A four-point electrical resistivity method for 
detecting wood decay and hollows in living 
trees
Ayodele O. Soge, Olatunde I. Popoola & 
Adedeji A. Adetoyinbo
European Journal of Wood and Wood 
Products
Holz als Roh- und Werkstoff
ISSN 0018-3768 
Volume 77 
Number 3
Eur. J. Wood Prod. (2019) 77:465-174 
DO I 10.1007/S00107-019-01402-'•
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A four-point electrical resistivity method for detecting wood decay 
and hollows in living trees
Ayodele O. Soge1,2 • Olatunde I. Popoola* 1 • Adedeji A. Adetoyinbo1
Received: 12 September 2018/Published online: 28 March 2019 
© Springer-Verlag GmbH Germany, part of Springer Nature 2019
Abstract
An accurate method of detecting wood decay and hollows or cavities in living trees is useful for risk assessment and main­
tenance of both forest and urban trees. This study presents the implementation of the four-point electrical resistivity method 
for the early detection of the presence, location and extent of wood decay and hollows in living acacia trees (Senna cilata 
L.). Electrical resistivity measurement of randomly selected living acacia trees and a freshly-cut acacia tree with decay and 
hollows were taken to obtain electrical resistivity profiles for sound, decayed and hollowed trees. A laboratory experiment 
was set up to replicate the resistivity pi ofiles. Wood decay and hollows were replicated at different depths in the laboratory 
prototype using good electrical conduc tors and insulators respectively. Resistivity profiles for the sound, decayed and hol­
lowed trees were obtained from the experimental and field results. The resistivity profiles were applied to detect decay and 
hollows of similar dimensions in living trees through resistivity curve matching. The electrical resistivity of the decayed 
acacia tree was markedly lowered by an average factor of 5 compared to that of the sound acacia tree. Likewise, the electri­
cal resistivity of the hollowed acacia tree was noticeably greater than that of the sound acacia tree by an average factor of 4. 
Wood decay and hollows modelled into the laboratory prototype were detected with relatively lower and higher resistivity 
anomalies respectively. The method indicated that 80% of the randomly selected living trees were sound, healthy trees, whilst 
20% had decay and hollow at the time of measurements. This method is suitable for early detection of decay and hollows 
in hardwood trees.
1 Introduction same market value as sound wood— wood decay causes 
a decrease in wood density (Beall and Wilcox 1987; G< nez 
Wood decay is the process by which wood is decomposed et al. 2018). Additionally, the ecological value of a lining 
by micro-organisms to provide nutrients for their survival tree in conserving the environment from the threat of climate 
(Harris et al. 2004). It has been repor:ed that wood decay change is significantly reduced by wood decay. Therefore, an 
within a tree trunk is a major cause of tree failure, which accurate method of detecting decay in trees would be useful 
frequently leads to loss of human lives and colossal damage for risk assessment and maintenance of living trees both in 
to property in urban areas (Johnstone e t al. 2010). Besides, the forest and urban settlement.
the economic value of a living tree is ieduced significantly Electrical resistivity methods were originally designed 
by wood decay since decayed wood would not attract the and are commonly used for ground survey (Herman 2001; 
Reynolds 2011). However, their viability in detecting wood 
decay and hollows in living trees has been reporter by 
Electronic supplementary material The online version of this researchers in various publications (Ostrofsky and Shcrtle 
article (https://doi.org/10.1007/s00107-019-0 402-1) contains 1989; Shortle 1990; Manyazawale and Ostrofsky 1992; 
supplementary material, which is available to authorized users. Butin 1995; Rinn 1999; Martin and Gunther 2013). The 
E  Ayodele O. Soge main factor determining the electrical resistance of a tree is 
sogea@run.edu.ng; aos2@alumni.leiceste ac.uk the concentration of mobile cations, which is usually very 
different between sound and degraded wood (Johnstone et al. 
