Ultrasonic tomography for trees inspection

Abstract

Trees have great importance in maintaining the environmental balance, in the release of oxygen and absorption of carbon dioxide, in the improvement of air quality, in the provision of shading, in the absorption of noise, in thermal comfort, in the prevention of floods and in the well-being of people. In the city of São Paulo, for example, there are approximately 650,000 tree species on the roads, of which about 23,000 have been removed in 16 months (2015 to April 2016), according to the Secretariat of Coordination of the Local Administration (SCS). Such removals are mainly based on visual inspections, which although very important, can be subjective. This subjectivity causes that, even with a large number of removals, the number of tree falls is high (1,466 in 5 months in the year 2016, according to SCS data), causing great material damages and, often, leading with victims. The most common reasons for tree fall are: inadequate pruning, lack of space for root development and support, age, fungal and termite diseases and attacks. There are extremely useful equipment in the international market to complement visual inspections, but they are rarely used in our country due to the high cost of purchasing and/or contracting specialized services. The propagation of mechanical waves is a very promising technique in the identification of deterioration in trees. The propagation of these waves is mainly affected by differences in elastic properties, impedance differences between propagation media and path changes. All these interferences cause, therefore, changes in the speed of propagation. The deteriorated wood will have its resistance and stiffness properties affected, thus causing variations of wave velocity when compared to sound wood. Timber will cause changes in the wave path, which seek the material medium to propagate, also affecting speed. Thus, the measurement of velocities in several routes in the section of a trunk allows to evaluate its variation and, by means of this variation, to identify possible deteriorated areas or inside. To facilitate visualization, speed variations are associated with colour. In practice, however, there is a limit of measured routes, so it is necessary to use an interpolation system to assign velocity values to the unmeasured routes and thus to fill in all the points corresponding to the trunk section, generating an image representative of the variations of velocity in its interior, denominated acoustic tomography. This inspection tool is still considered in development, having as technical challenge the acquisition of images that allow more effective detection, localization and obtaining the dimensions of deteriorated zones. The goal of this project is to develop ultrasonic tomography equipment for use in live tree inspections. As specific objectives, we highlight improvements in the processes previously developed by the team and the association of all these processes in an equipment that generates, in real time, an ultrasonic tomography image that represents the internal condition of the trunk under inspection. (AU)