Sonic Tomography

Decay in trees can weaken wood enough to increase the chance of mechanical failure. However, in many instances decay is not extensive enough to compromise the safety factor of the tree. The use of both Picus and Arbotom sonic tomography enables the extent of decay to be assessed without invasive testing such as resistograph drilling or boring, which damages trees and potentially impairs the tree’s ability to compartmentalise the decay.

Sonic tomography uses relative velocity of sound waves, induced across the stem of a tree, to construct a two-dimensional picture (tomogram) that shows zones of different sound-transmission properties. The zones are colour-coded and indicate degrees of degradation and the presence of hollows. One advantage of the Arbotom system is that the software that accompanies it enables the data collected during each test to be used to calculate strength loss.

Paul Melarange is probably the only tree consultant in the country to integrate the information gathered through sonic tomography with the ‘Tree Statics Model’. In tree statics, the ability of a tree to withstand wind loads of gale force is calculated by including the shape of the load-bearing structure (trunk and crown), the properties of green wood and the forces that occur in a gale-force wind gust. The influence of the reduced cross-sectional area, resulting from decay and measured by the tomography, can be incorporated into the tree statics evaluation. The combination of the two enables a more objective assessment to be made of the tree’s fracture stability. The result is presented as a safety factor, which can be used to indicate whether or not remedial action is required.