1 Department of Physics, University of Iba* ian, Ibadan, 2010). It has been observed that in the region adjacent to 
Oyo State, Nigeria wood decay, the concentration of cations in the wood would
2 Department of Physical Sciences, Redeemer’s University, 
PMB 230, Ede, Osun Stale, Nigeria
*£) Sppn iger
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4(36 European Journal of Wood and Wood Products (2019) 77:465-474
increase and therefore electrical resistance would decrease hardwood trees are amongst the most common urban trees in 
(Shigo 1991; Goh et al. 2018). the South-West region of Nigeria where this work was car­
According to Larsson et al. (2004), the invasion of the ried out. Moreover, these trees constitute a serious threat to 
stem by wood decay fungi causes move.ment of water to the human lives and property during strong winds that normally 
area of mycelia growth, thus mobilising metal ions released accompany heavy rainfall in the tropical zone.
by damaged tree cell. This results in a decrease in the resis­
tivity of the decayed tree compared with the sound tree 
(Shortle and Smith 1987; Goncz et al. 2018). A decrease in 2 M aterials and m ethod
electrical resistance for bacterial wet wc od was also reported 
by Nicolotti and Miglietta (1998). However, there are fungi 2.1 Field measurements
in trees that lower electrical resistivity of the trees signifi­
cantly without causing wood decay as-reported by Kucera Electrical resistivity values of living trees were estimated 
(1985). using the four-point electrical resistivity method. Forty aca­
The electrical resistivity methods are often called imped­ cia trees (S. alata L.) of diameter ranging from 45 to 110 cm 
ance methods. Electrical impedance tomography (EIT) were randomly sampled from various locations on Univer­
determines the electrical conductivity within wood tissue; sity of Ibadan campus, Ibadan, Nigeria. The random sam­
as moisture builds up in wood from fungal invasion, con­ pling of the trees was carried out between 1st May and 31st 
ductivity increases, thereby decreasing electrical impedance luly 2014. Vertical variation of resistivity with depth, also 
(Brazee et al. 2010). EIT gives an image of the resistivity known as vertical electrical sounding (VES), was carried out 
of the wood and can identify incipient decay, which would on the trees using the Schlumberger electrode configuration 
otherwise go undetected by sonic tomography (Brazee et al. with a reduced scale in centimetres (Reynolds 2011).
2010). However, the large number of sensors and sophis­ The implementation of the four-point electrical resistiv­
ticated electrical circuitry required, ar.d time-consuming ity method involved the use of four electrodes, which were 
nature of the measurements make EIT expensive, tedious arranged across the length of the tree stem (Appendix 1 in 
and unattractive for routine monitoring of decay (Goncz Electronic Supplementary Material). A direct current was 
et al. 2018). applied by two electrodes (C, C2) separated by distance 
Goncz et al. (2018) demonstrated that the electrical resist­ AB, and the potential difference between two points was 
ance method using four electrodes was highly reliable in measured by two electrodes (P|5 P2) separated by disttnce 
detecting red heart in beech trees (Fagus sylvatica) of diam­ MN, as shown in Fig. 1. Potential electrode separations of 
eter 40-60 cm. Red heart— a visual defect in beech wood— 4 cm and 6 cm were used while varying the current elec­
was detected with a measured voltage drop of 1/3 to 1/5 of trodes positions by 2 cm to increase the depth of current 
that measured on unaffected wood. However, the method penetration. Generally, the current electrode separation AB 
was unable to reliably determine the extent of red heart in is proportional to the depth of current penetration (Her­
beech trees. man 2001). Therefore, by increasing the current electiode
Larsson et al. (2004) implemented a four-point electrical 
resistivity (RISE) method to detect wood decay in living 
trees. Four-point measurements were made by passing a low- 
fi equency alternating current to the tree stem with one pair 
of electrodes, whilst measuring the vohage difference with 
another pair of electrodes. The effective resistivity of wood 
depends on water content and temperature making it difficult 
to use the resistivity of an individual tree for the detection 
of decay (Larsson et al. 2004). Instead, the resistivity of an 
individual tree was compared with that of other trees meas­
ured under similar conditions, i.e., temperature, humidity, 
site conditions and time of year. The major drawback of the 
four-point electrical resistivity method is that it does not 
provide information on the volume or location of the decay 
(Larsson et al. 2004).
