Partial coherence of transient ultrasonic fields in anisotropic random media: Application to coherent echo detection

In this paper the spatial coherence properties of the speckle noise backscattered from random media insonified by a broadband pulse are studied. The approach presented is an adaptation of the optical notion of coherence introduced by Zernike. How the statistical parameters (particularly the second-order moment) of the medium determine those of the speckle noise is studied. This is applied to fully or partially incoherent media, and media with anisotropic scatterer distribution such as unidirectional and cross-ply composites; also investigated is the possibility of detecting a coherent reflector among speckle noise through measurements of coherence length. Experimental as well as theoretical results are presented. Measurements were carried out with a linear array of 128 programmable transmitters/receivers. (C) 1997 Acoustical Society of America.