Anisotropic migration velocity analysis from residual moveout transformation in angle-domain common-image gathers

Pierre Jousselin and Biondo Biondi

Author has no known email address

ABSTRACT

We present the application of anisotropic migration velocity analysis (MVA) on synthetic data and on a real dataset acquired in the North Sea. The method we use to estimate anisotropic migration velocities is based on the computation and analysis of angle-domain common-image gathers (ADCIGs) in conjunction with anisotropic wavefield-continuation migration. Assuming flat reflectors and vertically transversely isotropic (VTI) media, we compute anisotropic velocity spectra from a residual moveout (RMO) transformation, based on the theoretical expression of the RMO in anisotropic ADCIGs as a function of perturbations in migration velocities.

In the first part of this paper, we briefly present the theory of ADCIGs and the derivation of the mathematical expression of the RMO function. In the second part, we analyze synthetic data with the method we propose to estimate anisotropic migration velocities. We demonstrate that our method is unable to resolve vertical velocities but nevertheless accurately estimates horizontal and normal moveout (NMO) velocities even for significant perturbations in the velocity model, provided the range of aperture angle is not too small. In the last part of this paper, we demonstrate on real data the validity of our estimation method, by showing it indicates the presence of velocity anellipticity in layers stratigraphically interpreted as anisotropic and for which well data do not match seismic data migrated under the assumption of isotropy.

Since approximating the subsurface as an isotropic medium can be geologically unrealistic and since isotropic migration methods can give results inconsistent with well data, anisotropic migration has become an important focus of research and is now widely used. However, superior image quality and accurate reflector positioning require good estimates of anisotropic parameters.

Today, migration velocity analysis (MVA) is most commonly employed to estimate isotropic migration velocity in complex media. However, it is still less mature for anisotropic applications, and most recent work has developed anisotropic MVA only in conjunction with Kirchhoff migration Tsvankin (2001). The mathematical relationship that links residual moveout (RMO) in anisotropic angle-domain common-image gathers (ADCIGs) to anisotropic migration velocity errors Biondi (2005a,b) opens the way to new anisotropic MVA methods that can be performed in conjunction with wavefield-continuation migration. These methods have the great advantage of being based on migrated gathers and allow the iterative estimation of anisotropic migration velocities.

In this paper, we present the results from computing anisotropic velocity spectra from RMO transformation in anisotropic ADCIGs assuming flat reflectors and vertically transversely isotropic (VTI) media. In the first part, we briefly present the theory of ADCIGs and the derivation of the mathematical expression of the RMO function. In the second part, we perform anisotropic MVA on synthetic data and analyze the accuracy and convergence of our estimation method. We demonstrate it does not resolve vertical velocities but nevertheless accurately estimates the horizontal and NMO velocities, even for significant perturbations, provided the range of aperture angle is not too small. We also show that when computing anisotropic velocity spectra as few approximations as possible must be made in order to attain accurate parameter estimates. In the last part, we perform anisotropic MVA on a real data set acquired in the North Sea. We demonstrate the validity of the method by showing it indicates the presence of velocity anellipticity in the layers stratigraphically interpreted as anisotropic, and for which well data do not match seismic data migrated under the assumption of isotropy.