160 Bassuk et al.: Ground-penetrating Radar Locates Tree Roots in Two Soil Media Under Pavement Arboriculture & Urban Forestry 2011. 37(4): 160–166 Ground-penetrating Radar Accurately Locates Tree Roots in Two Soil Media Under Pavement Nina Bassuk, Jason Grabosky, Anthony Mucciardi, and Gary Raffel Abstract. This study involved locating tree roots with a ground-penetrating radar (GPR) system and then examining excavated roots in the same soil volume to compare the accuracy of the GPR system with true root location. In 2003, Acer platanoides ‘Emerald Queen’ Norway maples were planted in trenches containing two compacted soils (native silt loam and CU-Structural Soil). The trenches were paved with 10 cm of concrete. In 2008, a GPR system consisting of a 900 MHz antenna mounted on a root-scanning cart was used to conduct linear scans on top of the concrete. Immediately after scanning, the concrete was removed for selected trees and whole root sys- tems were excavated (as an entire system attached to the tree trunk) using an air excavation tool. Regression analysis using mixed ef- fect models showed that the radar reliably predicted root presence in both the native and structural soils. The root count correlations were r2 = 0.76 and r2 = 0.81 for the native and structural soils, respectively. In the compacted native soil under concrete, the radar out- put overestimated the presence of roots at the minimum detection diameter but did provide a signal associated with root presence at this detection level. In the structural soil under concrete, the radar output reliably predicted roots with only slight overestimation. This study showed that GPR data reliably predicted the presence and locations of roots under the concrete pavement in two compacted soils. Key Words. CU-Structural Soil; Root Counting; Root Detection; Root Mapping; Root Morphology; Soil Excavation; Virtual Trench. The benefits of conserving trees on urban and community devel- opment sites in both new neighborhood developments and in the redevelopment of inner city areas are well known and increasing- ly quantified (Nowak and Dwyer 2007). Economic benefits such as the reduction of heating and cooling costs, increasing property values and environmental benefits, such as reducing the urban heat island, buffering wind, reducing glare, abating noise, and improv- ing water quality by reducing storm water runoff, are just some of the ways trees improve our lives. Even with the best of intentions, however, trees are often sacrificed in the face of new development. It is difficult to know where tree roots are located to avoid damag- ing them during construction. Ground-penetrating radar (GPR) is a candidate technology for noninvasively establishing subsurface structural roots layout and creating detailed morphology maps. GPR is an established noninvasive (i.e., nondestructive) in- spection method that has been used worldwide for more than thirty years to locate subsurface objects such as pipes, utili- ties, and other engineering and environmental targets (Neal 2004). One of the main worldwide uses of GPR is in concrete inspection, where the integrity of the structurally support- ing rebars is examined along with the integrity of the concrete matrix itself (Daniels 1996; Conyers and Goodman 1997). GPR has been shown to successfully locate tree roots noninva- sively and in three dimensions in forest soils (Hruska et al. 1997; Butnor et al. 2001; Stokes et al. 2002; Butnor et. al. 2003). The tool has been used to a limited extent underneath pavement (Cermák et al. 2000, cited by Stokes et. al. 2002), while suggesting the need for verification of radar output by direct root excavation. This re- ©2011 International Society of Arboriculture search has shown that GPR can detect roots in favorable (nonclay) soils and a root density estimate can be produced, but detailed root morphology maps were not presented. Although this technology has a long history of use in archaeology and engineering to locate antiquities and utilities (Conyers and Goodman 1997), the practice of using it to map roots in urban soils, which can be compacted, layered and discontinuous, is comparatively new. Before this tech- nology can be used to its fullest, ‘ground truth’ studies need to be undertaken to explore the limits and resolution of GPR as a tool for locating tree roots on development sites and under pavement. GPR measurement as a method of mapping tree roots has several advantages over other methods: 1) it is capable of scan- ning root systems of large trees under field conditions in a rela- tively short time, 2) it is completely noninvasive and does not disturb the soils or damage the trees examined, 3) being non- invasive, it allows repeated measurements that reveal long- term root system development, 4) it allows observation of root distribution beneath hard surfaces (e.g., concrete, asphalt, bricks, pavers, roads, buildings), and 5) its accuracy is suffi- cient to detect structural roots with diameters as small as 1 cm. GPR inspection employs electromagnetic waves, which will reflect, refract and/or diffract from the boundary in a predict- able manner when encountering a boundary between objects with different electro-magnetic properties (Daniels 1996). The electromagnetic material property that creates the contrast and causes reflections is the dielectric (ε), which is a dimension- less quantity relating to the material’s behavior when subjected to an electric field. The larger the difference between the di-
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