Calibrating the impact of root orientation on root quantification using ground-penetrating radar

Abstract

Background and aims Ground-penetrating radar (GPR) has provided a non-invasive means for field root investigation. However, the horizontal cross angle (x) of root orientation intersecting a survey line considerably impacts the amplitude area (A) reflected from a root and impairs the accuracy of GPR-based root quantification. Prediction of A(90°) (the value of A scanning at x=90°) from multiple A(x) measurements could correct such impact. Previous method of A(90°) prediction focused on target roots at field point scale. The aim of this study is to develop a method to predict A(90°) at field plot scale. Methods A(90°) was predicted by a pair of A(x) measured at two arbitrary scanning lines together with an estimated soil background amplitude area. Three independent datasets were employed to test the proposed method. The field experiment included radar data collected for six roots of Caragana microphylla in a sandy-clay soil at four cross angles (30°, 45°, 60°, and 90°). The sand box experiment included radar data for 12 dowels at 13 cross angles (0°to 180°, in 15°steps). The simulation experiment included A(x) of 46 simulated roots at 13 cross angles (0°to 180°, in 15°s teps).Results For all experiments, A(90°) was accurately estimated. Root orientation could also be determined.After correcting the impact of cross angle, the accuracy of root diameter estimation improved. Correlation coefficient between actual and estimated root diameters increased from 0.77 to 0.81, with RMSE declining from 9.53 to 7.05 mm.Conclusions A method of correcting the influence of root orientation on root GPR signal at the field plot scale has been established. This method enhances root quantification using GPR.

Tree Root Automatic Recognition in Ground Penetrating Radar Profiles Based on Randomized Hough Transform

“…As a linear target, the reflection amplitude of root is influenced by the root direction relative to the antenna [38,39]. Tanikawa [40] and Guo [32] analyzed the impact of horizontal azimuth angle on root detection using GPR and pointed out that horizontal orientation significantly influences both the amplitude and the opening angle of reflected hyperbolic signals, which supports our results (Figure 5c). However, few studies have focused on the combined effects of both horizontal and vertical inclination of roots in GPR detection.…”

“…Trench I-III were originally designed for the study of root orientation by Guo et al [32], with 4 m length and 0.5 m depth. Figure 1a shows trench I as an example.…”

The quest for understanding phenotypic variation via integrated approaches in the field environment

“…Soil temperature and the amount of nonliving organic material also are known to influence GPR signal; therefore, extensive calibration must be undertaken, especially for nonuniform soils. Even in ideal conditions (relatively dry, sandy soils), careful comparisons with excavated root systems have not shown strong correlations, especially for roots growing downward (Zenone et al, 2008;Guo et al, 2015a). To date, GPR has not been shown to be capable of detecting fine (approximately less than 2 mm) roots, which may constitute the majority of root length for many plants (Pierret et al, 2005;Brown et al, 2009).…”

Experimental Evaluation of Several Key Factors Affecting Root Biomass Estimation by 1500 MHz Ground-Penetrating Radar

“…However, these studies have only utilized either single direction scanning transects [7,[19][20][21][25][26][27]29,30,42,44,45] or perpendicular, two-direction scanning transects [28,41,43]. It has been shown that the angle at which the GPR antenna crosses over a root affects the amplitude of the reflected waveform, thus affecting the estimated size of the root [46][47][48]. Guo et al [48] examined a three-direction method to correct for non-perpendicular root orientation relative to the GPR transects, but did not extrapolate their data to system level biomass estimation.…”

“…It has been shown that the angle at which the GPR antenna crosses over a root affects the amplitude of the reflected waveform, thus affecting the estimated size of the root [46][47][48]. Guo et al [48] examined a three-direction method to correct for non-perpendicular root orientation relative to the GPR transects, but did not extrapolate their data to system level biomass estimation. No research has been conducted to determine if models built using four scan directions have a higher potential to reduce variability based on root orientation compared to models based on fewer transect directions.…”