWI imaging (FWII), wherein the final image is directly constructed from the velocity model by calculation of reflectivity (Mifflin et al., 2021). The prevailing industry direction is to use FWI imaging resulting from the use of full Elastic implementation of FWI. Another option, which we advocate for here, is to use the full Elastic wave equation as the basis for implementation of PSDM. Using RTM as the underlying algorithm, this results in Elastic reverse time migration (ERTM).
In this work we present the application of ERTM using a proprietary OBN survey recorded over the Shenzi field in deep-water Gulf of Mexico. Over the years, many geophysical applications and various acquisition setups were used to better image the Shenzi field (Howard 2007, Mifflin et al., 2021). This includes narrow azimuth, rich azimuth and OBN acquisitions as well as Acoustic RTM PSDM (ARTM) and Acoustic FWI imaging (FWII). Application of ERTM using Shenzi OBN data can now be evaluated and compared to acoustic-based imaging (both ARTM and FWII) performed using the same OBN data over the field.
Read the full article here.