Si is deduced from the electric signal that enters the acquisition chain and also accounts for all the filtering effects
not related to ultrasonic phenomena. It is considered as an input parameter of the model. u and T refer respectively
to the displacement vector and to the stress tensor projected on the normal of the defect of ultrasonic fields. The
indexes Em and ReF correspond to two configurations. Em is the ultrasonic field radiated by the emitting probe
when the flaw is absent. ReF is the ultrasonic field radiated by the receiving probe when it is acting as an emitter and
when the flaw is present.