Electrical resistivity imaging of tree trunks

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Abstract
An electrical resistivity technique using a ring array of needle electrodes is applied to image the internal electrical structures of the trunks of living standing trees and trunk disks. Measured electrical resistance data are inverted using a 2D iterative finite‐element algorithm which incorporates the cylindrical geometry of the trunk. The technique is successfully tested using synthetic models showing that the resolution obtained when mapping anomalous zones inside the trunk is higher for the dipole‐dipole configuration than for the Wenner configuration. Measurements on a trunk column show a well‐established inverse relationship between the imaged resistivity and the moisture content determined from wood cores.Healthy tree trunks always show a concentric ring structure with a central maximum of resistivity, which decreases towards the outside, corresponding to an increase in moisture from the inner dry heartwood to the outer wet ring of sapwood. Asymmetry in the ring structure is interpreted as the influence of branching or the direction of sunshine and wind. Strong local resistivity anomalies are related to infections causing wet, rotting or dry cavities. The resistivity of the heartwood is a maximum in olive and oak trunks, intermediate in young fruit trees and a minimum in cork oak trunks that are considered to be wet. The resistivity imaging technique can also monitor the dynamic process of sapflow if adequate tracers are used.