Using Ground-Penetrating Radar for Noninvasive Tree Assessment

The TreeRadar Unit (TRU™), manufactured by Tree Radar, Inc., of Silver Spring, Maryland, is a novel application of ground-penetrating radar (GPR) that can be used to assess trees noninvasively. The TRU can be used to assess the trunks and roots of trees 20 inches and larger. It also can, with a specific antenna, be used on tree branches and smaller trees. The primary uses we have seen for it in our practice have been for tree-risk assessment and tree management during construction. However, we also have used it in tree-condition assessment generally and in tree- and hardscape-conflict assessment, detecting roots behind a wall or under a hardscape.

While GPR is a well-established technology, the differentiator in the TRU is the software. It is important to note that TreeRadar is not an X-ray and it does not “see” inside the tree or ground; the results of scans are not photos of what you would see if you cut the tree down or dug a trench. The hardware and software are geared to detect tree moisture content. You can scan over concrete or asphalt and detect tree roots beneath. The software will differentiate between roots, rebar, conduit, etc.

Photo 1: The sycamore with the trunk flagged at 1-foot elevation increments in preparation for trunk scanning. The root-scanning cart can be seen to the left. Unless otherwise noted, photos courtesy of Michael Galvin.

However, because it detects moisture content, there are some things a user needs to understand. When a scan says a root at a certain point is a 4-inch root, that means the biomass at that point of data collection is functioning as a 4-inch root. It could be a point at which two 2-inch roots cross each other below ground. It also could be a 6-inch root that is compromised and has reduced function. Art and experience combine with science to optimize the utility and interpretation of the TRU’s results.

Very important points!

There are a number of things one should always remember when deploying the TRU. They include the following key points.

Job hazard analysis (JHA) first. An antenna is not a chain saw, so it can be easy to think that no dangers pertain to using the TRU. However, this is not accurate. The trees we scan are often large and sometimes pose significant risk. It is always important to assess the site first and do a JHA. Look for hazards, including large roots and root flares, uneven terrain and obstructions to your scan lines. Practice your footwork in advance. It will be a lot easier to do so without the equipment. When you have the equipment, you will be focused on the next two items, so it is important to have a clear scan plan laid out in advance.

Photo 1: The sycamore with the trunk marked at 1-foot elevation increments
in preparation for trunk scanning. The root-scanning cart can be seen to the
left. Unless otherwise noted, photos courtesy of Michael Galvin.

Have a scan plan before you start. Whether you are assessing trunks, points or roots, you need to have a scan plan before you begin in order to get the data that will support decision-making. This is important for all scans, but is particularly critical for roots, as combining the scans in post-processing is more critical and complex for root scans.

Plug everything in before you turn anything on. Failing to do so can result in damage to the equipment. Check that all connections are made before turning on the components. Make sure everything is off before unplugging any component.

Make contact. There are two things that are critical to proper data collection: 1) The antenna needs to make good contact with the surface being scanned; and, 2) The survey wheel on the antenna needs to be rotating as data is being collected in order for the measurements to be accurate. Make sure the antenna is producing a signal in the tablet and the survey wheel is turning as you are collecting data.

Photo 2: An arborist standing inside the cavity after felling a tree at West Virginia University. Photo provided by WVU arborist Josh Pitts.

Additional considerations

There are items besides the TRU that you will need to operate it properly in the field, from measuring tools, such as a diameter tape and tape measure, to safety gear, such as a safety vest and hard hat, to marking tools, such as chalk, flagging or paint.

Remembering that antenna contact is important, be aware that some trees or portions of trees may be more difficult to scan. Trunks with heavy ribs and sinuses may impact contact. In such cases, you may need to perform 360-degree elevation scans, which will provide you 360 degrees of cross-sectional data, until you get to the root flare. You can then use the TRU in point-scan mode as a “virtual drill,” collecting readings at single locations as you would with a resistance drill. Slope, leaf litter and other debris on the ground can impact contact during root scans, so some ground preparation may be needed before scanning.

It is also important to understand that TreeRadar is an attractive nuisance. It is unusual, and people are very interested in it. However, stopping to answer questions during use can result in bad data or no data. A significant sequence of events has to occur properly in order to deploy the TRU successfully. These are best accomplished without interruption. Ask your client to conduct discussion or ask questions before or after, but not during data collection.

Image 2: Linear scan plan overlaid on site plan. Red lines are scans on asphalt, green lines are acans on turf. Arrows show direction of scan, important in post-processing. Black rectangles are vehicles that had to be worked around. Hatched circle is critical root zone, per local requirements. Circle denoted as 1341 is the subject tree.

Case studies

West Virginia University tree-risk assessment

The West Virginia University (WVU) campus had us perform Basic Tree Risk Assessments (ANSI A300 Level 2) on a number of trees in a high-traffic area in 2019. We were subsequently asked to perform an Advanced Tree Risk Assessment (ANSI A300 Level 3) on one of those trees, a 63-inch-diameter sycamore. The tree was believed to have been planted in 1819, pre-dating the university. It was, therefore, a very important tree to the institution and the community.

