Highly absorptive rocks usually have low velocity. Behind a low velocity pod, waves should be weakened by absorption. They should also be strengthened by focusing. Which effect dominates? How does the phenomenon depend on spatial wavelength? Maybe you can figure it out knowing that black on Figure 4c denotes low amplitude or high absorption, and black on Figure 4d denotes low velocities.
I'm inclined to believe the issue is focusing, not absorption. Even with that assumption, however, a reconstruction of the velocity v(x,z) for this data has never been done. This falls within the realm of ``reflection tomography'', a topic too difficult to cover here. Tomography generally reconstructs a velocity model v(x,z) from travel time anomalies. It is worth noticing that with this data, however, the amplitude anomalies seem to give more reliable information.