Complex Resistivity (CR) of Wood and Standing Treestivi t y [Ohm*m] 0.001 0.01 0.1 1 10 100 frequency [Hz] 0 0.5 1 1.5 2 2.5-p h ase [ ] oak samples axial 1 axial 2 axial 3 radial

NDTCE’09, Non-Destructive Testing in Civil Engineering Nantes, France, June 30th – July 3rd, 2009

Complex Resistivity (CR) of Wood and Standing Trees Tina MARTIN1 1 BAM Federal Institute for Materials Research and Testing, Berlin, Germany,

[email protected]

Abstract Complex resistivity (CR) has been used to investigate the properties and the extent of damage in wood and standing trees. Our laboratory experiments have proved that wood’s anisotropy influences its electrical behaviour. It is a well-established fact that wood exhibits the lowest resistivity amplitudes in the axial direction compared to that in tangential and radial directions. It has been shown that anisotropy affects not only the amplitude but also the phase. Phase values are the highest in the axial direction, particularly in the lower frequency range. The changes in resistivity due to fungi-induced damage were investigated in a long-term laboratory test. With progressing damage, both resistivity and phase decrease. The phase also shows sensitivity to the changes in the structure of wood cell. CR Tomography (CRT) has been employed in the field to detect fungi-infected zones in fallen trees. A number of factors such as the type and age of the tree, and the season seem to influence the CRT results. Among those, the effect of seasonal changes has been demonstrated here. Furthermore CRT can be used to characterise fungi-infected zones in the tree. Keywords Tree investigation, complex resistivity tomography, wood anisotropy, fungi, oak

Résumé La résistivité complexe (CR) a été employée pour étudier les propriétés et la stabilité du bois et des arbres vivants. Nos expériences en laboratoire ont vérifié que l'anisotropie du bois influence son comportement électrique. On sait que le bois montre les plus basses amplitudes de résistivité dans la direction axiale comparée aux directions tangentielles et radiales. Notre expérience a également démontré que l'anisotropie affecte non seulement l'amplitude mais également la phase. Les valeurs de phase sont les plus hautes dans la direction axiale particulièrement dans la gamme de fréquence inférieure. Des dommages Mycète-induits ont été étudiés par un essai de laboratoire de CR pendant une période dépassant un an. Pendant que les dommages progressent, la résistivité et la phase diminuent. La phase montre également la sensibilité aux changements de la structure de la cellule en bois. Le CRT a été utilisé dans le domaine pour détecter des zones mycète-infectées dans les arbres qui sont tombés. Un certain nombre de facteurs tels que le type d'arbre, l'âge et la saison ont un impact sur les résultats de CRT. L'influence des changements saisonniers sur un arbre de chêne sain a été montrée ici. Les tomogrammes de CR enregistrés au cours des trois dernières années montrent la sensibilité à l'humidité du bois.

1 Introduction Complex Resistivity (CR) is a conventional geophysical method which uses alternating current to measure the difference in the voltage within a broad frequency range (1 mHz to 1000 Hz). The frequency spectra can be interpreted using various models. It is possible to deduce information about environmental contamination from SIP data. Using CR, the amplitude and the phase of the complex resistivity provide complementary information about decay and the level of damage in wood. CR Tomography can then be developed as a modified


version of the geoelectrical tomography method [1] to investigate standing trees. The usually invasive point measurement methods used provide only punctual information about the decay. In contrast, tomographic methods can give imaging information about the entire plane investigated. Unlike Computer Tomography (CT) the application of tomographic methods in wood investigation is rare. Although CT [2] provides very good results, its application is rather complicated. Acoustic tomography has also been used for this purpose but the interpretation of its results has not yet fully been established [3].

2 Laboratory Measurements For the laboratory measurements we used the CR equipment SIP256c. We measured in the frequency range from 1 mHz to 100 Hz. The wood samples (diameter ~ 20 mm, length ~ 70 mm) were placed in a measuring cell with two taps for the current (outer taps, Figure 1 left) and two taps for the voltage measurements (inner taps), so it was measured in a 4-point-array. The first experiment will show the effect of the strong anisotropy in the wood’s complex resistivity. This anisotropy is caused by the naturally inhomogeneous structure of the trees. To understand the effect of fungi damage on complex resistivity, a long-term experiment with Daedalea quercina was carried out. It will be described afterwards.

