ANISOTROPIC MIGRATION-VELOCITY BASED ON INVERSION OF COMMON AZIMUTHALSECTIONS

A knowledge of the background velocity model is crucial to achieve the accurate reservoir description now expected from three-dimensional (3 -D) prestack imaging and inversion. Conventional methods for reconstru cting the background velocity model, like migration-velocity methods, often assume an isotropic subsurface and can yield inaccurate reservoi r descriptions when the subsurface contains anisotropic rock formation s. Here, we generalize the migration-velocity concept by (1) replacing migration with linearized inversion and (2) permitting the background velocity to be anisotropic. The scheme consists of scanning different anisotropic velocity models using a linearized inversion in the w-k d omain. As the anisotropic background velocity model is generally descr ibed by several elastic coefficients, it is important to adopt an effi cient scanning procedure. We have chosen to work with common azimuthal sections. For a given common azimuthal section, we sequentially scan two parameters: normal move-out velocity and the anisotropic parameter known as anellipticity. These two scans allows us to reconstruct an a zimuthally isotropic velocity model. The procedure is then repeated fo r different common azimuthal sections; each common azimuthal section l eads to a new azimuthally isotropic velocity model if the medium is az imuthally anisotropic. The number of common azimuthal sections, and th erefore the 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 sect ions are needed for an orthorhombic medium.