Schematic illustration of the banana–doughnut mechanism of wavefront healing. In this example, a wave-speed anomaly of diameter a=4λ is situated at a distance S=10λ from the source s. The three panels show the relation of the anomaly (white circle) to the 3-D traveltime sensitivity kernel K of a τ=25 s wave, as the position of the receiver r is shifted from R=2λ to R=12λ to R=30λ, representing the propagation of the wave. The size of the doughnut hole in the vicinity of the anomaly increases with increasing propagation distance, so that the traveltime shift δT_bdk decreases with time.