Application of an ultrasonic tomographic technique for detecting defects in standing trees

The purpose of this study was to evaluate defects in trees using an ultrasonic tomographic technique. In order to simulate different degrees and kinds of defects, a hole was chiseled in the central and side areas of trunk cross-sections and then the ultrasonic velocities were measured in multiple directions (eight equidistant ultrasonic test points and 56 independent travel time measurements) and analyzed using ultrasonic equipment. Results revealed that the highest coefficient of determination was found between the residual disc diameter ratio and the ultrasonic velocity in the virtual radial direction. Strong correlations between the drop in the relative ultrasonic velocity and residual disc diameter ratio were observed in the virtual radial direction. Moreover, the ratio of ultrasonic velocity along paths D (a three-point interval between two measured points) to B (a one-point interval between two measured points) decreased with increase in the ratio of the hole-to-disc area. However, values became more diverse for specimens with a hole chiseled in the side area as the ratio of the hole-to-disc area increased. In situ experimental results showed that differences in the mapped colors and ultrasonic velocities between the hole area and its surrounding area were obvious, and when the ratio of the hole area to the cross-sectional area exceeded 2.8%, the hole was detectable by ultrasonic tomography. Therefore, the location and size of the hole in the tree trunk cross-section could be detected by this ultrasonic tomographic technique.