NORSAR-3D uses the Wavefront construction method to model the seismic events (see chapter 2.4 in the User's Guide) The wavefront construction method mimics a true wave propagation through a model. The event attributes at a receiver position are calculated on basis of the wavefront parameters, not single rays. The single rays are just approximated ray paths traced with take-off angles estimated from the wavefront construction. In models with strong velocity gradients and/or complex structures, these approximated ray paths may miss the target.