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Ultrasound imaging for sustainable and rational management of standing trees in urban areas

Espinosa Moreno, Luis Fernando (2019) Ultrasound imaging for sustainable and rational management of standing trees in urban areas. Doctorado thesis, Universidad Nacional de Colombia - Sede Bogotá.

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The tree plays a major ecological role in modern cities. The management of the plants is the subject of requests from urban operators: the diagnosis is essentially visual, even when the extent of internal damage and the associated hazard cannot be precisely evaluated by simple observation. Ultrasonic imaging methods allow answering biological questions related to the adaptation of the tree to exogenous constraints, such as pathogenic attacks, presence, and type of internal damages, the extent of degraded or traumatized areas. The major scientific issues are linked to the image production (reconstruction of the intrinsic parameter from a set of measurements) and to the image interpretation (discrimination for detection of alterations and its positioning). The overall aim of this thesis was to develop an ultrasonic imaging method for the diagnosis of the internal condition of urban trees. The scientific objectives were to develop numerical models to study the factors of influence on the propagation of ultrasonic waves in the cross-section of a tree and to propose an image reconstruction solution, suited for orthotropic materials, allowing the discrimination and positioning of decay. The development of a protocol for the acquisition, processing, analysis and interpretation of ultrasound tomography signals and images is of great importance for wood science. Obtaining reliable and interpretable images is a recurring demand from urban operators. Initially, to set-up the ultrasonic chain of measurement, a comparative experimental study was done to choose the excitation signal parameters, such as shape, temporal duration, and frequency response, and then the choice of a suitable time-of-flight determination technique. Then, we were concerned on evaluating the influence of the orthotropic condition of wood on the propagation of ultrasonic waves, by performing a time-of-flight (TOF) estimation using a raytracing approach, a method used in the field of exploration seismography to simulate wavefronts in elastic media. The anisotropy of wood in the radial-tangential plane influenced the wave velocity depending on the direction of propagation, that led to deformed wavefronts compared to the perfectly circular wavefronts for an isotropic case. The paths from each receiver to the transmitter in the wood presented a curvature, therefore the trajectories differed from the straight-line distance obtained for an isotropic case. A numerical comparison was made using the Finite Elements Method (FEM); the TOF estimates and wavefronts agreed with those of the raytracing approach. A similar experimental validation was performed. Wood sections from two species were tested. Defects in the wood were simulated by drilling holes. The shape of TOF curves computed using the raytracing algorithm and those obtained from the experiments were in good agreement. Defects located in the center of the trunk presented larger TOF variations compare to defects located in off-centered positions. Thus, off-centered defects would be more difficult to determine and characterize by tomographic inversion. Then, we were interested in the influence of the wood orthotropic condition on the tomography image reconstruction process (inverse problem) and how it should be adapted to the standing tree constraints. For wood, the ray paths between the ultrasonic transmitter and the receivers are not straight as for isotropic media; therefore, the image reconstruction method should be adapted to deal with curved rays. The proposed method considers the orthotropy property of wood material, performing an iterative process that approximated the curved rays. A slowness function was defined for every pixel and a nonlinear regression allowed the mapping of the inner elastic constants. Initially, four numerical configurations were tested representing real cases usually found in standing tree monitoring. The reconstructed images using the proposed method were compared with a straight-ray reconstruction method (filtered back projection algorithm), highlighting a more detailed identification and quantification of the inner state of the anisotropic structure of the trunk. Then, the inversion procedure was tested using wood samples from two species for three different configurations: a healthy case, a centered defect case, and an off-centered defect case. As for the numerical study, the proposed method resulted in a more accurate defect representation when compared to a straight-ray reconstruction, especially for the case of centered defects., Les arbres urbains jouent un rôle écologique, sanitaire et esthétique majeur dans les villes modernes. Ces bénéfices incluent entre autres la réduction de la pollution de l’air, l'abri et nourriture à plusieurs espèces d'oiseaux et petits animaux, la protection contre la chaleur, et la mise en valeur du paysage. Cependant, certains risques sont associés aux arbres urbains : la chute d'un arbre (ou d’une branche) peut causer des dommages aux passants, aux véhicules et aux habitations. L’évaluation des risques associés aux arbres dans les villes est souvent à l’initiative des opérateurs urbains. Le diagnostic est essentiellement visuel, alors que l'ampleur des dégâts internes et le danger associé ne peuvent pas être évalués avec précision par la seule observation. Fréquemment, les arbres présentant une décomposition interne semblent sains. Répondre à la gestion des plantations urbaines nécessite donc l’utilisation de méthodes de diagnostic interne des arbres in situ. Des techniques modernes peuvent être utilisées pour minimiser les risques associés à la chute d’arbres ; des progrès significatifs dans le matériel de diagnostic ont été réalisés, ainsi que dans les formules et les directives d'évaluation des arbres dangereux., Los árboles urbanos desempeñan un importante papel ecológico, sanitario y estético en las ciudades modernas. Estos beneficios incluyen reducir la contaminación del aire, brindar refugio y alimento a muchas especies de aves y animales pequeños, proteger contra el calor y mejorar el paisaje. Sin embargo, algunos riesgos están asociados con los árboles urbanos: la caída de un árbol (o rama) puede causar daños a los transeúntes, vehículos y casas. La evaluación del riesgo de los árboles en las ciudades a menudo está a cargo de operadores urbanos. El diagnóstico es esencialmente visual, mientras que la extensión del daño interno y el peligro asociado no pueden evaluarse con precisión mediante una simple observación. Con frecuencia, los árboles con descomposición interna parecen sanos. La respuesta a la gestión de las plantaciones urbanas requiere, por lo tanto, el uso de métodos de diagnóstico interno in situ para los árboles. Se pueden utilizar técnicas modernas para minimizar los riesgos asociados con la caída de árboles; ha habido un progreso significativo en los equipos de control y diagnóstico, así como en las fórmulas y pautas para evaluar árboles peligrosos.

Tipo de documento:Tesis/trabajos de grado - Thesis (Doctorado)
Colaborador / Asesor:Prieto Ortiz, Flavio Augusto and Brancheriau, Loïc
Información adicional:Doctor en Ingeniería Mecánica y Mecatrónica.
Palabras clave:Wood, Ultrasound, Nondestructive evaluation, Anisotropy, Madera, Ultrasonido, Evaluación no destructiva, Anisotropía
Temática:6 Tecnología (ciencias aplicadas) / Technology > 62 Ingeniería y operaciones afines / Engineering
Unidad administrativa:Sede Bogotá > Facultad de Ingeniería
Código ID:72920
Enviado por : Luis Fernando Espinosa Moreno
Enviado el día :19 Julio 2019 16:30
Ultima modificación:19 Julio 2019 16:30
Ultima modificación:19 Julio 2019 16:30
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