1. A seismic imaging method that comprises: migrating seismic survey data with an anisotropic velocity model to obtain common angle image gathers; processing the common angle image gathers to obtain depth residuals along one or more horizons; determining subsurface opening angle sensitivity kernels for the horizons independent of shot location and based on the anisotropic velocity model and a Green's function at an image point; inverting the depth residuals using the determined sensitivity kernels to obtain velocity errors; applying updates to the velocity model based on the obtained velocity errors; and displaying a representation of the survey region to a user based at least in part on the updated velocity model.