The aim of this study was to detect toe presence, location Fig. 1 Generalised form of electrode configuration in resistivity sur­
and extent of the resistivity anomalies due to wood decay veys. C, and C2 are the current electrodes whilst P, and P, arc the 
potential electrodes. MN, AM, MB. AN NB, and AB are the separa­
and hollows in living acacia tree stems (Senna alata L.) tions of the electrodes. AB is the current electrode separation whilst 
using the four-point electrical resistivLy technique. These MN is the potential electrode separation
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I  . m j i f i u f  h p e i py
European Journal of Wood and Wood Products (2019) 77:465-474 467
separation, more of the injected current will flow into greater 
depths. The sort of electrode separations (Appendices 2 and 
3 in Electronic Supplementary Material) used in this study 
requires the use of tiny electrodes to p'event a short circuit 
which may occur if conventional electrodes of typical thick­
ness 14 mm, designed for soil resistivity measurement, were 
used. Tiny electrodes of thickness 3.32 mm were locally 
fabricated for this research work to accommodate the small 
electrode separations. The electrode penetration depth in the 
tree was limited to 5 cm to prevent injury to the tree stem 
and fungi attack. Electrical resistance values were measured 
at different points on the trees using Miller 400D digital 
resistance meter— commonly known as earth resistivity 
meter for shallow ground survey. The geometric factor K of 
the electrode configuration was estimated simultaneously 
using Eq. (1).
J _ A - ( - L - 1 Y
A AM M B) \A N N B j .
where AM, MB, AN and NB are the separations of the 
electrodes as shown in Fig. 1 (Reynolds 2011). Further, the 
apparent resistivity values pa were con puted as
P, = RaK (2)
where Ra is the measured resistance value of the tree (Reyn­ Fig. 2 Freshly-cut logs of decayed and hollowed acacia wood
olds 2011).
Electrical resistivity values of a log of acacia tree of stem 
diameter 40.5 cm and comprising decayed, hollowed and the trees as described under subsection 2.1. The resistivity 
sound segments were also obtained. The tree was cut down plot of the sound tree w'as compared to that of the labora­
after showing visual evidence of internal decay (Fig. 2). tory prototype for similarity in profile. Once a similarity in 
The resistivity measurement was taken on the log of wood, profile is established, the resistivity profile of the laboratory 
immediately after the tree was cut down, as it was done on prototype serves as a replica for the sound tree.
living trees. Furthermore, the extent of wood decay or hollow was 
modelled into the laboratory prototype that serves as a 
2.2 Laboratory measurements replica for the sound tree. This was carried out by using 
good electrical conductors, for example, copper wire lumps 
A. laboratory experiment was set up based on the field meas- (Fig. 3a) because wood decay process is normally accom­
mements to further investigate the e lec ts of wood decay panied by an increase in electrical conductivity due to 
and hollows on the electrical resistivity profiles of sound increasing concentration of mobile cations in the decayed 
acacia trees. This was carried out by modelling wood decay region (Nicolotti et al. 2003; Goh et al. 2018). A copper 
and hollows at different depths in a 1; moratory prototype, wire lump of thickness 4 cm and length 20 cm was placed 
which serves as a sound tree replica. The electrical resistiv­ at 5-cm, 10-cm, 15-cm, and 20-cm depths, from the centre 
ity profiles of the laboratory prototype were applied to detect of the modelled decay to the laboratory prototype surface. 
