Building tilted transversely isotropic depth models using localized anisotropic tomography with well information

Abstract

Tilted transverse isotropy (TTI) is increasingly recognized as a more geologically plausible description of anisotropy in sedimentary formations than vertical transverse isotropy (VTI). Although model-building approaches for VTI media are well understood, similar approaches for TTI media are in their infancy, even when the symmetry-axis direction is assumed known. We describe a tomographic approach that builds localized anisotropic models by jointly inverting surface-seismic and well data. We present a synthetic data example of anisotropic tomography applied to a layered TTI model with a symmetry-axis tilt of 45 degrees. We demonstrate three scenarios for constraining the solution. In the first scenario, velocity along the symmetry axis is known and tomography inverts for Thomsen’s [Formula: see text] and [Formula: see text] parame-ters. In the second scenario, tomography inverts for [Formula: see text], [Formula: see text], and velocity, using surface-seismic data and vertical check-shot traveltimes. In contrast to the VTI case, both these inversions are nonunique. To combat nonuniqueness, in the third scenario, we supplement check-shot and seismic data with the [Formula: see text] profile from an offset well. This allows recovery of the correct profiles for velocity along the symmetry axis and [Formula: see text]. We conclude that TTI is more ambiguous than VTI for model building. Additional well data or rock-physics assumptions may be required to constrain the tomography and arrive at geologically plausible TTI models. Furthermore, we demonstrate that VTI models with atypical Thomsen parameters can also fit the same joint seismic and check-shot data set. In this case, although imaging with VTI models can focus the TTI data and match vertical event depths, it leads to substantial lateral mispositioning of the reflections.

Moveout inversion of wide-azimuth P-wave data for tilted TI media

“…As pointed out by Bakulin et al (2010), the assumption of the symmetry axis being perpendicular to the reflector can become too restrictive when tectonic processes and sedimentation occur together. Also, for stress-induced anisotropy in sediments near salt bodies, the symmetry is largely controlled by the principal stress direction which is not necessarily aligned with the normal to the bedding.…”

“…It is common to put constraints on the symmetry-axis orientation using a priori information (Charles et al, 2008;Huang et al, 2008;Bakulin et al, 2010). If TI layers were rotated by tectonic processes after sedimentation, the symmetry axis typically remains perpendicular to the layering, which means that its tilt m and azimuth b coincide with the dip / and azimuth w of the reflector, respectively.…”

“…Allowing for the symmetry-axis tilt results in more plausible velocity models for sedimentary formations in complex geological settings including fold-and-thrust belts and subsalt plays (Vestrum et al, 1999;Behera and Tsvankin, 2009;Bakulin et al, 2010). P-wave velocities and traveltimes in TTI media can be expressed through the symmetry-direction velocity V P0 and Thomsen (1986) anisotropy parameters e and d defined with respect to the symmetry axis.…”

Geological and Rock Physics Constraints in Anisotropic Tomography

“…Anisotropic models remain a challenge, because they are not constrained by surface-seismic data alone. We are able to build useful anisotropic models locally, where we can combine borehole seismic data and surface seismic data, provided we have good illumination around the VTI or TTI axis of symmetry (Bakulin et al, 2010a(Bakulin et al, , 2010b. Away from wells, we must find other constraints to build models that will correctly depth our seismic images.…”

Joint migration velocity analysis of PP- and PS-waves for VTI media

“…Prestack depth migration (PSDM) and reflection tomography in the migrated domain are common tools in P-wave imaging (Stork, 1992;Wang et al, 1995;Adler et al, 2008;Bakulin et al, 2010). Most current PSDM and migration velocity analysis (MVA) algorithms account for transverse isotropy with a vertical (VTI) or tilted (TTI) symmetry axis.…”