Anisotropic multimode joint Rayleigh- and Love-wave inversion Edan Gofer 1 , Scott Leaney 2 , Ranjan Dash 3 , and Hermes Malcotti 3 , Jim DiSiena 4 1 Schlumberger, 2 SLRDC, 3 Chevron Energy Technology Corporation, 4 formerly with Chevron Energy Technology Corporation Summary Obtaining anisotropic near surface elastic properties is of interest for engineering, environmental, and geophysical exploration purposes. State of the art depth imaging workflows are anisotropic. While surface-wave inversion has been used for building near surface velocity models for depth imaging, by itself and jointly with first arrival analysis, it is being carried out assuming the subsurface is isotropic. There is a need to incorporate anisotropy in surface-wave inversion and to pursue joint estimation of VS and transversely isotropic parameters. In this study, we demonstrate multimode joint estimation of VS and transversely isotropic parameters, which can provide near surface anisotropy for different applications. Introduction The accuracy of the near-surface velocity model is one of the main factors that impacts the quality of the seismic image at the depth of interest, a few kilometers into the subsurface (Keho and Kelamis, 2012). While surface-waves are routinely attenuated in seismic processing as they overlay the reflections (Strobbia et al., 2011), inversion of surface- wave dispersion curves has been used in near surface velocity model building for the purposes of depth imaging and static corrections (Boiero et al. 2013). In general, Rayleigh-, Scholte-, and Love-wave dispersion curves are inverted assuming the subsurface is isotropic (Socco et al., 2010). In an isotropic media, Rayleigh/Scholte-wave and Love- wave phase velocities are a function of the vertical and horizontal S-wave velocities, respectively (Aki and Richards, 1980). As a result, when seismic data is measured with multicomponent source and acquisition system, dispersion curves of the two wave types are inverted separately (Socco et al. 2011). Results of two separate inversions may not always be consistent. Moreover, the isotropic assumption may be invalid as the near subsurface can be anisotropic. For example, vertical loading will cause stress induced anisotropy in soft unconsolidated sediments (Nur and Simmons, 1969; Walton, 1987; Sayers, 2002). Fine scale layering can induce vertical transverse isotropy (VTI) (Backus, 1962) and fractured rocks can exhibit azimuthal anisotropy (Schoenberg and Sayers, 1995). To characterize these rocks accurately, the anisotropy must be taken into consideration. In this paper, we present an anisotropic multimode joint Rayleigh- and Love-wave inversion. We analyze the effect of elastic anisotropy on surface wave phase velocities, the sensitivity of different surface wave modes to model parameters and show inversion results from a field example. Model parameterization A VTI media is defined by five independent elastic constants C11, C33, C55, C66, and C13 with C12=C11–2C66. To assist in analyzing seismic reflection normal moveout (NMO) velocities and amplitude versus offset (AVO), several notations have been employed. Herein, we use Schoenberg et al. (1996):   ≡√  33  ;  ≡√  55  (1)   ≡  11 − 33  11 + 33 ;   ≡  66 − 55  66 + 55   ≡ ( 11 − 55 )( 33 − 55 ) − ( 13 + 55 ) 2 ( 11 − 55 )( 33 − 55 ) where Cij are the elastic stiffness constants and  is the density. In a VTI media, Rayleigh-wave phase velocity can be written in implicit form as (, ,  11 , 33 , 55 , 13 , , ℎ) and the Love-wave phase velocity as (, ,  55 , 66 , , ℎ), where f and k are the frequency and wavenumber. Cij, , and h are the elastic stiffness constants, the density, and the thickness of an individual layer in the media, respectively, (Anderson, 1961). Note, Rayleigh-wave phase velocities are a function of four out of the five independent elastic VTI constants and Love-wave phase velocities are a function of two out of the five. Implicit form with Schoenberg et al. (1996) notations is: • Rayleigh-wave phase velocity: (, ,   ,  ,  ,  , , ℎ) • Love-wave phase velocity: (, ,   ,  , , ℎ) Phase velocity sensitivity to model parameters To better understand the effect of elastic anisotropy on surface wave dispersion curves and how it changes as a function of depth, we calculated sensitivity of both Rayleigh- and Love-wave phase velocities of different modes to model parameter perturbation. Figure 1 shows the VTI model and the modeled fundamental mode and two higher modes. Rayleigh-wave dispersion curves were analyzed with respect to vertical S-wave velocity (VS), EP, and EA (Figure 2) and Love-wave dispersion curves were 10.1190/segam2021-3590447.1 Page 1856 © 2021 Society of Exploration Geophysicists First International Meeting for Applied Geoscience & Energy Downloaded 09/22/21 to 54.80.106.143. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/page/policies/terms DOI:10.1190/segam2021-3590447.1