The least-squares inverse problem is formulated as follows. A constant-offset l2 misfit energy functional can be defined as
where is a recorded constant offset section. Minimizing (3) with respect to and leads to two coupled normal equations. The equations can be decoupled by the stationary phase (high-frequency) approximation, in which the major contribution to (2) occurs near the specular point when .In this case, the equation can be solved independently of , and the result can be backsubstituted into the original normal equation for .