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This study assesses the reliability of a non-destructive method for determining the in situ distribution of tree coarse roots within a scaled distance 6-fold the DBH by comparing the results with the actual 3D root architecture revealed by invasive methods. The root architecture of 22-year-old olive trees was determined non-destructively with a Root Detector device (Fakopp Enterprise Bt) using sonic speed and directly by a 3D digitizer (Fastrak, Polhemus) after soil removal. The radial and vertical distributions of the coarse root biomass and diameter in the soil as determined by the 3D digitizer were correlated with the root map detected by sonic speed. A highly significant correlation was observed between the coarse root biomass distribution and the sonic speed within 30 cm of soil depth, but this correlation decreased with increasing distance from the trunk, up to 120 cm. No correlations were observed between sonic speed and root diameter. The Root Detector was able to map the coarse roots of the olive tree in the soil environment, but only under certain conditions. First, root detection was more efficient within 30 cm of soil depth, provided that more than 35% of the total biomass of lateral roots occurs within this depth range. Second, the distance of 120 cm from the trunk, scaled as 6-fold the DBH, may be considered the threshold over which the sonic speed and the detection of roots markedly decreased. Third, Root Detector technology is unable to detect root size in terms of geometric parameters such as root diameter.