162 Bassuk et al.: Ground-penetrating Radar Locates Tree Roots in Two Soil Media Under Pavement since the tool could not move through the tree trunk. GPR mea- surements recorded all root-like signals down to 90 cm. Radar files were divided into sections of output related to the tree pit opening, henceforth termed “scan sections.” The process re- sulted in eight scan sections per tree replicate (Figure 1), four corner sections, and four side sections surrounding a square tree pit opening. Because the scans along the length of the pavement overlapped across the width of the pavement, each corner of the tree root zone included two root signal counts. GPR scan- ning used a standard setting for the soil system under pavement and an amended setting for the CU-Soil to account for min- eral and porosity differences, to keep depth of field accurate. Immediately after scanning, the concrete was removed from Figure 2. GPR Roots Inspection System. The SIR-3000 Control Computer is at the top with its sun shield. The radar antenna is the module in the white tub (which maintains contact with the ground surface). An encoder wheel rubs against the cart’s right rear wheel to provide distance information for automatic data col- lection. Calibration of a GPR system is a critical pre-inspection step for roots detection because the signal strength (amplitude) of waves propagating through a medium decreases with increasing depth and causes deeper objects to be less detectable. The SIR- 3000 system has a means for automatically compensating for this depth-dependent amplitude loss irrespective of the soil surface – either bare or covered (e.g., concrete, asphalt, bricks, pavers). The amplitude calibration procedure involves positioning the antenna at various locations within the inspection area, observing a root reflection on the display, and allowing the SIR-3000 to automati- cally adjust its gain (amplitude) profile to maximize the reflected amplitude from the root. This is repeated at multiple locations and the inspector selects the profile that provides the best overall fit. The dielectrics of the natural soil and the CU-Soil gravel mix were each empirically found by: 1) locating a root with the GPR system, 2) driving a measuring rod into the soil to find the exact depth, and 3) adjusting the dielectric param- eter on the SIR-3000 until the depth scale on the display agreed with the actual depth. This was done at multiple lo- cations within the inspection area and the dielectrics were averaged. The average values found were 13 for the natural soil and 9 for the CU-Soil, which gives a velocity of 8.5 cm/ nsec and 10.2 cm/nsec for the natural soil and CU-Soil, re- spectively. The 20% higher radar speed in the CU-Soil was most likely due to the higher density of small air pockets. The GPR system was set to automatically record a radar wave for every 5 mm of movement along each scan line. This high data collection density, along with the wave’s beam width, enabled a root or root cluster to be observed multiple times which enhanced detection sensitivity during data analysis. A series of 59 cm wide scans over the concrete paving (be- tween the wheels of the scanning cart unit) were collected by two long scans running the length of the pavement on either side of the tree pit openings, and short scans across the width of the pavement on either side of each tree. As a result, the tree root system was scanned on all four sides of the tree pit opening. Roots in the center opening of the sidewalk were not scanned ©2011 International Society of Arboriculture the trenches for the first five trees in each scanned treatment panel. In a protocol similar to Stokes et al. (2002), shoots were removed near ground level and whole root systems were exca- vated as an entire system attached to the tree trunk using an air excavation tool. The root system was not mapped to test depth inferences between root system and radar depth output. Given the homogenous layers installed under the pavement, mapping of pipes and rubble was not needed. The orientation of the trunk was marked so as to compare it to the GPR scan data when counting roots. After excavation, the root system was skeleton- ized by removing all roots less than 0.8 cm in basal diameter and all root axis apices when tapering to a diameter less than 0.8 cm (Figure 3). The remaining large roots were counted in each section comparable to where the GPR unit made its mea- surements. No roots that were in the 0.5 m × 0.5 m opening around the tree trunk were counted. A guide frame constructed of cardboard was placed over the root system to identify that area. GPR root signals were compared to root counts. Five trees per sidewalk treatment were scanned and excavated, providing 40 sample zones per treatment. Mixed effect models in regres- sion analysis were conducted in JMP v.8 to provide a ground- truth test. Regression plots for data presentation were developed in Minitab 14.2. Since the goal was to evaluate the use of GPR scans in pavement sections as a method of estimating root coloni- zation, GPR data was used as the fixed effect, and the excavated root count as the dependant variable. Random effects included tree individual and pavement section type (the compacted soil versus the compacted CU-soil). The slope of the model out- puts were tested against a presumed slope of 1.0 as a calibra- tion guide (one root signal for one root excavated). Paired data were used to explore any discrepancies between radar output and root count. Pavement types were compared for root counts after five years of growth. The mean value for the overlapping scan sections at the corners of each pit were used in the analysis. RESULTS During excavation, it was noticed that no roots were present in the base layer of either treatment. Sample zone root counts and radar output variances within treatment were tested and found to be equal. Differences between radar output and associated root counts in sample zones were found to be normally distributed in the CU-Soil treatment, but not in the compacted soil treatment. One Way Analysis of Variance of radar output showed an average of 3.3 roots in compacted soil sidewalk sam- ple zones, compared to 5.7 roots in CU-Soil sidewalk zones (P < 0.001), which agreed with actual count data of
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