Near Field Measurement Models

Abstract

In Chap. 12 we derived the Thompson-Gray measurement model using both a direct approach and an approach based on reciprocity relations. In either case, we assumed the incident and scattered waves could be treated as quasi-plane waves modified by appropriate diffraction correction terms and the scattering amplitude was assumed to be slowly varying so that it could be evaluated along a fixed set of incident and scattered directions. Although the quasi-plane wave (or paraxial) approximation of the incident and scattered fields is typically a very good assumption even in the near field, the combination of this assumption with the slowly varying scattering amplitude assumption can cause the Thompson-Gray models developed in Chap. 12 to break down in the near field. This breakdown is particularly severe for very specular scatterers (such as cracks or flat-bottom holes). In this chapter, we will construct near field measurement models based only on a high frequency, small flaw assumption. These models will reduce to the Thompson-Gray models found previously when the directions of the waves and the scattering amplitude both do not vary significantly from their values along a set of fixed rays in the transmission and reception processes.