wood decay and hollows in living trees through resistivity Similarly, electrical insulators, for example, plastic cylin­
curve matching. The laboratory prototype was fabricated as ders, were used to create hollows in the laboratory proto ype 
a wooden hollow cylinder of height 60 cm and diameter (Fig. 3b) since hollows or cavities in a tree do not conduct 
50 cm and filled with compacted sawdust or wood dust, electricity (Larsson et al. 2004). A hollow of diameter 6 cm 
obtained from wood without decay cr defects, to mimic and height 21 cm was modelled into the laboratory proto ype 
stems of living trees. The stem perimeter of thickness 10 cm at depths 4 cm, 12 cm, and 20 cm, from the centre of the 
was modelled with compacted wet sawdust, whilst the stem modelled hollow to the laboratory prototype surface.
centre of thickness 15 cm was replicated with compacted Resistivity anomalies were introduced into the labora­
dry sawdust. Vertical electrical sounding (VES) was carried tory prototype at different depths, and VES was carried out 
out on the laboratory prototype the same way it was done on to detect both the presence and location of the anoma ies.
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468 European Journal of Wood and Wood Products (2019) 77:465-474
A laboratory experiment was also carried out to deter­
mine the effects of tree decay and hollow size on the resistiv­
ity profile. Tree decay of various size was modelled into the 
laboratory prototype replicating a healthy acacia tree, using 
copper wire lumps of different thicknesses— 5 cm, 8 cm and 
12 cm, inserted at depths of 5 cm, 6.5 cm, and 8.5 cm from 
the centre of the wire lumps to the laboratory prototype sur­
face, respectively. Hollows of different diameters— 8 cm, 
10 cm and 14 cm, were also modelled into the laboratory 
prototype at 4-cm, 5-cm, and 7-cm depths from the centre of 
the hollows to the laboratory surface, respectively. VES was 
carried out to detect both the presence and location of the 
resistivity anomalies created by the modelled wood decay 
and hollows.
3 Results
3.1 Resistivity values of sound acacia trees 
and their replica
Table l shows the resistivity values of four sound acacia 
trees of similar diameter sited at the same location. The 
F ig .3 A laboratory prototype with a a copper wire lump inserted to resistivity values of the laboratory prototype are included 
model wood decay, b a plastic cylinder inserted to model an hollow for comparison. The resistivity values of the sound acacia 
trees show a major trend— a series of low resistivity values 
followed by a series of high resistivity values. The low resis­
The effect of the anomalies on the resi rtivity profiles of the tivity values represent the stem perimeter of the tree whilst 
prototype was determined by comparing the resistivity plots the high resistivity values correspond to the stem centre of 
obtained before and after anomalies "ere  introduced. The the tree. The diameter of trees l and 2 is 54.5 cm whilst that 
resistivity profiles of the laboratory prototype with anoma­ of trees 3 and 4 is 50.4 cm. Figure 4 shows that the resis­
lies serve as the replica for those of liv ing trees with decay tivity profiles of the acacia trees and that of the laboratory 
and hollows. prototype are identical. The resistivity values of all the forty
Table 1 Resistivity values of AB/2“ (cm) MNh (cm) Resistivity (Qm)
four acacia trees with s i m i l a r ____________________________________________________________________
diameter Tree 1 Tree 2 Tree 3 Tree 4 Mean value Laboratory prototype
4 4 46.276 61.167 50.611 60.790 54.711 45.427
6 4 40.464 70.120 61.575 77.409 62.392 61.827
8 4 46.464 70.686 56.549 73.513 61.803 63.146
