Anisotropic migration-velocity based on inversion of common azimuthal sections

L.T. Ikelle
Schlumberger Cambridge Research, High cross, Madingley Road,Cambridge CB3 0EL, United Kingdom

Abstract:

A knowledge of the background velocity model is crucial to achieve the accurate reservoir description now expected from 3D prestack imaging and inversion.Conventional methods for reconstructing the background velocity model, like migration-velocity methods, often assume an isotropic subsurface and can yield inaccurate reservoir descriptions when the subsurface contains anisotropic rock formations. Here, we generalize the migration-velocity concept by (i) replacing migration with linearized inversion and (ii) permitting the background velocity to be anisotropic. The scheme consists of scanning over different anisotropic velocity models using a linearized inversion in the -k domain. As the anisotropic background velocity model is generally described by several elastic coefficients, it is important to adopt an efficient scanning procedure. We have chosen to work with common azimuthal sections. For a given common azimuthal section, we sequentially scan over two parameters: normal moveout velocity and the anisotropic parameter known as anellipticity. These two scans allows us to reconstruct an azimuthally isotropic velocity model. The procedure is then repeated for different common azimuthal sections, each common azimuthal section leads to a new azimuthally isotropic velocity model if the medium is azimuthally anisotropic. The number of common azimuthal sections, and thereforethe number of azimuthally isotropic velocity models, needed to reconstruct an azimuthally anisotropic velocity model is dependent on the type of symmetries. For example, only three common azimuthal sections are needed for an orthorhombic medium.

AGU Index Terms: 0935 Seismic methods; 0910 Data processing; 7203 Body wave propagation; 7203 Body wave propagation
Keywords/Free Terms: migration-velocity, anisotropy, common azimuthal data, anellipticityA

JGR-Solid Earth 96JB02296