Reflection moveout inversion in azimuthally anisotropic media

Using the elliptical variation of P-wave normal-moveout (NMO) velocity with azimuth, measured in three different source-to-receiver orientations, we can obtain the vertical velocity Vpvert, anisotropy parameter δ(v) and the azimuth a of the symmetry-axis plane in a transversely isotropic medium with a horizontal symmetry axis (HTI medium). Parameter estimation from this az-imuthal variation is quite sensitive to the angular separation between the survey lines. The accuracy in estimating the parameter cy, in particular, is also sensitive to the strength of anisotropy. In order to maximize the accuracy and stability in parameter estimation, it is best to have the azimuths for the three source-to-receiver directions 60° apart. An azimuthally anisotropic layer overlain by an azimuthally isotropic overburden (as might happen for fractured reservoirs) should have a relative thickness (in time) to the total thickness of at least 0.2 in order to obtain acceptable estimates of the medium parameters, provided that the azimuthal variation in the interval NMO velocity within the layer is about 10% or higher.