High-resolution ultrasonic imaging of wood

Ultrasonic nondestructive evaluation holds the promise of significantly improving predictions of the load bearing capacity of wood. The inhomogeneity, anisotropy and natural variability of wood pose a formidable challenge to the realization of that promise. As a first, qualitative step in correlating known wood mechanical properties and their natural patterns of variability with parameters of ultrasonic wave propagation, high resolution ultrasonic images of transmitted and reflected energy are presented. The experiments were carefully designed to observe only bulk waves. The results show that surface echoes and backscatter from subsurface features contain useful and as yet untapped information on local wood properties and structure. Transmitted signals were found to always contain more than one image of the outgoing pulse, indicating another source of information so far overlooked. The high spatial sampling density used here allows the generation of images in which the human eye can distinguish familiar features even in the presence of significant noise. Combining qualitative information on wood properties and their distribution with high resolution imaging can be used to remove noise and extract image features one knows to be present by appropriate application of image processing. Received 23 October 1996