Drone-mounted GPR surveying: flight-height considerations for diffraction-based velocity analysis

Abstract

Recent studies highlight the potential of the drone platform for ground penetrating radar (GPR) surveying. Most guidance for optimising drone flight-heights is based on maximising the image quality of target responses, but no study yet considers the impact on diffraction travel-times. Strong GPR velocity contrasts across the air-ground interface introduce significant refraction effects that distort diffraction hyperbolae and introduce errors into diffraction-based velocity analysis. The severity of these errors is explored with synthetic GPR responses, using ray- and finite-difference approaches, and a real GPR dataset acquired over a sequence of diffracting features buried up to 1 m in the ground. Throughout, GPR antennas with 1000 MHz centre-frequency are raised from the ground to heights < 0.9 m (0-3 times the wavelength in air). Velocity estimates are within +10% of modelled values (spanning 0.07-0.13 m/ns) if the antenna height is within ½ wavelength of the ground surface. Greater heights reduce diffraction curvature, damaging velocity precision and masking diffractions against a background of subhorizontal reflectivity. Real GPR data highlight further problems of the drone-based platform, with data dominated by reverberations in the air-gap and reduced spatial resolution of wavelets at target depth. We suggest that a drone-based platform is unsuitable for diffraction-based velocity analysis, and any future drone surveys are benchmarked against ground-coupled datasets.