Spatial coherence of ultrasonic speckle in composites.

Pulsed echography in a scattering medium generates speckle noise. Its random nature implies a statistical processing of the backscattered signal. The second-order statistics, such as autocorrelation, can provide information about the structure of the medium and help detect a possible defect. The spatial autocorrelation function is related to the spatial coherence of the backscattered waves, which determines the speckle noise level. Specifically, this spatial coherence approach is applied to anisotropic fiber-reinforced composites. The authors present theoretical as well as experimental results and show how the autocorrelation function changes with the orientation of the layers inside the composite, thus pointing out its anisotropy.