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Getting to the Root of the Project
Using Ground Penetrating Radar for Belowground Root Detection

By Robert Booty, Registered Consulting Arborist


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Root damage can cause a wide array of structural problems if left unchecked. If an area under inspection sits beneath hard surfaces such as asphalt or concrete, these need to be removed first for access to the soil. There are a number of methods used to excavate and identify roots in the soil for study. To some degree root damage occurs during all of these procedures. For example, an air-spade uses compressed air to move soil from around roots. Hydro excavation or vacuum excavation removes soil with pressurized water in conjunction with a vacuum that sucks the liquefied soil into a waiting tanker truck. The most common method for inspecting root damage is manual excavation. However, Registered Arborist Robert Booty has taken a different approach inspecting root damage with technology meant for the battle field, radar.


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In this root inspection procedure diagram, the operator can get a cross sectional view of where the roots are. As the antenna is moved along the ground every 2/10ths of an inch a radio signal is released into the soil at a predetermined depth set by the technician.


It's eight o'clock in the morning, the phone rings, and the caller tells you there is a 90-year-old redwood tree (Sequoia sempervirens) cracking the concrete slab of his backyard patio and garage. The caller wants the roots cut because he believes it will solve his problem.

Many trees could be preserved if there were a widely available and reliable way to determine where their root systems are below ground. This would help better manage trenching activities and minimize root damage. We would gain an understanding as to how far away our tree protection fencing should be on a construction site. If and when problems arise, such as having to decide between a tree and hardscaping, knowing where the roots are helps us better assist the homeowner, and provide recommendations that benefit everyone involved.

Current methods used, to physically expose roots for inspections
There are a number of methods used to excavate and identify roots in the soil for study. All of these procedures are invasive, and to some degree cause damage to the tree. The use of compressed air to move soil from around roots is common. Hydro excavation or vacuum excavation is the use of pressurized water in conjunction with a vacuum that sucks liquefied soil into a waiting tanker truck. A procedure called "potholing" sometimes is used to visually inspect roots. Individual inspection holes are produced within a targeted area. One can then utilize air, water or hand digging to expose roots for these inspections. The most widely used method for root inspection is manual excavation using a pick or shovel. If the area of your inspection is under hard surfaces such as asphalt or concrete, these are removed first for access to the soil.

The use of non-destructive technology for root inspections
If you were to conduct a root study, whether it involves trenching for a water line or inspecting damage to a garage floor. Would it not be to your advantage to formulate your decisions from data obtained from a non-invasive source if that were possible, one that didn't require excavation? If you could simply collect your data and walk away, as if you were never there, then it truly would be the optimal situation for your client and the tree.

Ground Penetrating Radar
The use of non-invasive 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, allowing readings of the soil beneath to identify roots and their depth on a large scale. This technology has proven to be a very effective tool for the arborist.



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Root damage can cause a wide array of structural problems if left unchecked. If an area under inspection sits beneath hard surfaces such as asphalt or concrete, these need to be removed first for access to the soil. There are a number of methods used to excavate and identify roots in the soil for study. To some degree root damage occurs during all of these procedures. For example, an air-spade uses compressed air to move soil from around roots. Hydro excavation or vacuum excavation removes soil with pressurized water in conjunction with a vacuum that sucks the liquefied soil into a waiting tanker truck. The most common method for inspecting root damage is manual excavation. However, Registered Arborist Robert Booty has taken a different approach inspecting root damage with technology meant for the battle field, radar.


How does it work?
GPR is an established technique that has been used for many different applications worldwide 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. It is an acronym, which stands for radio detection and ranging (RADAR). Its uses today seem endless. For example, when you look at the weather report, you are looking at weather radar, which tells you where and when the heaviest amounts of precipitation will fall in your area. The radar wave, as it passes through the clouds, measures the density of the moisture in them and the speed they are traveling. Provided with this information, meteorologists will know approximately when it will start raining and how much rain will fall. Radar is used in aviation, automobiles, law enforcement, tree risk assessment and locating objects belowground.

