23 1. Wave propagation optimum fit between the synthetic and the field data is obtained. This procedure is called full waveform inversion (FWI). FWI is discussed in a specific chapter (chapter 5). The wave equation is used in specific processing procedures such as tomography and migration. Box 1.2 Wave equation for a 2D seismic profile. Works such as Achenbach (1973), Dobrin (1988) and Quiblier (1997) should be consulted for more complete information on synthetic seismogram calculation. 1.2 Seismic methods When P or S-waves strike the interface at the critical angle ic, head waves or refracted waves are generated. This only occurs when a wave perturbation passes from a medium with velocity Vi to another with velocity Vi+1 which is greater than Vi and at the critical angle given by sin(ic ) = Vi / Vi+1. The critical angle ic is the criterion for differentiating the various seismic methods associated to body wave propagation (Box 1.3): • i<ic : the method is seismic reflection • i=ic : the method is seismic refraction • i>ic : the method is wide angle reflection. In wide angle reflection there is no transmitted energy, only reflected.
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