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 backs cattered 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; correlatio n is studied between signals acquired in two different positions. A st udy investigates how the correlation function of the medium determines that of the speckle noise. This is applied to fully or partially inco herent media, and media with anisotropic scatterer distribution such a s unidirectional and cross-ply composites; we show that it is possible to extract information about the medium from a measurement of the bac kscattered field correlation length. The influence of correlation leng th on defect detection is discussed. Experimental as well as theoretic al results are presented, they are found in good agreement. (C) 1998 A coustical Society of America.