10 4 44.636 68.009 52.176 70.874 58.924 79.922
12 4 1847.256 2704.911 1880.243 2759.889 2298.075 1140.513
14 4 1854.796 2774.655 1915.115 2789.734 2333.575 1508.170
16 6 2988.597 4077.159 3265.686 3443.814 3443.814 2100.638
18 6 3116.460 4046.371 3116.460 4448.495 3681.947 3040.484
20 6 3265.686 4167.637 3029.312 4665.265 3781.975 3296.128
22 6 3502.247 4109.203 3113.947 4787.787 3878.296 3473.620
24 6 3683.832 4191.482 3153.719 4986.650 4003.921 3890.250
26 6 3873.961 4338.414 3277.561 5256.764 4186.675 4061.485
“Current electrode half separation 
bPotenli'il electrode separation
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European Journal of Wood and Wood Products (J 019) 77:465-474 469
Table 2 Resistivity values of a decayed-hollowed acacia tree
AB/2a MNb Resistivity (Dm)
(cm) (cm)
Sound tree segment Decayed-hollowed
segment
Side A Side B Side A Side B
4 4 64.748 50.611 12.912 11.875
6 4 77.409 61.575 13.105 12.273
8 4 67.858 56.549 14.219 13.851
10 4 69.805 52.176 15.302 14.625
12 4 2144.137 1880.243 5621.488 5731.472
14 6 1990.513 1915.115 6032.421 680G.638
16 6 2968.805 3265.686 7889.668 5623.105
Fig. 4 Resistivity plots of acacia trees of similar diameter and their 18 6 2940.531 3116.460 8606.318 6609.824
replica 20 6 3041.753 3029.312 9591.880 7264.529
22 6 2997.079 3113.947 10,572.536 8314.141
“Current electrode half separation 
Potential electrode separation
10000
1000
Tree Resistivity Mean Value (Om)
100 ;
Fig. 5 Resistivity of laboratory prototype ve.sus mean resistivity of 
acacia trees
10
0 5 10 15 20 25
Current Electrode Half Separation, AB/2 (cm)
randomly selected acacia trees are presented in Appendix 4 
in Electronic Supplementary Material. Fig. 6 Resistivity plots of a decayed-hollowed and sound segments of 
Furthermore, the resistivity profiles of the sound acacia an acacia tree
trees correlated strongly with that of tne laboratory proto­
type replicating the sound trees (r2 = 0.9298), as shown in 
Fig. 5. Therefore, the resistivity profile of the laboratory Fig. 6 as a sharp decrease in resistivity values in the region 
prototype was a true replica of the sound acacia trees of of the resistivity plots corresponding to the stem perim­
similar diameter. eter. The resistivity values of the decayed stem-perimeter 
varied between 12 and 15 Qm, whilst those of the sound 
3.2 Resistivity values of a decayec-hollowed acacia stem-perimeter ranged between 51 and 77 Qm. The length 
tree and diameter of the decayed section were 20 cm and fi cm 
respectively. The sound tree segment is of stem diameter 
Electrical resistivity values of a decayed-hollowed acacia 40.5 cm and length 50 cm. Table 2 also shows the presence 
tree of stem diameter 40.5 cm are presented in Table 2. The of a cavity or hollow in the stem centre of the acacia tree as 
acacia tree comprised three sections—decayed, hollowed represented by a series of higher resistivity values compared 
and sound segments. Table 2 shows tie  presence of wood to those of the sound acacia tree. This is equally shown in 
decay in the stem perimeter of the acacia tree as represented Fig. 6 as a sharp increase in resistivity values in the stem 
by a series of lower resistivity valuer compared to those centre section of the resistivity plots. The resistivity values 
of the sound acacia tree. This trend is clearly displayed in of the hollowed stem-centre varied between 5622 and 10,573
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470 European Journal of Wood and Wood Products (2019) 77:465-474
O.m, whilst those of the sound stem-ce itre ranged between values of the laboratory prototype with a hollow created at 
1880 and 3114 £2m. The length and diameter of the hollowed different depths in the laboratory prototype.
