Correlation length of ultrasonic speckle in anisotropic random media: Application to coherent echo detection

In a recent paper [Derode and Fink, J. Acoust. Sec. Am. 101, 690-704 (1997)], a technique for studying spatial coherence properties of backscattered speckle noise by means of a transducers array was described. In this paper another approach is presented. Here, a single transducer, focusing inside a scattering medium, is moved in a plane; correlation is studied between signals acquired in two different positions. A study investigates how the correlation function of the medium determines that of the speckle noise. This is applied to fully or partially incoherent media, and media with anisotropic scatterer distribution such as unidirectional and cross-ply composites; we show that it is possible to extract information about the medium from a measurement of the backscattered field correlation length. The influence of correlation length on defect detection is discussed. Experimental as well as theoretical results are presented, they are found in good agreement. (C) 1998 Acoustical Society of America.