Decay Detection Techniques   Tree Sway Monitoring

The Picus sonic tomography unit uses the relative velocity of sound waves induced across the stem to compose a colour-shift image. There is a correlation between the velocity of the sonic energy and the density of the wood; higher velocities are shown as brown shades on the tomogram, while slower velocities result in a shift through green to maroon and blue/white shades.

Examples of a tree with no decay (on the left) and one with extensive decay and hollowing are shown below:

Additional information can also be gained through the use of a complementary instrument, the Picus Treetronic electrical resistance tomograph. This uses analysis of a low-voltage electrical field induced into the stem to compose a tomogram based on relative electrical resistance. When used in conjunction with the sonic tomograph the Treetronic can provide additional information about decay just below the level of assessment and the presence of benign features inside the stem (such as trapped bark and hidden stem unions) that can lead to erroneous sonic results.

Sonic tomogram (on the left) and Electrical resistance tomogram measured on the same tree:

The sonic image shows no decay at the level of inspection; however the electrical resistance image indicates decay around the edge of the stem just below ground level (see blue arrows).

Information about the depth of residual sound wood can be established using the IML Resistograph. Depending on the model used, the instrument records the drilling resistance of a fine drill bit to a depth of 30-40cm. On the example below decay is indicated at a depth of 20cm by a very distinct drop in drilling resistance:

Tree Sway Monitoring   Decay Detection Techniques
A very new method of monitoring tree stability in windy conditions has recently become available in the UK and again Harraway Trees is the first company to offer the service in this country.

IML Tree Motion Sensors (TMS) are used in pairs, one at the base to measure tilt and another higher on the stem to act as a control, and left on the stem during periods of moderate to high wind for up to two weeks to record the movement of the stem in ambient conditions. Recent research in Australia using the system has provided guidelines on levels of acceptable movement.

The results can be displayed as a graph of movement against time and as a plan view of the spatial movement of both sensors:

Tree surveys to BS5837: Trees in relation to construction  
We supply tree surveys and associated documents to accompany both large and small planning applications. For further details of this and all our other services please contact John Harraway who will be pleased to discuss your specific requirements.
Email: John Harraway T: 01903 756153 / 07831 651090