The presence of a lens anomaly in a velocity model results in a variety of ray paths, the most interesting of which is a development of a triplication in the wavefront. This multi-arrival traveltime phenomenon typically occurs when a negative velocity anomaly is present. The intriguing issue is that triplication can also occur when we have positive anomalies.
Figure 5 shows rays and corresponding wavefronts that were obtained using conventional raytracing in the depth domain (black curves), and using the equivalent raytracing in the -domain (gray curves) through a VTI model with =0.1. The velocity model is shown in the background with a negative velocity anomaly that has a peak of -1.0 km/s. The result is a noticeable triplication that develops soon after the rays pass the anomaly. Despite the triplication, the results of raytracing in the two domains (depth and time) are similar.
Figure 6 also shows raypaths through an anomaly. The anomaly now is in , and it is positive. Therefore, the background is an model, with =0 everywhere other than in the anomaly. Again, the black curves correspond to solutions of raytracing in the depth domain, while the gray curves correspond to raytracing in the -domain. Triplication, smaller than that associated with the velocity perturbation, occurs in the wavefront. Velocity-wise this medium is homogeneous; it is that is causing the severe bending of the rays! The rays with larger propagation angles from the vertical are the most influenced by the anomaly.