It’s eight o’clock in the morning and your phone rings. The caller says he has a 90-year-old redwood tree (Sequoia sempervirens) in his backyard that is cracking the concrete slab of his patio and garage floor and would like the roots cut.
This type of scenario plays itself out on a daily basis across the country.
Inspecting the situation, the arborist may think: How can I be sure where the roots really are? Which ones are contributing to the damage? How many are there, and how deep are they? Can I install root barrier and not jeopardize tree stability? Should I recommend the tree be removed?
Not having the ability to see beyond the concrete slab makes answers to these questions difficult, even for an experienced arborist.
Traditionally, a reasonable conclusion is obtained after concrete removal and soil excavation is performed. In the end, the damage may just be due to normal ground movement and settling. Hopefully your initial recommendation didn’t involve root pruning or tree removal.
So many trees could be preserved if we had a reliable way to determine where their root systems are below the ground. This would help us better manage trenching activities and minimize root damage. We would have an enriched understanding of how far away tree protection fencing should be on a construction site. When problems come up, such as trees verses hardscapes, knowing where the roots are helps with decision-making and providing recommendations that benefit everyone involved.
If you were to conduct a root study, whether it involves trenching for a water line or inspecting damage to a garage floor, wouldn’t it be to your advantage to make decisions based on data obtained from a noninvasive source that didn’t require excavation? Collecting data and walking away as if you were never there would truly be the optimal situation for your client and the tree.
The use of noninvasive ground penetrating radar has been successfully used for belowground root mapping worldwide for over a decade. It has the ability to penetrate hard surfaces such as asphalt and concrete and then continue into the soil to identify roots and their depth on a large scale. This technology has proven to be a very effective tool for arborists.
GPR is an established technique that has been used for many different applications for over 30 years. Radar is an object-detection system that uses electromagnetic waves — specifically radio waves — to identify the range, altitude, direction or depth, and speed of both moving and fixed objects.
Although radar imaging will not produce a picture of roots, it will provide images of predicted root locations based on how this technology interacts with roots within the soil profile.
Applying GPR in the field
Nondestructive radar imaging has opened doors that previously were unavailable to arborists using traditional root locating methods.
Roots are a very complex and not a typical inspection application for radar. TreeRadar Inc., Silver Spring, Maryland, has developed a specialized software package to interpret and map the 2D and 3D subsurface root layout, density and morphology using GPR. Training and regular use are essential for success with this equipment and software.
TreeRadar’s root analysis software, TBA, takes root data collected from the field and processes it, ultimately generating root mapping results and various data reports.
TBA is designed exclusively for root detection, so it filters out unwanted objects the radar finds in the soil profile, such as gopher and squirrel tunnels, rocks, metal or pipes. The finished product only shows predicted root targets in the final data. This analysis procedure can be performed in the field using a laptop computer or back at the office as you develop a report. The average time to perform a normal root inspection in the field is less than one hour. The individual line scans are recommended to be kept at 40 feet or less in length. The investment in this type of equipment is generally less expensive than the cost of most invasive procedures.
At this time, the software cannot identify or display the different sizes of roots the radar encounters in the soil profile. This important feature is continually being developed and is very near completion.
Currently, the use of two different antennas to classify roots into two size categories, pending the completion of the sizing algorithm, is available. A 900 MHz antenna emits high frequency radio signals that are capable of penetrating the soil to a depth of a little more than 3 feet. As it does this, it will begin targeting roots that are a quarter inch in diameter or greater.
A 400 MHz antenna does the same but produces a lower frequency that will identify larger diameter structural roots. This antenna begins targeting structural roots that are 1 inch in diameter and greater, and it has a depth range capable of 12 feet. This is the antenna of choice when it comes to construction because of its root size and depth capabilities.
This antenna combination is useful when the arborist needs to calculate the percentage of structural root loss from neighboring trees prior to the construction of a basement or deep trenching. The use of these two antennae will provide more information to the arborist on root size as the software continues to develop.
In the GPR report, the results are displayed as a virtual trench view of the soil profile from the collected root data. The software also can provide a topdown spatial view utilizing a 3D connect-the-dots feature creating a conceptual line drawing that de picts how the root system may look below ground according to the field-collected data. Root data can be overlaid on a site map establishing a visual tree protection zone for a construction site.
Moisture detection benefits
One of the advantages of mapping roots using GPR involves the method this technology uses to locate them below ground. As discussed earlier, radar is an object-detection system, and roots are principally identified by their internal moisture.
Healthy roots have a strong reflection; compromised roots produce a weak reflection. Roots that have been severed or are dead (lacking moisture) have no reflection and will not be displayed in the final analysis results.
This type of information is crucial to the arborist when performing level 3 risk inspections involving trees that have displayed trunk decay problems.
From this data, the arborist can quickly see that there may also be stability issues involving a compromised root system.
The optimal way to view this type of data is in the spatial top-down or aerial view feature of the software. From this view the arborist can visually see the spatial layout of areas where roots would normally and should be found but are missing.
At times the area being scanned can pose a problem. Some soils can be composed of buried fill materials of different kinds. For example, consider a home built on the side of a mountain. To create a usable backyard, old building materials from the construction site might be discarded down the hill. Then, top soil is brought in to finish and level off the area into a usable landscaped area.
When this area is scanned, the radar signal will travel through the soil with no problems until it encounters the mixed foreign materials. Then the data becomes unreliable at the lower level containing the fill. At times, it can be difficult to know what conditions we are dealing with that could affect data collection until they are encountered.
Tile glaze, often used as a finish on floor or pool tiles, can be manufactured containing crushed stone or metal. Radar signals reflect from these materials and penetration is not possible. The only exception is if the tiles and stone are small. If the grouted areas are large, the radar signal will penetrate through the grout area and provide the root information needed. As the radio signal passes through the grout and into the soil, it spreads out much like a flashlight beam, allowing it to capture roots under the glazed tiles or stone of the floor.
Heavy clay soils can also effect radar signals. High concentrations of clay in various soil layers can decrease the amplitude (attenuates) of the reflected radar wave as it travels back through the soil.
This means that as the signal/noise ratio gradually decreases or weakens due to the clay, root detection becomes more difficult.
However, there are now enhanced signal processing filters built into the TBA root analysis software that considerably increase these signal/noise ratios and make the desired reflectors (roots) much easier for detection in clay soils.
Looking beyond The experienced arborist has more choices today than ever before about how to obtain needed information. It is expected that arborists use their knowledge and experience when determining a solution for their clients. Today, as arboriculture moves deeper into the world of high-tech, ground penetrating radar is just one of the arborist’s tools of choice.
When you are called upon to inspect concrete damage, your first thoughts may lead you to think, “Are these cracks from tree roots, or is this just normal ground movement and settling?” Through GPR technology, it is now possible to look beyond what is visible and find the answer to that question.
The author is Robert Booty, a consultant at Arborist OnSite Horticultural Consulting Inc., San Jose, California, and Registered Consulting Arborist 487 and International Society of Arboriculture Tree Risk Assessment Qualified. He can be reached at firstname.lastname@example.org.