Application of an ultrasonic tomographic technique for detecting defects in standing trees Cheng-Jung Lin a , Yu-Chien Kao b , Ta-Te Lin c , Ming-Jer Tsai b, * , Song-Yung Wang b , Lang-Dong Lin d , Ya-Nan Wang b , Ming-Hsun Chan e a Division of Forest Utilization, Taiwan Forestry Research Institute, Taipei, Taiwan b School of Forestry and Resource Conservation, College of Bio-Resources and Agriculture, National Taiwan University, Taipei, Taiwan c Department of Bio-Industrial Mechatronics Engineering, National Taiwan University, Taipei, Taiwan d Department of Forest Products Science, National Chiayi University, Chiayi, Taiwan e The Experimental Forest, College of Bio-Resources and Agriculture, National Taiwan University, Nan-Tou, Taiwan article info Article history: Received 20 July 2007 Received in revised form 27 September 2007 Accepted 27 September 2007 Available online 24 June 2008 Keywords: Ultrasonic tomographic technique Defect detection Trees abstract 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 corre- lations 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. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Detecting deterioration in trees, particularly internal decay that does not have external indicators, is an important issue for forest management. Various nondestructive evaluation (NDE) techniques, based on different concepts, have been used to detect deterioration in trees in order to identify hazardous trees, to prevent the spread of decay, and to improve stand conditions. Sounding a tree by striking it with a tool can be used to detect advanced decay or hollows inside the trunk, but this method is not effective on large trees with thick bark (McCracken, 1985). Visual tree assessments are successfully supported by stress wave and ultrasonic wave- based evaluations. Stress and ultrasonic wave transmission times or attenuation in wood have proven to be effective parameters for detecting and estimating deterioration in tree stems and wood structural members (Hoyle and Pellerin, 1978; Hoyle and Rutherford, 1987; Mattheck and Bethge, 1993; Ross and Pellerin, 1994; Yamamoto et al., 1998; Lin et al., 2000; Pellerin and Ross, 2002). However, the one-dimensional stress wave test provides limited information about defects in trees. Reliable defect evalua- tion for imaging internal characteristics in logs and trees is possible by X-ray and neutron radiography, computed tomography (CT), and magnetic resonance (MR) (Habermehl et al., 1986; Hailey and Morris, 1987; Holoyen and Birkeland, 1987; Wagner et al., 1989; Oja et al., 2000). These techniques can provide one- to three-di- mensional spatial locations of various defects and internal wood characteristics, but their application to trees has been limited be- cause of the high costs, fear of X-rays and gamma rays, strong regulations, and the control of radiation sources associated with their use. Some NDE techniques have recently been developed for tomo- graphic investigations. Tomography allows the reconstruction of a cross-section through an object by means of measurements performed on the object surface and by measuring the energy that passes through the object itself (Worthington, 1984; Nolet, 1987; * Corresponding author. Tel.: þ8862 33 664 641; fax: þ8862 23 686 335. E-mail address: tmj@ntu.edu.tw (M.-J. Tsai). Contents lists available at ScienceDirect International Biodeterioration & Biodegradation journal homepage: www.elsevier.com/locate/ibiod 0964-8305/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ibiod.2007.09.007 International Biodeterioration & Biodegradation 62 (2008) 434–441