Figure 1: Left: Measuring cell for laboratory measurements. Right: Assembly for field measurements.

2.1 Anisotropy To estimate the degree of the influence of anisotropy on the amplitude and the phase of complex resistivity, laboratory measurements were carried out on oak along the main directions axial (along the growth direction), radial (perpendicular to the growth direction) and tangential (along the annual ring). The results are shown in Figure 2. The axial samples show the lowest resistivity (Figure 2a). The resistivity of the radial samples is more than two times greater (around 410 Ωm) than that of the axial ones. However, the greatest resistivity was observed in the tangential samples, for which resistivities were measured almost four times greater than those for the axial samples. Anisotropy influences not only the amplitude, but also the phase measurements (Figure 2b). The strongest phase effect was observed in the axial direction. The radial samples show much less phase effect and the tangential samples show almost none at all. The peak of the phase curves is also different for different directions. The axial peak occurs at a frequency of about f = 0.01Hz while the radial phase peak occurs around f = 0.1Hz. For the tangential curves no distinguished phase peak is detectable. Summing up, the anisotropy of wood affects both the amplitude and the phase of the complex resistivity. Because of the considerable anisotropy-induced differences in resistivity and phase, it is important to record the direction along which the field measurement have been taken.


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Figure 2: Resistivity and phase directional measurements for oak samples

2.2 Long-term-experiment on fungi-infected wood Daedalea quercina is a brown-root producing fungus damaging mainly the heartwood of the trees. For this experiment, 50 wood samples along the radial and axial direction were cut, sterilized, oven-dried and injected with the fungus and an agar-malt-solution in a Petri dish. After six weeks the first four samples (two axial and two radial ones) were extracted from the Petri dish, weighed and their complex resistivity was measured. From then on, four new samples were extracted and investigated every other week. After each measurement, the samples were oven-dried again to obtain the mass loss caused by the fungi. The change in resistivity and phase at the frequency of the phase peak over the elapsed time are shown in Figure 3. First, the resistivity of the samples decreases with time. But, after 22 weeks (for radial samples, Figure 3c) and 25 weeks (for axial samples, Figure 3a) the resistivity of the samples begins suddenly to increase. In phase measurements, this abrupt change is however not visible. The phase decreases as a function of time over the whole period of the test. The decrease is more explicit for radial samples (Figure 3d) than for axial samples (Figure 3b). These results show that resistivity is very sensitive to the effect of moisture. Fungi need moisture to grow and develop which they extract from the air and the agar-malt-solution, therefore resistivity first decreases. After 22 –25 weeks the resistivity increases. There are two possible reasons for the phenomena: by this time the fungi have decomposed everything they were able to or they did not manage to adsorb any more moisture so they died early and the wood dried. The results in Figure 4a of the same samples indicate furthermore that a big change in the resistivity only occurs when the wood moisture content is low (< 50%). For higher moisture contents, the changes are marginal. However, the phase decreases over the entire infection time. That can also be seen by the reduction of the mass loss of the same infected samples in Figure 4b. The reduction of the mass loss amounts to the decomposition of the wood caused by fungi. That means that the phase is sensitive to changes in the structure of the wood cell which is caused by the fungi.


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Figure 3: Resistivity and phase for axial (red triangles) and radial (blue crosses) wood samples infected by the fungus Daedalea quercina for different infection times.

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Figure 4: Resistivity vs. wood moisture (left) and phase vs. change in the mass loss for axial and radial samples.

3 Field measurement The laboratory measurement system described above can be extended to a 24-channel system applicable for the field tomography measurements. Steel nails were used as electrodes and applied at an equidistant pitch around the perimeter of the trunk (Figure 1 right). To reduce the duration of the test, a frequency range of 0.1 to 10Hz was used. The measured data were inverted by the reconstruction program DC2dTree [4]. Starting with a homogenous seed model, the program uses an iterative approach to reconstruct the inner structure to match the resistivity data. In the current version it is possible to include the geometry of the trunk in the reconstruction program but not the anisotropy and the three-dimensionality of the tree. Ignoring the anisotropy can cause big measurement differences (compare section 2.1). But for


tomography measurements along a horizontal section of the trunk the current propagation is similar in radial and tangential direction. Although there are differences between these directions, they were negligible for the inversion. Along with the CRT measurements, the drill resistance test was also conducted. The equipment used IML-Resi E500 measures the mechanical strength (density) of the wood. In this method a very long needle is driven into the tree. The mechanical resistance is then measured by the power that is necessary to drive the needle into the tree. Driving the needle into strong wood takes more power (higher amplitude) than into infected, i.e. soft wood.