In the root inspection procedure diagram above, this is how the radar looks for roots below ground, as the antenna is moved along the ground every 2/10ths of an inch a radio signal is released into the soil at a predetermined depth set by the technician.

As this signal encounters a root, it is primarily reflected off the moisture within it. The signal then travels back to a receiver inside the antenna with surprising accuracy. The data is then recorded to a field computer. This is how radar distinguishes the difference between roots and other materials that might be buried alongside the root system . As an example if you were using a fish finder to locate fish, the sonar waves would bounce off the air bladder within the fish, giving you its location and depth. Ground-penetrating radar does the same thing, only radio waves are used.

Although radar imaging will not produce a picture of a root or roots, it will provide images of predicted root locations based on how this technology interacts with roots within the soil profile.



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The GPR software can also provide a top-down spatial view, which creates a conceptual line drawing depicting how the root system may look belowground according to the field data collected. In this example, the root data was over-laid on a site map establishing a visual tree protection zone (TPZ) for a construction site.


How root data is displayed in your report
The above plate illustrates the results from an actual project. In your report this would be displayed as a virtual trench view of the soil profile from your collected root data. You would view this as if you were looking through a window. To illustrate, think of using a backhoe to dig a trench. As you dig, tree roots are encountered at various levels in the soil profile, after you have completed your trench you are able to walk down and stand at the bottom.

Looking up at the earthen wall you are able to see all the severed tree roots from your trenching activities protruding from the soil at the various depths of your trench. As you look at each x on the above plate, it represents your earthen wall showing severed root locations. Each colored X represents a different depth where the root is located. In the results for the above project the radar identified the concrete thickness and location of the roots in relation to the bottom of the concrete slab. The two vertical green lines to the right on the plate are markers physically placed on the field computer by the technician during the scanning, as he moves the antenna over cracks in the concrete surface.

Finding roots under a concrete slab does not mean they are the source of the problem, in most cases, this is expected. However, they may be in an area and depth that cause little concern. The identification of problem roots proves is invaluable when putting the root damage puzzle together.

As the results are further analyzed, shallow roots are identified within the first seven inches of soil, one root appears to be touching the slab. The presence of the two vertical green-line markers indicates the concrete was visibly cracked in those areas at the time of the scan. These shallow roots likely indicate soil has been displaced upward and is now applying pressure on the underside of the slab. Root studies have indicated trees have the ability to affect hard surfaces. As the diameter of a root increases in size, soil is displaced, hydraulically moved above and around the root. To illustrate, the axial and radial pressure exerted by roots is substantial (Feldman). For instance a 15-inch segment of root 1 inch in diameter exerts 175 psi, this force is capable of lifting a concrete sidewalk slab 10 feet long and 4 feet wide (Dunn).



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The GPR software can also provide a top-down spatial view, which creates a conceptual line drawing depicting how the root system may look belowground according to the field data collected. In this example, the root data was over-laid on a site map establishing a visual tree protection zone (TPZ) for a construction site.


Different ways you can view your collected field data
In addition to the virtual trench view we discussed, the software can display data so it can be viewed as an aerial or spatial top-down view. Here you can view all roots or even isolate your root data search according to their three individual respective root depth categories. With this feature you can adjust the software to only view roots within the first eight inches or eight to sixteen inches from the surface.

The software also can provide a top-down spatial view, utilizing a 3D connect-the-dots feature that creates a conceptual line drawing, depicting how the root system may look below-ground according to the field data collected. In the case studies below (lower right) the root data was over-laid on a site map establishing a visual tree protection zone (TPZ) for a construction site.

Ground Penetrating Radar, how it's being used in the field
Non-destructive radar imaging has opened doors that previously were unavailable to arborists using traditional root locating methods.



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The arborist seen here is scanning and mapping roots over a mosaic stone driveway. This was part of a tree protection project that involved preserving this historic 100 year old Moreton Bay Fig in preparation for future construction.


