Our next example concerns a zero-offset GPR dataset over a lava flow region. In this situation, diffraction focusing is the only option available for migration velocity analysis. The data depicted in Figure 11 show many diffractions spread over the entire dataset. A few obvious ones are at x=22 ft, t=27 ns, at x=28 ft, t=22 ns, and at x=35 ft, t=23 ns.
We follow the same procedure for migration velocity analysis as the one described for the preceding example. Figure 12(a) shows the initial image obtained by migration with a constant velocity of 0.2 ft/ns, and Figure 12(b) shows the final image obtained after velocity update. We can notice that the image has been vertically compressed, since the velocity update indicated a faster velocity, and most of the diffractions have been collapsed.
Figures 13 and 14 are detailed views of the initial and final images and slownesses at various locations of interest. Figure 13 shows collapsed diffractions in the left part of the image. We can also observe features with better continuity in the updated image than in the original image, for example at x=20-24 ft and z=2 ft in Figure 13(a,c). Likewise, Figure 14 shows a better focused image than in the original, for example at x=34 ft and z=1.8 ft in Figure 14(a,c).