By Frank Rinn, Heidelberg, Germany
Stress wave tomography on trees with multiple sensors is a widely used method for tree quality inspection . Additional to tomography on trees, a root diagnostic device has been developed using the stress wave method. Since 2004 such measurements are applied and interesting results and experiences have been collected. This approach is increasingly being employed by arborists to detect roots near infrastructure, where trenching needs to occur, and for installation of cables near trees. This article outlines this technique and presents an application study.
A stress wave tomograph measures the time of flight of stress wave impulses between multiple sensors, which function as emitter and receiver . In the case of cavity or decay located in between the delaminated area stress wave impulses make deviations and hence are received with delay by the receiver sensors (Fig. 1). On basis of these delays the software calculates stress wave velocities of the cross section. This coloured tomogram reveals the mechanical connectivity of the wooden cross section .
Damages in urban trees often rise from the roots. This is the reason why stress wave tomography on trees is mostly applied just above ground height around the root collar. For root analysis an additional sensor on a metal pole is connected to the stress wave sensors around the tree circumference (Fig. 2). When hitting the pole with a hammer, mechanical impulses are induced into the ground on selected spots around the tree. If there is a sufficiently thick and intact wooden root in the ground below the measurement point this impulse can be transmitted to the tree where the other sensors receive it. By this means the operator changes his position around the tree with different distances to the tree. Consequently a line graph from the operator position to the tree is displayed showing on the one hand the position from where the impulse was sent to the tree and on the other hand the velocity of the stress waves.
The metal pole is mostly positioned towards the main roots of the root collar. The measured stress wave impulses are assigned to different colours (Fig.3).According to different ground conditions (lawn, concrete, pavement) stress wave velocities differed in previous measurements between 100 and 500 m/s. Onconcrete or pavement, velocities tend to be higher. Furthermore the detection range of one impact/hit is higher. In soft ground, roots have been detected up to a depth of 30 to 50 cm. When the soil is compacted, the depth of detection can be up to 1m.Ultimately, such measurements provide a line graph which shows the area around the tree where roots are found and which velocity of the impulse to the tree base is measured. In the case where there is no root detection roots are either absent or too small in diameter (fine roots).
On a forested slope property with several old oak trees a house construction was planned. The responsible administration demanded a proof that no thick anchoring roots are damaged by the construction. Therefore sensors have been positioned on the stem base of the oak trees. Impulse measurements from the trees towards the planned edge of the building pit have been conducted (Fig.4). A later executed excavation shaft confirmed the measurement results: in the relevant area only fine roots have been detected, which conformed with the condition of not damaging thick anchoring roots(Fig.5).
The main application area of this method described in this paper is the proof of where roots are extending – often in combination with planned or past building measures. Up to now it is not possible to derive tree standing stability calculations out of the results but in combination with the local conditions, a rough estimation of the anchoring root area can be achieved.
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Arboradix® is a root detection device developed by Frank Rinn, founder or the company Rinntech in Heidelberg, Germany tobe used in combination with the Arbotom®stress wave tomograph. Rinntech is supported in Australia by ICT International.