  In seismic imaging, understanding the relationship between
wavefront-propagation velocity and time-interval velocity is crucial for
achieving optimal resolution. However, this task becomes even more challenging
when considering anisotropic situations. To accurately account for the
influence of anisotropy on wavefronts, it is essential to have a solid grasp of
the underlying physics. Unfortunately, the anisotropy model that best describes
the medium is often unknown. To address this issue, we utilize paraxial-ray
theory in a ray-centered coordinate system to study the wavefront phenomenon.
This approach allows us to develop explicit expressions that describe the
physics of the problem. Using this theoretical framework, we can accurately
generalize the relationship between time-migration rays and Dix velocity by
incorporating the velocity-spreading factor for general anisotropic media. This
factor lets us determine the type of anisotropy present in the medium.
Moreover, the velocity-spreading factor provides valuable information for
various applications, including model building, time-imaging, and time-to-depth
conversion. Overall, the presented theoretical framework offers a comprehensive
understanding of wavefront propagation in anisotropic media, which can aid in
improving the knowledge of the phenomena that form seismic images.