3.1 Seasonal Influence As expected, seasonal changes proved to have a great influence on the CRT results. Because of the dependency of the resistivity on water content (wood moisture) and temperature, the tomograms change considerably with the change of the season. The tomograms for a typically healthy oak measured in summer and winter are shown in Figure 5 (frequency f = 0.1Hz). In summer the oak has an outer ring of low resistivity (Figure 5a) and low phases (Figure 5b). That is the area of the sapwood where the nutriment and water transport (sap flow) take place. The compact and lignificated heartwood is characterized by a zone of high resistivity and high phases. In the middle of the tree there is again a good conducting zone characterized by low phases. That occurs because of the storage of the good-conducting phenolics (tanning agent) during lignification. In the winter time, while the tomograms maintain their ring-like structure, the resistivities (Figure 5c) are much higher than those measured in summer time. Furthermore, the sapwood ring can no longer be recognized because the tree has no sap flow in the cold season. The phase tomogram (Figure 5d) shows only a slight change because the phase is not as sensitive to the moisture as resistivity.

Figure 5: Tomograms of a healthy oak in summer and winter. The typical oak has a ring-like structure in resistivity and phase.


3.2 Fungi-infected Oak Figure 6 shows an example of a fungi-infected oak. As shown in Figure 6a there is a hole in the centre of the tree. Around the hole the wood is also infected and is already brittle. The hole produces a zone of very high resistivities (Figure 6b) and very low phases (Figure 6c). Around the hole the fungi are still progressing, so the resistivities as well as the phases are fairly low. The healthy parts of wood are characterized by moderate (~300 Ωm) resistivities and high phases. The drill resistance measurements used also indicate a huge weak zone in the centre of the tree. The results of drill resistance are generally in agreement with those of CRT. However, on the left border the drill resistance still indicates strong wood, but the electrical resistivity and phase yield that the border is already infected by the fungi. So the wood on this border might be already infected by the fungi but the decomposition is still going on.

Figure 6: Photo with the drill resistance results (a), tomograms of resistivity (b) and phase (c) of a fungi-infected tree.

4 Conclusions Our laboratory measurements have demonstrated that the anisotropy of wood has a significant influence on complex resistivity. Also, progressive fungi infection affects the resistivity characteristic, which can be used to differentiate between healthy and damaged wood. Resistivity tomography measurements were also conducted in the field. Because of the correlation between resistivity and wood moisture, the seasonal changes were indicated clearly. Measurement results on fungi-infected trees demonstrated the applicability of the method in non-destructive testing for tree investigation.

References 1. U.Weihs, V. Dubbel, F. Krummheuer and A. Just. Die elektrische Widerstandstomographie. Forst und Holz, volume 54, pages166 - 170, 1999. 2. A. Habermehl and H.-W. Ridder. Computer-Tomographie in der Forstwirtschaft und Baumpflege (part 1). DGZfP-Zeitung 55, pages 48 - 55, 1996. 3. S. Rust, S. Franz, M. Minke, I. Schumann, and A. Roloff. Schalltomographie zur Erkennung von Fäule und Höhlungen an stehenden Bäumen. Stadt + Grün, pages 50 - 52, June 2002. 4. Th. Günther. Impedanztomographie an Bäumen unter Berücksichtigung der Baumform. Manual zu DC2dTree, 2005.

IntroductionLaboratory MeasurementsAnisotropyLong-term-experiment on fungi-infected wood

Field measurementSeasonal InfluenceFungi-infected Oak

Conclusions

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Complex Resistivity (CR) of Wood and Standing Treestivi t y [Ohm*m] 0.001 0.01 0.1 1 10 100 frequency [Hz] 0 0.5 1 1.5 2 2.5-p h ase [ ] oak samples axial 1 axial 2 axial 3 radial