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Migration velocity analysis and imaging for tilted TI media

Tilted transversely isotropic (TTI) formations cause serious imaging distortions
in active tectonic areas (e.g., fold-and-thrust belts) and in subsalt exploration.
Here, we introduce a methodology for P-wave prestack depth imaging in TTI
media that properly accounts for the tilt of the symmetry axis as well as for
spatial velocity variations.
For purposes of migration velocity analysis (MVA), the model is divided into
blocks with constant values of the anisotropy parameters ǫ and δ and linearly
varying symmetry-direction velocity VP0 controlled by the vertical (kz) and
lateral (kx) gradients. Since estimation of tilt from P-wave data is generally un-
stable, the symmetry axis is kept orthogonal to the reflectors in all trial velocity
models. It is also assumed that the velocity VP0 is either known at the top of
each block or remains continuous in the vertical direction. The MVA algorithm
estimates the velocity gradients kz and kx and the anisotropy parameters ǫ
and δ in the layer-stripping mode using a generalized version of the method
introduced by Sarkar and Tsvankin for factorized VTI media.
Synthetic tests for several TTI models typical in exploration (a syncline, up-
tilted shale layers near a salt dome, and a bending shale layer) confirm that
if the symmetry-axis direction is fixed, the parameters kz, kx, ǫ, and δ can
be resolved from reflection data. It should be emphasized that estimation of ǫ
(with known VP0) in TTI media requires using nonhyperbolic moveout for long
offsets reaching at least twice the reflector depth. We also demonstrate that
application of VTI processing algorithms to data from TTI media may lead to
significant image distortions and errors in parameter estimation, even when tilt
is moderate (e.g., 20-30◦). The ability of our MVA algorithm to separate the
anisotropy parameters from the velocity gradients can be also used in lithology
discrimination and geologic interpretation of seismic data in complex areas.