Image registration guided wavefield tomography for shear-wave velocity model building

Multicomponent acquisitions offer the opportunity to form elastic migration images and to estimate elastic parameters of the subsurface. Compared with better constrained P-wave velocity inversions, it is more difficult to estimate the S-wave velocity due to strong nonlinearities introduced by converted S-waves. We have developed an iterative S-wave velocity inversion method guided by image registration. Given an accurate P-wave velocity and a simple initial S-wave model, we form P-P and P-S images using elastic reverse time migration. We use image registration to find the shifts between the P-S and P-P images. The S-wave velocity model could be updated iteratively by minimizing the differences between the original and the fractionally warped P-S images. A simple layered model and a modified Marmousi model are used to demonstrate the viability of the new method. In both examples, high-quality P-S images, as well as smooth S-wave velocity models, are inverted efficiently with a homogeneous S-wave initial model. INTRODUCTION Multicomponent seismographs have been deployed in global seismology for decades to estimate the earth’s elastic properties, such as Pand S-wave velocities. In hydrocarbon exploration, the S-wave velocity, coupled with the P-wave velocity, provides crucial information for lithology estimation and reservoir characterization (Stewart et al., 2002). Multicomponent imaging methods have been proposed in the time and depth domains. Herrenschmidt et al. (2001) compare several converted-wave imaging approaches using real data, and show that prestack time migration provides interpretable results when lateral velocity variations are not significant. Kuo and Dai (1984) propose Kirchhoff elastic-wave depth migration based on KirchhoffHelmholtz-type integrals. Hokstad (2000) presents multicomponent Kirchhoff migration using the survey-sinking concept. These raybased methods are likely to fail when ray theory breaks down in complex media as does acoustic Kirchhoff migration (Gray et al., 2001). One-way migration methods can also be extended to elastic applications. Wapenaar and Haime (1990) propose separating wave modes on the surface before one-way migration in isotropic media. Yan and Sava (2008) advocate an alternative procedure to separate the wave modes during the vector wavefield propagation and reconstruction in elastic reverse time migration (ERTM). Shang et al. (2012) and Sava (2011) extend the ERTM concept to passive source applications, e.g., in teleseismic or microseismic studies. Accurate velocity models are necessary for elastic depth migration (Alkhalifah, 2003). P-wave model building is relatively mature and robust. On the other hand, S-wave velocity or VP∕VS ratio estimation is less well understood. A converted-wave migration velocity model is often obtained in the time domain by tuning the VP∕VS ratio (Fomel et al., 2005; Hale, 2013). For example, S-wave velocities can be estimated by registering corresponding P-P and P-S reflections in time-migrated sections. Assuming that the P-wave velocity is correct, the time shifts between P-P and P-S events can be transformed into VP∕VS ratio corrections. However, this method suffers from the limitations of time migration in handling lateral inhomogeneity, and it does not provide an accurate interval S-wave velocity model with which to depth migrate the data. Du et al. (2012a) propose a joint migration velocity analysis in the angle domain for P-P and P-S depth images. However, they use a Kirchoffbased migration, which is likely to break down in complex structures. Yan and Sava (2010) present a wave-equation migration velocity analysis (WEMVA) method that finds the S-wave velocities and P-S depth-migration images simultaneously, but the computation Manuscript received by the Editor 6 August 2014; revised manuscript received 24 November 2014; published online 27 April 2015. Earth Resource Lab, Cambridge, Massachusetts, USA. E-mail: [email protected]; [email protected]; [email protected]. Formerly Earth Resource Lab, Cambridge, Massachusetts; presently Shell International Exploration and Production, Houston, Texas, USA. E-mail: [email protected]. Aramco Research Center, Houston, Texas, USA. E-mail: [email protected]. © 2015 Society of Exploration Geophysicists. All rights reserved. U35 GEOPHYSICS, VOL. 80, NO. 3 (MAY-JUNE 2015); P. U35–U46, 13 FIGS. 10.1190/GEO2014-0360.1