Sonic Tomography for Tree Decay in Greater Victoria

What is sonic tomography?

Sonic tomography, also called acoustic or stress-wave tomography, uses sound to look inside a tree trunk without cutting it open. Small pins, often roofing nails, are tapped just through the bark so their points touch the outer wood (sapwood). The pins are spaced around the trunk at one height, and a sensor is attached to each one.

Each sensor is tapped lightly with a small hammer, and the time the sound takes to reach every other sensor is recorded. Sound travels more slowly through decayed wood than through solid (healthy) wood, and it has to travel around hollows. Software uses these travel times, and the measured distances between the sensors, to build a cross-section picture called a tomogram. Its colours show where solid wood and decay are likely to be.

How it fits into an advanced tree risk assessment

Every assessment starts with a visual inspection. That can turn up signs of internal decay, such as fungal growth, cavities, seams or swelling, but it can't show how far the decay goes. Oakstead uses sonic tomography in advanced (TRAQ Level 3) tree risk assessments to fill that gap. A tomogram shows the likely shape of decay across the whole cross-section, which lets the arborist judge how much load-bearing solid wood is left.

A tomogram is not a photograph of the wood. It shows how sound moved through the trunk, and research has found some limits:

• Internal cracks force sound onto a longer path, so a crack can look like a large area of decay.
• Early decay, before hollows form, may not show clearly.
• The picture is less detailed close to the bark.

So Oakstead's arborist reads the tomogram alongside the visual findings and, where needed, resistance drilling at selected points to tell decay apart from cracks. Tomography doesn't decide on its own whether a tree is safe. Research shows that experienced, TRAQ-qualified arborists can still rate the same tree differently after viewing tomograms. So the results feed into a professional judgement rather than replacing it. The findings go into the written report, with recommendations to retain, prune, monitor or remove the tree.

Sonic tomography and resistograph testing

The two methods answer different questions and are often used together. Resistograph testing (resistance drilling) drives a fine needle into the wood and records a detailed reading along that single line. A drop in resistance shows decay in the needle's path. Sonic tomography gives a picture of the whole cross-section and is less invasive, but it is less precise at showing exactly where decay is and, sometimes, how much there is. One study found this order worked well: a visual inspection first, then tomography to find where decay is likely to be, then drilling at the points the tomogram flags. Our Tree Resistograph Mapping page covers drilling in more detail.

When sonic tomography is useful

• Large, valued or heritage trees showing signs of internal decay, where limiting wounds to the tree matters
• Trees near homes, driveways, play areas or other high-use spaces
• Trunks where decay may be uneven, so a few drilling lines alone could miss part of it
• Trees that need a documented advanced assessment for a tree cutting permit or tree management plan

Price

Advanced (TRAQ Level 3) tree risk assessments, which can include resistograph and sonic tomography testing, start from $1,200 before GST (Goods and Services Tax).

To ask whether sonic tomography is right for your tree, call Oakstead at (250) 888-9222 or request a consultation through the form on our Get a Quote page, and we'll get back to you.