Localized anisotropic tomography with well information in VTI media

Abstract

We develop a concept of localized seismic grid tomography constrained by well information and apply it to building vertically transversely isotropic (VTI) velocity models in depth. The goal is to use a highly automated migration velocity analysis to build anisotropic models that combine optimal image focusing with accurate depth positioning in one step. We localize tomography to a limited volume around the well and jointly invert the surface seismic and well data. Well information is propagated into the local volume by using the method of preconditioning, whereby model updates are shaped to follow geologic layers with spatial smoothing constraints. We analyze our concept with a synthetic data example of anisotropic tomography applied to a 1D VTI model. We demonstrate four cases of introducing additionalinformation. In the first case, vertical velocity is assumed to be known, and the tomography inverts only for Thomsen’s [Formula: see text] and [Formula: see text] profiles using surface seismic data alone. In the second case, tomography simultaneously inverts for all three VTI parameters, including vertical velocity, using a joint data set that consists of surface seismic data and vertical check-shot traveltimes. In the third and fourth cases, sparse depth markers and walkaway vertical seismic profiling (VSP) are used, respectively, to supplement the seismic data. For all four examples, tomography reliably recovers the anisotropic velocity field up to a vertical resolution comparable to that of the well data. Even though walkaway VSP has the additional dimension of angle or offset, it offers no further increase in this resolution limit. Anisotropic tomography with well constraints has multiple advantages over other approaches and deserves a place in the portfolio of model-building tools.

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.…”

Application of the Rock Physics Model in Anisotropic Seismic Velocity Model Building and Quantitative Reservoir Structural Uncertainty Analysis: Gulf of Mexico Case Study

“…Traditionally the process is manually done by fixing vertical velocity and picking anisotropic parameters to flat common imaging gathers. Recent examples of localized tomography (Bakulin et al, 2009) demonstrated that anisotropic parameters can be solved simultaneously at well locations with check shot transit times.…”

Migration Velocity Analysis for TI Media in the Presence of Quadratic Lateral Velocity Variation

“…(Alkhalifah and Larner, 1994;Alkhalifah et al, 1996). Migration velocity analysis (MVA) has been extended to heterogeneous transversely isotropic media with a vertical (VTI) and tilted (TTI) symmetry axis (e.g., Sarkar and Tsvankin, 2004;Biondi, 2007;Behera and Tsvankin, 2009;Bakulin et al, 2010bBakulin et al, , 2010c. Numerous field examples demonstrate that application of prestack depth migration (PSDM) with anisotropic MVA yields significantly improved images in the Gulf of Mexico and other regions (Calvert et al, 2008;Huang et al, 2008;Neal et al, 2009;Bakulin et al, 2010a).…”