Ultrasonic waves in wood: a ray-tracing approach

Urban trees present several benefits at ecological and sanitary levels in modern cities. In order to manage this resource, urban operators need tools to evaluate their phytosanitary state. Aiming to analyze the inner structures of trees without altering their condition, non-destructive imaging methods have been proposed. In this study, we are concerned on evaluating the influence of orthotropic behavior in wood on the tomography image reconstruction using ultrasonic waves, by performing time-of-flight (TOF) estimation using the raytracing approach, a method used mainly in the field of exploration seismography to simulate wave fronts in elastic media. Mechanical parameters from six wood species and one isotropic material were defined and their wave fronts and corresponding TOF values were obtained, using the proposed raytracing method. If the material presented anisotropy, the ray paths between the emitter and the receivers were not straight; therefore, curved rays were obtained for wood and the TOF measurements were affected. We were interested in using also the Finite Elements Method (FEM) to study the propagation of elastic waves in wood to compare and validate the results obtained with the proposed raytracing approach. FEM model resulted in TOF estimations for a wood trunk equivalent to those obtained with the raytracing approach. Wave fronts in both cases agreed and TOF comparison resulted in a relative difference between the two models smaller than 2%. Finally, to study the effect on the process of tomographic image reconstruction from the TOF measurements, the filtered back-projection algorithm was applied. This reconstruction method is extensively used in straight ray tomography; nevertheless, also it is commonly used in wood acoustic tomography. Discs without inner defects for isotropic and wood materials (Spruce sample) were tested: isotropic material led to a flat color image; in the case of wood, a gradient of velocities was obtained. Then, centric and eccentric defects were evaluated, both for isotropic and orthotropic cases. From the results obtained for wood, when using a reconstruction algorithm intended for straight ray tomography, the images presented velocity variations from the border to the center that made difficult the discrimination of possible defects inside the samples, especially for eccentric cases. (Texte intégral