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

摘要

Seismic images of the earth's interior can be significantly distorted by complex wave propagation effects arising from 31) 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 im-plicitly assume that the effects of 3D structural velocity and azimuthal anisotropy are separable, whereas in fact, both ef-fects are coupled together in the seismic data. In the presence of strong azimuthal velocity anisotropy, this can lead to sig-nificant errors in seismic velocity estimation 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 sub-surface geologic features for hydrocarbons, geothermal energy, CO_2 sequestration, and other important geophysical imaging applications. Our approach simultaneously addressed velocity structure and azimuthal anisotropy by development of an ellip-tic dip moveout (DMO) operator. We combined the structural-velocity insensitivity of isotropic DMO with elliptic moveout representative of azimuthal velocity anisotropy. Forward and adjoint elliptical DMO operators were then cascaded together to form a single elliptical moveout (EMO) operation, which had 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 azi-muthal 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 demonstrated that EMO can reduce structural dip image errors of 100-20° or more for realistic azimuthal velocity anisotropy values at far offsets.