Elliptical dip moveout (EMO) for 3D seismic imaging in the presence of azimuthal anisotropy

Abstract

Seismic images of the earth?s interior can be significantly distorted by complex wave propagation effects arising from 3D structural velocity variations, combined with the presence of azimuthal velocity anisotropy within some of the rock layers. Most image processing techniques attempt to separate and compensate for both of these phenomena sequentially; they rarely address both simultaneously. These approaches implicitly assume that the effects of 3D structural velocity and azimuthal anisotropy are separable, when in fact both effects are coupled together in the seismic data. The presence of strong azimuthal velocity anisotropy can lead to significant errors in estimated seismic velocity and degraded quality of subsurface images, especially for large source-receiver offsets, wide azimuths and steep geologic dips. Such imaging errors can greatly increase the uncertainty associated with exploring, characterizing, developing and monitoring subsurface geologic features for hydrocarbons, geothermal energy, CO2 sequestration, and other important geophysical imaging applications. Our approach is to simultaneously address velocity structure and azimuthal anisotropy by development of an elliptic dip moveout (DMO) operator. We combine the structural-velocity insensitivity of isotropic DMO with elliptic moveout representative of azimuthal velocity anisotropy. Forward and adjoint elliptical DMO operators are then cascaded together to form a single elliptical moveout (EMO) operation, which has a skewed saddle-like impulse response that resembles an isotropic azimuthal moveout operator. The EMO operator can be used as a prestack data conditioner, to estimate azimuthal anisotropy in a domain that is relatively insensitive to 3D velocity structure, or to compensate and map the data back to its original prestack domain in its approximately equivalent isotropic wavefield form. We show that EMO can reduce structural dip image errors of 10-20º or more for realistic azimuthal anisotropy values at far offsets.