Two defect detection systems

Sonic tomography

Sonic tomography is a non-invasive defect detection system which involves using the relative velocity of sound waves, using sensors, across the stem of a tree. The output of such systems is an image showing the different sound transmission properties over a cross section of a tree. The image is coloured and the colours indicate levels of sound transmission (i.e sound travels faster through degraded, soft and hollow wood). Well-known systems include the PiCus and Arbotum models.

While this system is good for assessing levels of decay in the main stem, it does have some limitations. The most obvious from a practical point of view in that the sensors can only be practically placed from the tree base to around 2m. Any higher would require ladders or scaffolding. It’s therefore good for a certain area of the stem but nowhere else. Also if the sensors are slightly out, a developing decay point could be missed. For the many fungi that cause base and principal root rot for example, while the sonic tomographer could pick up in the area located it would not pick up decay just below, just above or within the upper roots. To be absolutely sure then, the retesting may need to be done several times.  

Another claimed issue is that the measurement of density is relative rather than absolute which can skew and misrepresent the results (Slater, 2018).

This device is expensive, has more parts and may be more difficult to setup on trees with irregularly shaped stems (which is be more likely on the trees requiring use of this tool – the older trees).

Improvements include a less cumbersome system with fewer parts. There has also been research looking into combining the sonic tomograph with other tools to overcome limitations in accuracy.

  • Fig 18. Sensors used to measure sound transmission (Channel instruments.com)
  • Fig 19. Image output from the PiCus sonic tomograph (BHA trees)

Resistograph

The resistograph is a tool which uses a very fine drill to penetrate the tree and a trace is recorded which shows the level of resistance. The softer the wood, the less resistance. Again this is useful for assessing the integrity of stems and the bases of trees and possible surface roots avoiding excessive removal of earth and the results are true values.

Some of the reported shortcomings are that there can be inaccuracies based on flexion of the drill bit and that, as the drill takes the path of least resistance, smaller diameter drill bits are more likely to showed skewed results as the drill deviates around harder wood or knots for example (Slater, 2018a). It also only surveys exactly where the drill is used and therefore its possible pockets of developing decay may be missed. The results have also be open to some error depending on user error – for example the user applying pressure whilst using the tool.

A shortcoming is that the tool is invasive and that is reported to create openings and allow the ingress of disease or creates pathways through barriers (Kersten & Schwarze, 2005). A situation that could be exacerbated through continual testing, highlighting another issue in that drill placement is key and therefore this tool potential needs to be used by a more experienced practitioner.

  • Fig 20. Using a resistograph (Rinntech.de)
  • Fig 21. Output of a resistograph shown against its testing subject (Comet.ca)