The Phase-shift operator in a prestack Fourier-domain is described by the double-square-root equation, which is a function of the angular frequency ,the midpoint rayparameter px, the offset rayparameter ph, and the velocity, v. Constant-velocity prestack migration, with output provided in two-way vertical time (), in offset-midpoint coordinates Yilmaz (1979), is given by:
and , , and are the horizontal midpoint wavenumber, the horizontal offset wavenumber, and the vertical wavenumber, respectively. In this paper, I will freely alternate between the half offset, h, and the full offset, X, in representing the offset axis, where X=2h. The phase factor , for isotropic media, is defined as
For VTI media, the phase factor is given by a more complicated equation Alkhalifah (1997b),
Kirchhoff migration is typically applied by smearing an input trace, after the proper traveltime shifts, over the output section in a summation process. To obtain the response of inserting a single trace into the prestack phase-shift migration [equation (3)], we multiply the input data by a Direc-delta function in midpoint and offset axes as follows
Inserting this equation into equation (3) provides us with the migration response to a single input trace given by
The number of integrals in Equation (7) can be reduced by recognizing areas in the integrand that contribute the most to the integrals in kh and kx. Since the integrand is an oscillatory function its biggest contributions take place when the oscillations are stationary, when the phase function is either minimum or maximum. This approach is referred to as the stationary phase method (Appendix C). The stationary points (px and ph) correspond to the minimum or maximum of equation (8). In fact, the phase has a dome-like shape as a function of px and ph (see Figure 1). Thus, to calculate the stationary points, we must set the derivative of equation (8) with respect to ph and px to zero, and solve the two equations for these two parameters. An easier approach is discussed next and in Appendix A.