DETECTION OF DECAY AND HOLLOWS IN LIVING TREES USING RESISTIVITY METHOD WITH MODIFIED SCHLUMBERGER ELECTRODE CONFIGURATION

Abstract

Decay and hollows in stems of Living Trees (LT) are responsible for some tree failures which may lead to loss of lives and damage to property, especially during stormy weather. This possible disaster and attendant economic loss could be prevented if such defects are detected early for timely intervention in cutting down the affected trees. Several attempts have been made to detect the Location, Extent of Decay and Hollows (LEDH) in LT using resistivity method with minimal success owing to the electrode configuration adopted. Therefore, this study was undertaken to detect LEDH in LT. Eighty LT comprising forty candle trees (Senna alata L. Roxb.) and forty almond trees (Terminalia catappa L. Roxb.) were purposively selected within the University of Ibadan campus. The Resistivity Profiles (RP) were obtained from resistivity measurements for the selected LT, freshly-cut healthy, decayed and hollowed tree stems. The resistivity method implemented involved the use of an earth resistivity meter and a modified form of Schlumberger electrode configuration, which employed tiny electrodes with the spacing scaled down to centimetre range. A laboratory experiment was set up using three wood fabricated hollow cylinders filled with compacted sawdust to mimic stems of LT. The correlation between the RP of healthy LT and that of healthy tree replica was determined. The RP of healthy, decayed and hollowed trees were replicated in the Laboratory Prototypes (LP). Wood decay was modelled by inserting copper wire lumps into the LP at depths 5.00, 10.00, 15.00 and 20.00 cm from the centre of the modelled decay to the LP surface. Hollows were replicated in the LP using a plastic cylinder, at depths 4.00, 12.00 and 20.00 cm from the centres of the modelled hollows to the LP surface. The replicated RP were compared to those of selected LT to detect LEDH in LT. Data were analysed using descriptive statistics and ANOVA. The Mean Resistivity Values (MRV) of decayed, healthy and hollowed trees were. 13.52±1.11, 62.59±8.61, 7388.17±1564.58 Ωm for candle trees; and 14.23±1.78, 171.24±33.43, 12430.70±1410.79 Ωm for almond trees. The sharp decrease in MRV of decayed trees may be due to mobile cations in the decayed region. The rapid increase in MRV of hollowed trees could be attributed to the non-conductivity of electric current by the hollows. The RP of healthy LT correlated strongly with that of healthy tree replica (r 2 mean=0.956 for candle trees and r 2 mean=0.998 for almond trees). Hence, the LP was a true replica of healthy LT. The resistivity values of the LP ranged between 45 and 80 Ωm. Wood decay replicated in the LP were detected as resistivity anomalies of 11–17 Ωm representing a decrease by a factor of four compared with iii the healthy tree replica. The embedded hollows were detected as resistivity anomalies of 155– 271 Ωm representing an increase by a factor of three compared with the healthy tree replica. The location and extent of the resistivity anomalies corresponded to LEDH in LT of similar dimensions. The resistivity method with modified Schlumberger electrode configuration detected the location, extent of decay and hollows in living trees. The method would assist in non-invasive urban tree management