Deborah Ellis, Consulting Arborist and Horticulturist

Specialist in the Diagnosis of Plant Problems
in Horticulture, Arboriculture and Agriculture

Technical information on PiCUS tomography

PiCUS System in use

The PiCUS Sonic Tomograph is used to investigate the internal condition of a tree using sound waves. A series of nails are installed around the tree at the measuring plane where visual inspections have identified defects requiring further investigation. These nails become the measuring points and are used to send or receive sound waves. The distances between the measuring points are carefully measured and recorded by the field technician. The sound waves are generated by a hammer tapping on one of the nails. The PiCUS instrument measures the time of flight of sound waves between the sending point and the other receivers. The software calculates the apparent sonic velocities (distance/time) and draws a "velocity" or "density" map of the tree by combining the measured tree geometry with sonic data recorded during the assessment. The sonic velocity can be correlated with wood densities and therefore with the soundness of the wood.

The PiCUS Sonic Tomograph won the Technology Award 2000 from the German Federal State of Excalibur.

Structural Analysis

Municipalities and private owners are held liable for damages caused by their trees. Most often trees break into pieces or fall over because of decay caused by fungal infections. To avoid risks the safety of large trees should be assessed on a regular basis.

The PiCUS® Sonic Tomograph is an instrument designed to detect decay and cavities in standing trees in a minimally-invasive manner. The velocity of sound waves in wood depends on the modulus of elasticity and the density of the wood itself. The PiCUS unit tracks the speed of these waves. Most damage and disease causes fractures, cavities, or rot that can reduce the wood’s elasticity and density.

Measurement using the PiCUS Instrument

The PiCUS Sonic Tomograph consists of 5 to 20 sonic sensors. These sensors are spaced out evenly around the circumference of the trunk or branch of concern. They detect sound waves or a sonic impulse induced by an electronic hammer and propagated through the wood. This sketch displays the basic working principle, in that sound waves cannot take a direct path through the wood (red dotted line) if there is a cavity or other defect between the transmitter and receiver.

Advantages

How a PiCUS Sonic Tomogram (SoT) is recorded

1. The level, number, and positions of measuring points on the tree is determined by the arborist on site.
To determine the measuring level a thorough visual inspection of the tree is conducted looking for external signs of internal defects, such as fungus growth, cracks, cavities, damaged bark, etc. A mallet tap test is used to find superficial areas of loose bark, and to listen for differences in the sound and produced by the mallet strikes that may indicate internal structural defects.

2. Measuring the geometry of the tree at the measuring level
The geometry of the tomograph level is calculated using measurements between measuring points around the tree with an electronic caliper to give an accurate shape of the tree.

PiCUS Sensors on Tree

3. Taking sonic measurements
A sonic impulse is generated on each measuring point (nail) and recorded on the sensors at the other points. The electronic hammer (photo on the left) allows to use fewer sonic sensors than measuring points. This enables the PiCUS system to measure a large size range of trees.

Sonograph of Oak

4. Calculating a Tomogram
The sonic tomogram of a tree can be calculated on site using a number of functions for data analysis and presentation. The size and location of defects is shown using distinct colors. Tomograms of a tree can also be displayed in 3-dimensional graphics.

There are assistance-functions that help to analyze the tomogram. The tomogram (jpg-image) can by copied into other programs.

Limitations

Cracks in wood are real barriers for the sonic waves. They appear in the tomogram much larger than they really are and may thus lead to incorrect conclusions about the tree. To identify “star shaped” cracks, the PiCUS software we use contains a CrackDect Function. Other inspection methods, such as the Treetronic electric impedance tomography or a mechanical Resistograph Test are also used to further image and analyze the interior status of the tree.

Treetronic - Electrical Impedance Tomography (EIT) for trees

The PiCUS Treetronic uses electric current/voltage to examine the tree. The resulting measurements are displayed in a two-dimensional map showing the apparent electrical impedance of the wood, called an Electrical Impedance Tomogram (EIT). The electric impedance - or resistance - of the wood is influenced most of all by the water content, cell structure and chemical elements which change according to the status of wood. The Treetronic can detect such changes by measuring how the heterogeneous wood “bends” the electrical field.

When used in combination with a Sonic Tomograph, an EI tomogram offers more information about the tree. The EIT helps to analyze the type of damage, and it is often possible to distinguish between cavities and ‘wet’ diseased wood.

The sketch below shows standard and optional components of the PiCUS Sonic Tomography system.


Recording Sonic Data

The standard method of measuring the sonic data is to attach one sensor to each measuring point (nail). The time of flight of sound waves generated by tapping each nail is recorded at each of the other sensors and used to calculate the tomograph. The sketch below illustrates this method. Blue dots indicate the sonic sensors on each measuring point.



Advanced Recordings with the PiCUS Electronic Hammer

2D View of a Tree

Tomograph images are generated at the site as soon as the measurements are completed. A number of functions are available for data analysis and presentation. Measurements of residual walls and the extent of decayed areas can be taken directly from the tomograph image. The images can also be saved as digital files that can be exported into other software and used for presentations and reports.

Tree cross section and sonographs