segment were 28 cm and 14 cm, respectively. The copper wire lump inserted at different depths in the 
laboratory prototype to replicate wood decay in a tree stem 
3.3 Laboratory results was responsible for the comparatively low resistivity val­
ues recorded at the various current electrode half separa­
Table 3 comprises the electrical resistivity values of the tions, AB/2, where the resistivity anomaly was detected, 
laboratory prototype with a copper wire lump inserted at as shown in Table 4. For comparison, the resistivity val­
different depths to model tree decay, and which introduced ues of the laboratory prototype representing a sound tree 
anomaly into the resistivity profile of the prototype. The (i.e. without anomaly) are also included. Figure 7 shows 
resistivity values of the laboratory prototype representing a the resistivity plots of the laboratory prototype with an 
sound tree (i.e., without anomaly) are also included for com­ anomaly (i.e. modelled tree decay) and without anomaly. 
parison. Similarly, Table 4 contains the electrical resistivity The resistivity anomaly produced by the modelled tree decay
Table 3 Resistivity values AB/2“ (cm) MNh (cm) Resistivity (fim)
of laboratory prototype with 
a copper wire lump inserted 5-cm depth 10-cm depth 15-cm depth 20-cm depth No anomaly
at various depths with the 
centre of the wire lump as the 4 4 11.214C 39.155 41.025 42.852 45.427
reference point 6 4 15.253c 57.111 56.294 57.209 61.82''
8 4 17.388° 59.896 60.583 61.172 63.146
10 4 65.273 21.468° 76.192 77.105 79.922
12 4 910.275 296.186c 1012.621 1020.726 1140.513
14 4 1206.896 385.943° 1315.022 1321.165 1508.170
16 6 1853.310 1901.681 521.267° 1840.198 2100.638
18 6 2591.041 2652.175 752.139° 2605.429 3040.484
20 6 2907.586 2835.022 831.271° 2801.226 3296.128
22 6 3168.315 3214.334 3120.182 782.943° 3473.620
24 6 3497.220 3571.939 3563.142 886.752° 3890.250
26 6 3834.090 3953.136 3845.396 1010.031° 4061.481
“Current electrode half separation 
bPotentiul electrode separation 
cDetection point of the copper wire lump
Table 4 Resistivity values of AB/2“ (cm) MNb (cm) Resistivity (£2m)
laboratory prototype with a 
modelled hollow at various 4-cm depth 12-cm depth 20-cm depth No anomaly
depths with the centre of the 
hollow as the reference point 4 4 155.139° 57.222 59.382 45.427
6 4 206.358° 73.901 76.701 61.821
8 4 215.219° 79.529 84.526 63.146
10 4 271.382° 93.018 98.711 79.922
12 4 1250.336 3546.251° 1251.728 1140.513
14 4 1629.765 4832.318° 1641.302 1508.17.0
16 6 2251.221 6935.102° 2280.119 2100.638
18 6 3152.032 10042.641° 3246.058 3040.484
20 6 3401.714 3412.927 10388.652° 3296.128
22 6 3587.352 3591.536 11215.716° 3473.620
24 6 4002.159 4002.172 12502.481° 3890.250
26 6 4179.172 4185.263 12995.004° 4061.485
“current electrode half separation 
bPotenti;d electrode separation 
cDeteetion point of the modelled hollow
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European Journal of Wood and Wood Products (2019) 77:465-474 471
10000 of decrease or increase in resistivity values depends or: the 
thickness of the copper wire lump or the diameter oi the 
hollow respectively. Table 5 and Fig. 9 show that the largest 
decrease in resistivity values of an average factor 6.4 was 
observed for 12-cm thick copper wire lump placed at 8.5- 
cm depth, with the centre of the wire as the reference point; 
followed by 8-cm thick copper wire lump, placed at 6.5-cm 
depth, with an average factor of 5.1; and the lowest decrease 
was observed for 5-cm thick copper wire lump, placed at 
5-cm depth, with an average factor of 3.8. Likewise for the 
hollow, Table 6 and Fig. 10 show that the largest increase in 
resistivity values of an average factor 5.6 was observed for 
14-crn hollow diameter placed at 7-cm depth, with the centre 
of the hollow as the reference point; followed by 10-cm hol­
low diameter with an average factor of 4.0, placed at 5-cm 
depth; and the lowest increase was observed for 8-cm ho low 
Fig. 7 Resistivity plots of the laboratory prototype with a copper wire 
lump at 5-cm, 10-cm, 15-cm, and 20-cm depths from the centre of the diameter with an average factor of 3.1, placed at 4-cm depth. 
modelled decay to the surface of the laboratory prototype The experimental results also showed that as the conducting 
medium and hollow increase in size, the detection poin's of 
the anomalies also widen.
