Measurement setup and execution
At the height of the trunk level to be investigated, the sensors of the measuring instruments are evenly distributed around the tree trunk. They are mounted on thin measuring pins that must be in contact with the outer wood body.
The linear distance of the measuring pins from each other is recorded ideally using electronic calipers, which enables the estimation of a simplified image of the trunk geometry. The exact measurement of the distances between each of the measuring positions is an essential prerequisite for the results of the measurement to be reliable (Rust 2017). In sonic tomography, an electronic hammer is used to trigger a sound pulse successively at each measurement point. The speed of travel of the sound within the measurement plane is recorded with high precision by all sensors.
In electrical resistance tomography, four electrodes are involved in each individual measurement, one “current-sensing” and one “voltage-sensing” pair of electrodes (Weihs et al. 2007).
Based on the measured distance between the measuring points and the measured transit times, the apparent sound velocity and the apparent resistivity can be determined. The connected measuring station evaluates, stores and presents a visualisation of the data collected.