256 Moore and Ryder: Ground-Penetrating Radar to Locate Tree Roots In other instances, relative distance and depth have been considered to be sufficient, and so absolute depth was not determined (Hruška et al. 1999), which is more likely to be the practice for arborists using GPR in the field. In many field situations, arborists would be concerned more with root detec- tion, rather than the accuracy of depth predictions and so would be prepared to trade ease and speed of operation against the accuracy of depth. The misdetection of roots was most likely due to poor resolution and confusing wavelets that showed up on the scan. If the object of investigation is smaller than the wavelength, then it can be easily missed (Wielopolski et al. 2000; Gibson and George 2004). Transmission and retransmission losses are unlikely as the GPR device was rolled across the ground at no more than 10 mm from its surface (Daniels 1996). This particular GPR model was able to determine the location of coarse tree roots with diameters down to 10 mm at shallow depths, but at greater depths there were increasing errors, both in the spatial position horizontally and vertically, as well as the misdetec- tion of roots and the creation of phantom roots. In the scans for the experiment on paired roots, GPR detected the presence of roots, but even at 200 mm depth, the signal did not always reveal the presence of two roots (Figure 9). Butnor et al. (2001) found that there can be signal conflict when there are multiple roots present, as one can mask another or change the shape of the hyperbola. Evidence of replicates was missing in all of the scans even though there was a 300 mm gap between replicates, which should have allowed for discrimi- nation. The optimum resolution for the GPR, using a 900 MHz antenna set to ‘standard soil’, according to manufacturer’s specifications, is 26 mm, which is in good agreement with Conyers and Cameron (1998). The device should have been suitable for detecting buried roots of between 10–15 mm diam- eters, and so the choices of using roots 20–30 mm in diameter at gaps of 20 mm, 40 mm, and 80 mm were across the published range for the instrument. It is likely that this margin of error would be acceptable to practicing arborists trying to locate tree roots. The experiment on Pistacia chinensis in situ was conducted to test GPR under field conditions where soils were uncontrolled and less than ideal for GPR use. The soil around these roots was not unconsolidated fill, like that used to bury the roots ©2015 International Society of Arboriculture in trenches, and there was no risk of root desicca- tion. Dead and decaying roots were undetectable in forest GPR studies (Butnor et al. 2001), and read- ings changed as sand placed in pits was allowed to settle over a four-week period (Barton and Montagu 2004). Bassuk et al. (2011) found an increase in the number of misdetected roots in compacted soil compared to CU-Structural Soil™. Misdetection of roots could result from signal loss, antenna spreading loss—involving the inverse fourth power of distance for a point reflector (the root) (Daniels 1996) as it gets deeper (farther away) the signal will spread more and be more wide-ranging with a much weaker signal return- ing to the receiver—or attenuation and scattering losses occurring as the wave moves through the medium (Daniels 1996). Misdetection of roots is most likely due to poor resolution and confusing wavelets showing up on the scan. Unless scanning occurs under optimal conditions, if the object of investigation is smaller than the wavelength, then it can be easily missed (Wielopolski et al. 2000; Gibson and Geirge 2004). However, transmis- sion and retransmission loss are unlikely as the GPR device was rolled across the ground at no more than 10 mm from its surface (Daniels 1996). The detection of phantom roots, particularly in the outer trench, suggests that the GPR was either detecting other subsurface features (e.g., stones, wood, utility pipes), or there was interference from aboveground objects resulting in false root detection. There were only three roots to be found in total on Side 4 (Table 4), which was next to a vehicle path that over the years has become com- pacted. Highly compacted soils present a physical barrier to root growth (Craul 1992), and it is not surprising for there to be few roots in this area as root growth is generally opportunistic and fol- lows the path of least resistance (Perry 1982). Trees growing near roads have been found to have asymmetric root systems (Cermák et al. 2000). The differences between the predicted and the observed number of roots, particularly in the 2.5 m trenches, would suggest that the post-processing algorithms and filters were insufficient to remove noise, clutter, and unwanted objects. During the Air- Spade excavation, some debris (stones and wood) were identified but too few to account for the predic- tion of 52 roots. Conversely, in the one meter trench,
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