100000
3.4 Discussion
-£• 10000 
a In this study, a steep rise in electrical resistivity values from 
the stem perimeter to the stem centre was observed as sh own 
in Fig. 4. This trend was equally reported by Bieker and 
4-cm depth Rust (2010), who estimated sapwood and heartwood width 
->■#— 12-cm depth in Scots pine (Pinus sylvestris L.) trees in lower Saxony, 
—♦ “ •-20-cm depth 
—♦— No anomaly Germany, using electrical resistivity tomography. All tomo­
grams displayed a distinct pattern of low resistivity ai the 
stem perimeter and high resistivity in the stem centre with 
10 a steep increase in resistivity in between (Bieker and Rust 
o 5 10 15 20 25 30 2010). In addition, a similar pattern was observed by Man- 
Current Electrode Half Separation, AB/2 (cm) yazawale and Ostrofsky (1992), claiming that sound sap- 
wood usually measures lower internal electrical resistance 
Fig. 8 Resistivity plots of the laboratory prototype with a modelled 
hollow at 4-cm, 12-cm, and 20-cm depths from the centre of the mod­ (IER) than heartwood. A marked pattern of lower resistivity 
elled hollow to the laboratory prototype surface values in the outermost stemwood, and high resistivity val­
ues in the inner stemwood of conifers was reported by Guyot 
et al. (2013). Thus, the steep rise in electrical resistivity val­
was detected as a mismatch in the resit; ivity profiles caused ues from the stem perimeter to the stem centre implied that 
by a sharp decrease in resistivity values at the location of the acacia trees presented in Fig. 4 were healthy sound trees.
the copper wire (Fig. 7). Moreover, the corresponding cur­ The in-situ measurement of the resistivity values of 
rent electrode half separations, AB/2, constitute detection freshly-cut acacia trees with decay and hollow showed that 
points of the anomaly (or modelled tree decay). Additionally, decay causes a significant decrease in the resistivity values 
the hollow modelled in the laboratory prototype at various of a sound tree whilst hollow results in a large increase ir the 
depths resulted in very high resistivity values recorded at the resistivity values. This result correlates with the reports by 
detection points of the anomaly as presented in Table 4 and some researchers who claimed that lowered electrical resist­
graphically in Fig. 8. ance and resistivity are strong indications of early decay 
The results of the experiment carried out on the effect process prior to its visualization (Smith and Shortle 1988). 
of the size of decay and hollow on the resistivity profiles Additionally, Larsson et al. (2004) reported that cavities or 
are presented in Tables 5 and 6, respectively, and graphi­ hollows increase stem resistivity because they do not con­
cally in Figs. 9 and 10. The results showed that the extent duct electricity.
