The inverse of the cross product operator LLT acts as a filter for the data space. Each element Aij of (LLT)-1 measures the correlation between a data element di and another data element dj. The computation of each element Aij requires the evaluation of an inner product in the model space. Since the model space is regularly sampled, the inner products for several imaging operators can be computed analytically which leads to a fast and affordable evaluation of the elements of LLT. This is the case for the solution we present in this paper.
Let's consider an irregularly sampled input of n seismic traces and let Lm,di be the operator that maps trace di into the model space m. The operator that performs the inverse mapping is therefore LTm,di. In matrix notation, we write the cross-product matrix LLT as
where A(hi,hj) is AMO from input offset hi to output offset hj and, I is the identity operator (mapping from hi to hi). Comforming to the definition of AMO Biondi et al. (1996), A(hi,hj) is the adjoint of A(hj,hi); therefore, the filter is Hermitian with diagonal elements being the identity and off-diagonal elements being AMO transforms.
This is the fundamental definition of A that will allow a fast and efficient numerical approximation of its inverse, and thus of the whole prestack imaging inverse problem. Being a narrow operator, the cost of applying AMO to prestack data is almost negligible compared to other imaging operators such as prestack migration. When the azimuth rotation or the offset continuation is small, as is the case for geometry regularization problems, the size of the operator is very small, but also has a strong curvature. Biondi et al 1996 discussed the design of an efficient implementation of the AMO operator that avoids aliasing and simultaneously takes advantage of the opportunity for saving computation by properly limiting the spatial extent of the numerical integration.