© 2004 Heron Publishing—Victoria, Canada
Detection of tree roots and determination of root diameters by ground penetrating radar under optimal conditions
Craig V. M. Barton (1, 2) and Kelvin D. Montagu (1)
1. Forest Research and Development Division, State Forests of NSW, P.O. Box 100, Beecroft, NSW 2119, Australia / 2. Corresponding author (craigba@sf.nsw.gov.au) / Received December 10, 2003; accepted February 20, 2004; published online October 1, 2004
Summary
A tree’s root system accounts for between 10 and 65% of its total biomass, yet our understanding of the factors that cause
this proportion to vary is limited because of the difficulty encountered when studying tree root systems. There is a need
to develop new sampling and measuring techniques for tree root systems. Ground penetrating radar (GPR) offers the potential
for direct nondestructive measurements of tree root biomass and root distributions to be made. We tested the ability of GPR,
with 500 MHz, 800 MHz and 1 GHz antennas, to detect tree roots and determine root size by burying roots in a 32 m3 pit containing damp sand. Within this test bed, tree roots were buried in two configurations: (1) roots of various diameters
(1–10 cm) were buried at a single depth (50 cm); and (2) roots of similar diameter (about 5 cm) were buried at various depths
(15–155 cm). Radar antennas were drawn along transects perpendicular to the buried roots. Radar profile normalization, filtration
and migration were undertaken based on standard algorithms. All antennas produced characteristic reflection hyperbolas on
the radar profiles allowing visual identification of most root locations. The 800 MHz antenna resulted in the clearest radar
profiles. An unsupervised, maximum-convexity migration algorithm was used to focus information contained in the hyperbolas
back to a point. This resulted in a significant gain in clarity with roots appearing as discrete shapes, thereby reducing
confusion due to overlapping of hyperbolas when many roots are detected. More importantly, parameters extracted from the resultant
waveform through the center of a root correlated well with root diameter for the 500 MHz antenna, but not for the other two
antennas. A multiple regression model based on the extracted parameters was calibrated on half of the data (R2 = 0.89) and produced good predictions when tested on the remaining data. Root diameters were predicted with a root mean squared
error of 0.6 cm, allowing detection and quantification of roots as small as 1 cm in diameter. An advantage of this processing
technique is that it produces results independently of signal strength. These waveform parameters represent a major advance
in the processing of GPR profiles for estimating root diameters. We conclude that enhanced data analysis routines combined
with improvements in GPR hardware design could make GPR a valuable tool for studying tree root systems.
Keywords:
GPR, nondestructive root measurement, radar antenna, root biomass.