How difficult is this technology to use? What are its limitations?
The main use for GPR world-wide is non-invasive subsurface inspection, concrete/rebar inspection. Other uses involve geophysical detection of "regular" objects - irrigation and utility pipes, graves, septic tanks, underground aquifers, pipe leaks, etc. Roots are a very complex and a "non-regular" inspection application. TreeRadar Inc., produces the only GPR system currently available on the market. They have developed a specialized software package to interpret and map 2D and 3D subsurface root layouts, density and morphology.

Training and regular use are essential for success when using this equipment and software. TreeRadar Inc., Silver Spring, Md., has developed special software called TBA(TM) (root analysis software). Root data collected from the field is processed through this software and ultimately generates the root mapping results. This analysis procedure can be performed in the field using a laptop computer or back at the office as you develop your report. This is the only high volume root locating equipment on the market.

The use of this technology is generally less expensive than the cost of most invasive procedures discussed at the beginning of this article.

At this time the software cannot identify or display the many different sizes of roots radar waves encounter in the soil, this important feature continues to be in development and is very near completion. The only way to determine root size is by using two different antennas to classify roots into two size categories. The completion of a sizing algorithm is still underway. A 900 MHz antenna which emits high frequency radio signals, is capable of penetrating the soil to a depth not much greater than 36 inches. As this happens, it will begin targeting roots that are a quarter-of-an-inch in diameter or greater.

The use of a 400 MHz antenna does the same but produces a lower frequency that will identify larger diameter, structural roots. This antenna begins targeting roots that are one inch in diameter or greater and has a depth range capable of twelve feet. This antenna's depth capabilities make it the antenna of choice when it comes to construction. This increased range is when called on to calculate the percentage of structural root loss from neighboring trees prior to the construction of a basement or deep trenching. These two pieces of equipment will provide information to arborists on root sizes while software continues being developed.



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The root mapping team performing a structural root study on a hillside. The antenna is being lowered by rope, mapping tree root structures down to five feet.


One of the advantages of mapping roots using GPR involves the method this technology uses to locate roots belowground. Roots are primarily detected by measuring their moisture content. As discussed earlier, radar is an object-detection system. 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 your final analysis results.

This type of information is crucial to the arborist when performing level-3 risk inspections involving trees that have in the past, displayed trunk decay problems. This data can be used to quickly see that there could also be stability issues to consider, from 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 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 all different kinds. To illustrate; a home is built on the side of a mountain, to create a usable backyard; old building materials from the construction are discarded down hill. Then topsoil is brought in to finish and level off the area and convert it to a usable landscaped area.

When this area is scanned the radar signal will travel through the soil with no problems until it encounters the foreign material. When this occurs the data becomes unreliable at the lower level containing the fill. It is unknown what conditions could affect data collection until they are encountered.



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A large redwood, not seen in the photo, caused structural damage to the concrete of this pool in Sacramento, Calif. The root mapping team scanned the wall of this pool to determine the extent of the root growth and damage.


Tile glaze, often used as a finish on some floor or pool tiles, at times are manufactured containing crushed stone or metal. Radar signals reflect from these materials and will not allow penetration. The only exception to this are areas paved with small stone or tile in conjunction with large grout areas. The radio signal as it passes through the grout and into the soil spreads out much like a flashlight beam, in this way it captures roots under the glazed tiles or stone of the floor. This was the case involving the mosaic stone driveway at the above project in Santa Monica California.

Heavy clay soils can also have an affect on radar signals. High concentrations of clay in various soil layers can decrease the amplitude of the reflected radar wave as it travels back through the soil. This means that as the signal to 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(TM) root analysis software that considerably increase these signal to noise ratios and makes the desired reflectors (roots) much easier for detection in clay soils.

The experienced arborist has more choices today than ever before about how he will obtain needed information. It is expected that he use his knowledge and experience when determining a solution for his client. Today as arboriculture moves deeper into the world of hi-tech, ground penetrating radar is just one of those choices.

The rebar found in this concrete garage floor below creates no problems during a root inspection.

Although your first instinct when you see cracks in concrete is to suspect root damage, it is possible that what is seen is due to normal ground movement and settleing. You won't know what you're looking at until you can see past what is visible.


Robert Booty, Registered Consulting Arborist 487 and ISA



As seen in LC/DBM magazine, September 2017.






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