Acoustic tomography in relation to 2D ultrasonic velocity and hardness mappings

Abstract

Acoustic tomography is an emerging nondestructive testing (NDT) technology for tree decay detection in both urban community and production forest. Many field studies have been conducted to assess the applicability and reliability of the technique in such applications. Although investigations into urban trees showed great success using acoustic tomography to detect moderate to severe internal decay within the trunk, detection of early stage of decay using such technology still constitutes a challenge. This study was aimed to evaluate the capability of acoustic tomography by determining the relationships between acoustic tomograms and two-dimensional mappings of ultrasonic properties and end-hardness of the trunk cross sections. A freshly cut black cherry (Prunus serotina) log was used to simulate a tree trunk and tested in the laboratory. Time-of-flight (TOF) acoustic tomography measurements were conducted at three different heights (10, 30, and 50 cm). A disk was then cut from each height and subjected to ultrasonic and mechanical evaluations. The results indicated that the acoustic shadows in the tomograms revealed internal structural defects that were at the same locations and in similar magnitudes as the wood property mappings of the disks. However, no good correlations were found between apparent acoustic velocity of the tomograms and the measured ultrasonic velocity and end-hardness of small cubic samples. This analysis indicates that TOF acoustic tomography lacks the sensitivity to low-velocity features of decayed areas and thus has limited capability in detecting early stages of decay in trees.