Coordinate-independent angle-gathers for wave equation migration

We formulate angle decomposition, during and after wavefield extrapolation, independent of the choice of coordinate systems. The key idea is to describe the source-receiver separation during wavefield extrapolation as a vector quantity. This formulation links the reflection angle to the magnitude of the offset vector, thus reducing the cost and storage associated with angle decomposition. This coordinate-independent formulation eliminates the difficulties encountered by traditional angle decompositions for steeply dipping reflectors, and it can also be used for extrapolation techniques in general, non-Cartesian coordinate systems.