We performed a series of trunk scans one foot apart from an elevation of six feet down to one foot. The smoothness of sycamore bark facilitated this process. We also performed root scans. Due to the configuration of the land cover around the tree, we performed linear scans rather than circular scans. While most of the rooting area near the trunk was turf, there was a hedgerow close to the trunk with a sidewalk behind it. (Photo 1) We ran the turf scans with a soil calibration, the sidewalk with a concrete calibration and omitted the hedgerow. We stitched the lines together in post-processing.

Image 3: TreeRadar root-scan data. Top row, from left: All roots detected by size; roots by size from surface to 12-inch depth; roots by size from 12- to 24-inch depth; roots by size from 24- to 36-inch depth. Bottom row, from left: Root density at same depths as top row. This tells us most of the roots and, particularly, most of the larger roots, are in the 12- to 24-inch depth range, with larger roots under turf but higher densities under the asphalt.

The root scans showed good root distribution and density, but few structural roots of significant size. The trunk scans were also troubling. Many veteran trees are able to persist despite having very little wall thickness remaining, but this tree exceeded all thresholds for loss of cross-sectional volume in my experience and in the literature. (Image 1) We recommended removal based on scan results, the frequency of use of the target area and the inability to alter target areas or uses.

After the tree was felled, the campus arborist reported 4 to 6 inches of wood in the shell, as the TRU reported. The arborist who felled the tree was 5-feet, 10-inches tall. He stood in the hole in the stump and only his head was visible – the area beneath the trunk was void to a depth of approximately 5 feet. (Photo 2) While root density was good, there were no roots of significant size detected.

5 M Street SW, Washington, D.C., heritage tree preservation

We were retained by Bohler DC to provide tree-preservation planning support on this project. A new building is to be erected on the site of a current parking lot. There is an American elm that is a street tree adjacent to the site. This tree is protected under D.C. law due to its location (street tree) and its size; at 43-inches in diameter, it is considered a heritage tree and cannot be removed unless it poses a risk.

Our initial assignment was to scan the root system on site within the planned limit of disturbance. Due to the logistics of the site, we performed line scans. There were two different land-cover types within the designated critical-root-zone (CRZ) area to be scanned: turf and asphalt. We also had to work around some vehicles that were not supposed to be in that area of the lot that day. The scan plan is shown in Image 2 and the scan results in Image 3.

The root scans were then mashed up with the building plan and the tree’s CRZ and structural root zone (SRZ) per local code to modify the design to accommodate the tree and minimize impacts to the SRZ and understand impacts to the CRZ. (Image 4)

Image 4: Draft concept drawing to reconfigure building to accommodate heritage tree. Structural root zone is smaller circle (0.5-foot radius per 1-inch diameter at breast height [DBH]) and critical root zone is the larger circle (1.5-foot radius per 1-inch DBH). Image courtesy of Bohler DC/Gensler.

Johns Hopkins University champion Eastern redcedar

Johns Hopkins University (JHU) has an Eastern redcedar tree that has been recognized as a Baltimore City champion tree for its species. (Photo 3) The tree is imperiled by planned improvements to the campus. The JHU’s Facilities Department retained us to move the tree to another location on campus so it will not be lost.

Photo 3: Champion Eastern redcedar (Juniperus virginiana) on the Johns Hopkins University campus. Photo courtesy of Daniel Bunner.

In addition to performing plant-health-care treatments to prepare the tree for eventual digging, we also used the TRU to map the root system of the tree prior to root pruning. The root size and density scans are shown in Image 5. Despite having multiple land-cover types in the mapped area, we were able to perform circular scans and stitch the data together in post-processing to create a root map. This will help us optimize the size of the root ball, using the 22-foot minimum suggested by ANSI A300 (Part 6) Annex F and the 32-foot scan area of actual root detections. JHU Facilities staff had previously engaged us to use the TRU for root mapping for tree-risk assessment and tree-preservation planning related to the construction of a building on another portion of the campus, so staff were familiar with the decision support the tool can provide.

Image 5: Analysis of the root-scan data for the Eastern redcedar in Photo 3 showed that almost all
of the roots were at depths of 6 to 24 inches, that they were unevenly distributed and that there
was significant root more than 30 feet from the tree. Photo courtesy of Daniel Bunner.

Summary

TreeRadar is not a point-and-shoot technology. It is complicated to operate the device and to process and interpret the data. Like any such tool, the results should be interpreted by an arborist who has some understanding of how the technology works. We prefer the TRU as a diagnostic tool, as it:

  • Is one of the only truly noninvasive such tools;
  • Can provide both cross-sectional and radial trunk data;
  • Can be used to assess both trunks and roots;
  • Can be used on a variety of land-cover surfaces; and,
  • Can be used in situations where use of pneumatic excavation is not possible or desirable, either due to a concrete, asphalt or other surface covering the roots, or where making soil airborne results in a regulated health hazard and requires extensive mitigation measures.

Michael Galvin, registered consulting arborist (RCA), is director of SavATree Consulting Group, part of SavATree, an accredited, 36-year TCIA member company headquartered in Bedford Hills, New York.

Daniel Bunner, CA, TRAQ, is a consulting arborist with SavATree and the company’s primary TreeRadar operator.

Leave a Reply

Your email address will not be published.

Click to listen highlighted text!