Integration of ultrasonic tomography and GPR full waveform inversion for internal decay detection in tree trunks

Decay-induced necrosis and structural failure of tree trunks in urban environments pose significant risks to public safety. Ground penetrating radar (GPR) has gained widespread recognition for tree trunk decay inspection due to its high efficiency. Full waveform inversion (FWI) of GPR data emerges as a high-resolution technique that quantitatively estimates the permittivity distribution of subsurface structures by iteratively minimizing discrepancies between observed and simulated waveforms. However, the accuracy of FWI imaging heavily depends on the quality of the initial velocity model, which remains a critical challenge in GPR-based tree trunk analysis. This study presents an integrated approach combining ultrasonic tomography and FWI to enhance the detection and imaging of internal decay within tree trunks. Ultrasonic tomography is employed to generate a refined initial velocity model for FWI by analyzing variations in propagation velocity of ultrasonic wave through the tree trunk, thereby improving the accuracy of inversion results. The proposed method is validated through laboratory and field experiments, demonstrating its effectiveness in detecting and characterizing internal decay defects. Compared to conventional ultrasonic tomography, the integrated approach provides significantly improved imaging resolution and accuracy. These findings highlight the potential of complementary non-destructive testing (NDT) techniques for precise tree health assessment, offering a reliable solution for urban tree management and environmental monitoring.