Acoustic wood tomography on trees and the challenge of wood heterogeneity

Abstract The assessment of tree stability requires information about the location and the geometry of fungal decay or of a cavity in the interior of the trunk. This work aims at specifying which size of decay or cavity can be detected non-destructively by acoustic wood tomography. In the present work, the elastic waves that propagate in a trunk during a tomographic measurement were visualized by numerical simulations. The numerical model enabled to systematically investigate the influence of fungal decay on tomographic measurements neglecting the heterogeneity of wood. The influence of wood heterogeneity was studied in laboratory experiments on trunks. The experiments indicated that the waveforms of the measured signals are by far more sensitive to the natural heterogeneity of trunk wood than the travel times, thereby making waveforms unsuitable for decay detection. Thus, it is recommended to further develop the travel time inversion algorithms for trunks and to neglect the information in waveforms or amplitudes. Fungal decay is detectable if the influence of the decay is distinguishable from the influence of the heterogeneity. It was found from the numerical analysis that the cross-section of a cavity, which is larger than 5% of the total cross-section of the trunk, can be detected by acoustic wood tomography.

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