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472 European Journal ofWood and Wood Products (2019) 77:465-474
Table 5 Resistivity values AB/2" (cm) MNb (cm) Resistivity (£2m)
of the laboratory prototype 
with copper ware lumps of Thickness: 5 cm 8 cm 12 cm Without anomaly
length 15 cm and of different 
thicknesses 4 4 23.192“ 16.018“ 12.521“ 81.336
6 4 31.837“ 25.259“ 20.398“ 128.032
8 4 34.011“ 26.713“ 21.526“ 135.201
10 4 130.528 28.164“ 22.073“ 142.439
12 4 142.916 139.825 23.641“ 153.150
14 4 155.281 150.692 141.058 162.320
16 6 241.389 238.213 230.629 249.380
18 6 5530.211 5500.964 5452.122 5825.692
20 6 5890.813 5872.158 5831.806 6157.670
22 6 6136.079 6119.172 6080.514 6425.308
24 6 6293.512 6280.361 6235.119 6577.804
26 6 6520.758 6505.019 6461.682 6816.513
“Current electrode half separation
bPotential electrode separation
“Detection point of the copper wire lump
Table 6 Resistivity values of AB/2“ (cm) MNb (cm) Resistivity (£2m)
the laboratory prototype with 
hollows of length 21 cm and of Diameter: 8 cm 10 cm 14 cm Without anomaly
different diameters
4 4 250.217“ 327.913“ 458.174“ 81.336
6 4 392.944“ 516.286“ 719.634“ 128.032
8 4 413.528“ 545.925“ 753.211“ 135.201
10 4 432.016“ 578.782“ 797.735“ 142.439
12 4 195.592 618.011“ 855.159“ 153.150
14 4 201.127 226.825 914.052“ 162.320
16 6 276.152 299.672 335.821 249.380
18 6 5938.371 5976.521 6080.913 5825.692
20 6 6264.892 6315.243 6420.285 6157.670
22 6 6531.533 6598.726 6697.019 6425.308
24 6 6698.219 6748.119 6851.725 6577.804
26 6 6929.047 6985.038 7095.814 6816.513
“Current electrode half separation 
bPotenti;.i electrode separation 
“Detection point of the modelled hollow
The electrical resistivity values of the decayed acacia tree determining the location of tree decay and hollows with 
were considerably decreased by an average factor of 5 in similar resistivity anomalies given the detection points of 
comparison to those of the sound acacia tree. Likewise, the the anomalies. Apart from showing the locations of the 
electrical resistivity values of the hollowed acacia tree were resistivity anomalies created by the copper wire lump and 
noticeably greater than those of the sot nd acacia tree by an hollows, the laboratory results may also provide informa­
average factor of 4. This result is similt r to that reported by tion on the extent of the resistivity anomalies—an equiva­
Larsson et al. (2004), who indicated hat Norway spruce lent of the extent of decay and hollows in living trees. The 
trees with decay had a resistivity lowered by a factor of 2 detection points of the resistivity anomalies expressed in 
compared to sound trees. terms of the current electrode half separation AB/2 cor­
The depths of placement of the modelled tree decay respond to the diameters of the copper wire lump (i.e., the 
and hollow in the laboratory prototype can be matched wood decay replica) and hollows since AB/2 is propor­
with the detection points AB/2. This .vould be useful for tional to the depth of current penetration (Herman 2001).
4L Springer
UNIVERSITY OF IBADAN LIBRARY
European Journal of Wood and Wood Products (2019) 77:465-474 473
in the laboratory prototype. There was a strong agree­
ment between the field results and the laboratory results. 
Hence, the resistivity profiles of the laboratory prototype 
with modelled wood decay and hollows serve as a sort of 
benchmark for the early detection of the presence, location 
and extent of decay and hollows of similar dimension in 
living acacia trees. Wood decay and hollows in living trees 
could be detected by matching the resistivity profiles of the 
trees under investigation with those of the laboratory pro­
totype with modelled decay and hollows. However, there 
are fungi activities in wood causing a notable reduction 
in electrical resistivity values of trees without resulting in 
decay. This constitutes a limitation to the use of electrical 
resistivity values exclusively for detecting wood decay in 
Fig. 9 Resistivity plots of the laboratory proiotype with copper wire trees. In addition, seasonal changes and all other factors 
lumps (replicating wood decay) of thicknesses 5 cm, 8 cm and 12 cm influencing the moisture content of the living tree have 
inserted at 5-cm, 6.5-cm, and 8.5-cm depths respectively from the 
centre of the wire lump to the laboratory prototype surface an impact on the results of the resistivity measurements 
and thus, should be considered especially when evaluating 
absolute values and not only relative changes.
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