Figure 1 shows at the bottom the image perturbation caused by the ideal slowness perturbation shown in the top panel in Figure 2. is created by subtracting the reference image from the perfectly focused one .

We take the image perturbation shown in the bottom panel of Figure 1 and compute the corresponding slowness perturbation using Equation (1). Figure 2 shows in the middle the result we obtain by applying the adjoint of the operator to the image perturbation in Figure 1, and at the bottom the result of applying the least-squares inverse of to the same .

Despite the inherent vertical smearing caused by the limited angular coverage, the slowness perturbations are nicely focused at their correct locations. Obviously, the result obtained with the least-squares inverse is much better focused than the one obtained by the simple adjoint operator, although we have only used the zero-offset and not the entire prestack data. The simple backprojection (top panel in Figure 2) creates ``fat rays,'' also discussed by Woodward (1992) and Sava (2000).

11/11/2002